Stuck On What To Do Next To Fix My 94 Gmc. Help Please.

theflatlander · April 17, 2011

I have a 94 GMC 1500 2wd manual 4.3l vortec tbi. Bought this truck from a friend who change the plugs and wires and he never got it to run right again. I went through it myself and changed the plugs, wires, rotor, and cap to make sure it was done right. Well this didnt solve the problem of the rough running and idleing issues.

Next thing i checked was the fuel filter and replaced it I also check fuel pump fuse and relay. I ended up replacing the relay because at the moment i had no way to test it and it was only 10 bucks. Well this still hasnt took care of my issue yet. I made a test gauge set up and removed the fuel filter to use the test gauge in its place. The way i understand is the truck should have 50-60lbs standing pressure with 9-13lb's running pressure. Well when i had some one hit the key, when the fuel pump primed it only jumped up to like 2lbs of pressure so in my mind this is saying the pump is bad. I also couldnt hear the fuel pump prime some of the time. I went fourth with starting to drain the gas after i removed the test gauge i put a hose of the supply line before the filter and was hopeing by turning on the key the pump would try to prime itself and i could drain the gas this way. Of course that didnt work. I tried to start the truck just to see what would happen and the fuel pump started throwing out the gas like crazy but as soon as I let off the key from starting it stoped pumping the gas...

This had me questioning if it was a fuel pump problem at all. I jumped the relay on the ODB1 and waited for it to blink 12 three times like you are suppose too and it came back with no error report. Im still thinking its the fuel pump. I have checked as much of the wiring, and vaccum lines that i can and everything seems to be good. The other thing is some times i can get the truck to start and some times I cant but at the same time I can always get it to start with a secondary fuel source through the throttle body. Should I be looking at replacing some valves and sensors before I tear into the fuel pump? Is it possible the computer is reading the problem?

Im looking for any help or suggestions at all guys. No one in the town Im in seems to be too familiar with the tbi setup 4.3l vortecs. if you want to pm me or i could also to talk to you over the phone. If you want to know anything else feel free to ask. I tried summing up all of what I did as best as I could.

thanks for your time.

rdnkcpa · April 18, 2011

Have you checked the intake manifold gaskets? Compression check? How many miles on are on the truck? I think these are also known for having distributor problems with high mileage. One of the experts can chime in on that.

I'd stop throwing parts at it and try to diagnose it. Put a fuel pressure gauge on it and see what you get. Not sure on the 94's but there should be a shrader valve near the back of the engine where you can hook a pressure guage up to.

theflatlander · April 19, 2011

Have you checked the intake manifold gaskets? Compression check? How many miles on are on the truck? I think these are also known for having distributor problems with high mileage. One of the experts can chime in on that.

I'd stop throwing parts at it and try to diagnose it. Put a fuel pressure gauge on it and see what you get. Not sure on the 94's but there should be a shrader valve near the back of the engine where you can hook a pressure guage up to.

thanks for the info. checked the fuel pressure and it was about 2 psi going to install a new fuel pump and check into the ignition sytem a little bit more since I already replaced the rotor and the cap.