High RPM's on start up

[boomdriver]

EFI sensor = TPS (throttle position sensor)?

 

[Nick90vetteguy]

.....I'm soo lost with the post !!!:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W:W

 

[joshwilson3]

1. Did you reset the ECM?
2. Did you reset the IAC after resetting the ECM?
At startup, the ECM drives the IAC to move 160 steps. Normal hot idle IAC steps is around 20 for reference. ECM then uses targeted idle speeds based on several input variables (resistance in the coolant sensor is one of them). Even if the coolant sensor was reporting resistance for 0*F on a 80*F day, the ECM never targets the high level of RPMs being reported. However, if the IAC is sticking, the ECM may open it to 160 steps, yet it can't close fast enough when instructed by the ECM. Having scanner data would really help isolate this. I've given up on cleaning IACs, I replace them when rebuilding/cleaning throttle bodies.

I'm gonna work on those checks and resets over the next few weeks while I also look for vacuum leaks. Then I'll drive it around for a while to let the ECM reset itself and see what that does.

Though, it may could be the IAC. As when I had the throttle body off a year ago, the IAC was clean on the inside. So may have been replaced at some point by the PO. But someone mentioned putting some lithium grease on the needle thing of the IAC while it was out to help keep it from sticking. So, I put some of that on there. So maybe that could of gumed up or something a year later when it sat for a month.

 

[TedC]

Don't waste your time trying to clean or lub the IAC. It may look fine, and it may look like it just needs a cleaning, but there is no easy way to test how it operates in responsiveness and stepping accuracy. If you want a smooth running TPI, a properly setup TB is critical. The TPS needs to set up right with voltages smooth as silk when the plates open and close. Most times the TPS is checked at the sensor connectors at idle and WOT. This is quick way to setup and test a TPS, but leaves open other possible TPS related issues. The ECM can't control idle properly unless the IAC is set properly. Minimal air speed must be checked first. It provides the base engine rpm (around 475 RPM) that the ECM adds additional RPMs to. The ECM manages the RPMs by controlling the IAC. This is the most over looked area in properly setting up the TB. On a stock L98, if the min air speed is set properly, and the timing is set properly, the number of IAC steps is around 20 with a fully hot engine at idle. You need a scanner to get this infomation. I worked on a throttle body that was showing 0 steps because the min air speed was set too high. Idle quality was crap because even with the IAC fully closed the ECM couldn't bring down the overall idle speed to spec (someone screwed around with the min air speed adjustement and had carefully reinstalled the cap so even the dealership didn't check it out properly). I feel your issue is in the electronics, not a vacuum leak. Without scanner data, I'm only guessing. Get yourself an OBD-I scanner on ebay (around $100), and master the TB setup. Until I did this myself, I didn't feel confident with working on L98 engine management.

 

[ex-x-fire]

GMs are known to act funny w/ bad grounds, there probably a couple on the heads at the firewall. A FSM will help you find em. Really all you need to do on those hidden ones is loosen & retighten them(unless corroded badly).

 

[boomdriver]

GMs are known to act funny w/ bad grounds, there probably a couple on the heads at the firewall. A FSM will help you find em. Really all you need to do on those hidden ones is loosen & retighten them(unless corroded badly).

The one (ground) that you;re interested in is on the right side head just below the valve cover...almost directly under the wiper motor, east of the oil pressure sender. Kinda hard to get at without removing something.

 

[joshwilson3]

Don't waste your time trying to clean or lub the IAC. It may look fine, and it may look like it just needs a cleaning, but there is no easy way to test how it operates in responsiveness and stepping accuracy. If you want a smooth running TPI, a properly setup TB is critical. The TPS needs to set up right with voltages smooth as silk when the plates open and close. Most times the TPS is checked at the sensor connectors at idle and WOT. This is quick way to setup and test a TPS, but leaves open other possible TPS related issues. The ECM can't control idle properly unless the IAC is set properly. Minimal air speed must be checked first. It provides the base engine rpm (around 475 RPM) that the ECM adds additional RPMs to. The ECM manages the RPMs by controlling the IAC. This is the most over looked area in properly setting up the TB. On a stock L98, if the min air speed is set properly, and the timing is set properly, the number of IAC steps is around 20 with a fully hot engine at idle. You need a scanner to get this infomation. I worked on a throttle body that was showing 0 steps because the min air speed was set too high. Idle quality was crap because even with the IAC fully closed the ECM couldn't bring down the overall idle speed to spec (someone screwed around with the min air speed adjustement and had carefully reinstalled the cap so even the dealership didn't check it out properly). I feel your issue is in the electronics, not a vacuum leak. Without scanner data, I'm only guessing. Get yourself an OBD-I scanner on ebay (around $100), and master the TB setup. Until I did this myself, I didn't feel confident with working on L98 engine management.

