IAC question. For my Z-28.
Thread Starter
Banned
Joined: Mar 2004
Posts: 3,577
From: Sandy VJJville
IAC question. For my Z-28.
Update edit for newcomers to this thread.
In hopes of figuring out this problem I have replaced the following:
-Ignition coil
-Optispark
-MAF sensor
-MAF sensor pigtail
-cleaned the IAC valve
-plug wires have < 5k miles on them
-spark plugs have < 15k miles on them.
-I pulled a couple spark plugs to inspect them. They are not fouled.
-Found a burnt O2 harness and repaired it. It did not solve the problem.
-MAP sensor
-IAT sensor
-IAC valve
**NOTE** The problem only occurs in closed loop. Once it goes from closed loop to open loop, it's fine. If I unplug the O2 sensors and force it to remain in open loop, it runs fine.
__________________________________________________ ______________________________
This is for the 97 Z-28 in my sig.
I did some searching but didn't find the answer I was after.
Would a bad IAC valve cause problems while driving the car? I'll explain:
At idle, my car will be sitting at ~1050 RPM on the tach and then after a minute or so it will drop to anywhere between 600-800 RPM. Then it will surge up to ~12-1400 RPM and drop back to normal. It will continue to do this every 30 seconds to a minute or so.
When I drive the car, if I'm going along steadily holding consistent throttle pressure, I'll feel the engine start to choke and it'll drop a few RPM and MPH. After a few seconds it will stop choking and it'll go back to normal. This cycle repeats every 30 seconds to a minute or so. So to me, it seems like it's doing the same thing in gear that it does at idle. Are these symptomatic of a bad IAC valve? I did the IAC position reset last night but it didn't have any effect.
I can't tell any IAC readings because I don't have access to a scanner right now. I had it scanned a few weeks ago but I didn't get the numbers then because I didn't think about checking for them. I mentioned this in one of my other threads, but while the car was on the scanner, when it would lose RPM, the "PCM status" LCD would turn off which meant that it had stopped signaling the scanner. After it would surge up in RPM, the LCD would come back on. Why would my PCM be going offline like that?
Also, I just replaced the optispark last week so I would doubt that's the issue. Plugs and wires are less than 5k miles old.
In hopes of figuring out this problem I have replaced the following:
-Ignition coil
-Optispark
-MAF sensor
-MAF sensor pigtail
-cleaned the IAC valve
-plug wires have < 5k miles on them
-spark plugs have < 15k miles on them.
-I pulled a couple spark plugs to inspect them. They are not fouled.
-Found a burnt O2 harness and repaired it. It did not solve the problem.
-MAP sensor
-IAT sensor
-IAC valve
**NOTE** The problem only occurs in closed loop. Once it goes from closed loop to open loop, it's fine. If I unplug the O2 sensors and force it to remain in open loop, it runs fine.
__________________________________________________ ______________________________
This is for the 97 Z-28 in my sig.
I did some searching but didn't find the answer I was after.
Would a bad IAC valve cause problems while driving the car? I'll explain:
At idle, my car will be sitting at ~1050 RPM on the tach and then after a minute or so it will drop to anywhere between 600-800 RPM. Then it will surge up to ~12-1400 RPM and drop back to normal. It will continue to do this every 30 seconds to a minute or so.
When I drive the car, if I'm going along steadily holding consistent throttle pressure, I'll feel the engine start to choke and it'll drop a few RPM and MPH. After a few seconds it will stop choking and it'll go back to normal. This cycle repeats every 30 seconds to a minute or so. So to me, it seems like it's doing the same thing in gear that it does at idle. Are these symptomatic of a bad IAC valve? I did the IAC position reset last night but it didn't have any effect.
I can't tell any IAC readings because I don't have access to a scanner right now. I had it scanned a few weeks ago but I didn't get the numbers then because I didn't think about checking for them. I mentioned this in one of my other threads, but while the car was on the scanner, when it would lose RPM, the "PCM status" LCD would turn off which meant that it had stopped signaling the scanner. After it would surge up in RPM, the LCD would come back on. Why would my PCM be going offline like that?
Also, I just replaced the optispark last week so I would doubt that's the issue. Plugs and wires are less than 5k miles old.
Last edited by shock6906; Feb 23, 2008 at 10:33 AM.
Registered User
Joined: Sep 2007
Posts: 81
It could be the IAC valve,I was having very similar problems and I didnt have a way to test, so I pulled it out and it had a lot of carbon debris that restricted the little spring that is on the sensor not allowing it to open/close correctly.I would take it out clean it and see if that helps.
Oh ,by the way clean out the port where it fits into the throttle body,as it will have carbon debris in it as well.
Thread Starter
Banned
Joined: Mar 2004
Posts: 3,577
From: Sandy VJJville
Alright, I'll check that out this afternoon. Any other suggestions anybody? My PCM has a custom tune. Do you think there's any possibility that it could be a bad tune causing my car to run like this? It never had these problems before the tune and I never got much of a chance to drive it after with the car being in the body shop for so long. I also had just done the header install and was tracking down burnt wires, etc so I'm not sure if the tune would be the problem or not.
Registered User
Joined: May 2004
Posts: 230
my car kinda does this too. it will idle well then after about 20-30 minutes of driving it will not want to idle, and backfire in between shifts or when I get off the gas. I figured its the IAC as well. I will probably just buy a new one. I will update you bro, to see if that fixes the problem - but I heard to just clean it out as well, but I figure I should just buy a new one and not half *** it especially if there are a lot of miles on the car.
Thread Starter
Banned
Joined: Mar 2004
Posts: 3,577
From: Sandy VJJville
Well I cleaned the IAC valve up. Got all kinds of gunk off of it. Made absolutely no difference. Now what? I'm at a loss here and I'm frustrated with this damned car like you wouldn't believe.
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Joined: May 2004
Posts: 230
im sorry bro, because your thread made my car run a hellofa lot better.
After taking off the TB I noticed that the IAC bolts were barely on, with pretty much both of them not screwed in and the IAC having a millimeter gap. So I bought a new one, put it in, drives like a dream. Idles so damn well without that rich smell or erratic idle, and when getting off the gas, it doesnt backfire.
Good luck man, hope you figure it out!
After taking off the TB I noticed that the IAC bolts were barely on, with pretty much both of them not screwed in and the IAC having a millimeter gap. So I bought a new one, put it in, drives like a dream. Idles so damn well without that rich smell or erratic idle, and when getting off the gas, it doesnt backfire.
Good luck man, hope you figure it out!
Registered User
Joined: Mar 2004
Posts: 626
From: scottsdale, az
I have a similar problem, except mine idles at about 900 but then it will start to idle aorund 400-600 and try to stall. i am going to work on it tomorrow and hopefully I will discover a solution that will help you
Thread Starter
Banned
Joined: Mar 2004
Posts: 3,577
From: Sandy VJJville
Thanks man. Please let me know because this is really getting to me. I'm just ready to have this car running right or it's time to sell it. I haven't driven it more than a couple weeks total since April due to header install after-effects and a never ending saga getting it painted. I'm just about over it.
