Help! Rough Idle--intermittent

[FForward]

Hi Everyone,

I'm new here posting, but have been reading all yours hoping to find a solution. I have an '89 auto that started running rough and surging. It threw a code 32 and I changed the EGR, no change at all. No codes either after changing it. I took it to a few shops and got no where. I replaced the whole fuel system, pump, filter, and injectors along with a new relay. My fuel pressure is right on and steady. I also replaced spark plugs, wires, complete distr., computer and chip. Everyone that has checked it said it was all up to specs. No one could find anything that was off. Plus there are no codes present. Gas mileage is below normal, but it runs great, and just has that little miss and surge. When it was put on a scope it showed little spikes between the larger ones. I was told it was running lean also. Some days it seems worse than others. Fuel system is new, ignition is new, vacuum lines are new, not much else to replace. It does it in both open and closed loop, so I know it can't be sensors. I did notice the air coming out of the exhaust tips seem to be less than I would think. My truck has more flow. I don't think the cats are clogged because it will rev and no loss of power at WOT. It has me and several mechanics baffled. I know several people on here have similar problems as mine, but haven't seen where they solved the problem. Could the ESC be the problem, or the MAF? I am at my wits end with this. I'm afraid to drive it any distance for fear it will quit on me and not restart. That only happened once and it was the injectors at fault. Its starts good and runs good, but is not right. Stopped at a light it will miss slightly, then surge, it feels like someone bumped me in the rear when it does. Hasn't quit on me but I don't trust it. Can anyone shed some new light on this, or have any suggestions. Gm mechanics are unable to find it, hope you guys can help me.....thanks for any info you might have....your my last resort. I know you guys are very good at solving problems and this is a great forum.

 

[John Robinson]

My FSM on a code 32 also says that an O2 sensor can throw a code 32. Also the TPS in reading my manual can cause a rough idle. You have replaced almost everything in the way of sensors but you do not say anything about cleaning the ground points. These cars are very sensitive to having clean grounds.

The ECM can only respond to the signals it gets from the sensors. If the O2 sensor tells the ecm it is lean the ecm will try to richen the mixture to correct that condition. Since the ECM gets it input from the grounds then a bad ground will drive the ecm crazy. When you get a code it does not mean the part is bad it only means the ecm is not happy with that circuits input. You need to check the FSM for the correct ground voltage for the O2 and TPS sensors. If they are at spec then your problem is in the wiring if not then it is the sensor.

I would be helpful to know the year of the car and the mileage.

And Welcome to the CAC

 

[boomdriver]

Sure sounds like a vac leak...

those do not set codes, and can drive the ECM crazy trying to compensate.

Another possible cause is electrical, either grounds, or rotten wiring in the injector harness. This causes weak or cross signals that make the misfire seem to move around from one cyl to the next.

MAF is always something to consider but theres usually a code associated with that.

I would'nt pull my hair out over what the current crop of mechanics are saying...most could not decribe what actually happens inside an engine because they were only trained to read the display on the diagnostic tools. We have the FEW real techs/mechanics that exist in the Corvette world right here.

I see you changed the EGR but does it work? The EGR solenoid has to work before the EGR valve can do anything. I'd sure put a vac on the EGR while its idling and see if that had an effect on the rough idle or misfiring. Theres no assurance that a replacement part is good these days....

Have any vac lines been replaced/disturbed? Certain lines MUST go back to the exact fitting they came off of, becasue some things are "port" vacuum that should only pull at higher rpm, like the throttle body fitting to the EGR and purge can. Make sure all those are in place and no broken lines. Those hard plastic lines are real good for breaking.

 

[93Rubie]

I feel your pain, I have been hunting down some problems for a guy with a very very sick 87 Buick GN, we got 'er running the best it has ran in years but still needs more TLC and money.

Dumb question but do you have access to a scan tool and if you do what do the IAC counts look like?
Also, is the throttle body been cleaned as of recent and/or the IAC? A carboned up IAC could cause this problem.

Also, check your PCV system for proper operation, especially the PCV Valve. If you pull it off at idle, and put your finger over the end it should snap back.

Take heart anything can be fixed, time, tools, knowledge, and maybe some parts can fix all.

 

[vetteboy86]

Since the issue happens regardless of open or closed loop, the cause is common to both modes of operation.

