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Pesky Problem

 
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Baywing
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PostPosted: Wed Jan 20, 2010 8:25 am    Post subject:  Pesky Problem Reply with quote

As I'm not sure where the problem lies, I'm posting it here. If the mods feel it belongs elsewhere, feel free to move it.
This is an intermittent issue (the worst kind) that seemed to start around the time I had my first transmission rebuilt (~85K). I've since had a factory rebuilt tranny installed and the problem is still there, so I think that would point to something else. Either the first re-build place messed something up, or it's all a co-incidence.
I've got a 2000 LS with the 3.8L. Car always starts fine, but after 30 seconds of running, the engine rpms start hunting. This only happens when the car has been run and then parked for several hours. The 2 typical scenarios are going to work in the morning, car is fine. 15 mile run, 1/2 highway, park car at work for 7-8 hours. Car starts fine, but after 30 seconds, engine starts hunting. If I'm in P or N, the engine just hunts, tach shows rpms go from almost 0 to 1500+/- back and forth very fast. Engine doesn't quit. This lasts about 5-15 seconds then as fast as it started, the rpms return to normal. If the car is in D when this happens (foot on brake) the car bucks wildly, but again, doesn't stall. If the car is moving (in D) power goes flat, no amount of pedal will get it moving but again, engine never dies. After the 5-15 second period, power returns and the car is fine. The other scenario is after a long (150 mile) highway drive, parked for 10 hours then I get the hunting 30 seconds after start-up. I can drive for hours, shut down for an hour and restart without issue or just do small hops (<5 miles), so the common factors seem to be longer runs with longer park times. (But not too long as I can come home from work (15 miles) put the car in the garage for 15 hours and it's fine the next day.
My first thought is that it is something like a vapor lock where fuel is missing from a part of the line that works it's way to the engine after about 30 seconds. Next on the list would be the egr followed by a temp sensor with a short at a critical location. The second two don't add up though as I would think they would be more constant.
I have tried different gas, from brands to stations even in different states and have not noticed any changes.
I'll add that there have never been any codes with this and the car has passed all emissions tests.
Sorry for the long post, but I'm just trying to describe things fully.
Any ideas?
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snook
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2004 Chevrolet Impala
1985 Chevrolet Blazer


PostPosted: Wed Jan 20, 2010 9:53 am    Post subject:   Reply with quote

Idle air control valve(IAC)? Dirty MAF? Checked for vacuum leaks?Try those,,,,,
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Baywing
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PostPosted: Wed Jan 20, 2010 12:38 pm    Post subject:   Reply with quote

Thanks. I'd think both of those (IAC, MAF) would throw codes, they did back on my 92 K-5 which had OBD1. OBD2 supposedly monitors more functions.
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