tracking down strange idle characteristics
Thread Starter
Supreme Member
Joined: Mar 2001
Posts: 3,306
Likes: 77
From: Fl
Car: 5.3L turbo 2800lbs RWD
Engine: Prefer 3L Iron & 5.3L Aluminum
Transmission: 4l80e
Axle/Gears: 3.512
tracking down strange idle characteristics
very strange thing going on with the idle.
initial startup, the throttle body is not open enough to start the motor. i have to adjust it open more, or hold it open. when it starts up the IAC motor takes up the slack by opening to like 150-170. so lets say i adjust the throttle open enough to drop the IAC to like 15-20.
about 5 minutes later, the idle is up to about 1500 RPMS. the IAC, of course, drops down to 10(lowest) and the idle is super high. I then have to back off the throttle plates to drop the idle back down to 800~ish, where the IAC picks control back up.
Then after driving for about 10 minutes, i come to a stop and put the car in neutral right?
the idle will be fine for about 45 seconds, then suddenly it will kick up to like 1400 RPMS again. the IAC does not even move at this point, so its not letting in more air. after about 30 seconds of 1500 RPMS, the idle drops back down to 850 RPMS, where i initially set it after fire up.
and it keeps doing this, so long as its running. All the vacuum ports in the plenum are plugged except the regulator vacuum and the brake booster vacuum, so its not somthing letting in air. according to the computer, when the idle changes the pulse width is changing too, but thats because the MAP is changing and so is the RPM. I did some data logging and it just isnt consistant with anything i have found.
initial startup, the throttle body is not open enough to start the motor. i have to adjust it open more, or hold it open. when it starts up the IAC motor takes up the slack by opening to like 150-170. so lets say i adjust the throttle open enough to drop the IAC to like 15-20.
about 5 minutes later, the idle is up to about 1500 RPMS. the IAC, of course, drops down to 10(lowest) and the idle is super high. I then have to back off the throttle plates to drop the idle back down to 800~ish, where the IAC picks control back up.
Then after driving for about 10 minutes, i come to a stop and put the car in neutral right?
the idle will be fine for about 45 seconds, then suddenly it will kick up to like 1400 RPMS again. the IAC does not even move at this point, so its not letting in more air. after about 30 seconds of 1500 RPMS, the idle drops back down to 850 RPMS, where i initially set it after fire up.
and it keeps doing this, so long as its running. All the vacuum ports in the plenum are plugged except the regulator vacuum and the brake booster vacuum, so its not somthing letting in air. according to the computer, when the idle changes the pulse width is changing too, but thats because the MAP is changing and so is the RPM. I did some data logging and it just isnt consistant with anything i have found.
Thread Starter
Supreme Member
Joined: Mar 2001
Posts: 3,306
Likes: 77
From: Fl
Car: 5.3L turbo 2800lbs RWD
Engine: Prefer 3L Iron & 5.3L Aluminum
Transmission: 4l80e
Axle/Gears: 3.512
Ah, I think i found it.
The knock sensor is picking up knock and retarding the timing.
this drops the idle speed, and heats up the exhaust.
plug #3 is sitting on the exhaust because its so close.
the extra heat destroyed the heat shield, and ate the plugwire, grounding it to the exhaust.
the knock sensor adds back in the timing, but its too late the plug had melted to the exhaust and now its missing.
then the knock sensor would pick up knock again and retard the timing, low idle. add it back in, high idle.
sucks right? sheesh. How much power would i lose if i dented the #3 header in about... half an inch.
The knock sensor is picking up knock and retarding the timing.
this drops the idle speed, and heats up the exhaust.
plug #3 is sitting on the exhaust because its so close.
the extra heat destroyed the heat shield, and ate the plugwire, grounding it to the exhaust.
the knock sensor adds back in the timing, but its too late the plug had melted to the exhaust and now its missing.
then the knock sensor would pick up knock again and retard the timing, low idle. add it back in, high idle.
sucks right? sheesh. How much power would i lose if i dented the #3 header in about... half an inch.
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