What information is used for open and closed loop? TPS is a possibility, they do ware, and if the problem is happening at idle it's possible there is a spot on the wiper that is worn out. This would show on the scans though. How well were the scans looked at? You mentioned the EST, what did the timing look like on the scan.

My thinking is that a vacuum leak would cause the surge and idle issues all the time, rather than intermittently. Since the issue occurs in open and closed loop, the trigger occurs when the motor is cold or warm.

Like has been already stated, there are a lot of folks here willing and able to contribute.

 

[FForward]

My FSM on a code 32 also says that an O2 sensor can throw a code 32. Also the TPS in reading my manual can cause a rough idle. You have replaced almost everything in the way of sensors but you do not say anything about cleaning the ground points. These cars are very sensitive to having clean grounds.

The ECM can only respond to the signals it gets from the sensors. If the O2 sensor tells the ecm it is lean the ecm will try to richen the mixture to correct that condition. Since the ECM gets it input from the grounds then a bad ground will drive the ecm crazy. When you get a code it does not mean the part is bad it only means the ecm is not happy with that circuits input. You need to check the FSM for the correct ground voltage for the O2 and TPS sensors. If they are at spec then your problem is in the wiring if not then it is the sensor.

I would be helpful to know the year of the car and the mileage.

And Welcome to the CAC

Thanks so much for the quick reply. The car is an '89 vert with 74k miles. I've owned it since '92 and it was always garaged when not in use. I had a heated garage I kept it in. I've done most all the work on it myself and do have some knowledge of mechanics. I don't have a FSM, I have Hayne's manual. I see from other posts that grounding points are a common problem on these cars. I know about the one by the oil filter and I will give that a look. All the mechanics that looked at it did have it hooked up to the latest equipment. Everything showed up to specs according to them. I don't know what they tested, but they did spend a lot of time looking. They also test drove it. There are no codes thrown at all. I was told it was running lean, and my thoughts went to the O2 sensor. I haven't replaced that yet, and thought maybe the mechanics would have checked it. The TPS was checked by me and the mechanics both and found to be Ok. Nothing could be found in the fuel system or ignition with all the testing. I am hoping its a simply matter of a bad ground. My wiring all looks in good shape, and so do the connectors. I can't rule out that it isn't in the wiring, but everything that can be seen or reached from topside has been gone over more than once. I will look at the ground at the oil filter for sure. Thanks for the help!!! Bill

 

[FForward]

Sure sounds like a vac leak...

those do not set codes, and can drive the ECM crazy trying to compensate.

Another possible cause is electrical, either grounds, or rotten wiring in the injector harness. This causes weak or cross signals that make the misfire seem to move around from one cyl to the next.

MAF is always something to consider but theres usually a code associated with that.

I would'nt pull my hair out over what the current crop of mechanics are saying...most could not decribe what actually happens inside an engine because they were only trained to read the display on the diagnostic tools. We have the FEW real techs/mechanics that exist in the Corvette world right here.

I see you changed the EGR but does it work? The EGR solenoid has to work before the EGR valve can do anything. I'd sure put a vac on the EGR while its idling and see if that had an effect on the rough idle or misfiring. Theres no assurance that a replacement part is good these days....

Have any vac lines been replaced/disturbed? Certain lines MUST go back to the exact fitting they came off of, becasue some things are "port" vacuum that should only pull at higher rpm, like the throttle body fitting to the EGR and purge can. Make sure all those are in place and no broken lines. Those hard plastic lines are real good for breaking.

Thanks for the input and quick reply. I've replaced a lot of the vacuum lines and was looking for a possible leak. I didn't find any. I sprayed all the vacuum lines with water, hoping to see a change in idle. Nothing changed! I did have the car running while I pulled some vacuum lines loose. The idle dropped for a second, then went back to idling with no change. The ECM just adjusted for it. The EGR valve was replaced and is in good working order. I did pull the hose off of it and blocked it back at the solenoid to see if that was the problem. No difference at all! Thinking a bad solenoid might pull it in when it shouldn't. I'm going to check for a bad ground and see if thats the cure. One of the mechanics that looked at it and spent a day testing, was a certified Vette Tech. He was at a loss for excuses why he couldn't find it. Wants me to give him the car for a few days so he can hook up all the gauges and drive it around. I spent enough money with them testing and finding nothing. I thought I'd be better off posting here and following the suggestions than listening to them. I am going to check the grounds and see what happens with that. Thanks for the suggestions and input.......Bill

 

[FForward]

I feel your pain, I have been hunting down some problems for a guy with a very very sick 87 Buick GN, we got 'er running the best it has ran in years but still needs more TLC and money.