I appreciate it. I'll do the checks and resets first and drive it around for a while and go from there. I do know that tab is missing for the idle air screw, so that has been messed with.

When I had the IAC off last year, it looked new. So the PO must of had that replaced at some point as I was expecting it to be carboned up.

Autozone has a rental scan tool, but it is OBD-II. Dang. I'll look around for a cheap OBD-I scanner.

 

[TedC]

I picked up an AutoXray 3000 on ebay for less than $90 including shipping. It was brand new. Also included the SW so I could upgrade the firmware. It has paid for itself several times.

 

[joshwilson3]

I picked up an AutoXray 3000 on ebay for less than $90 including shipping. It was brand new. Also included the SW so I could upgrade the firmware. It has paid for itself several times.

Is the kind of scanner you are talking about the one where you just read it off the LCD screen, or does it need to hook up to a laptop? As I don't have a laptop right now.

 

[TedC]

Handheld unit. I only used a laptop to upgrade the firmware.

 

[joshwilson3]

Handheld unit. I only used a laptop to upgrade the firmware.

I read into scanning the counts for the IAC. And learned the counts you read come from the ECM. So, you don't actually see what the counts the IAC is actually doing, just how many counts it is being told to do.

So, if you are reading the counts from the ECM, how do you know the IAC is or isn't sticking since you can't read what it is doing?

It looks like to me the IAC is one of those you have limited tests for, and just have to replace it and hope it fixes the problem. I do plan on ohming it as I found you can do some ohm tests. And if that fails, then you know it is bad. Other than that, I'll do the resets and if there still is the high rev. I'll pull the IAC and clean it and go from there.

 

[TedC]

I read into scanning the counts for the IAC. And learned the counts you read come from the ECM. So, you don't actually see what the counts the IAC is actually doing, just how many counts it is being told to do.

So, if you are reading the counts from the ECM, how do you know the IAC is or isn't sticking since you can't read what it is doing?

It looks like to me the IAC is one of those you have limited tests for, and just have to replace it and hope it fixes the problem. I do plan on ohming it as I found you can do some ohm tests. And if that fails, then you know it is bad. Other than that, I'll do the resets and if there still is the high rev. I'll pull the IAC and clean it and go from there.

BINGO!!! That is why I don't waste any time trying to clean them. If the TB is removed for cleaning, then I replace both the TPS and IAC. The ECM depends on a perfectly working TPS and IAC. When a L98 TB is set up properly, man it runs great. Don't forget, engine timing and min air speed also play an important role in this. The ECM is programmed expecting some specific base settings that it has no control of, yet C4 owners like to tinker with these settings thinking they can out smart the ECM or they just don't know any better.

 

[joshwilson3]

BINGO!!! That is why I don't waste any time trying to clean them. If the TB is removed for cleaning, then I replace both the TPS and IAC. The ECM depends on a perfectly working TPS and IAC. When a L98 TB is set up properly, man it runs great. Don't forget, engine timing and min air speed also play an important role in this. The ECM is programmed expecting some specific base settings that it has no control of, yet C4 owners like to tinker with these settings thinking they can out smart the ECM or they just don't know any better.

I'm gonna set the base idle to 450 rpm. Though I've read some mention 550 rpm.

