Chevrolet Repair: 92 chevy silverado running problem, throttle position sensor, chevy silverado

Question
I am driving a 92 silverado with a 4.3 v6, 4sd/OD manual Trans 4wd. When it sits for say several day the problem occurs.  Cold start-up all seems fine, engine warms up and then it seems to develop at hesitation or flat spot at lower rpm's. If you accelerate it will hesitate the feel a little rough while revving toward higher rpm range. You stop and restart the engine after say getting coffee and the problem will still be there. Also if it is in neutral and you give it throttle is will have a flat/dead spot then work it way up the rpm band.  If you let the truck sit a while an hour or two more times than not the problem will disappear and if you  
drive everyday there seems to no symptoms.. It has new plugs, wires, cap/rotar, and filters.  Any thoughts?

Answer
Hi Doug,
Your truck has a bunch of sensors that feed info to the ecm, which in turn adjusts the air/fuel mix and timing for optimum efficiency.

However....those sensors and adjustments are not done till the engine reaches operating temperature.
Prior to reaching operating temp, default values are used, which allow the vehicle to run fine, but just not optimum.
Probably be kinda rich at low rpm's, and lean at high rpm's, not great power, but perfectly acceptable for normal driving.

Hmmm...sounds like something they should have left off, but really, it normally works fine, and is transparent to the operator usually.

So, in your case, it sounds like you are running fine on the defaults, which would mean your plugs, wires, and fuel system are all fine.
It is a sensor problem we need to find.

The sensor that comes to mind is the throttle position sensor.
When you press on the gas pedal, it is connected by cable to the throttle body, and actually physically opens the throttle plate, allowing more air into the engine. If you open it wide open, you dump a massive amount of air in there.
Well, attached to the side of the throttle body, and connected to the shaft of the throttle plate, is the TPS, or throttle position sensor.
Not used when cold, but when the engine is at operating temp, and in closed loop, that is the gizmo that indicates to the ecm how much air is being fed to the engine, and the ECM feeds the correct amount of fuel through the injectors, and adjusts the timing accordingly.

The oxygen sensor in the exhaust then takes readings, and with those signals, the ecm fine tunes that mixture.
But your description sounds just like the old carburetor symptom of the bad accelerator pump, where the engine gets the slug of air without corresponding slug of fuel, and lean stumbles.

That TPS can be tested easily on the engine, with either a scan tool....the best way, or with a good ohm meter.
Smooth regular readings throughout the travel is what is desired.
Autozone will probably loan you a scan tool, on their loan-a-tool program, or maybe even walk out there and test it for you.

If a sensor is disconnected, it should store a fault code, and light the check engine light, but in the 92 area, you could have a dirty or bad TPS and not even get a code, if it gave some correct readings.

Fuel pressure should be tested before just running out and buying any parts, since low fuel pressure could also cause a stumble, although that should cause low power at normal operating temp also, but I would test it.

The coolant temp sensor is another item that can affect operation, and can be tested, but sounds to me like it is probably working correctly, with your cold and warm description.

Here is a helpful place for you, in case you haven't tried it yet.

Go to www.autozone.com
Enter your vehicle information.
Browse through the repair and troubleshooting stuff.

It is all free.
I have no desire to send you to Autozone....I use very little stuff from there. But that link is extremely useful.

Good luck,
Van