Idle searching only when hot
Thread Starter
Staging Lane
Joined: May 2007
Posts: 95
Likes: 2
Idle searching only when hot Hey guys, still trying to work the bugs out of my 1998 vette after an engine rebuild. On cold start up, the engine idles great...then when it warms up a little bit, maybe after 5-10 minutes, the idle will go from 800-1200 back and forth constantly. Its worse when driving, coming to a stop light, the RPMs will hang at 2,000 until completely stopped, then will suddenly drop down to 1,000 and continue idle hunting. I am not sure where to go from here....
Things I have replace so far
-PCV valve and replaced all PCV hoses with 3/8th fuel hose. The PCV system itself is properly routed, I spent hours of research to verify the set up.
-Oxygen sensors and wiring has been replaced with Bosch sensors and good quality wiring harnesses.
-Intake and throttle body are on air tight.
-Intake manifold gaskets are new
-MAP sensor replaced, strapped down tightly with several zipties
-rear manifold 1/8th vacuum tube held 20psi with mityvac
-fuel evap system is inplace and tight with new hoses
-AIR and EGR are both deleted w/ headers.
-rear o2's deleted in PCM
-PCM tune by Frost
Things I have replace so far
-PCV valve and replaced all PCV hoses with 3/8th fuel hose. The PCV system itself is properly routed, I spent hours of research to verify the set up.
-Oxygen sensors and wiring has been replaced with Bosch sensors and good quality wiring harnesses.
-Intake and throttle body are on air tight.
-Intake manifold gaskets are new
-MAP sensor replaced, strapped down tightly with several zipties
-rear manifold 1/8th vacuum tube held 20psi with mityvac
-fuel evap system is inplace and tight with new hoses
-AIR and EGR are both deleted w/ headers.
-rear o2's deleted in PCM
-PCM tune by Frost
Last edited by Ironembraced; Mar 1, 2012 at 07:42 PM.
codes?... apparently you have a problem being in closed loop, it could be your maf. you could try and clean yours or borrow one from a friend. you can also disconnect your O2 sensors and see if it solves the problem.
Thread Starter
Staging Lane
Joined: May 2007
Posts: 95
Likes: 2
No codes unfortunely...i will try your suggestions...thanks...
by the way...by unplugging my O2 sensors, is there anything I can prove by doing this? Couldnt it still be a vacuum leak even if the problem quits after disconnecting the O2?
by the way...by unplugging my O2 sensors, is there anything I can prove by doing this? Couldnt it still be a vacuum leak even if the problem quits after disconnecting the O2?
Last edited by Ironembraced; Feb 26, 2012 at 05:06 PM.
Thread Starter
Staging Lane
Joined: May 2007
Posts: 95
Likes: 2
Well, after it warms up, the O2 sensors (I assume?) are sensing a lean condition at 800 or so RPM, so then the PCM immediately adjusts to add a slight bit more fuel to bring it to 1200 RPM, then letting it back down to 800 RPM, etc and repeating itself in and endless loop.
I don't know 100% that is what is happening, but it's my best guess. Basically the PCM thinks my AFR is jacked up at idle and it's trying to compensate for that.
From most of what I've researched...this is typically caused by a vacuum leak in most cases. I haven't seen too much evidence of it being caused by oxygen sensors, and they typically leave a check engine light when they malfunction.
My best guess besides a vacuum leak (because I can't find it for the life of me) was the MAF. But still....the MAF seems to work fine before the PCM goes into closed loop.
So that maybe would point more towards my throttle being out of adjustment? I don't know...the C5 vette doesn't have an idle control valve, and I don't know how the heck I could verifiy proper adjustment on my throttle plate & actuator.
Another idea I had was my IAT sensor could be off a bit....
Or my fuel injectors could be dirty, causing me to run lean?
I don't know 100% that is what is happening, but it's my best guess. Basically the PCM thinks my AFR is jacked up at idle and it's trying to compensate for that.
From most of what I've researched...this is typically caused by a vacuum leak in most cases. I haven't seen too much evidence of it being caused by oxygen sensors, and they typically leave a check engine light when they malfunction.
My best guess besides a vacuum leak (because I can't find it for the life of me) was the MAF. But still....the MAF seems to work fine before the PCM goes into closed loop.
So that maybe would point more towards my throttle being out of adjustment? I don't know...the C5 vette doesn't have an idle control valve, and I don't know how the heck I could verifiy proper adjustment on my throttle plate & actuator.
Another idea I had was my IAT sensor could be off a bit....
Or my fuel injectors could be dirty, causing me to run lean?
Last edited by Ironembraced; Feb 26, 2012 at 11:32 PM.
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this^...it will mostly be a faulty sensor or just a dirty one...it could be your O2s btw..even if they are new, tons of people have O2s issue in less than a month. so start with things that are easy to diagnose first...i hate problems with no engine codes! good luck!
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Verdad Gallardo O2 sensors do not always throw a code, actually about 50% of the time i have seen O2's go bad or not function at their best and not throw a code. if id does not fault consistently enough for the computer to consider it is a problem it will not trip the code.
Are you running rich?
If so, it sounds like you have unmetered air entering the engine.
Check for air leaks.
One recent thread here found a faulty brake power booster.
I have/had the same exact trouble as you.
In taking apart the engine for a lack of oil pressure, I found the intake manifold runners had no gaskets.
It's a long story, but I can't wait to get everything back together to see how it really runs!
Good luck.
If so, it sounds like you have unmetered air entering the engine.
Check for air leaks.
One recent thread here found a faulty brake power booster.
I have/had the same exact trouble as you.
In taking apart the engine for a lack of oil pressure, I found the intake manifold runners had no gaskets.
It's a long story, but I can't wait to get everything back together to see how it really runs!
Good luck.
Thread Starter
Staging Lane
Joined: May 2007
Posts: 95
Likes: 2
Are you running rich?
If so, it sounds like you have unmetered air entering the engine.
Check for air leaks.
One recent thread here found a faulty brake power booster.
I have/had the same exact trouble as you.
In taking apart the engine for a lack of oil pressure, I found the intake manifold runners had no gaskets.
It's a long story, but I can't wait to get everything back together to see how it really runs!
Good luck.
If so, it sounds like you have unmetered air entering the engine.
Check for air leaks.
One recent thread here found a faulty brake power booster.
I have/had the same exact trouble as you.
In taking apart the engine for a lack of oil pressure, I found the intake manifold runners had no gaskets.
It's a long story, but I can't wait to get everything back together to see how it really runs!
Good luck.
No, I don't believe I am running rich. But I am glad you mentioned something about the brake booster, that was my next step in troubleshooting I think. I noticed the other day, that my hoses connection to the booster is a little bit loose feeling and is looking aged. I know for a fact my intake gaskets are good, I just had the intake off a little while back to change the MAP sensor, and put brand new gaskets.
