GM TBI Fuel problem

I bought a 91 Firebird with a no start condition as an engine donor for my challenge project. All appears well with it, except the fuel pump won't pump fuel. It'll run on starter fluid. Here's what I've done so far:

1. Checked the fuel pump/ecu fuse. Its good
2. Tested the wiring between the battery and the fuel pump relay. The power wire is hot, and the signal wire powers up for a few seconds when turning the ignition on, and stays on when cranking the motor.
3. Did a resistance test on the relay, didn't do a load test as I don't have enough gator clips.
4. Spent way too much time replacing the fuel pump, since that's what the other symptoms were pointing towards.
5. Replaced the relay, in case the old one was defective somehow.

There was a small hoard of nut shells on the drivers side area of the hatch where the seat belt bolts up, so a mouse or some other critter was in there at some point. Combined with the fact the fuel gauge reads full when there is not a full tank of gas (I had initially assumed a bad sender, since its was a 25 year old GM car), I am guessing that some wiring has been chewed on.

Before I start ripping out the interior to look for damaged wiring, or just hotwire the fuel pump, is there anything else I should be checking for?

Hot wire fuel pump at relay and check pressure with gauge.

Listen at tank to hear pump run.

Filter?

Check relay and fuel pump for ground.

bentwrench wrote: Hot wire fuel pump at relay and check pressure with gauge. Listen at tank to hear pump run. Filter?

Been listening for the pump, unless the original and emplacement are the quieted pumps in the world, they are not firing up. Will see if I can get my wife to listen with the hatch open, though.

Afraid I can't test for fuel pressure, at least not without picking up an adaptor, since GM didn't put a schrader valve, I'd need to track down an adapter. Had someone looking over the tbi unit when I first tried to start it, and they did not mention seeing any fuel coming out, though.

Edit: I also replaced the in tank filter.

In reply to Dusterbd13:

Is the case of the fuel pump grounded, or do I need to test at the wiring connection?

Yes

Since your gas gauge indicates "full" all the time, I believe that you've lost the ground to the fuel pump/gas tank float unit. I have a similar problem with my '89 L98 recently. Turn out the internal ground connection was corroded from years of exposure to ethanol and water. Had to replace the whole assembly unit (float sensor & fuel pump) to correct the problem.

To see if this is your problem, do a continuity check between the internal ground and the fuel pump and then the ground and the tank sender unit. A bad internal ground circuit will show no continuity and/or the circuit will disconnect if the wiring connector is moved while testing. Here's a link to a similar problem as discussed on FFRAX.

Hope this helps.

Check the ignition module?

Bobzilla wrote: Check the ignition module?

I'd think it wouldn't run on the starting fluid if the ignition is bad?

In reply to MrBeachcomber:

Hmm, some of the connections looked pretty cruddy when I had it apart, but I was in a hurry as I was losing light. I'll have to take a look at this.

Make a jumper and put power to the pump. I believe the grey wire is the +12v, black is for sure ground, and purple is sending unit signal wire.

First thing I would jump is black to ground, it may be that simple. I kicked up a stick once with my tbi truck and hit the fuel pump ground wire just right that it ripped off the screw on the bed crossmember. That was a fun roadside diagnosis

eastsidemav wrote:

Bobzilla wrote: Check the ignition module?

I'd think it wouldn't run on the starting fluid if the ignition is bad?

Interesting thing.... you can sometimes still get spark but no fuel from these damn things. Ask me how I know. No fuel pump and no injector pulse, but spark. Tested the Ign mod, it came up almost good, replaced and golden. stupid GM injection.

MrBeachcomber wrote: Since your gas gauge indicates "full" all the time, I believe that you've lost the ground to the fuel pump/gas tank float unit. I have a similar problem with my '89 L98 recently. Turn out the internal ground connection was corroded from years of exposure to ethanol and water. Had to replace the whole assembly unit (float sensor & fuel pump) to correct the problem. To see if this is your problem, do a continuity check between the internal ground and the fuel pump and then the ground and the tank sender unit. A bad internal ground circuit will show no continuity and/or the circuit will disconnect if the wiring connector is moved while testing. Here's a link to a similar problem as discussed on FFRAX. Hope this helps.

