Mechanics

IDLES ROUGH, HESITATES, AND SOMETIMES DIES BECAUSE RPM FALLS TOO LOW

1994 Chevrolet Silverado • 230,000 miles

When I start the engine it idles fine to begin with. I put it in gear and start driving, and by the time I get to the end of the street it has been stalling. I start it again, and it idles low and rough. When I give it the gas it hesitates badly.

I have replaced the IACV, TPS, ignition coil, cap, and rotor. A couple of months ago I rebuilt the TBI (including new fuel pressure regulator). I removed the EGR valve to examine for carbon buildup in the manifold passages beneath it and they were clear. The EGR valve itself has been replaced within the past few years. The coolant temp sensor on top of the manifold next to the water inlet is also fairly new at around six months old.
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Alkaline
November 23, 2011.




Is fuel pressure within spec? Have you checked for vacuum leaks? Any trouble codes?

The CEL came on twice during the past dozen or so times that I've driven it. On both occasions, the light reset/turned off after I turned the truck off. It is my understanding that I can't retrieve the code if the CEL has gone out. Is that correct?

I have visually inspected all the vacuum lines and can find no leaks.

Fuel pressure is something I've never checked. How can I do this?

Tiny
Alkaline
Nov 23, 2011.
If it is a hard code, it will stay in memory. If you can, give it a try to see if anything is showing. Also, if this happens when driving, you can use a live scanner to monitor the systems to see what is happening.

OK, just checked the code. It gave me a 44.

Tiny
Alkaline
Nov 23, 2011.
That is for a lean exhaust. Most likely a vacuum leak. Also, it could deal with the temp sensor or low fuel pressure.

Just checked for vacuum leaks using some spray carb cleaner. Didn't find any leaks. I also just now replaced the fuel filter.

It is still running rough and almost died when I got to the end of the street like it has been, but it might be running just a tad bit better than it was. I can't really know for sure until later on today/tonight when the temperature outside drops. Cold temperatures exacerbate the problem.

Since it is worse when it is cold outside, does that favor the problem being a bad temperature sensor vs. A bad fuel pump?

Tiny
Alkaline
Nov 23, 2011.
It could be either. Pumps can act funny in cold weather too.
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Will be replacing the fuel pump tomorrow. I'll update this thread with results.

Tiny
Alkaline
Nov 25, 2011.
Before replacing it, check the pressure it is producing. It's a lot of work and money if it isn't bad.

I think I may have found the problem. I think the temp sensor connector wasn't making good electrical contact. I removed the connector and applied some dilectric grease to the contacts. (I had replaced this sensor a few months ago and did not use any dilectric grease on it at that time). When I took it for a test drive around the block it ran fine. I'm soooo glad I looked at that before I dropped the tank and replaced the pump! I'm cautiously optimistic that this has fixed the problem. We shall see.

Tiny
Alkaline
Nov 25, 2011.

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