One thing I think has been overlooked is the throttle stop screw. Many people mistake this for an idle speed adjustment from the days of carburetors. It is not. It is there to adjust the throttle plate to just close when your foot is off the gas. It should not allow the throttle plate to close tight enough to stick in the bore or damage the bore, but it also should close completely.
When this is misadjusted, the TSP will not sense that your foot is off the gas pedal, therefore the computer does not attempt to control the idle, which means that the IAC never gets any signals from the computer.
But, as high as your idle is, it really sounds like a serious vacuum leak to me. I doubt that your PCM (computer) is part of the problem.
30 year old Toyota Camrys have a vacuum operated high idle system that opens the throttle a bit while cranking. It then releases and closes once vaccum starts. This is in addition to the IAC. If that were to get stuck …
To the best of my knowledge, through the throttle plate and through the IAC port. Here are some pictures of the throttle body, two of them are pictures after different cleaning sessions, but these are photos I had in my phone
I’ve tried blowing smoke (vape as I had no smokes on me) into the brake booster when the engine was cold and when it was high idling and both times I couldn’t find any leaks
As far as I can tell I only have the IAC, I’ve tried moving the cable on the throttle too when high idling and it seems loose as it should be. It’s really starting to seem to me like the IAC port is opening weirdly after driving for a bit in the heat. Again it has been replaced, I still haven’t had a chance to block the IAC port with IAC off during idle. I’m hoping to get the free time to do this soon
If I had a throttle stop screw though, wouldn’t adjusting it affect my regular idle? The high idle always starts after some time of driving, especially faster on hot days
You know, there is a vacuum line (you can see in above pictures) that is after the IAC it looks like… Maybe something to do with this? I should maybe pull it off and block, or pinch it during high idle
Edit, I googled it and it seems to be evap canister purge valve
When first starting engine cold it does idle a bit rough (steady rpm but slightly shaky feeling car)until warmed up. Maybe this is something I’ve overlooked in my searching
Further edit, research suggests people have had this issue with the purge valve sticking open and causing high idle. Tomorrow I will drive until high idle, unplug the vacuum from the purge valve to the intake and block it and see if rpm drops. When I first start the engine cold too sometimes it stalls the first time. Also the kids often ask me why they smell gas (I smoke, weak sense of smell). Couldn’t both these issues point to evap problem?
Gas tank vapors would come back out through the air filter if the purge valve stuck open, especially on hot days or after filling the tank. The purge valve doesn’t operate until the engine is warm and the car is driven. That explains why there is a delay. The purge valve edit: isn’t closing again. The canister valve opens as it should and air is sucked in through the canister causing high idle.
There must be some check valve or such to prevent the engine vacuum from sucking all the vapors out of the gas tank and collapsing the tank in the event that the canister valve doesn’t open right?
I’m going out in a couple hours to test out if it’s evap related. I don’t fully understand but it sounds like the vent valve itself can both get stuck open and become a vacuum?
So maybe this purge is staying open when it shouldn’t? Or should the purge be open when running and maybe the vent isn’t closing causing a vacuum leak through the vent, into the canister and up through the purge? Again I’m gonna determine in a couple hours if it is the evap system by blocking the intake port from the evap system, but if I do determine that is the issue, I’ve only learnt what I know about evap systems since last night so I’m not sure if it would be the purge or vent I would want to focus my attention to
Edit: okay yes, to my understanding the vent is normally open, but the purge is continuously monitored, so if my purge eventually sticks open after driving a while, I would have a continuous vacuum leak through the vent,into canister, through purge, and into intake, from what I can gather? Either way if the purge was stuck open and the vent opened wouldn’t this create a vacuum causing the vent to not be able to close? Anyway I’m getting way ahead of myself, I’ll isolates evap here today and if that fixes it I’ll replace the purge
The idle air control motor should be able to compensate for purge valve open/closed, a stuck purge valve shouldn’t cause the engine to race. The vent solenoid closes to test the fuel system for vapor leaks, this has nothing to do with your idle speed.
Yes the purge is normally closed and the vent is open. The purge opens to purge during driving, and the vent closes to test the EVAP system for leaks. Pre 1998 or 97 vehicles wouldn’t have a vent valve. If the purge is stuck open it would suck extra air in through the tank and fuel cap vent, and also through the canister like it’s supposed to.
