High RPM's on start up

[TedC]

A new IAC should be at the proper length, if it is not, then you adjust it as per provided instructions. No need to set it perfectly. The IAC reset procedures calibrates the IAC. You just don't want the pintle to score the TB when installing. It should be torqued down to spec with the provided thread sealant.

I would never reuse a IAC that the pintle what removed. The IAC pintle was meant to move internally, not externally. You DO NOT clean an IAC pintle by shooting it out.

 

[joshwilson3]

A new IAC should be at the proper length, if it is not, then you adjust it as per provided instructions. No need to set it perfectly. The IAC reset procedures calibrates the IAC. You just don't want the pintle to score the TB when installing. It should be torqued down to spec with the provided thread sealant.

I would never reuse a IAC that the pintle what removed. The IAC pintle was meant to move internally, not externally. You DO NOT clean an IAC pintle by shooting it out.

Ok, I guess all the write ups on IAC cleaning are wrong then. I had read for years of plugging the IAC back up after you pull it, and shooting the needle into a bag. I was just thinking of doing that to clean off the lithium grease I put on last time.

 

[joshwilson3]

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.

When I had the IAC out last year, it looked new. I let the needle shoot out per the IAC cleaning instructions that are all over the place so you can clean the entire pintle.

I'll watch it when I turn the AC on and off and see what it does. But generally, I don't notice any problems at any other time other than the RPM spike at start up. The only other problems I could think of is something around 3 times in the past I had a ruff start up where the RPM's kept wanting to die and I had to keep my foot on the gas to keep it from dying for several seconds.

I believe a scan only shows you the counts the ECM sends to the IAC. So, even if you have the counts, you still don't know what the IAC is doing. Someone had theorized you would have the same counts if you have a known bad IAC or a good IAC. So, it looks like to me that reading the counts from the ECM won't tell you anything if the IAC is good/bad or working/not working.

 

[joshwilson3]

I let it idle and get to 200 degrees. It was a steady 7XX at idle.

Turn A/C on, and the idle either stayed at 7XX or dropped to 6XX for a few seconds.

Turned A/C off and the idle either stayed at 7XX or raised to 8XX for a few seconds. Probably 3 times the idle raised to 10XX for a few seconds when turning the A/C off.

I checked my Firebird, and it pretty much does the same thing. Drops slightly when turning on the A/C, and raises slightly when turning off the A/C.

Boomer, is it possible you have it backwards? Since both of my GM cars do the opposite of what you were saying should happen when you turn on/off the A/C.

 

[boomdriver]

I let it idle and get to 200 degrees. It was a steady 7XX at idle.

Turn A/C on, and the idle either stayed at 7XX or dropped to 6XX for a few seconds.

Turned A/C off and the idle either stayed at 7XX or raised to 8XX for a few seconds. Probably 3 times the idle raised to 10XX for a few seconds when turning the A/C off.

I checked my Firebird, and it pretty much does the same thing. Drops slightly when turning on the A/C, and raises slightly when turning off the A/C.

Boomer, is it possible you have it backwards? Since both of my GM cars do the opposite of what you were saying should happen when you turn on/off the A/C.

nope...heres why..

a/c drags the idle down, a/c add drag and slows it down. a Happy IAC will bump the rmp UP when the a/c is turned on to prevent stall out from low RPM. And when the load is removed from the idlying motor, that frees up a few HP so the motor will surge up in rpm for a second as the a/c is turned off. Even as the eng raises up a few rpm when the a/c is turned off, the IAC should close to reduce the rpm when the load or the strain is removed,.

 

[joshwilson3]

nope...heres why..

a/c drags the idle down, a/c add drag and slows it down. a Happy IAC will bump the rmp UP when the a/c is turned on to prevent stall out from low RPM. And when the load is removed from the idlying motor, that frees up a few HP so the motor will surge up in rpm for a second as the a/c is turned off. Even as the eng raises up a few rpm when the a/c is turned off, the IAC should close to reduce the rpm when the load or the strain is removed,.

Ok, so should the RPM's go up or down when you shut the A/C off?

"When the a/c is turned off the rpm should drop a bit or remain steady,."

"And when the load is removed from the idlying motor, that frees up a few HP so the motor will surge up in rpm for a second as the a/c is turned off."

It looks like both quotes contradict each other as one says the rpm should drop, then the other says the motor will surge up in rpm.

When the AC is turned on, the RPM's drop a bit cause of the extra load, then the RPM's raise back up. Makes sense as the AC puts a load and bogs the motor down till the IAC adjusts and raises the RPM's back to idle.