Dumb question but do you have access to a scan tool and if you do what do the IAC counts look like?
Also, is the throttle body been cleaned as of recent and/or the IAC? A carboned up IAC could cause this problem.

Also, check your PCV system for proper operation, especially the PCV Valve. If you pull it off at idle, and put your finger over the end it should snap back.

Take heart anything can be fixed, time, tools, knowledge, and maybe some parts can fix all.

Thanks for the reply. The car was scanned and tested, but have no idea what any results were, other than the mechanic telling me everything was on the money. It was said to be running lean. I had replaced all the injectors recently because they went bad. While doing that the throttle body was cleaned along with the IAC. I reinstalled it according to the directons in the book. It does set up the idle when the A/C is turned on. The PCV works normal and most of the things that could be checked or tested has been done. The car was never abused and is always taken care of and kept in like new condition. The paint, interior, and top are still the orignal. The undercarriage is spot less. Its an awesome car, drives like new, handles great, and usually ran great untill this happened. I will not give up till I get get it running great again. Love the car!!! Thanks for the suggestions and input. This forum is great!

 

[FForward]

Since the issue happens regardless of open or closed loop, the cause is common to both modes of operation.

What information is used for open and closed loop? TPS is a possibility, they do ware, and if the problem is happening at idle it's possible there is a spot on the wiper that is worn out. This would show on the scans though. How well were the scans looked at? You mentioned the EST, what did the timing look like on the scan.

My thinking is that a vacuum leak would cause the surge and idle issues all the time, rather than intermittently. Since the issue occurs in open and closed loop, the trigger occurs when the motor is cold or warm.

Like has been already stated, there are a lot of folks here willing and able to contribute.

Thanks so much for the reply. Yes it occurs in both modes, and somedays its not so bad, then other days is worse. Its been scanned at two different shops, one by a Vette Tech and neither place could find anything. Everything within specs. The results weren't seen by me, but I was told it was running lean and thats all they could tell me. I did check the TPS with a digital meter and its smooth thru its range, no flat spots. Everyone seems to mention the ground wires and I haven't checked them yet. Everything topside has been gone over more than once by different people including me. It could be a ground under the car or maybe a bad wire. I thought maybe the ESC might be causing the problem or the O2 sensor causing it to run lean. The mechanics said they didn't see anything wrong there either. Timing is set to factory specs and is steady with a light on it. Has us all stumped, as we went over all the possible and obvious things. I will be checking the grounds and hopefully that will make a difference. Oh, I did check for vacuum leaks, I replaced a lot of them and sprayed water on them to check for leaks. I posted here because I have been reading the other posts and seen the help that was available here. I really enjoy this forum and the people here. I plan on contributing in the future, with some of my experiences. Thanks for the suggestions and input....its well appreciated.......Bill

 

[John Robinson]

There is another ground to check also. If you follow the ground wire form the TPS back into the wiring harness it will come to a bundle of ground wires that are all connected together. This is were the grounds from the TPS, Maf, Water pump temp, and a couple of others are all connected in a bundle. On my 93 mine was wrapped in tape and over the years it had corroded the connection and was interfering with the ECM being able to get good signals from those sensors. Just for fun the next time the car is acting up try moving all the wires that go to the sensors and see if the engine changes how it is running.

While doing the grounds don't forget to do the two on each side of the engine on the frame rails and then take the battery out and do the two hidden behind it.

From a personnel note on my 93 last year I went through a period of my car not running well. I found the wires to the TPS were broken and after I spliced and covered them with heat shrink I then found I had bad pins in the connector so I went to a salvage yard and got wire and connectors from some cars there. I then removed the wires form the salvage and there pins and installed them in my connectors and then I went back into the wiring harness to connect them with solder and heat shrink. If you look at the size of those wires they are very small and with time and heat and vibration then will break.

