No start TBI - fuel pressure drops to zero after priming

[shift5]

Hey fellas -

Got a no start condition on my '94 TBI 350.

-Truck ran great 3 days ago. Drove 60 miles. No issue.
-When I turn the key to run, the pump primes, and I get about 12 PSI of fuel pressure. Should be plenty.
-After it's done priming it drops to about 8 psi, then bleeds to zero within a couple seconds
-Cranking the engine seems to have no impact
-Engine starts on starting fluid and stalls after a couple seconds
-Has oil pressure

Any ideas?

 

[PlayingWithTBI]

When I turn the key to run, the pump primes, and I get about 12 PSI of fuel pressure. Should be plenty.
-After it's done priming it drops to about 8 psi, then bleeds to zero within a couple seconds

It's supposed to prime for ~2 seconds then shut off and lose pressure almost immediately, unlike on a Vortec which has a check valve. TBI does not have one. Then, when you start cranking the engine, it should turn on the fuel pump relay and give you your 12 PSI again. You also have an oil pressure switch that will turn on your fuel pump once it sees ~20 PSI+.

If it runs with starting fluid, it appears your fuel pump isn't coming on. There's a fusible link feeding them and then an in-line fuse after them. I'd check those 1st.

 

[Schurkey]

If the pump primes, the ECM has the ability to run the pump via the fuel pump relay. This is not likely a fuel pump problem, or a fuel pump relay problem. If the fuse was popped, the pump wouldn't run at all.

It may be a fuel pump wiring problem; the pump should run if the engine has oil pressure. But that assumes the pump isn't running based on the ECM getting an RPM signal from the ignition module. Verify by connecting a scan tool, see what the RPM is during cranking.

Are you sure there's no fuel pressure during cranking? Perhaps you have fuel pressure, but no injector spray. That could be a fuel injector wire harness problem, an ignition module problem, or an ignition module-to-ECM wire harness issue.

 

[PlayingWithTBI]

This is not likely a fuel pump problem, or a fuel pump relay problem. If the fuse was popped, the pump wouldn't run at all.

My bad, you're right, WTF was I thinking?

 

[Schurkey]

My bad, you're right, WTF was I thinking?

Ehhhh...just yesterday I was giving K1500 advice to a guy with a C1500.

Brain farts happen.

 

[dave s]

I actually had this issue when I joined up here on my 94. Ended up going away after I swapped in a new TPS. I think it was sensing a flooded condition and wouldn't let the pump come on. Maybe.

I did my fair share of "throwing the parts at it" before I settled in and started searching for hours about how the old TBI works. After rebuilding my TBI and installing the CFM technologies fuel block and gauge to monitor fuel pressure, replacing fuel pump, and replacing fuel pump relay, I gave up and started reading. I actually read about the TPS on an RV site where a guy was having my exact issues. Starting fluid and truck runs forever. Shut off and you're toast. I'm not saying that's your problem, but it did solve my starting issue. Maybe.

Now about that same time, my ignition module gave out. I replaced it with a cheap one from Advanced. Didn't last long and truck wouldn't start. Even with starting fluid. Waited a week and it started right up. WTH...So I put in another module. Same crap. Worked fine couple weeks then nothing. Satrting fluid and it fired right up. Finally replaced both module and coil with new AC Delco parts and never had another problem.

I'm not knowledgeable enough to know the answer to this problem but I always wondered if the module could cause the fuel pump not to work when trying to start? Once it got a sniff of the starting fluid it would run. It seems like the "it runs if you use starting fluid to get it going" should be an easier diagnosis but it always seems to have many causes. I really would like to hear more on this issue.

 

[Schurkey]

I always wondered if the module could cause the fuel pump not to work when trying to start? Once it got a sniff of the starting fluid it would run. It seems like the "it runs if you use starting fluid to get it going" should be an easier diagnosis but it always seems to have many causes. I really would like to hear more on this issue.

Absolutely. The module can fail to send an RPM signal to the computer. With no RPM signal, the computer doesn't turn on the fuel pump via the fuel pump relay.

However, if you start the engine on starting fluid--or just crank it long enough to build oil pressure--the oil pressure switch bypasses the relay, so the fuel pump runs even without an RPM signal to the ECM.

I'd expect poor fuel economy and low power due to the ECM not controlling spark advance like it should--but the truck should run and drive.

my ignition module gave out. I replaced it with a cheap one from Advanced. Didn't last long and truck wouldn't start. Even with starting fluid. Waited a week and it started right up. WTH...So I put in another module. Same crap. Worked fine couple weeks then nothing. Satrting fluid and it fired right up. Finally replaced both module and coil with new AC Delco parts and never had another problem.

The usual cause of repeat ignition module failures is a partially-failed ignition coil.

Any time I replace an ignition module, I test the coil with an ohmmeter AND a spark-tester calibrated for HEI ignition systems. An ohmmeter can tell you if an ignition coil is BAD. It cannot tell you that the coil is GOOD. Thus the need for the calibrated spark-tester.

 

[haroldwca]

You might also consider two things. One is that the fuel pressure regulator may have failed. It is just a rubber diaphragm and spring in the throttle body. The throttle body itself may look intimidating (or it may not, depending on your DIY-ness) but it's simpler than a Holley (I prefer Rochesters, but that's another story). You can have it off the intake and disassembled in a manner of minutes. If the rubber diaphragm is compromised or the spring is broken or crusty, you will not be able to hold adequate fuel pressure.

Also, try this check. Unplug the connector to one of your fuel injectors. Contact the terminals of a 9V battery to the terminals on the fuel injector. You should hear a click, which indicates an operating injector. No click = bad injector. They are about $40 at a discount auto parts store. If the injector checks out as stated, use the battery to activate the injector while you have someone crank the engine. If it runs (albeit, poorly - you are only on one injector), your injectors are not being fired by the ECU.

I'm significantly more inclined to think it's the regulator or a bad injector, rather than an ECU problem. I know that nothing I've said guarantees success, but maybe something I said will help. Good luck !

 

[PlayingWithTBI]

One is that the fuel pressure regulator may have failed. It is just a rubber diaphragm and spring in the throttle body

He said he gets 12PSI with KOEO so, I would think the pressure regulator's diaphragm is still good.

 

[Schurkey]

If the diaphragm failed, wouldn't there be gasoline pouring out the bottom of the regulator?