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.

 

[udidwht]

I thought the big blocks were 30psi?

1994 and 1995 454 TBIs were 26-32psi. Yours should be 9-13psi

 

[scott2093]

1994 and 1995 454 TBIs were 26-32psi. Yours should be 9-13psi

arigato

 

[Flynssr]

When talking vac leaks, a simple test to spray suspected leak areas with like WD 40 or something of the same. The engine should idle better until the liquid is sucked in. At the same time, you should see the liquid being soaked into the leak area. I too have been chasing a erratic idle in my 1990 454 C3500. But the last time I was able to work on it, I was replacing the O2 single wire, only to find the wire is missing in the wire bundle. Next work trip is to trace the wire down. Good luck on your fix and keep us updated.