 

[TedC]

This is where a scanner comes in really handy. You want around 20 IAC counts with a fully hot engine at idle. With the scanner you can watch the counts and RPM. On stock L98s, with timing set to spec, you should end up with 20 IAC counts with RPM adjusted between 450-500 RPMs. If you can't acheive these levels then something else could be wrong. A vacuum leak cause by worn throttle shafts can be detected. Just remember that when you crank open the throttle plates to increase min air speed, then the ECM will reduce IAC counts when you have the IAC reconnected. DO NOT USE dash tach to set min air speed. I think GM issued a FSB stating min idle speed could be set higher. I've worked on several L98 with intermittent stalling. Once we cleaned up the TB, installed a new TPS and IAC, set timing and set min air speed for 20 IAC counts, the difference was night and day. One C4 had the TPS set too low at WOT and would only reach 3.5 volts using th egas pedal. I reset timing to spec, checked min air speed, and played with the TPS until I got idle voltage set right and 4.6 volts at WOT. That vette with auto and 2.59 rear axle could now burn rubber.

 

[joshwilson3]

This is where a scanner comes in really handy. You want around 20 IAC counts with a fully hot engine at idle. With the scanner you can watch the counts and RPM. On stock L98s, with timing set to spec, you should end up with 20 IAC counts with RPM adjusted between 450-500 RPMs. If you can't acheive these levels then something else could be wrong. A vacuum leak cause by worn throttle shafts can be detected. Just remember that when you crank open the throttle plates to increase min air speed, then the ECM will reduce IAC counts when you have the IAC reconnected. DO NOT USE dash tach to set min air speed. I think GM issued a FSB stating min idle speed could be set higher. I've worked on several L98 with intermittent stalling. Once we cleaned up the TB, installed a new TPS and IAC, set timing and set min air speed for 20 IAC counts, the difference was night and day. One C4 had the TPS set too low at WOT and would only reach 3.5 volts using th egas pedal. I reset timing to spec, checked min air speed, and played with the TPS until I got idle voltage set right and 4.6 volts at WOT. That vette with auto and 2.59 rear axle could now burn rubber.

Oh, I planned on using the dash tach to set min air speed. So, I guess I need to skip that step and just do the other reset/checks? I've read other articles before on setting min air speed, and didn't see anything that you need something special to set it as I say most must use the dash tach.

 

[boomdriver]

I bought a new IAC motor a couple months ago....$35 at the local parts store. Just make sure the seat is clean in the TB and get all the old gasket off the TB so the new IAC screws in and seals. The pintle should be set in the range that it needs, but its worth a quick check...its just a measurement from the IAC seat to the pintle,. it comes with a diagram of how to do this. Basically the pintle shaft screws in/out with the motor. If too long when mounted it can be damaged and never work right. If too short, it will never seat and there will always be an air leak in the idle circuit. The base range is around 1/4" so its easy to get it set close enough for the motors stroke.

If your TB has a coolant bypass, another place to look for an air leak is in the TB bottom coolant chamber. That has a very fine seal between idle air passages/coolant and outside air.

 

[joshwilson3]

I bought a new IAC motor a couple months ago....$35 at the local parts store. Just make sure the seat is clean in the TB and get all the old gasket off the TB so the new IAC screws in and seals. The pintle should be set in the range that it needs, but its worth a quick check...its just a measurement from the IAC seat to the pintle,. it comes with a diagram of how to do this. Basically the pintle shaft screws in/out with the motor. If too long when mounted it can be damaged and never work right. If too short, it will never seat and there will always be an air leak in the idle circuit. The base range is around 1/4" so its easy to get it set close enough for the motors stroke.

If your TB has a coolant bypass, another place to look for an air leak is in the TB bottom coolant chamber. That has a very fine seal between idle air passages/coolant and outside air.

No coolant bypass. When I had the throttle body off last year, I replaced the gaskets. Though the OEM gaskets were rubber, so no clean up. Just wish the new gasket set that I put on was also rubber.

I checked the TPS via foot pedal, and that checked out. I ohmed the IAC, and that was good. Reset the IAC. Disconnected battery for a while. Checked timing which was fine. And pretty much no change. So, next I'll spray the throttle body and air passage with throttle body cleaner and see what that does. If no change, then I'll see if I can pop the IAC off without having to pull the throttle body.

But I didn't think you could put the pintle in too far? I know you don't want it out too far, but when I had it off a year ago, I put it in as far as it would go. But I believe the IAC resets itself every time. So, I don't think you could put the pintle in too far.

