92 LT1 Rough Idle (Consistent)

[92BlackVette]

152,000 Miles - No new changes - Almost all OEM

Condition: Engine shakes, and feels rough, not noticeable when cold or <100 degrees F

1.) Checked Ohms on Fuel Injectors: 23,23,23,23,24,23,23,24
2.) Fuel pressure 40 lbs static, > 42 lbs dynamic (increases with throttle)
3.) No Codes / Hyper-chip ThemoMaster Power Chip <-- 50K miles on it
4.) No arcing on wires at night, Spark plugs - not removed
5.) RPMS do not lobe at idle, they are 100% consistent at 700 RPMs
6.) At Highway speeds there are no noticeable problems (700 -2.5K RPMS).
7.) Spark plugs/wires changed 52K miles ago, Bosch (Platinum) type.
8.) Started 3 days ago, and the engine never dies.
9.) With Engine turned off, it takes 10 minutes to drop to 30 lbs of fuel pressure
10.) Coil, Optispark are 100% OEM
11.) The tone of the exhaust changed around the time the rough idle condition started. I run no mufflers and always have, so I am use to the "not hearing a cell phone issue" inside the cab with this setup. The tone changed, and I can tell; I drive this car daily! The best way to explain it, is it went from a Corvette with no mufflers sound (cut offs), to a 2006 Dodge Ram 5.7 exhaust tone, or a LT1 Corvette w/ no cats, no resonator, and no mufflers exhaust tone. - No exhaust sounds or rattling from cats. This is only noticed when RPMS are between 1.5K - 3K RPMS, not during idle
12.) There is a noticeable smell of gas from the exhaust, like it is running too rich, but that has been going on for longer then 3 days, more like 6 months.
13.) Fuel regulator diaphragm shows no traces of gas, nor do any vacuum hose on the right bank

*As you can tell I have been searching the forum---->;help

 

[Schrade]

You done your homework - that's for sure... and your numbers are TOO good for 150k.

Injector resistance is high - cold or hot reading. Not OEM injectors? Fuel pressure should be lower for 150k, unless the pump is hi-capacity, or adjustable FPR?

Any specs on the chip? Like longer pulsewidth? Most chips start there...

Opti has a low and high resolution signal. If the high res is 'going blind' from age, it might be doing a bad job lighting the fire without a code. How's the running temps? Any cooler?

Some DTC's won't light SES. Stick a paper clip in the ALDL anyway, watch it cycle through the modules...

Idle sound like mild cam with low rumble?

 

[92BlackVette]

You done your homework - that's for sure... and your numbers are TOO good for 150k.

I read every post I could before submitting, and I am here daily.

Injector resistance is high - cold or hot reading. Not OEM injectors? Fuel pressure should be lower for 150k, unless the pump is hi-capacity, or adjustable FPR?

The Ohm reading was taken when the engine was hot, too hot to hold your forearm against the alternator (~150 degrees F). The entire fuel delivery system is 100% OEM, including the objects above. Would you recommend a reading when the engine temp reads 55 F? - Typical starting temp.

Any specs on the chip? Like longer pulsewidth? Most chips start there...

The chip will be removed for troubleshooting (currently installed), but Hyper-tech doesn't offer details about technical specifications. Here is the link to the exact chip installed: http://www.ecklers.com/product.asp?pf_id=26957&dept_id=1750

Opti has a low and high resolution signal. If the high res is 'going blind' from age, it might be doing a bad job lighting the fire without a code. How's the running temps? Any cooler?

The temperature on the highway (70 MPH) is between 178 - 182 F. The car isn't moved in the morning until the temperature is > 100 F. Of course in hardcore city drive < 192 F.

Some DTC's won't light SES. Stick a paper clip in the ALDL anyway, watch it cycle through the modules...

For some reason, with the chip installed, if I try in check the SES codes (SES light is always off), the secondary fans run to full speed and codes fail to blink. I use this handheld device that does the same thing as the paper clip (bought before I new about the paper clip) which came with a book.

Idle sound like mild cam with low rumble?

The idle feels like you mentioned above. However between 1.5K - 3K RPMS, it sounds like my cats go missing all the sudden. To be honest, I welcome the new tone but somethings changed! And I didn't do it either. The only performance mods I have are no mufflers, K&N, Chip.

