Stumble on rev-down, any ideas?
08-03-2002, 08:20 AM
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Stumble on rev-down, any ideas?
OK Time for my semi-annual post, maybe I'll actually get it fixed this time:
My engine stumbles on rev-down. Basically, any time it is coming down from a brief rev peak.
Example 1: Starting. It cranks, starts OK, revs up (from the initial start) comes back down, stumbles (RPM drop), and then recovers and idles like it should (idles high for example if it is cold, or idles regular if it's warm).
Example 2: Revving. If I am sitting in gear at idle (automatic tranny), then tap the throttle (such as moving up a car-length in traffic) and let off immediately, RPMs drop below idle as it stumbles, the recovers and idles fine again.
When I replaced my timing chain I noticed it was slightly better. After playing with my timing I noticed the following: advanced timing = less stumble, retarded timing = more stumble. However, if I advance timing until I the stumble is barely noticable, I get serious knock (20+ counts when revving sitting still). So I can't fix it just by advancing the timing.
The only other thing I have noticed is that it seems worse when the engine is very cold (I.E. starting in the winter). Paradoxically, it also seems worse when the engine is very hot (inching along in a traffic jam in July).
It is least noticable when the engine is at good operating temperature or a little below (140-200 say).
Any ideas?
My engine stumbles on rev-down. Basically, any time it is coming down from a brief rev peak.
Example 1: Starting. It cranks, starts OK, revs up (from the initial start) comes back down, stumbles (RPM drop), and then recovers and idles like it should (idles high for example if it is cold, or idles regular if it's warm).
Example 2: Revving. If I am sitting in gear at idle (automatic tranny), then tap the throttle (such as moving up a car-length in traffic) and let off immediately, RPMs drop below idle as it stumbles, the recovers and idles fine again.
When I replaced my timing chain I noticed it was slightly better. After playing with my timing I noticed the following: advanced timing = less stumble, retarded timing = more stumble. However, if I advance timing until I the stumble is barely noticable, I get serious knock (20+ counts when revving sitting still). So I can't fix it just by advancing the timing.
The only other thing I have noticed is that it seems worse when the engine is very cold (I.E. starting in the winter). Paradoxically, it also seems worse when the engine is very hot (inching along in a traffic jam in July).
It is least noticable when the engine is at good operating temperature or a little below (140-200 say).
Any ideas?
08-03-2002, 09:41 AM
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Car: 1992 Camaro RS
Engine: 355
Transmission: T56
Axle/Gears: 3.73:1
Just some wild guess's:
Idle air control valve?
Old ignition system (I had a similar problem when my wires were bad)?
Well, actually, do kind of fuel system do you have?
Idle air control valve?
Old ignition system (I had a similar problem when my wires were bad)?
Well, actually, do kind of fuel system do you have?
08-03-2002, 04:50 PM
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It's an L03 (TBI). Plug wires are MSD SuperConductors, about two years old. Nothing visibly wrong with them. Also I have no trouble at high RPMs which I believe is where ignition problems tend to be more pronounced.
Idle air control... I have replaced it with no change.... I could try it again I suppose. But it appears to function at least somewhat, when I ground the diagnostic terminal on the computer it closes all the way, and when the engine is running it is not closed. I don't know of a more thorough test to try. I've tried cleaning the passage repeatedly with no effect (passage was not visibly dirty last time I looked).
Idle air control... I have replaced it with no change.... I could try it again I suppose. But it appears to function at least somewhat, when I ground the diagnostic terminal on the computer it closes all the way, and when the engine is running it is not closed. I don't know of a more thorough test to try. I've tried cleaning the passage repeatedly with no effect (passage was not visibly dirty last time I looked).
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