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I have a 1999 Buick Century and it is driving me nuts. It starts right away, runs great for about 30 sec or until I get to the end of my driveway, and then goes to hell for about 6 blocks. You have to mash the gas on and off just to keep it running. All of this with no codes. After that, it runs like a top. If you shut it down after a short run, it will do it again, but less of the bad timewise. If you have got it good and hot, and run into a store no problems, but if it sits for 15 min…does it again. I have changed every acronym that I know of (MAF, MAP, IAC, TPS, Fuel regulator, Fuel Pump, Fuel Filter. Checked vacuum lines, Coil Packs, and plugs. None of these has changed anything. I would park the thing if it wasn’t such a good runner otherwise and I don’t have a couple of hundred bucks in new parts in it!.
Could a bad O2 sensor cause this? What about a partially plugged catalytic converter? Good ideas on testing either one of these?
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O2 sensors are not in operation during open loop (cold start, engine below operating temperature) and a plugged cat would give you problems independent of engine temperature. There are a couple of things to check, though, the ambient air temp sensor (IAT) and engine coolant temperature sensor (ECT) should be equal and within a few degrees of the actual ambient air temperature when the engine is cold and not running. The temperature sensors give information to the computer to enrich the fuel mixture when the engine is in open loop and below operating temperature (generally ~192F). Based on faulty data, the computer could be over-enriching the mixture when the engine is cool, which means the engine needs to pull in more air to achieve stoichiometry. This would fit the behavior you are describing and also explain why it doesn’t affect starting and disappears when the engine is hot. If you do not have an OBDII scan tool that can give you live data, you should get one. Some are available for under US$20 on Amazon.com. These days it is very difficult to troubleshoot engines without one.
A bad O2 sensor should trip a CEL
A partially clogged exhaust may be at fault, but if that were the case, it should be constant and not correct itself.Perhaps a fault in the BCM. or a sensor giving bad input to the BCM.
GM wasn’t noted for outstanding computers back then.I almost think, and it sounds like, the low scale fuel mapping has gone wonky on you at cold temp.
I have seen temp sensors reek havoc on how an engine runs.Well it was the O2 sensor (apparently).
My Chevy mechanic at the dealership didn’t think it was the O2 sensor, but to eliminate the cat, he said he would run a boroscope down the O2 sensor port to look at the cat. Cat was good, but when he was putting it back together, He looked at the connection for the O2 sensor and noticed one of the prongs was powdery white. He thought that was odd, so he put some dielectric grease in the receptor, buttoned it back up, runs like a dream…….Go Figure.
When I first saw this I thought it could be a bad egr valve. It would be hard to detect unless you have a cpu in the car for a diagnostic machine. My 02 Ford Focus had a bad egr valve and it would shut off and there would be a rough idle as well as major power loss.
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