LT4 High Idle Problem
Thread Starter
Instructor
Joined: Apr 2010
Posts: 154
Likes: 0
From: Miami Florida
LT4 High Idle Problem
Ok car runs fine otherwise no performance issues or CEL.
Idles at around 800 to 900. Actually 900 and drops 100 when I smach the steering wheel, eitherway a bit too high.
No apparent vacum leaks and already sprayed card cleaner around intake and throttle body with no change to rpms.
So, why does smacking the steering wheel cause the guage to drop 100rpm?
As far as the idle in general, the LT4 does not have an EGR, however there are pipes coming out of the exhaust manifolds and into the intake, These pipes have round pancakes that probably contain a diaphram. Can these things be leaking air into the manifold and what are they called?
Also, I did pull out and clean the idle air control but did not really notice any improvement. How can this be tested?
Thanks
Idles at around 800 to 900. Actually 900 and drops 100 when I smach the steering wheel, eitherway a bit too high.
No apparent vacum leaks and already sprayed card cleaner around intake and throttle body with no change to rpms.
So, why does smacking the steering wheel cause the guage to drop 100rpm?
As far as the idle in general, the LT4 does not have an EGR, however there are pipes coming out of the exhaust manifolds and into the intake, These pipes have round pancakes that probably contain a diaphram. Can these things be leaking air into the manifold and what are they called?
Also, I did pull out and clean the idle air control but did not really notice any improvement. How can this be tested?
Thanks
Drifting
Joined: Jun 2010
Posts: 1,853
Likes: 8
From: Westerly RI
Not familiar w/ '96 specifically (have '92 LT1) but could be the idle speed drops 100 when you turn the steering wheel due to loading the engine via the p/s pump.
Others have had high idle problems due to excess air going thru the throttle body throats because the throttle plates leak. You could try testing this by taping over the front of the tb but not covering the IAC port, then see if the car idles normally. (Do not rev above idle with the tb covered, the engine might inhale the tape)
Others have had high idle problems due to excess air going thru the throttle body throats because the throttle plates leak. You could try testing this by taping over the front of the tb but not covering the IAC port, then see if the car idles normally. (Do not rev above idle with the tb covered, the engine might inhale the tape)
Race Director
Joined: Oct 2005
Posts: 11,766
Likes: 175
From: Rolla Missouri
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800-900 rpm range is about normal.
Just going by the factory tach, my '96 LT4 idles in the 800-900 rpm range.
Since that tach does not have a very good scale for rpms down that low it's hard to say though.
It's just below the 1k mark on the tach.
But it does not drop any when you hit the steering wheel either.
Just going by the factory tach, my '96 LT4 idles in the 800-900 rpm range.
Since that tach does not have a very good scale for rpms down that low it's hard to say though.
It's just below the 1k mark on the tach.
But it does not drop any when you hit the steering wheel either.
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
I posted this in the Scan/Tune section as well, but my '96 LT4 keeps throwing the P507 code. I have cleaned everything, in and out, from the air filter to the intake, but keep getting the code.
The car runs great, so the code is annoying. When driving around, if I push in the clutch to coast to a light, the engine is slow to come back to idle. It will usually run around 1200 rpms, before settling down to 850rpms after a few seconds of not moving. Is this what other's experience?
I did find some Camaro owners saying their cars do the gradual rpm down, but they are also saying a 650rpm idle is normal.
Thanks.
The car runs great, so the code is annoying. When driving around, if I push in the clutch to coast to a light, the engine is slow to come back to idle. It will usually run around 1200 rpms, before settling down to 850rpms after a few seconds of not moving. Is this what other's experience?
I did find some Camaro owners saying their cars do the gradual rpm down, but they are also saying a 650rpm idle is normal.
Thanks.
Thread Starter
Instructor
Joined: Apr 2010
Posts: 154
Likes: 0
From: Miami Florida
I posted this in the Scan/Tune section as well, but my '96 LT4 keeps throwing the P507 code. I have cleaned everything, in and out, from the air filter to the intake, but keep getting the code.
The car runs great, so the code is annoying. When driving around, if I push in the clutch to coast to a light, the engine is slow to come back to idle. It will usually run around 1200 rpms, before settling down to 850rpms after a few seconds of not moving. Is this what other's experience?
I did find some Camaro owners saying their cars do the gradual rpm down, but they are also saying a 650rpm idle is normal.
Thanks.
The car runs great, so the code is annoying. When driving around, if I push in the clutch to coast to a light, the engine is slow to come back to idle. It will usually run around 1200 rpms, before settling down to 850rpms after a few seconds of not moving. Is this what other's experience?
I did find some Camaro owners saying their cars do the gradual rpm down, but they are also saying a 650rpm idle is normal.
Thanks.
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
Primal, thanks for the logical analysis. That has me thinking, like Frankie D, that my IAC is not functioning properly.
Frankie, I cleaned it, and all the passages, really well, so it may be time to do the replacement. I hate replacing parts without knowing if they are bad, but I am running out of options.
I'll keep the thread updated.
Frankie, I cleaned it, and all the passages, really well, so it may be time to do the replacement. I hate replacing parts without knowing if they are bad, but I am running out of options.
I'll keep the thread updated.
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Burning Brakes
Joined: Apr 2006
Posts: 1,218
Likes: 7
From: Burleson Texas
It would be beneficial to data log this when it happens.
I had a PCV valve hose that was squishy that was causeing the same thing on my car. I replaced all the vacuum related ruber and pcv, but still didnt fix the issue. I pulled the intake manifold and replaced the gaskets, that fixed it for a while, but a few months, it was back again. After clearing the code 3 times it never came back. UNTIL I disconnected the battery, then the code re-appeared. Cleared it 3 times and it stayed away.
After dataloging my situation, I found nothing out of spec when the code was set. I never did figure out the cause.
I do know, when I went through the entire engine, and reprogrammed my PCM, the code never re-appeared.
I had a PCV valve hose that was squishy that was causeing the same thing on my car. I replaced all the vacuum related ruber and pcv, but still didnt fix the issue. I pulled the intake manifold and replaced the gaskets, that fixed it for a while, but a few months, it was back again. After clearing the code 3 times it never came back. UNTIL I disconnected the battery, then the code re-appeared. Cleared it 3 times and it stayed away.
After dataloging my situation, I found nothing out of spec when the code was set. I never did figure out the cause.
I do know, when I went through the entire engine, and reprogrammed my PCM, the code never re-appeared.
Racer
Joined: Jan 2006
Posts: 323
Likes: 4
From: McKinney Texas
Fixed. My replacement IAC arrived in the mail today, and has fixed the high idle. Now when I coast, the engine rpm drops to about 1000rpm, then to about 800rpm when stopped. Before it would only drop to around 1500rpm, and take longer to get the 800.
Hopefully I will never see the P507 again.
Thanks.
Hopefully I will never see the P507 again.
Thanks.
