Engine Problems

courtesy of Bob, The Auto Answer Man

I heard that you are the man to ask when it comes to car problems. I have a 1994 Chevy Lumina 3.1 v6. I recently welded on a new muffler as the old one was all banged up.

While disconnecting the battery, the positive terminal was so corroded that it sheered off when we were trying to loosen the terminal bolt. So I had to buy a new battery. I went to Advance Auto Parts and they matched the numbers on the battery with what they had in stock. I only had $50 on me so I opted for a weaker battery that they assured me would work fine on the car.

I went back to the car and cleaned up the terminals. While leaving the battery disconnected, I welded on the new muffler. After I connected the battery again, everything came to life after having the battery removed for about four hours.

I started the car, and a second later it stalled. So I started it again, and it stalled. Started it a third time and this time I kept my foot on the gas a little. The car continued to run so I pulled it off the ramps. When I took my foot off of the gas, the car seemed to rev up and down. It went from under 500 rpm to 2000 rpm. I noticed that the lights would dim when the revs dropped to 500 rpm and brighten again when the revs got higher. The battery volt meter on my dash also jumped around. When the engine dropped to 500 rpm, the volts would decrease and then get higher when the revs got higher. We all thought it was the ECM just reprogramming for the lower back-pressure muffler.

Driving the car is bad. When I come to a stop, the car will stall unless you keep the revs up. I can now get the car to idle in all gears, but this is after some playing. I first start the car in park, and I hold the gas pedal to where the revs are just over 1000 rpm. Then, I stay that way for about 20 seconds and slowly release the gas completely, watching the revs bounce up and down from 500 rpm to 1000 rpm. In about 30 seconds, the revs level out to just under 1000 rpm. Next I shift it into reverse and hold down the brakes. The revs again bounce from 500 rpm to 1000 rpm, and finally settle off at just under 1000 rpm. I shift it into neutral and the same thing happens. Then I shift it into drive and the same thing happens. After shifting back into park, I release the brakes and the car will idle. That's my story. Any ideas? I look forward to hearing from you and I thank you for your time

Well, hopefully your welding didn't effect the ECU and that your troubles are just related to the battery being disconnected.

Disconnect the battery for about 20-30 minutes. This will basically wipe the ECU memory. Then, reconnect the battery and start the car. DO NOT TOUCH THE GAS PEDAL AT ALL! The car needs to relearn how to idle. It needs to determine the current position of the idle air control valve, and then how much movement makes how much RPM change. This procedure should take about 20 minutes. During this time, the car will idle up and down while it adjusts the IAC.

Now, if this doesn't solve the problem, then you may have more problems that you need to address. I would also look for leaky vacuum lines as they could cause this problem compounded by the need to relearn. Then, if all else fails, the ECU may need to be replaced. Sometimes, if not grounded in the correct spot, welding while on a car could damage the ECU. Were you "Mig" welding or "Tig" welding? I think the field produced by a TIG welder is much worse than the field produced by a MIG welder. I notice my cell phone goes whacko when I am near an active TIG welder (as well as my computer screen that is in my office more than 50 feet away from the welder).
Good Luck!
Drive Safely!
Bob, The Auto Answer Man

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