Then when you turn the AC off, it frees up hp and causes the RPM's to raise till the IAC adjusts to lower the RPM's back to idle.

So, it looks like to me the IAC on both GM cars are working fine on AC turn on/off. Other than maybe the couple of times the RPM's went up to 10XX on the Vette when shutting AC off.

I may pull the IAC out and clean it and see what that does. If no change, then I could get a new IAC from the parts store as they have that in stock. Then see what that does. If no change, then I could just return the IAC. I'll keep an eye out on a scanner, but doesn't look like I'll find one cheap since it is OBD I.

 

[John Robinson]

Just for kicks

Next time you start it cold and it is on high idle start shaking the wireing harness around the TPS. Shake the wires as they go into the TPS and other connectors in that area and see if the rpm change. Also do a search on the location of the grounds and clean them. Be sure to do the one on the bell housing that has three ground wires going to it because one of those is the ground for the ECM.

What you have sounds to me like an intermittent ground problem or broken wire.

Finally Check for a manifold leak.

 

[TedC]

ECM can sense some loads directly (hard wired), other times it just tries to maintain the programmed targeted speed. This varies by how the prom chip is programmed. ECM prom program is dependent on model year and type (auto/manual), so I would avoid trying to compare different models unless you are sure they have the same prom program. On my 1989, the ECM bumps up idle speed in park and neutral by 50 rpm. When the A/C cycles on, it adds 5 IAC steps regardless of gear. Notice it adds steps not RPMs. It does that for a reason. Adding steps provides for a fast react time by the ECM. It adds counts but it is still using basic RPM management, so it will alter the IAC counts as required. You can watch the ECM adding and subtracting 5 counts using a scanner. Plus, the scanner tells you if the A/C is cycled on or off as well so you can verify the hard wiring from is working properly. Need to be careful that we don't compare apples and oranges. Most engine wear occurs during starting (especially cold), and I would not want my engine reving over 2000 RPMs when I start it, running the risk of burning out engine bearings. At a minimum replace the IAC and reset the ECM and IAC. The best procedure is replace the IAC, check timing, check min air speed, check TPS (this changes with min air speed adjustements), reset ECM, reset IAC all with the help of a scanner. I would be checking for throttle plate shaft wear as well.

 

[joshwilson3]

Next time you start it cold and it is on high idle start shaking the wireing harness around the TPS. Shake the wires as they go into the TPS and other connectors in that area and see if the rpm change. Also do a search on the location of the grounds and clean them. Be sure to do the one on the bell housing that has three ground wires going to it because one of those is the ground for the ECM.

What you have sounds to me like an intermittent ground problem or broken wire.

Finally Check for a manifold leak.

I don't have a high idle problem. I have a high rev on start up that lasts a few seconds while it drops down to idle.

It is supposed to rev up some on start up, but mine revs up a little too high. I believe they call it a "flare up". Only other probably was a couple times on start up when the rpms' dropped, the rpm's hunted up and down some till it went into gear.

That is the only time I notice a problem, for a couple seconds on start up. Everything else is fine, put in drive it is fine. Drive around fine. Which is why I didn't really want to throw parts at it.

 

[joshwilson3]

ECM can sense some loads directly (hard wired), other times it just tries to maintain the programmed targeted speed. This varies by how the prom chip is programmed. ECM prom program is dependent on model year and type (auto/manual), so I would avoid trying to compare different models unless you are sure they have the same prom program. On my 1989, the ECM bumps up idle speed in park and neutral by 50 rpm. When the A/C cycles on, it adds 5 IAC steps regardless of gear. Notice it adds steps not RPMs. It does that for a reason. Adding steps provides for a fast react time by the ECM. It adds counts but it is still using basic RPM management, so it will alter the IAC counts as required. You can watch the ECM adding and subtracting 5 counts using a scanner. Plus, the scanner tells you if the A/C is cycled on or off as well so you can verify the hard wiring from is working properly. Need to be careful that we don't compare apples and oranges. Most engine wear occurs during starting (especially cold), and I would not want my engine reving over 2000 RPMs when I start it, running the risk of burning out engine bearings. At a minimum replace the IAC and reset the ECM and IAC. The best procedure is replace the IAC, check timing, check min air speed, check TPS (this changes with min air speed adjustements), reset ECM, reset IAC all with the help of a scanner. I would be checking for throttle plate shaft wear as well.

I'll probably end up trying out the IAC, as if it doesn't work. I can return it. I don't think it would be a throttle plate wear problem. Had the car for several years no problem. Shut it down for a month to replace some things, then when I started driving it after that is when I noticed the high revs on start up.