Tonight as I was checking the fuel pressure while the car was on, and sitting on the passenger tire, I could feel the roughness reverberating through the tire. Kind of reminds me of a slight miss. Because the exhaust is so loud you really can't hear it do this, rather when you sit in the car you would feel it.

 

[92BlackVette]

I started the car at 65 F and warmed up to 100 F like always, drove it up to 175 F. The car idled and sounded 100% normal in terms of the exhaust tone. There was no gas smell. I let it sit all day yesterday for the first time in the three days since it started.

So here is how I know for sure the car is idling rough, the exhaust tone will change between 1.5K - 3K RPMS (I rarely go above 3K), regardless for speed. The tone is relavent to RPMs not speed and sounds more deep, like the car is running 3in straight exhaust.

Once the issue starts again, the first step I will perform is to remove the chip and drive it right after to take that out of the question.

 

[Schrade]

Did you buy this car new? I've never seen injector resistance this high. You can't put that rap on the chip either, cause the chip doesn't change the coil windings in injector, and that's the impedance/Ohms readings. Sounds like 24 lbs fuel / hour injectors...

The K&N bothers me too; you sure you haven't gotten too much oil in the filter? Oil will pull onto the MAF filament, and wreak havoc on the computer air mass reading... Take the MAF out, and CAREFULLY clean the filament and clean it WELL. You can use a Q-tip, IF YOU"RE CAREFUL!!!

Does your scanner give a air mass measurement? Or a voltage equivalent? Or is it an ACTRON plug-in jumper scanner?

Putting a chip in is fine. BUT, you need to know that the burn is perfect BEFORE you put it in, WITH PCM data SAVED, and after you put in the chip, you need to have NEW data scanned and saved, so you know what the chip is doing. And that's even if you have the specs for the chip. If you don't, you got to start over again, to see waht's wrong.

Do you still have the OEM chip? If so, put it in, and stick in the paper clip, and check, AND CLEAR codes, including CCM codes procedure here.

Does your scanner do the same as the paper clip, with the chip in? Fan and all???

 

[92BlackVette]

Did you buy this car new? I've never seen injector resistance this high. You can't put that rap on the chip either, cause the chip doesn't change the coil windings in injector, and that's the impedance/Ohms readings. Sounds like 24 lbs fuel / hour injectors...

The K&N bothers me too; you sure you haven't gotten too much oil in the filter? Oil will pull onto the MAF filament, and wreak havoc on the computer air mass reading... Take the MAF out, and CAREFULLY clean the filament and clean it WELL. You can use a Q-tip, IF YOU"RE CAREFUL!!!

Does your scanner give a air mass measurement? Or a voltage equivalent? Or is it an ACTRON plug-in jumper scanner?

Putting a chip in is fine. BUT, you need to know that the burn is perfect BEFORE you put it in, WITH PCM data SAVED, and after you put in the chip, you need to have NEW data scanned and saved, so you know what the chip is doing. And that's even if you have the specs for the chip. If you don't, you got to start over again, to see waht's wrong.

Do you still have the OEM chip? If so, put it in, and stick in the paper clip, and check, AND CLEAR codes, including CCM codes procedure here.

Does your scanner do the same as the paper clip, with the chip in? Fan and all???

Did you buy this car new? I've never seen injector resistance this high. You can't put that rap on the chip either, cause the chip doesn't change the coil windings in injector, and that's the impedance/Ohms readings. Sounds like 24 lbs fuel / hour injectors...



I'll take pictures of all the readings when the issue returns....



The K&N bothers me too; you sure you haven't gotten too much oil in the filter? Oil will pull onto the MAF filament, and wreak havoc on the computer air mass reading... Take the MAF out, and CAREFULLY clean the filament and clean it WELL. You can use a Q-tip, IF YOU"RE CAREFUL!!!



I give it 2/3 quick sprays on the filter when I serviced it last ~10K miles ago. I don't have a MAF sensor but I do have an air-temp sensor, which could be cleaned in the same fashion.



Does your scanner give a air mass measurement? Or a voltage equivalent? Or is it an ACTRON plug-in jumper scanner?



ACTRON plug-in jumper scanner



Putting a chip in is fine. BUT, you need to know that the burn is perfect BEFORE you put it in, WITH PCM data SAVED, and after you put in the chip, you need to have NEW data scanned and saved, so you know what the chip is doing. And that's even if you have the specs for the chip. If you don't, you got to start over again, to see waht's wrong.