 

[John Robinson]

Tps

I have posted this several times over the years and some of the regulars are probably tired of my beating this drum all the time but perhaps this will help you understand the function of the tps and the importance of the grounds to this whole electrical system

Hope this sheds some light on the tps for you

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​

Throttle Position Sensor (TPS)
Copied from 1993 Service Manual
The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts
from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

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​

 

[LT4man]

I have posted this several times over the years and some of the regulars are probably tired of my beating this drum all the time but perhaps this will help you understand the function of the tps and the importance of the grounds to this whole electrical system

Hope this sheds some light on the tps for you

---

Throttle Position Sensor (TPS)
Copied from 1993 Service Manual
The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts
from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

---

John, you are right on!

SAVE the gosh darn WAVE!

 

[FForward]

Thanks for all suggestions and I will closely look at the grounds and TPS again. I'll do that this weekend and get back here with what I found. Thanks All!

 

[FForward]

Rough Idle and surging

Hi Everyone,

I put the car up on ramps and looked for the grounds underneath. By the oil filter, all I could find was a ground strap all by itself. I also couldn't find the ones from the frame rails on either side. I admit, I couldn't look around that much for them. We have had some bad rain storms here everyday, so I'm limited by the weather. I have a garage where the car is kept all the time, and usually work on it in there. I can't put the car up on ramps in the garage, because backing off the ramps they slide across the garage floor. I can only do it in the driveway. I will do a more thurough job looking as weather permits. I went over all vacuum lines again using the propane method and am convinced its not a vacuum leak. I also went over the plenum, runners and intake with the propane, nothing changed. I also went and wiggled all the wiring to the sensors and all the other wires, nothing changed. Because it misses in open loop as well as closed loop, would elimanate some sensors.....right? Which ones are active in open loop? Because the car is sensitive to grounds and the ECM reads the signals from the sensors to a ground, can't each sensor be resistance checked at the sensor connector to a grounding point. If there was any resistance between the connector ground lead and ground, that would indicate a bad or poor ground for that sensor. Effectively checking that wire its entirety, without going to the actual ground point, which is hard to get at or find. I would just need to know which wire on a sensor is a grounding wire. If there was resistance in that way of testing, it would mean either a bad ground or wire. If other senors that are connected to the same grounding point have resistance also at their grounds, that would mean the ground point is bad not the wiring. Does this sound reasonable? The TPS also being suspect for wear or a flat spot or anything abnormal would have showed up when tested. It lightly surges and slightly misses at all throttle positions when crusing at different speeds. It doesn't seem to miss or hesitate when accelerating. Just when cruising or idling at a steady rate is it detected. Fuel pressure is steady and constant and the fuel system checks out good. The pressure regulator for the fuel system has been disconnected to boost the running pressure for the upgraded injectors. The Multitechs were replaced with Bosch Gen. 3's which work better with the higher pressure than the normal pressure. The EGR is new and is working the way it should. I have applied vacuum to the valve and noticed the drop in idle, so I know its not stuck open. The ESC is orignal and I may replace it just to elimanate it. When the car was hooked up to a scope, the mechanic showed me a small spike between the larger spikes. He said it was caused by a bad distributor. I replaced the plugs, wires, and complete distributor and all the conponets for it. Nothing has solved the missing or surging. At one point before I started changing parts it ran worse than what it does now. So some improvement was seen. I have been driving it around and letting the ECM learn and adjust itself after the battery was disconnected. No improvements so far and the condition still exists after many starts and miles. Driveability isn't bad at all and I could live with the slight miss and surge, but gas mileage is also way lower than normal. I was getting about 17mpg city, now its 14.5 to 15mpg. That might be due to the new injectors. The fuel mixture is lean and I wonder about that. Maybe the O2 sensor is dirty or something, but don't beleive its in circuit in open loop.. Its been stumping many good mechanics, one a Vette tech at a speciality garage. I hope this gives you guys more information to think further and maybe help me still with other ideas. Thanks again for all the responces I have gotten so far....Bill

 

[John Robinson]

I believe you have a problem with an O2 sensor. You said that your mileage has dropped off and that is the same thing I noticed with my 93. Mine went from 17 in town to 10 or 11 but the car started and went through all the loops. When I replaced it the mileage went back up to 16.5. Currently my mileage has dropped off a bit and I will now have to replace the O2 on the other side.

You still need to clean the grounds as they have not been eliminated as a source of your problem. As you said all the things you have done have shown some improvement and that just tells me that it was time for a good engine tune.



Hope this will help you find the grounds. I think you should start with the ones under the hood and not worry about the ones in the interior parts of the car.