It would probably be good to get a scanner if I can find one cheap. Not gonna pay a ton of money for one. I've pretty much done all the resets and checks other than the min air speed since I don't have a scanner.

 

[boomdriver]

The reason for the a/c & idle observations was to see if there was actually any IAC operation by looking for some reaction, and there IS from what you say. That tells us that the IAC is not dead, but functional. Being of limited ability, it comes down to "it works, or it don't"...Unless something crawled inside the idle air passages and made a nest, there is nothing else in there to effect the way idle works. There are no other moving parts in the idle air passages.

When the a/c is turned off the engine will surge upward but at that instant the IAC will be trying to compensate, lowering the idle speed. The ideal reaction would be a perfectly stable rpm when turning the a/c on or off, but some small bump in the opposite direction is expected since there is some degree of delay in the way it operates. A bump up of 50 rpm for 1/2 second then settling down is pretty common. Point being, the IAC IS reacting to the engines demands, otherwise killing the a/c would increase idle and it would stay there. Turning on a/c would drag the idle down and it would stay down if the IAC were not working.

Your problem with rpm spikes is likely elsewhere if the IAC is working and it was not involved in any recent repairs, IMHO. There are still dozens of possibilities...TPS, TB plates, vac leaks, manifold leaks, rich fuel, HEI timing, thats why a scan would tell where to look. Its guessing without any hard evidence.

 

[joshwilson3]

The reason for the a/c & idle observations was to see if there was actually any IAC operation by looking for some reaction, and there IS from what you say. That tells us that the IAC is not dead, but functional. Being of limited ability, it comes down to "it works, or it don't"...Unless something crawled inside the idle air passages and made a nest, there is nothing else in there to effect the way idle works. There are no other moving parts in the idle air passages.

When the a/c is turned off the engine will surge upward but at that instant the IAC will be trying to compensate, lowering the idle speed. The ideal reaction would be a perfectly stable rpm when turning the a/c on or off, but some small bump in the opposite direction is expected since there is some degree of delay in the way it operates. A bump up of 50 rpm for 1/2 second then settling down is pretty common. Point being, the IAC IS reacting to the engines demands, otherwise killing the a/c would increase idle and it would stay there. Turning on a/c would drag the idle down and it would stay down if the IAC were not working.

Your problem with rpm spikes is likely elsewhere if the IAC is working and it was not involved in any recent repairs, IMHO. There are still dozens of possibilities...TPS, TB plates, vac leaks, manifold leaks, rich fuel, HEI timing, thats why a scan would tell where to look. Its guessing without any hard evidence.

Thanks. I'm gonna keep an eye out for a cheap OBD I scanner. Kinda sucks as the parts stores will let you rent a OBD II scanner.

I'm assuming with the scanner that I would first hook it up. Then start the car and then read what numbers I get when first starting it up?

 

[boomdriver]

Thanks. I'm gonna keep an eye out for a cheap OBD I scanner. Kinda sucks as the parts stores will let you rent a OBD II scanner.

I'm assuming with the scanner that I would first hook it up. Then start the car and then read what numbers I get when first starting it up?

That, I cannot help with...Thats gonna be a "read the instructions first" kind of deal.
I've been casually looking for an OBD-I myself. You don;t see them everyday. That system is just too outdated for anyone to be real interested in mfg lots of scanners. Most shops don;t even have the proper equipment to read OBD-I anymore,. Its all been traded in and the new machines are more universal in all car mfg, but only OBD-II and up, bascially 1993(?) or 1996(?) and newer.

 

[TedC]

I'm assuming with the scanner that I would first hook it up. Then start the car and then read what numbers I get when first starting it up?

You set up the scanner (battery operated) disconnected with proper info on year, model, and engine. Then connect it to the ALDL port using one of the supplied cables (in this case the GM OBD-1 cable). Power up ECM by turning ignition key on or starting engine. Scanner will start communications (serial data) and can collect data in either mode (on or running). For your situation, I would first check the ECM data in just the on position. Making sure readings like TPS voltage and coolant temps are correct. Next I would scroll the live data screen so I could watch IAC counts and RPMs as I start the engine. You now have a view into the ECM and idle management. If the RPM is high, but IAC count sare low, this suggests a vacuum leak. Similar hookup when checking min air speed but you must follow the procedure provided because the IAC needs to be fully extended and disconnected from ECM. This way you are only adjusting the base line RPM across the throttle plates, taking the IAC out of the picture. Once you reconnect the IAC, the ECM will manage idle speed, and you can monitor IAC counts which should be around 20 fully hot. Be very careful in which scanner you purchase. As reported some scanners that claim support for both OBD-2 and OBD-1, do not work on L98s, so do your reserach before buying. No issues with my AutoXray 3000 working on L98s.

