1991 Chevy Silverado

Vincent, I have a 1991 Chevy Silverado ¾ ton pickup with a 350, 700R4 transmission, 150,000 miles, TBI fuel injection, ABS on rear brakes, P/S, P/B, A/C, Cruise, steering box steering, (typical GM steering box.

The truck runs great. Has good power and runs strong but (you knew there had to be a reason I emailed you) when the ENGINE temperature reaches around 180° and above then the truck has an off idle stumble. The stumble is NOT effected by OUTSIDE air temperature. When the ENGINE temperature is around 140° or lower, the stumble is not present.

The OUTSIDE air temperature can be 30° degrees or 110° degrees and it makes no difference to the stumble. The stumble is only present when the engine temperature reaches 180° and above. The stumble gets worse as the engine temperature climbs.

I live in Cedar City Utah (where is gets cold in the winter) and only notice the problem in the summer months even though I drive the truck everyday year around.

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I think I can help you with this one. It sounds a lot like a bad Coolant Temperature Sensor (CTS). When they fail completely they will turn the Check Engine Light (MIL) on, but they don't often fail completely. Most often they start sending the computer wrong information. A cold engine needs more fuel to operate efficiently, a richer mixture. As the engine warms up, it's fuel requirements decrease, the mixture gets leaner. It is the job of the CTS to tell the computer what the engine temperature is so it can meter out the fuel accordingly.

Now if the CTS is telling the computer the engine is hotter than it actually is, the computer will lean out the fuel mixture to the point where the engine does not have enough fuel to operate properly. And cause the problem you are having. The fact that it is ONLY engine temperature that effects the problem would tend to confirm this.

I would suggest replacing the Coolant Temperature Sensor, it is cheap and easy to do, and see what happens. I think it will take care of the problem.

If not, there are a couple of other possibilities such as a dirty fuel injector(s) or a vacuum leak. You may also have an EGR valve that is not closing completely.

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Aug 10, 2006 9:48 PM

Stephen Tolliver :

I have a 1991 Chevy Silverado. It has just over 100,000 miles on it, and runs fairly well. My problem is that my ac isn't blowing cold air. It was blowing cool air fine, and just seemed to stop abruptly. The vent blows air, but it's warm. I've checked my fuses and they're fine. I noticed that the compressor isn't coming on when I try to use the ac. Also, the ac light flashes for a little bit when I turn the ignition on, which according to the manual, is the "low coolant indicator". Now this may be just that- that I need more coolant. But would the compressor not come on if that was the case? Any advice would be greatly appreciated.

Nov 2, 2006 6:41 AM

Vincent Ciulla :

The A/C System has a Low/High Pressure Switch to protect the A/C Compressor if the refrigerant is too low. It is usually located in the Accumulator Assembly (big silver thing under the hood by the firewall). If you unplug it and jump the connector terminals the A/C compressor should kick on. If it does then it is a low refrigerant level problem. I recommend taking it to an A/C shop to have the system checked for leaks and repaired before trying to refill it.

Vince Ciulla

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