Idle Problem

jordan · 08-28-2008, 12:12 AM

anyone ever had to replace the PCM for a high erratic idle problem?

Nate's Alero · 08-28-2008, 12:17 AM

damn.....maybe TPS? CPS? Timing? not a pcm for high idle....the car would not run...

jordan · 08-28-2008, 12:37 AM

Ya everything mechanically is working on the car fine according to the mechanic, IAC valve changed too, just the mechanic is having a hard time telling the computer to idle at proper RPM with the "steps" of the IAC valve and the car did some weird shifting yesterday with the low trac light on which cleared itself when i turned the car off and then turning it back on cleared the light. so he is kinda stumped too but he is thinking the PCM so im getting a newly "used" flashed PCM with newest software out of a grand am installed tomorrow and im hoping this fixes the problem.

PS no codes thrown out from computer no engine light on

01silveralero · 08-28-2008, 01:29 AM

i had a problem with mine which was solved by fixing the main vacuum hose to the throttle body.

jordan · 08-31-2008, 11:39 AM

ok heres the updated story... I got the computer and IAC valve replaced and the car still idles erratic and it'll be erraticly high wanting to go go go so to speak. when in drive it'll idle at about 1000 RPM and waver up and down when holding the break in in drive, and then i'll put it into park and it will idle even higher and waver at higher RPMS! one morning i started the car and it went up to 2250 RPMs before coming back down to 1100 to idle at in park mode. Weird i think. Damn car is annoying already. At the odd time it will idle normal. checked for vacuum leaks at the shop twice and nothing noted. no vacuum leaks either because the car idles normal still sometimes at about 2% of the time when stopped in drive.

Any educated guesses. A senor like MAP or Throttle Position Sensor or what else could it be?

PS: mechanically the car is great and driving on the hwy is fine with this problem. its just the idle problem

alerored04 · 08-31-2008, 12:02 PM

I would check the throttle body gasket for leaks, sounds like you are getting unmetered airflow causing a high idle. The pcm cannot correct for air that the maf is not reading. Best way to check this is to unplug the maf and see if anything changes. If so you have a leak or possibly a bad maf, but that would be last resort, those are pricey.

[ion] C2 · 08-31-2008, 12:05 PM

You don't replace the car's VCM for an idle issue... lol.

Could be the TPS. It's going to be a sensor, vacuum line, or intake sealing issue.

Also, we don't have MAFs.

jordan · 08-31-2008, 12:13 PM

what is MAF?

Note i have the 2.4litre engine

[ion] C2 · 08-31-2008, 12:19 PM

Mass Airflow Sensor, looks like a smaller throttle body that's a screen, measures how much air flows through it. Alerored04 suggested it may be a MAF-related issue, but that's not true.

How exactly did you check for vacuum/intake leaks? Spray starter fluid around connections/hoses/at the intake manifold -> head area and see if the RPMs go up.

jordan · 08-31-2008, 12:24 PM

hmmm Starter fluid. ok i'll give it a shot later today. i'm going to test myself the MAP and TPS sensors today. i got a manual on what voltages to test for when backprobing the sensors with a digital multimeter so ill give that a shot. so the starter fluid is ok if it sprays on stuff like the sensor outer shells and wires? im just worried about overspray. how detramental is that stuff?

and if there was a vacuum leak...why would the rpms be settle down sometimes and not? doesnt make sense to me. and the car on occasion would idle fine because i was saying earlier it would idle perfect at times 2-3% of the time

[ion] C2 · 08-31-2008, 12:33 PM

leakage areas temporarily sealing due to heat expansion or other conditions, bump knocking it into place, etc.

alerored04 · 08-31-2008, 01:17 PM

Yeah i did not look at the section, I assumed 3400, I fail. Sorry I can't help you much with 4 cyl problems.

jordan · 10-06-2008, 04:55 PM

Quote:

Originally Posted by [ion] C2

You don't replace the car's VCM for an idle issue... lol.

Could be the TPS. It's going to be a sensor, vacuum line, or intake sealing issue.

Also, we don't have MAFs.

Ya so update! Car's PCM was reflashed a second time and this time at the GM dealer only for $80 and problem with idle totally fixed. driving it for 3 weeks and no problems! so stuff your "LOL"s Turns out the PCM problem can cause the idle problem.

The reason being is because the computer wasnt properly telling the TPS to open at the proper amount. instead it was telling the TPS to idle at 3% which it should be at 1% open. so a reflash (reprogram) of the PCM fixed the problem. There....you just learned something.

[ion] C2 · 10-06-2008, 04:59 PM

..... o_O

I am 100% sure that it was not the computer. It will not randomly change values around that would cause a high or rough idle. Them re-sending the same data it had on before wouldn't solve anything. It's not like the table values deteriorate and disappear or change over time for idle..

jordan · 10-06-2008, 05:01 PM

Quote:

Originally Posted by [ion] C2

..... o_O

I am 100% sure that it was not the computer. It will not randomly change values around that would cause a high or rough idle. Them re-sending the same data it had on before wouldn't solve anything. It's not like the table values deteriorate and disappear or change over time for idle..

The reason being is because the computer wasnt properly telling the TPS to open at the proper amount. instead it was telling the TPS to idle at 3% which it should be at 1% open. so a reflash (reprogram) of the PCM fixed the problem. There....you just learned something.

What are you a mechanic? GM Tech?

Well it fixed it buddy so explain that. its just one of those things that the computer needs a reflash with the software

Bad99Olds · 10-06-2008, 05:09 PM

chris is about to own you in t minus a few mintures. wait for it... wait for it...

[ion] C2 · 10-06-2008, 05:12 PM

Quote:

Originally Posted by jordan

The reason being is because the computer wasnt properly telling the TPS to open at the proper amount. instead it was telling the TPS to idle at 3% which it should be at 1% open. so a reflash (reprogram) of the PCM fixed the problem. There....you just learned something.

No. The Throttle Position Sensor does not "open" any amount. It's a sensor that reads how much the butterfly plate in the throttle body is turned, and relays that information to the PCM to determine many different things that are dependant on TPS, such as shift speed, PE (Power Enrichment), and various other tables. There is no reason the PCM would or could "tell the TPS" to idle at whatever throttle percentage.

Idle RPM is controlled by Engine Coolant Temp, VE (Volumetric Efficiency) and MAP (Manifold Air Pressure).

Either your TPS was faulty, and they replaced it, or your IAC (Idle Air Control) valve was faulty, and they replaced that. There is NO reason they should have to re-tune your car based on mechanical failure.

Bad99Olds · 10-06-2008, 05:13 PM

owned.

jordan · 10-06-2008, 05:14 PM

hey im just saying what was done on the car and it fixed it. this ion guy is some 19year old punk that is laughing at stuff he knows nothing about.

Bad99Olds · 10-06-2008, 05:18 PM

apparently they breed them slower in canada now, because what he said is dead on. Chris knows more about that engine than you could ever hope to, hes done a shit ton of work on it. just walk away gracefully.