 

[MBDiagMan]

John,

Is this a high mileage car? The reason I ask is that I have driven mine to high mileage and along the way got into a similar situation as yours. It turned out to be the Throttle Body completely worn out. Once the throttle shaft and it's bearing surfaces wear, it lets the blades drag and will give all sorts of crazy idle results.

To check, simply raise the hood and when it has a high idle, push on the throttle lever and see if manually closing it puts the idle back to normal. If it does, you may very well need to have the throttle body rebuilt.

Good luck with it.

 

[joshwilson3]

John,

Is this a high mileage car? The reason I ask is that I have driven mine to high mileage and along the way got into a similar situation as yours. It turned out to be the Throttle Body completely worn out. Once the throttle shaft and it's bearing surfaces wear, it lets the blades drag and will give all sorts of crazy idle results.

To check, simply raise the hood and when it has a high idle, push on the throttle lever and see if manually closing it puts the idle back to normal. If it does, you may very well need to have the throttle body rebuilt.

Good luck with it.

92k miles.

Haha. I don't have a high idle problem at all. It idles fine and where it should. It is a start up flare that's the problem. As when you start it, the rpm's flare up some. But I'm getting a high flare up on start up. The RPM's shoot up a little too high for a few seconds when I start it, then fall back down to idle. Just a few times did it drop down then have a hunting idle till I put it in gear.

But no, I don't have a high idle problem. :W

 

[joshwilson3]

You set up the scanner (battery operated) disconnected with proper info on year, model, and engine. Then connect it to the ALDL port using one of the supplied cables (in this case the GM OBD-1 cable). Power up ECM by turning ignition key on or starting engine. Scanner will start communications (serial data) and can collect data in either mode (on or running). For your situation, I would first check the ECM data in just the on position. Making sure readings like TPS voltage and coolant temps are correct. Next I would scroll the live data screen so I could watch IAC counts and RPMs as I start the engine. You now have a view into the ECM and idle management. If the RPM is high, but IAC count sare low, this suggests a vacuum leak. Similar hookup when checking min air speed but you must follow the procedure provided because the IAC needs to be fully extended and disconnected from ECM. This way you are only adjusting the base line RPM across the throttle plates, taking the IAC out of the picture. Once you reconnect the IAC, the ECM will manage idle speed, and you can monitor IAC counts which should be around 20 fully hot. Be very careful in which scanner you purchase. As reported some scanners that claim support for both OBD-2 and OBD-1, do not work on L98s, so do your reserach before buying. No issues with my AutoXray 3000 working on L98s.

Thanks. I'm gonna try and get a scanner if I can get one at a good price.

Is there a place that tells you what your numbers are supposed to be at?

 

[TedC]

Not that I'm aware of. However, if you post your results, experts here at CAC are more than willing to share their years of knowledge.

 

[joshwilson3]

Not that I'm aware of. However, if you post your results, experts here at CAC are more than willing to share their years of knowledge.

I was looking around and saw a posting on resetting the IAC which is alot different than just holding the throttle open at start up for a few seconds. Have you seen this method?

1) With the engine off, ground the diagnostic terminal of the ALCL under the dash.
2) Turn on the ignition to ON but do not start the engine....wait at least 30 seconds.
3) With the ignition still on, disconnect the IAC electrical connector.
4) Remove the ground from the diagnostic terminal of the ALCL and start the engine.
5) Assuming you did not have to adjust the idle RPM's.....turn the ignition to OFF and reconnect the IAC valve connector.

 

[TedC]

Doesn't surprise me. I found diiferent procedures when investigating this as well. When I documented all of the steps provided earlier for throttle body adjustements, I stayed as true as possible to the GM FSM instructions. Specific to IAC reset, no changes required.
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
When you depress the pedal sligthly, the engine draws in extra air, more than what is required by the ECM to reach targeted (programmed) RPM setting. So the ECM extends the IAC pintle, trying to reduce IAC air flow. Since air flow across the plates is more than the ECM can manage, the ECM signals the IAC into full extension (closure). The ECM and IAC are now insync with each other. On restart, The ECM uses several variables and prom settings to signal initial IAC position. Post procedure a scanner should be used to verify the IAC counts.