Do you still have the OEM chip? If so, put it in, and stick in the paper clip, and check, AND CLEAR codes, including CCM codes procedure here.



Does your scanner do the same as the paper clip, with the chip in? Fan and all???



I may see if the chip is suppressing codes by it being installed. I am going to try to proactively remove it and check for codes with the ACTRON plug-in jumper scanner. The deal with the secondary fans worries me, but it didn't do that with the stock chip installed 2+ years ago.

 

[RP220]

Back to basics first.
Visually check to spark plugs. You may have one that is fouled or burnt. Either way you will not have a complete burn causing the fuel smell you mentioned. While your at it test the compression to make sure each cylinder is performing within range. The plug or reduced compression could cause the rough running.

If those both come up clean you may have an injector that is clogged or pissing into your cylinder. Run some fuel system cleaner through with a fuel mixture that is higher cleaner to fuel rate. Good luck finding which injector it is. I cant help there.

Next down the line is the o2 sensor. Yes this should have two. If one is out or eating shit then half your power plant will act like it has the flu.

Good luck!

 

[tmkassin]

A bad or carboned up EGR valve can cause this kind of issue. It won't always throw a code either.

 

[air1jpg]

Back to basics first.
Visually check to spark plugs. You may have one that is fouled or burnt. Either way you will not have a complete burn causing the fuel smell you mentioned. While your at it test the compression to make sure each cylinder is performing within range. The plug or reduced compression could cause the rough running.

If those both come up clean you may have an injector that is clogged or pissing into your cylinder. Run some fuel system cleaner through with a fuel mixture that is higher cleaner to fuel rate. Good luck finding which injector it is. I cant help there.

Next down the line is the o2 sensor. Yes this should have two. If one is out or eating shit then half your power plant will act like it has the flu.

Good luck!

There are 4 o2 sensors. Two sensors on each bank, one before and after the cat. If you re-install the OEM chip, your computer may through the code(s) and tell you which sensor(s) need to be replaced.

 

[Hib Halverson]

A 92 doesn't have four sensors, only two.

Did you check out the EGR system as suggested above?

Also, in what condition are all the ignition parts?

With 152,000 miles I'd look for mechanical problems, too. Do a compression test. If it turns up some questionable cylinders, back it up with a leakage test.

Also, try running three bottles of Red Line Complete Fuel System Cleaner, one bottler per tank of gas for three fill-ups.

 

[John Robinson]

A bad or carboned up EGR valve can cause this kind of issue. It won't always throw a code either.

On my 93 after I throughly cleaned the egr and was satisfied it was not malfunctioning I then ran the diagnostics from the fsm I have for the egr vacuum solenoid and found it to be bad. New part installed and the car ran much different because the vacuum was allowed to make the egr function correctly. And while I have mentioned vacuum check all the hoses and connectors for cracks or leaks with the mileage and age of the car it is possible you simply have a vacuum problem. I picked up about 3 feet of different size vacuum hose and then jumped around the vacuum hoses on the car one at a time until I found the ones that were leaking The first time I jumped the throttle after that I thought I had added 100 HP to the car.

 

[surfer93]

Test the IAC, it will go bad and can carbon up.
Your FI's should test 16-18 good and go lower when going bad, I have never heard them be so high.

 

[surfer93]

Did you pull the plugs to check for fouling?
Your fuel pressure is good, but it should not drop so fast. Have you checked the fuel pressure regulator?

 

[Schrade]

Did you pull the plugs to check for fouling?
Your fuel pressure is good, but it should not drop so fast. Have you checked the fuel pressure regulator?

Fast???

I think his FP is holding VERY well... Remember, the pressure decay rate (half-life) is INversely exponential. 10psi drop in FIRST 30 minutes = 5 psi drop in SECOND thirty minutes, 2.5psi drop THIRD thirty minutes, 1.25, ... .625, etc...

I think his is pretty good...

 

[92BlackVette]

***UPDATE***
So after two days of driving, the issue would no longer be considered consistent.

I have since removed the chip and run the vehicle for over an hour, check codes, none found.