This guy is on the right path.

I replaced a fuel pump on a third gen firebird for a family friend a couple years ago, and it wound up needing a pump & housing assembly due to a faulty internal ground wire. It was exactly as described in the quoted post, except that I don't think it was from corrosion.

Hmm, since this car is going to be getting scrapped (or sold as a shell) after I yank the engine, its tempting to just cut the in tank harness, and hotwire it to bypass all the failure points. Just means that during my test drive I'll have very nicely vented gas tank so I don't pinch the wires.

Edit: I may have some time to ponder this one, since the weather looks to be getting kind of crappy for the next several days. If nothing else, I'll switch to tearing the 4.3 out of the S10

All I can say is that I'm pretty sure the way to get the fuel tank out of a 3rd f-body to get to the fuel pump and sender is to jack it up so the axle is at full droop. I've seen people cut access panels into the floor instead though. (no there isn't one there from the factory)

Chadeux wrote: All I can say is that I'm pretty sure the way to get the fuel tank out of a 3rd f-body to get to the fuel pump and sender is to jack it up so the axle is at full droop. I've seen people cut access panels into the floor instead though. (no there isn't one there from the factory)

Yup, cut the access panel already, since it's unlikely I'll be trying to resell this thing. I was not going to drop the gas tank in the driveway in an Ohio winter, and risk breaking gas tank straps and any 25 year old rubber tubing.

Finally hotwired the fuel pump. It was definitely not running before, as it's easy to hear when running. Still, the car won't start. The security light goes out as it should, so VATS is not disabling it. It does run on starting fluid. Even with just the starter motor turning, it builds oil pressure, so I suspect the engine is good.

Any suggestions on things to check before I break down and order an adapter to test fuel pressure in a TBI system, since a Schrader valve would have cost GM a few extra cents?

You tee into the supply line at the fuel filter to test pressure.

The pressure regulator is in the rear of the throttle body and is rebuildable.

One more item - had my wife look down the throttle bore while I cranked the engine. She did not see any fuel coming out of the injectors. So, I'm guessing either no fuel pressure, or the injectors are not firing.

Got under it, and the fuel filter looks pretty new, as a matter of fact, I'm guessing it was replaced when the prior owner was trying to fix it. Checked the arrow on it, and it is oriented in the correct direction. Fuel lines are very crusty. I also checked the injector fuses, just in case. They were good.

Will probably get a hold of an adapter to check fuel pressure soon, and just go back to working on the S10 for now.

At this point, if I knew the transmission was good, I'd probably stop worrying about this, and just convert it to carb when I swap it onto the S10, but I really want to make sure the 700R4 is okay.

Do you have a noid light to test for injector pulse?

patgizz wrote: Do you have a noid light to test for injector pulse?

No I do not. I'm not even sure what that is. Is it something I can plug into the harness somewhere?

Edit: Looked it up. Tempted to pick one up locally, if I can find one. Harbor Frieght has a set, but I kind of doubt I'll need the others anytime soon.

eastsidemav wrote: Harbor Frieght has a set, but I kind of doubt I'll need the others anytime soon.

Chances are good that the whole set from HF is cheaper than buying the GM one on its own. I needed a specific Allen wrench once, and it was cheaper to buy a whole set at HF than the single size I needed at a hardware store.

In reply to EvanR:

The HF set is around $40, and I think I can get the individual one for Amazon for under half that. I'm still searching to see if anyone around here actually stocks it, but no luck so far.

Edit: looks like Autozone has a set for $25. Will probably run out this evening to get it, but may wait until tomorrow for any more testing.

In reply to MrBeachcomber:

FWIW MrBeachcomber is the SOLE reason we were able to get our 2016 Challenge vette up and running. He tirelessly assisted us long distance in troubleshooting a truly messed up fuel injection system. He helped us completely rewire and troubleshoot a TPI system that had suffered from nearly thirty years of peckerwood jake leg attempted repairs.

Listen to his advice. He really knows his stuff. We couldn't have made it without him.

THANKS AGAIN GREG!