1992 TBI 454 4L80E. Idle hunts in Park/Neutral but is smooth in drive.

[someotherguy]

Have you replaced the TBI base gasket? They're a little bit notorious for leaking after they get old. I wouldn't be surprised if mine was the original at 160K+ miles; I had a funky idle that got much better after replacing it.

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2nd pic came out a bit blurry but there's significant gasket material eroded away to the metal core.

While you're there you may as well replace the fuel line O-rings for safety's sake.

Richard

 

[Cheyennek1500]

I agree it appears I have high vacuum when in park/neutral. What causes high vacuum?
The MAP sensor chart appears to be correct.
The O2 voltage swings between 200mv and 900mv when the idle surge is happening in park/neutral.

Excellent.


Also, good.


Easier for me to think about this in terms of actual pressure, not voltage.
IF (big IF) this chart is accurate for your MAP sensor:
www.autozone.com/znetrgs/repair_guide_content/en_us/images/0900c152/80/08/c6/ae/large/0900c1528008c6ae.gif

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Your idle pressure is low, meaning high vacuum.


Fairly high altitude, I guess.


Open a huge vacuum leak, manifold pressure should go WAY up, leading to much higher MAP voltage. 3+ volts not "above 1 volt".


What is the narrow-band O2 sensor--the one the computer is reading--showing for voltage and cross-counts?

I agree it appears I have high vacuum when in park/neutral. What causes high vacuum?
The MAP sensor chart appears to be correct.
The O2 voltage swings between 200mv and 900mv when the idle surge is happening in park/neutral.

 

[Cheyennek1500]

Have you replaced the TBI base gasket? They're a little bit notorious for leaking after they get old. I wouldn't be surprised if mine was the original at 160K+ miles; I had a funky idle that got much better after replacing it.

You must be registered for see images attach

You must be registered for see images attach

2nd pic came out a bit blurry but there's significant gasket material eroded away to the metal core.

While you're there you may as well replace the fuel line O-rings for safety's sake.

Richard

I replaced it last night when I checked all the throttle passages for IAC and vacuum. Everything was clean. There was no improvement to the idle in park/neutral.