94 K1500 350 starts but won't run
 

I have a 94 K1500 350 Z71. It starts but will only run if the gas pedal is pumped. Before I start throwing parts at it, what would be the most likely reason for this? Just replaced the fuel filter which made no difference.

So I talked to my mechanic and he says the fuel pump is bad. That doesn't make sense to me because if the pump was bad why would pumping the accelerator keep the truck running? I can drive the truck by pumping the gas. It seems like it would more likely be a bad or dirty injector or bad pressure regulator. I guess I'm going to have to buy a fuel pump pressure tester to find out. Any other suggestions?

I'm currently working on my brothers 95 K1500 which is a TBI.

Here is what i found on my brothers since i started looking into it.
I was told "It will turn over but wont stay running unless i feather the pedal" so basically the same as what you are describing.

What i found in my brothers truck was actually very odd so its likely not your problem but i'll list what i found anyway just to toss some idea's around.

First things first, voltage supply to pump relay. Gotta prime the line so lets start there.
Voltage supply to coil for relay, 12V has to be there. Get a print and see what pin number goes to your fuel pump, on my brothers truck i believe it was pin 87 on the relay which was 12V and an orange wire which supplied voltage to both the Oil pressure switch and the fuel pump relay. Then there was the grey wire which also came from the oil pressure switch and the relay which was pin #30 i believe. Basically your fuel pump relay should energize(Run for about 15 seconds-aka=prime) once the key is in the on position which at that time should start your fuel pump by closing the set of contacts in the relay. This would allow the 12V to come from >>Battery +>>Fuse>>Fuel pump relay contacts>>Fuel pump. In my brothers truck i was not able to hear his pump run so after checking the voltage to the coil which was ok @12V i checked the supply from the fuse which was ok @12V and then of course at the grey wire which on his pickup can be accessed by the fuel pump due to a plug which is mounted to the frame which also is where the fuel pump ground wire was attached as well. I did not have 12 V at that grey wire which is my power feeding the pump. At this time i had someone else turn the key to the on position and realized the relay was switching, you should hear a click and also feel it with your finger positioned on the relay. Due to this not working i decided to check my ground. Came out to be 0.2 Ohms which is good right? Not always :-)

Ended up making sure the pump wasnt completely fried by jumping the orange wire to the grey wire on the oil pressure switch and on the relay base, the pump ran fine on both plug jump attempts.

So at this point i bench tested the relay and came to find that the relay was fine, Which is why i said not always to the ground.

As of right now i am about 14 hours in to tracing out the ground wire from my relay base because the relay wouldnt pull in due to a bad ground. Also ended up replacing oil pressure switch because i felt like being a nice brother and after the bench test (Using low pressure compressed air and a regulator to simulate oil pressure) turned out that the switch was bad anyway.

From what i saw in my brothers truck he likely has a low fuel pressure supply due to the fact that when i spiced into the ground wire and soldered in a new 12Gauge ground wire to the existing wire i found that i was able to run the pump and get the relay to work but i still am not getting fuel to the injectors like i should.

I rebuilt the TBI and i'm testing fuel pressure saturday so i'll let you know how that goes for me and see if i find anything interesting.

I'm in the search for the end of the ground wire for the fuel pump relay (Pin #86) because i am worried about another wire sharing that ground, if that is the case i may find my problem to not be a fuel pump pressure issue but to be a bad ground which is causing an open circuit on a differant componant.

I'll be sure to let you know what happens on my truck but if i were you i would definatly get a fuel pressure test kit and see what you have. The truck i'm working on will run w/12PSI to the TBI not sure what you will need, should be about the same.

The only other thing i would suggest you do if you have voltage to the pump and your relay is working would be to check your ground connection from your fuel pump to the frame, alot of rust can accumulate over the course of 20 years, you can pull it apart and file everything back down to bare metal and reconnect the ground and see if that was your issue.

Thanks a lot
 

Thank you for your extensive reply. Really helps. Yeah I just ordered a pressure tester but I will also check the electrical. When you rebuilt the TBI did that include the pressure regulator? I have 310,000 miles on this truck and it still doesn't burn an ounce of oil. Put a new fuel pump in it at around 200,000. When it quit it quit. So I'll start checking the electrical. Then the pump pressure. GM says normal pressure is 9-13#. We'll see.

Thanks again for your response.

I'm renting a pressure tester from autozone tomorrow. I'm spending 150 dollars to rent it and get a full refund when i return it. Although we know how autozone is, the gauge prly wont work well..lol.. but the kit comes with the TBI adapter which you'll need if you want to check pressure the the Throttle body.

I was looking into the pressure regulator rebuild and here is the funny part about that. If you go to a auto parts store and ask for the pressure regulator rebuild kit it will be around 60 dollars. If you get the Throttle body gasket kit it will come with all needed gaskets, the diaphgram for the pressure regulator (No spring included, must be a very expensive spring...haha) the fuel injector filters, 2 per injector. and the O-rings needed for injectors along with steel spacer for injectors. I paid 34 dollars for that kit here in wisconsin and weaver auto parts.

Typo...hehe corrected in red.

Found my ground splices, they are located on the left side of the distributor just before the harness clamp on the passenger side firewall. Total is 6 Ground wires spliced together, the corrosion in this connection was unbelievable. I cut everything free and put in a new connector and shink tubed everything several times. Wont have to worry about that ground anymore. I did not get a chance to check fuel pressure, i'll be doing that tomorrow. Took alot longer than i wanted to get all the wiring loose and inspect it and then get everything back together neatly.

You might try pulling the temp sensor connector and applying some contact grease. That's what fixed my '90 350. Started but idled horrible and belched black smoke. Seemed worse when it was cold outside.

Finally got to pressure test fuel pump. 5 pounds steady with engine running and pumping the throttle. Pressure less than half what it should be. Does that mean the pump is bad or could there be another reason?

Ok. Did another pressure test. Pinched off the return line and checked pump pressure. Still did not get over 5 pounds. According to another post, if the pressure had risen to at least 10 pounds it would indicate a bad pressure regulator. If pressure remains low it is probably a fuel pump going bad. Looks like I'll be changing fuel pumps. BTW, I bought a TBI rebuild kit from Rockauto parts for $28.36 no shipping no sales tax 2 day delivery and they're on the East coast and I'm on the West coast. Outstanding!