If I recall correctly, I think you don't want the pintle any further than 1 1/8" out.

 

[boomdriver]

No coolant bypass. When I had the throttle body off last year, I replaced the gaskets. Though the OEM gaskets were rubber, so no clean up. Just wish the new gasket set that I put on was also rubber.

I checked the TPS via foot pedal, and that checked out. I ohmed the IAC, and that was good. Reset the IAC. Disconnected battery for a while. Checked timing which was fine. And pretty much no change. So, next I'll spray the throttle body and air passage with throttle body cleaner and see what that does. If no change, then I'll see if I can pop the IAC off without having to pull the throttle body.

But I didn't think you could put the pintle in too far? I know you don't want it out too far, but when I had it off a year ago, I put it in as far as it would go. But I believe the IAC resets itself every time. So, I don't think you could put the pintle in too far.

If I recall correctly, I think you don't want the pintle any further than 1 1/8" out.

I get what you're saying but yes, you can lengthen the pintle shaft too long so when the motor iis screwed into the TB, the pintle hits bottom early and gets shoved back into the motor. it is supposed to screw in, not get jammed in.
Like I said, the new one comes with a chart and a measurement. 1/8" away from the motor base I think is the correct setting. It has 1/4" in range. If the pintle is in that range, its safe to install and the ecm will do the final; adjust. so thats pretty easy.

 

[joshwilson3]

I get what you're saying but yes, you can lengthen the pintle shaft too long so when the motor iis screwed into the TB, the pintle hits bottom early and gets shoved back into the motor. it is supposed to screw in, not get jammed in.
Like I said, the new one comes with a chart and a measurement. 1/8" away from the motor base I think is the correct setting. It has 1/4" in range. If the pintle is in that range, its safe to install and the ecm will do the final; adjust. so thats pretty easy.

I'll probably end up pulling the IAC out to check it out. Is it still okay to shoot the pintle out of the IAC after you pull it by grounding the diagnostic thing inside the car?

That's what I did last time when I cleaned it as I've seen it written that way. Then you rock the pintle back in.

But then I have seen some people say not to do that, but just pull it as is and spray it. But I had put some lithium grease on the pintle last time, so may be good to shoot the pintle out and clean that off.

 

[boomdriver]

I'll probably end up pulling the IAC out to check it out. Is it still okay to shoot the pintle out of the IAC after you pull it by grounding the diagnostic thing inside the car?

That's what I did last time when I cleaned it as I've seen it written that way. Then you rock the pintle back in.

But then I have seen some people say not to do that, but just pull it as is and spray it. But I had put some lithium grease on the pintle last time, so may be good to shoot the pintle out and clean that off.

If I remember right, you should not try to run the pintle out with the motor when its removed because it needs ecm signals to end its travel. Since it has limited range of adjustment, it can be hard to get it back in the right place. The last one that I messed with had to have the pintle pressed inward then twisted to adjust it in/out. If memory serves, there is nothing to be gained by running it out. All you care about is the pintle face & seat. A good soaking in carb cleaner or spray can do damage to the motor. If its that dirty and crudded up, its worth replacing. The FSM states that the proper measurement for instalation is 28mm or 1-1/8" from IAC motor gasket surface to pintle tip. They also point out that there are 3 pintle designs and the correct one must be used when replaced.
Seeing if the motor is working properly can only be seen with a scan (to see if the signal is there) then by actual engine operation. Since its a pretty precise little engine control it needs to be as near perfect as a new car.

One simple test of the IAC is to get a steady idle established if possible, then turn on the a/c.... then off after a few seconds. The a/c should cause the engine rpm to stay steady or even go up 50 rpm. That says the IAC is making the adjustment and reacting. When the a/c is turned off the rpm should drop a bit or remain steady,. When a/c lowers the rpm, that shows that there is little or no IAC circuit adjustment. That was the first tip that my old one was bad. It should make noticable idle changes when a/c is turned on,or when an auto-trans goes into gear or anytime there is a drag on the engine at idle that might lower it more than the ECM minimum, OR whatever the min set screw holds the plates at. IMHO the set screw should be backed off the throttle link enough so the ecm has the control of the airflow which must go thru the idle circuit so that other sensors and adjustments will be correct.