96 gmc 4.3 idles fine, runs great at wide open. It cuts out/misses bad when driving at cruise speeds Ideas?

I own a shop, and think this; It sounds like a bad throttle position sensor to me. The portion of the sensor that is used the most is in fact the cruse position. This sensor works like a volume control on a radio. As you turn it, an arm moves across a spring looking wire, either increasing the resistance, or decreasing the resistance of the wire (signal) to the ECM. The ECM regulates the fuel flow to the engine based on the resistance it reads from the sensor. A throttle position sensor very rarely sets a code due to the fact it varies by movement, and the computer has no way of knowing if you have your foot on the gas or not. To test this sensor; Its best to test it with an analog type meter on the ohms scale with the wiring harness unhooked at the sensor. Probe the terminals on a high ohms scale to determine the two that gives you a reading. Move the throttle at a slow steady pace while watching the meter for the needle to fall back. It should rise constantly through the full range of movement of the throttle, and not fall back any. If you hit a spot in which the needle falls back, then you have found the bad spot in the (rheostat) sensor.

Next thing I would check is the ground strap that runs from the engine to the firewall. If its missing, not hooked up, or making a bad ground, then you will have problems such as the one you are having.

Your problem could in fact be caused by a bad EGR valve, or vacuum leak some place.

I would also suggest removing the fuel filter to see if its plugged up. If the vehicle set for a while, then you might have trash in the tank that has plugged the fuel filter up. A tank thats empty, and sets for a period will form this flaky stuff on the inside that will clog up everything.

Finally; Check to see if the temprature sensor is working like it should. It may be bad.

Part 2:

Is it possible you have the distributor in the wrong notch? Put a timing light on the engine, and have a look at what its doing through the full range of the rpm's. I'm thinking you may have a worn out distributor, or its somehow timed wrong. Check to see if the timing is all over the place. Behind the glove box you will find a (brown wire) connector that must be unhooked when you set the ignition timing. The timing should set at 0 (zero) degrees @ idle, with this wire unhooked, and the engine at normal operating temp. Other things can make you think you have a fuel problem, such as a bad O-2 sensor. The distributor controls when the injectors fire, and the O-2 sensor controls how long the injector is open based on demand. Demand is determined by engine load, position of the throttle, and the temp sensor reading. Idle & cruise speed is regulated by the amount of air allowed into the engine through the (IAC) Idle Control Valve/Solenoid, so its possible there is a vacuum leak doing this. You must first rule out any chance of something in the ignition system doing it. Since you have pretty much covered your bases, attach a fuel pressure gage to the rail to determine that you in fact have the correct pressure throughout the range. A bad miss at cruise speeds really screams ignition problems. Don't be so convinced its a fuel problem. A fuel problem will show up throughout the rpm range, unless there is A: a vacuum leak B: a pressure loss C: Ignition timing out of range, and D: either the computer is bad, or its getting false readings from one or more information sensors. Since the engine was rebuilt, I'm inclined to think the distributor is somehow not installed right, or is moving the timing around at these rpm's. At wide open throttle there is little or no vacuum on the engine, and at idle there should be full vacuum. The EGR valve can do this exact thing, if its sticking. Unhook the EGR valve, plug off the vacuum hose to it, and drive the vehicle to see if it then does the same thing. If not, then you have the culprit. Use a propane torch with no fire. Move the torch around the entire top of the engine blowing propane around to see if the engine rpm's change when you do this. You may find a vacuum leak. If you have a Mass Airflow sensor/meter in the air induction system, it could be faulty. You can check this by tapping on it with the engine at idle, and if the engine stalls, its bad.

You can find many problems with an engine by running a vacuum test on it to see what the vacuum needle is doing. This is one of the best diagnostic tools a mechanic has. Remove the spark plugs and compare them to each other. Does one plug look different in the (electrode) gap area? I'm thinking you should rule out any mechanical cause, and between these two tests you may find the problem. Finally; do a compression test on each cylinder to rule out a sticking lifter or compression ring. If the engine was rebuilt, then you may have a valve that is adjusted wrong, and the spark plugs would show which cylinder is the cause.

Glad to help out, good Luck!!!

Some models were built with an incorrect EGR valve, which could cause your concern because EGR only operates at cruising speeds. GM released a recall #97024 in August '97. Check with your dealer to see if your vehicle applies.

sounds like tranny is jumping in and out of overdrive lock...ask how i know...1300 dollars later... 99 s10 4.3

easy way to figure it out w/no guessing would be to hook it up to a scanner,autozone will do this for free