So capping off evap system did nothing. Though while at a good healthy idle I unplugged IAC and it now idles fine, but it shifts late like this . Gonna try another IAC I guess and if not, go from there
Edit, gonna replace the harness because partsource has a ridiculous amount of them so he said they’re a popular item, if still have issue I’ll warranty the actual valve to them, if still not better then maybe ecu actually is to blame
When you refill the gas tank the fumes (containing both gasoline vapors and air) in the tank are pushed into the canister which grabs hold of the gasoline component, and the air is pushed out through the vent valve. The purge valve is closed during refilling to prevent gasoline being pushed into the engine during refilling, which would cause difficult starting. When engine is warm and driving, purge valve opens, & gasoline stored in canister is pulled into engine and burned. The vent valve remains open during this purge process so air can enter the canister to take the place of the gasoline. Every once is a while the computer decides to test if the evap system has any leaks. For that test it closes both the purge valve and the vent valve, making the evap system air-tight. Then it either applies vacuum or pressure (varies car to car) to check for leaks.
So I’ve determined the operation of the IACV is the issue, if I unplug it when at proper idle it will idle great forever, the delayed shift was in my head, the problem with running like this is once I turn the vehicle off I can’t restart it without the iac plugged in which is a pain obviously. I replaced the connector and warrantied the IAC and with the new IAC I still have the issue.
Could it be crankshaft sensor (I read that plays a part in IAC function) or ECU heating up and malfunctioning?
It sounds like your diagnosis is correct, the IAC is the problem; however the replacement IAC didn’t fix it. That means either the new IAC is also faulty (unlikely), or (more likely) there’s a problem between the ECU (engine computer) and the IAC, which is preventing the ECU from commanding the IAC properly.
If you can obtain the pertinent wiring diagram and post it here, I expect you’ll get some good ideas. Basically you’ll have to trace each wire , make sure the electrical connection is good. There’s some possibility the ECU is the problem. But that would be pretty unlikely unless the car was jump-started or used to give a jump-start to another car.
Crankshaft sensor? ECU heating up? Both seem unlikely.
One idea, find a shop with a Buick pro scan-tool, maybe there’s a testing fucntion for the IAC, scan tool commands IAC to move to a certain position, IAC is removed but still plugged in, so you can watch if it actually does.
I do happen to have a new crankshaft sensor I haven’t installed, I’m just thinking maybe because the random stalls sometimes when starting? Might be a shot in the dark. Alternately now that I know for sure it’s an issue with the IAC, if I take it to a shop at least they won’t be spending hours looking at everything else. One comment I read the guy claimed to have to replace alot of ECUs in GM from people “popping the driver” by unplugging the IAC while running, which has been done many times.
The car has 250,000km and previous owners and being in the cold Canadian prairies I can guarantee there were jump starts done both ways.
Do you think it’s even worth my time to replace that crankshaft sensor, or would I be better off to take it to a shop with the knowledge of IAC malfunctioning and their specialized tools?
If I had this same problem I wouldn’t replace the crank sensor on a hope b/c that seems like a very unlikely cure and it introduces another variable into the equation, a complexity as a diy’er I wouldn’t want.
It’s possible unplugging the IAC when the engine is running could damage a circuit in the ECU. Generally safest to only unplug or plug vehicle electrical connectors with engine off & battery negative disconnected. There’s probably an electro-magnet in the IAC used to create a magnetic field, the force needed to move the IAC into position. A powered up electro magnet represents stored energy, and when it is disconnected that energy has to go somewhere. It might go back to the ECU and damage a diode there. A “driver” is usually a software component in tech-parlance, but I suppose the part of the circuit on the ECU that operates an electro-magnet could be called a “driver” also. (There are usually special diodes called “snubbers” in circuits that power up electro-magnets, to prevent damage to the circuit if the magnetic field collapses b/c it loses power. But sometimes they aren’t up to the job. Or they got damaged the first time it happened, and couldn’t prevent the damage the next time it happened. )
Suggest to take your car to a well-recommended experienced shop having a pro-level scan tool capable of testing the ECU/IAC interface. Once you know what the actual problem is, isolated to a set of faulty parts , you can replace the parts yourself if you like.
If the ECU is faulty, probably will never know if unplugging the IAC is what caused it, the jump starts, or just bad luck.
Okay, thanks for the information. When I’m back from work in a few weeks I’ll take it to a shop that seems up to task to isolate the issue then see if it’s something I’m comfortable working on (pretty much anything except the wiring)