I would like to entertain the compression checking, leak down test if the issue was still consistent but it has the attributes of an EGR/IAC/Vacuum/Spark Plug/Bad Gas

I will look into the fuel cleaner - Red Line Complete Fuel System Cleaner

How exactly do you clean the EGR? Replace it? Its $102.18 for EGR and Vacuum Soleoid, so I would like to exhaust all options before shotgunning. I have replaced both of those after a code was thrown, for each of those parts, 100,000 miles ago.


I did notice that the vacuum lines seem to pull apart from their elbows/90's effortlessly which is concerning, considering they are there to hold vaccum, no crack observed though.
I could consult the FSM if needed....

 

[KANE]

On my 93 after I throughly cleaned the egr and was satisfied it was not malfunctioning I then ran the diagnostics from the fsm I have for the egr vacuum solenoid and found it to be bad. New part installed and the car ran much different because the vacuum was allowed to make the egr function correctly. And while I have mentioned vacuum check all the hoses and connectors for cracks or leaks with the mileage and age of the car it is possible you simply have a vacuum problem. I picked up about 3 feet of different size vacuum hose and then jumped around the vacuum hoses on the car one at a time until I found the ones that were leaking The first time I jumped the throttle after that I thought I had added 100 HP to the car.

False air, that is air not calculated by a MAF or MAP and not taken into consideration otherwise by the ECM, can create a slew of driveability related issues.

92BlackVette- I'm going to go out on a limb... and ask if the car just recently started to run rough. Is that correct? If so, then I don't think wrong injectors are the case. If it was, you'd likely have had these problems from the get go. Then again, if they were wrong- they could exacerbate the current problems you are having- but they would not be a root cause if it recently started running rough.
​

Regardless, false air from vacuum lines may be worth taking a look at.

Another question- have you altered the throttle blade set screw for minimum idle air or replaced any any engine or sensor components recently?

 

[92BlackVette]

False air, that is air not calculated by a MAF or MAP and not taken into consideration otherwise by the ECM, can create a slew of driveability related issues.

92BlackVette- I'm going to go out on a limb... and ask if the car just recently started to run rough. Is that correct? If so, then I don't think wrong injectors are the case. If it was, you'd likely have had these problems from the get go. Then again, if they were wrong- they could exacerbate the current problems you are having- but they would not be a root cause if it recently started running rough.​

Regardless, false air from vacuum lines may be worth taking a look at.

Another question- have you altered the throttle blade set screw for minimum idle air or replaced any any engine or sensor components recently?

I haven't altered anything on the car short of the air cleaner, added a chip (currently removed) and removed the mufflers. All of this was done around 50K miles ago. No recent changes.

-No changes to any screws have been made.

-The vaccum lines are very questionable, and almost seem to be too loose at the connectors to hold any vaccum.

-The issue started last Monday, for the past two driving days there have been no probems, like the orginal.

-I found a lot of vacuum replacement parts at advanceautoparts.com so I will explore that option.

 

[92BlackVette]

***Update***
The car was hard to start this morning, this afternoon was worse. The car took .75 seconds this morning whereas this afternoon was like 1.0 second. [I am counting this like 1-mis-sis-sip-pi]

 

[Schrade]

***UPDATE***
So after two days of driving, the issue would no longer be considered consistent.

I have since removed the chip and run the vehicle for over an hour, check codes, none found.

I would like to entertain the compression checking, leak down test if the issue was still consistent but it has the attributes of an EGR/IAC/Vacuum/Spark Plug/Bad Gas

I will look into the fuel cleaner - Red Line Complete Fuel System Cleaner

How exactly do you clean the EGR? Replace it? Its $102.18 for EGR and Vacuum Soleoid, so I would like to exhaust all options before shotgunning. I have replaced both of those after a code was thrown, for each of those parts, 100,000 miles ago.


I did notice that the vacuum lines seem to pull apart from their elbows/90's effortlessly which is concerning, considering they are there to hold vaccum, no crack observed though.
I could consult the FSM if needed....

There's a very specific highly technical procedure for EGR servicing...

Carbon solvent sprayed directly into the vac tubes and the valve ports, and let it soak. Follow up with bong cleaner. (edit: oops; I mean pipe cleaner).

Tip:
Keep your mouth closed, and safety glasses on. When that crunchy carbon grit gets into your chewing gum, it does wonders on tartar-control.

 

[92BlackVette]

I am going to hit the EGR service manual process in book two of the FSM.