One last thought, being that there was a stored code for rear o2 circuit low, is it possible that a clogged cat would somehow cause the ecu to think it needs to idle higher?
Well now I can give the auto wrecker the proper variant and see if they have one too if I need it
Plus I didn’t do a relearn yet, but now that the battery is disconnected it’ll force one. I guess it’s possible the new IAC is good but not learnt, but I’m not holding my breath
Gonna put it back together, Unplug the upstream o2 (maybe that’s why it slightly shakes at idle, maybe the intermittent high idle)and see if it returns, if it does I’ll just drive without the IAC until I can take it to a shop lol.
Another thing of note is it seems almost nothing triggers a CEL in this thing. Unplugged o2 sensor, nothing, unplugged IAC, nothing, faulty MAF or MAF unplugged, nothing,
Not even pending codes when read by a scan tool?
A manufacturer specific scan tool that can command for testing the IAC would be very helpful in this case.
The devices has 4 connections. 2 would be for the motor, and the other 2 for some kind of speed sensor or variable resistor to send feedback to the ECU.
The possibility of the ECU not commanding the IAC to adjust to idle speed still exists. As someone already said, if it doesn’t recognize that your foot is completely off the throttle it won’t try to adjust the idle. A problem with the throttle position sensor could do this, but that wouldn’t explain why the problem starts later on after the car is warmed up. There is no reason to adjust the IAC to make the idle higher after starting.
Why won’t it start without the IAC connected? Can’t you increase the throttle a bit while cranking like with old cars?
Yes I managed to start it without the IAC and press on gas to get it running but once I let off it stalled instantly. I’m gonna try a test drive with the o2 sensor disconnected as a last effort, then I will take it to a shop when I’m back from my work shift.
The throttle position sensor has been replaced already, old one was in rough shape.
When I got the vehicle it had two stored (pending?) codes, one for TCC clutch(was a side effect of dead MAF) and one for the rear o2 sensor.
The only time I actively got a CEL was after changing my injectors and one of them was seated improperly and shooting fuel everywhere, but once I reseated it and reset the battery it was gone. Other than that, you can unplug basically any sensor in the vehicle and not get a CEL, this coupled with the IAC commanding open when not required is what leads me to think it COULD possibly be a faulty ECU. We also seasonally get from -45 Celsius to +35 Celsius here, I don’t know if these temperatures could cause issues over 20 years
The o2 sensor attempt is because judging by the state of alot of other parts I believe this is the original o2 sensor, with 250,000km, and to my understanding the upstream o2 doesn’t come into play until engine is warm and it sounds like if it’s detecting a rich condition it could be opening the IAC to balance the ratio.
Crankshaft was an idea because of one comment I read suggesting that the ECU uses data from the crankshaft sensor to determine IAC values, that and the occasional stall on first start.
Realistically with the amount of isolating systems and replacing parts (I had some money and wanted to just get alot of parts off rockauto under the same waybill) I think I’ve eliminated most of the usual suspects.
It is definitely an odd situation and if the o2 sensor does not turn out to be the cause, I will plug it back, unplug the IAC at proper idle again and see if I can restart the car (I was in a parking lot in a rush when I had the issue starting it with iac unplugged) and wait to get to a shop
If the o2 sensor is the culprit I have both new o2 sensors in the trunk and will pay someone to do the upstream because I can’t for the life of me get it out or it would be changed already.
hmmm… that’s interesting. I don’t know that that indicates a problematic ECU though. Unplugging the coolant temp sensor or MAP on my OBD I Corolla turns on the CEL immediately as I recall. Not sure about the O2 sensor though. Every car uses its own CEL-deciding algorithm. One thing is certain, when the CEL turns on, it annoys the car’s owner. There seems to be a tendency among the manufacturers to delay turning it on until the problem is confirmed over several drive cycles, avoids turning the CEL on due to a temporary glitch.
In order to keep track, most ECU’s maintain three lists of diagnostic codes: pending, current, and history. The pending ones are awaiting confirmation of an actual problem from more drive cycles before being moved to “current” status and turning on the CEL. Check the instructions that came w/your scan tool to see if it is possible for you to see all three lists.
I doubt the O2 diagnostic code is related to the high idle problem. However the best strategy for solving engine problems is to fix everything that is causing diagnostic codes, starting with the code with the lowest number, before worrying about problems that aren’t causing diagnostic codes. This helps prevent chasing your own tail.
You are correct, I drove without the o2 sensor for two drive cycles and it finally came on, I didn’t know about the pending thing so that lowers my suspicion of ECU. Alas the o2 sensor was not the issue but it was worth trying I think. Also the relearning idle was not the issue. I’ve also noticed when I put it in neutral or park sometimes it steps down a few hundred rpm at a time but likes to go back up again. I’ve booked it into a shop in a few weeks. Hopefully they can figure it out, with IAC unplugged it seems to have weird issues with shifting late
While waiting for the shop to schedule you in, you might try to find a used oem IAC from a wrecked LaSabre, could provide some clues.
Adjusting it may affect your idle. There is a range on the TPS (throttle position sensor) that tells the computer to take over the idle. As long as the adjustment is in this range, the idle will not be affected. Once you adjust outside this range, then it is just like the idle adjustment screw on a carburetor, and the idle speed will vary with temp and other factors.
Do you still have your old IAC. If so, put it back in. GM can be very sensitive to OEM parts. IIRC, in one of your posts above you said that the new IAC was aftermarket.
I had a Saturn once and it got the infamous P0509 code for idle too high (it has to idle 200 rpm above normal for 2 minutes in gear). It was idling at around 1000 rpm instead of the spec 750. I put in an aftermarket IAC and it went to around 1800. I took it back and got another one and it did the same thing. I put the old IAC back in and the idle returned to the 950-1000 range. Then I learned about the intake manifold gasket issue, which was NOT on the list of possibles.
I currently have a Silverado and almost every aftermarket part I used on it did not fit and did not work properly. Even high quality aftermarket parts from reputable companies fail.
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Engines that idle at too high rpm or that fail to crank the engine with the key in “start” are two of the most annoying car problems imo.
I will certainly order an oem one and give it a shot. These two are bwd or something .