 

[G Winter]

I thought I read that you had installed new injectors. They shouldn't go bad that quickly if you have. I replaced my injectors with the Bosh III from Fuel Injector Connection Both Jon and his son Jon are great. My car never ran as good as it does with those injectors.
What you discribe actually sounds a little like plugs or wires but you did state the scans indicated a lean. Be sure you don't have any play in the throttle body shaft. You can only check this with engine off, forward and rearward is the direction on wear.


Glenn

 

[FForward]

Thanks for the reply!

I don't need a regular tune up....everything was replaced. New cap, new dist, new wires, new plugs....all new. Only thing that was carried over was the coil. I'm thinking it could be the O2 sensor also, but is that in the circuit when the engine is in open loop? I know the O2 sensor is used for mixture control and mine is running lean. Could the O2 be good so it doesn't throw a code, but maybe dirty. It happens in both open and closed loop. Thanks for the diagram, I saved it and will use it to help locate the grounds. Is the '89's any different in there grounds that other years? I looked by the oil filter and couldn't find only a ground strap. I didn't have much time to play with it this weekend, but will spend more time on it during the week ahead. Again thanks for your help....

 

[FForward]

I thought I read that you had installed new injectors. They shouldn't go bad that quickly if you have. I replaced my injectors with the Bosh III from Fuel Injector Connection Both Jon and his son Jon are great. My car never ran as good as it does with those injectors.
What you discribe actually sounds a little like plugs or wires but you did state the scans indicated a lean. Be sure you don't have any play in the throttle body shaft. You can only check this with engine off, forward and rearward is the direction on wear.


Glenn

Hi Glenn,

Yes I did install new injectors the same ones you have from FIC and Jon is great to deal with. My car ran better than ever with them. I talked to Jon after installing them and having the same problem thought maybe I got a bad one. He sent me a whole new set even tho he said he was sure they were good. I swapped them out with no difference, so I know the injectors are good. The whole fuel system is new, the ignition is all new except for the coil and ESC. I only have 74k miles on the car. What ever is causing the slight missing and surging is not anything obvious, as everything has been gone over not only by me but 2 mechanics. One of them is factory trained on Vettes. After all the testing, everything was found to be right to specs. Only thing found was a lean mixture and a small spike between the large spikes on a scope. That was suppose to be from a bad dist. or so we thought. A new dist was put in with all new parts, but hasn't been back on the scope since then. It has me and the mechanics stumped. Thats why this forum is my only hope. Thanks for the reply....much appreciated.

 

[John Robinson]

Thanks for the reply!

I don't need a regular tune up....everything was replaced. New cap, new dist, new wires, new plugs....all new. Only thing that was carried over was the coil. I'm thinking it could be the O2 sensor also, but is that in the circuit when the engine is in open loop? I know the O2 sensor is used for mixture control and mine is running lean. Could the O2 be good so it doesn't throw a code, but maybe dirty. It happens in both open and closed loop. Thanks for the diagram, I saved it and will use it to help locate the grounds. Is the '89's any different in there grounds that other years? I looked by the oil filter and couldn't find only a ground strap. I didn't have much time to play with it this weekend, but will spend more time on it during the week ahead. Again thanks for your help....

On my 93 there are (if my memory is right) three wires on the bell housing ground post. I can not say for sure but I believe the 89 would be the same. The flat ground strap is the ground for the ECM.

 

[FForward]

I had a chance today to do a little more on the car . I did check the throttle shaft for play and found some slight movement. When I checked for a vacuum leak prior with propane, I also did it around the whole the throttle body and shaft. Nothing changed the idle while doing it. Wouldn't a leak at the shaft be compensated for by the ECM? I know when I pulled a vacuum hose loose from the fuel regulator the idle dropped for only a split second then went back to regular idle again. The ECM compensated for the extra air intake I beleive. I still had the slight miss and surge while the hose was removed and back at normal idle. I also checked the wire at the TPS that goes to ground. From the connector pin to a good engine ground. There by checking its continuety and resistance to ground. I had only minamal resistance and attribute that to the wire itself. I'm thinking all the grounds from other sensors that are in that harness must also be good if they are gounded at the same point. The other grounds I haven't got to yet, the ones that are on the bell housing, I'm going to replace the o2 sensor and will have to get it back up on the ramps for that and will get to those other grounds that are on the bell housing. I can't see or reach them from topside. Thanks for the replys and keep them coming, I appreciate them all....Thanks Guys.