I have a 1991 GMC S-15 Jimmy 2WD with the 4.3L V6 and a 5 Speed manual transmission. I have approximately 260,000 miles on the drive train. Lately, I have had a problem with jerky and sluggish acceleration.

I first thought that the clutch might be on its way out, but I ruled that out this morning when I couldn't keep the motor running. The night before I had replaced the distributor cap and rotor, and I noticed that the distributor had a lot of corrosion. Also, fearing that I damaged the distributor shaft or a pickup coil when removing the old rotor, I replaced the distributor. I then had the ignition timing set by a professional mechanic. At first, things seemed to be working, but the problem soon returned. I replaced the ignition coil and alternator (alternator because the bearings were failing in its pulley). Still, no luck. I checked for any apparent vacuum leaks, but did not find any. I haven't been able to burn of the tank of fuel, so I can't rule out bad gasoline.

Symptoms are as follows: during acceleration, the engine sounds and feels like it's trying to die. There is a noticeable loss in power that continues until I am able to build the revs past 4000 rpms (just guessing by sound, no tachometer on this model) and then all the torque of the motor kicks in and lurches the vehicle forward. By this time, however, it's time to shift gear and start all over again. Earlier, I took the car on the highway at 70mph for 10 minutes. Afterwards, it seemed to be working fine. I parked the car for a couple of hours and am now back to square one.

The injectors seem to be spraying fuel into the manifold correctly, though I can't be entirely sure it's the correct amount. I have a new air filter. At this point I am concerned that the either the fuel filter or fuel pump might be failing. The filter isn't too big a deal, but the pump will be a pain and a bit beyond my skill. Any advice anyone can offer would be appreciated.

Try cleaning yor EGR valve (carb and choke cleaner) if that helps you may want to replace it but I recommend cleaning first due to the cost of a new one,is located in front of the engine on the top is very accesible there is only 2 bolts to it be careful not to snap them and if you are careful you may be able to reuse the gasket even do a new one is recommended

Replaced the spark plugs and plug wires. The plugs in the motor had a gap of approx. 0.020" when they should have been at 0.045". Don't know how this happened since I correctly gapped them prior to last installation. Started the motor and had the same problem as the other day: engine would idle fine for approx 30-60 seconds and then try to die. Just before the engine stalls, the idle kicks back up to idle and then the whole cycle repeats. Unplugged the Throttle Position Sensor (TPS), results as follows:

Engine idle steady, but lower than normal. Check Engine light on (duh!), but the engine is running better without the sensor. I will replace the sensor tomorrow (approx. $35). If that doesn't work, I'll try cleaning the EGR valve like you (56F100) suggested.

This has been frustrating because the problem happened so suddenly. Also, I've had this vehicle 5 years and never had a major problem. The only time it ever "broke down" was once when the Distributor cap failed, and once when the clutch cylinders failed. Hopefully it will be working fine again tomorrow. Will update on results.

Inspect for vacuum leak at the throttle body gasket: remove the air cleaner and prop the throttle plates open. Look down the bore and if the gasket has pulled in, even slightly, replace it. Clean the entire housing and passages, too.
Let us know.

Replaced Oxygen sensor. Removed the throttle body and cleaned thoroughly with carb/choke cleaner. Throttle body not that dirty considering its age. Replaced the throttle position sensor. Reassembled throttle body onto manifold with new gasket. Reconnected all sensors, fuel lines, and vacuum lines. Started motor and noticed immediate increase in power at idle. Hearing a distinct whistling from somewhere, but I cannot pinpoint it. Took the car on the road and experienced definite loss of power in the mid range of all gears as well as the engine struggling. I am at a loss. Still have to check the EGR valve, just trying to put it off. If the EGR valve and vacuum solenoid check out, I don't know what to do.

Checklist:

1. Replaced Distributor, Distributor cap, and distributor Rotor
2. Had Ignition timing done by professional
3. Replaced Ignition Coil
4. Replaced Alternator
5. Replaced Spark Plugs and Wires -- Gap set to 0.045"
6. Replaced Fuel Filter
7. Replaced Oxygen Sensor
8. Cleaned throttle body and replaced throttle body gasket
9. Replaced Throttle Position Sensor

If the Ignition Control Module is not new, replace it:

Ignition Module/Control Unit by ACDelco - Advance Auto Parts (http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Ignition-Module-Control-Unit-ACDelco_3964256-P_194_R%7CGRPTUNEAMS_347789924___)

Hearing a distinct whistling from somewhere, but I cannot pinpoint it.
Distinct whistling is bad.
Carefully inspect around the area you just serviced. Something is out of place.

While the engine is warmed up and idling, take some of that choke/carb cleaner and, with the straw, apply very small shots of that around the throttle body area. Listen for the engine to rev while it burns off the cleaner. That should tell you where the vacuum leak is located.

Have you tested fuel pressure/volume?

I'm taking a break for now. The car is being very frustrating. The Ignition Control Module is new and came as part of the distributor. A diesel mechanic where I work also suggested spraying choke/carb cleaner or WD40 in small amounts to check for vacuum leaks... and I will try that next, followed by the EGR valve if necessary. Thanks everyone for your help. Will post results as soon as I get anything done.

Back again! I took a break for awhile because the problem was becoming increasingly frustrating. I checked and cleaned my EGR valve, but the valve was not that dirty and was functioning properly. It also turns out that the bearings on the new distributor were bad and the shaft of the distributor was allowed to wobble. I replaced the distributor (again! ) and had my timing reset. Everything seemed fine until the car started dying again. I added half a bottle of octane boost (per my mechanic) and things seemed better for a day or two. The problem increased in severity to the point where I could not keep the car running. That is when I discovered something interesting:

When the car starting dying/gasping, I noticed the fuel spraying from the injectors died into a trickle. My mechanic and I checked the fuel pump relay, but it was good. We then conducted a fuel pressure test:

According to my information, normal fuel pressure for this motor is approximately 9-13 psi, however, the fuel pressure gauge indicated a maximum fuel pressure of 0.5 psi! We can hear the fuel pump running, so I think a check valve in the pump has failed. Under $100 to replace the fuel pump (pump, wiring harness, filter screen). Will get that done as soon as possible and report back with the results. I do think this will fix the problem, since all my symptoms can be explained by low fuel pressure. Thanks everyone for your help and advice. Also, a special thanks to TxGreaseMonkey.

Check your gas cap. Make sure it is venting

Check the gas cap. Make sure its not plugged for air,. need too be vented