GM-GMC: problems starting truck, diesel owner, gm diesel

Question
truck : 1991 chev 3/4 ton 2500
engine : 305 v8

truck dies/stalls if it isnt being rev'd very high.  Basically I cant move it from where it sits, because as soon as I take my foot off the gas it dies, and its nearly impossible to start.  It turns over fine. I had it at the shop and they thought it was the battery/alternator.  So today I had the battery, alternator, tensionor, and the belt replaced ($500) and its still doesnt work.  It turns over now how ever.  A friend suggested it maybe the timing, another suggested fuel filter or pump.  I am not currently working and credit card is practically out of room, so I have to make the next step corrrectly...any help would be great


Answer
Sorry to hear about your problem.  Unfortunately, it can be caused by several different systems in your vehicle.  I order of increasing cost, they are as follows:

1) A restricted/clogged fuel filter.  I am a GM diesel owner and I do not keep up with changes to gas engines.  If your vehicle has fuel injection, there is a stainless steel canister that is placed in the the fuel line between the tank and the engine.  If your vehicle has a carburetor, the filter is usually located right where the fuel line enters the carb.  If you do not know when the fuel filter was changed last, or if it has been more than a year, you should change it.

2) If your vehicle is fuel injected, the fuel pump is in the tank.  There are tests for pressure and volume (fuel flow) that can be done to verify proper operation.  I do not know what the test values are.  If you have a carburetor, there is usually a mechanical fuel pump located on the passenger front side of the engine right above the oil pan.  They are very reliable by they do wear out after the vehicle has accumulated many miles.

3) A carbureted engine can have a host of problems caused by the carburetor.  Fuel injected engines can have problems with sensors.  Your vehicle has OBD (On-Board-Diagnositc) capability and the codes can be checked with a hand-held tester.  AutoZone and other auto parts stores will often check them for free.

4) Timing chain and gears:  GM V-8s use a cam gear with nylon coated teeth.  After many miles, the teeth wear down and the chain stretches.  This can cause a situation where the chain "jumps" a tooth or more on the gear and puts the engine's pistons out of time with the camshaft and valves.  A compression test of all cylinders, or a vacuum test with the engine running, can confirm this problem.  Low, even compression across all cylinders and/or low vacuum readings indicate a timing problem.

5) Though I am mentioning this last, it actually should be the first check performed:  Ignition systems are usually the culprit when a gas engine won't start or stay running.  Electronic ignition systems are more reliable than the old points and condenser system, but your vehicle still uses a distributor, rotor, and spark plug wires.  All of these items, along with the spark plugs, should be verified as working correctly before blaming anything else.