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1999 Lumina no power when cool, runs fine hot

I have a 1999 Chevy Lumina, 3.1 ltr V-6.



The car starts fine and idles fine.

While cool, has no power when you try to accelerate. While warming up, has no power after an upshift. Once warmed up, the car runs great. Problem is intermittent: some days it runs fine.



Took to a reputable mechanic, and he could not find the problem. Tried installing a new cranckshaft sensor - no change. Suggested new computer.



Took to a Chevy Dealer - diagnostics showed only that O2 sensor should be replaced. They cleaned the throttle body and Idle Air Control. No change. We repleced the Upstream O2 sensor ourselves - no change.



The following have been done or replaced with no change:

Spark Plugs replaced,

Ignition wires replaced,

PCV Valve,

Ignition Module,

Fuel Filter,

Fuel Pressure was tested - okay,

Ignition Relay was checked - okay.

Vacuum tested - about 19", steady.



So we are open to suggestions!



Thanks.



The problem might be with the coolant temp sensor for the computer. This is a primary input to the computer so it can adjust the proper fuel mixture for the engine operating temp. If this sensor has failed where it’s telling the computer the coolant temp is at 200 degrees when the engine is cold, the engine will run too lean. This can cause a lack of power from the engine until it comes up to operating temperature.

Tester

Thanks, Tester.

This makes a lot of sense. We received the same suggestion at the parts store this afternoon, so we already replaced it. The old one brake apart while attempting to get the wiring harness disconnected, and it looked pretty nasty inside. We did a resistance test on both the old and new sensor, and both gave very similar values indicating roughly the same ambient temperature of 90 degrees. So first indications are that it was somewhat okay.

The car was running fine today both before and after we replaced the sensor. Took it for a test drive after replacing it, and it was terrific.

But the intermittent nature of this problem has been frustrating. Some days it is fine, other days it is not. So time will tell now. I am hoping that perhaps the housing of the old sensor was cracked (although we could not see that), and it might have been affected by dampness on the East coast of Florida where my son lives, but it was nice and dry on the West Coast today where we were working on it. When cool and damp, it shorts out, and as the engine warms up and dries it back out again, it resumes proper operation. Just guessing here.

If this is not it, out next thought is the MAP sensor.

Thanks again for your help! --MinnLoon

By “ignition wires”, do you mean the “spark plug wires”?
Carefully check the voltage, and/or ohms, of the tps (throttle position sensor). Voltage spikes, especially just off idle, can cause erratic engine behavior. The engine computer depends upon the tps to tell it when, how much, and how fast, the throttle (gas pedal) has been moved. It uses this information (and that from other sensors) to control fuel and spark.

Thanks for your reply. Yes, I did mean the spark plug wires.

So to update everyone interested, and maybe to help someone else, we finally found and fixed the problem over Thanksgiving. Wanted to wait a while to make sure we solved it before posting this.

It turned out to be the cable that runs from the ignition module down the back of the engine, near the exhaust manifold, to the 7x crankshaft position sensor. Over the years, the heat and vibrations had melted off the protective casing of the cable, and the insualtion of the wires in about 5 places. So depending on conditions (moving the cable, or humidity) the cable was shorting out. We would poke around looking at things, and unknowingly move the cable away from the engine block (or manifold), and it would run great for a while. Either moisture would short it out until heat dried it sufficiently, or the cable would move and touch the engine block or manifold, and short out and cause the engine to run poorly.

The car has run perfectly since we replaced that cable 3 weeks ago.

Thanks to tester and hellokit for your ideas!

Cheers. --MinnLoon