TBI 454 idle problem

You've got classic symptoms of the fuel pump being driven from (only) the oil pressure switch.

HOPE that you have a failed relay. The contacts inside them can burn and then no current flows. Easy enough to swap relays from another circuit long enough to see if the pump primes.

HOWEVER, you could have a wire-harness problem, or even a failed ECM or ECM wire harness problem.

If the ECM or ECM harness failed, the relay doesn't get the trigger signal it needs to turn on the power to the fuel pump.

If the relay or relay harness fails, the relay doesn't turn on or there's an interruption in the power supply to the pump.

So the pump is driven by the oil pressure switch; but not until the engine cranks long enough to build oil pressure. While the engine is cranking, battery voltage is reduced, so the fuel pump starves for power.

Fix the relay problem first, then verify voltage and current to the pump with the engine running and alternator charging. Measure the voltage AS CLOSE TO THE PUMP AS YOU CAN ACCESS THE HARNESS. Excess voltage drop in the circuit could lower fuel pressure by starving the pump of power.

It's pretty typical that the pump is only supplied with 12-ish volts when the alternator is producing 14+ volts. GM uses under-sized wire in the pump circuit. Make sure the GROUND for the fuel pump has no more than 1 1/2 volts; again we'd like to see zero volts on the ground side, but skimpy wiring creates some resistance. The pump runs on about 3 volts less than system voltage; about 2 volts lost on the supply side, and about one volt lost on the ground. (Measured within about one foot of the fuel tank)

The voltage drop from harness corrosion INSIDE the tank is "invisible". I always replace the in-tank wire harness whenever I stuff a fuel pump into the tank.

Schurkey said:

You've got classic symptoms of the fuel pump being driven from (only) the oil pressure switch.

HOPE that you have a failed relay. The contacts inside them can burn and then no current flows. Easy enough to swap relays from another circuit long enough to see if the pump primes.

HOWEVER, you could have a wire-harness problem, or even a failed ECM or ECM wire harness problem.

If the ECM or ECM harness failed, the relay doesn't get the trigger signal it needs to turn on the power to the fuel pump.

If the relay or relay harness fails, the relay doesn't turn on or there's an interruption in the power supply to the pump.

So the pump is driven by the oil pressure switch; but not until the engine cranks long enough to build oil pressure. While the engine is cranking, battery voltage is reduced, so the fuel pump starves for power.

Fix the relay problem first, then verify voltage and current to the pump with the engine running and alternator charging. Measure the voltage AS CLOSE TO THE PUMP AS YOU CAN ACCESS THE HARNESS. Excess voltage drop in the circuit could lower fuel pressure by starving the pump of power.

It's pretty typical that the pump is only supplied with 12-ish volts when the alternator is producing 14+ volts. GM uses under-sized wire in the pump circuit. Make sure the GROUND for the fuel pump has no more than 1 1/2 volts; again we'd like to see zero volts on the ground side, but skimpy wiring creates some resistance. The pump runs on about 3 volts less than system voltage; about 2 volts lost on the supply side, and about one volt lost on the ground. (Measured within about one foot of the fuel tank)

The voltage drop from harness corrosion INSIDE the tank is "invisible". I always replace the in-tank wire harness whenever I stuff a fuel pump into the tank.

Click to expand...

Great info, thanks!

I swapped the relay with a spare, no change. I've got an old car battery so I threw it on the charger overnight, and this morning both relays tested good. Both engaged, continuity across the switch pins. Used the multimeter to check the socket, coil pins were good and switch pins received power when the ignition was set to key on/eng off, then it dropped to zero as it should. Had to use the stethoscope to actually hear the clicking, but the relays are getting signal from the ignition switch and 12v to the fuel pump circuit properly.

Long way of saying: the problem is after the relay in the circuit. And it's already 106* out, so that's it for today. Time to dig into wiring diagrams and look for pictures of where connections and grounds are so I can minimize time under the truck tomorrow.

Excellent plan. Good work so far.

It's been a while since I posted an update. Got quarantined after testing positive for COVID, had to catch up on work, then had a work trip out of town. And my garage time has been split between the truck and replacing an oil pan gasket on a C2 Corvette (and forgetting how the starter wiring goes, thanks to all the aforementioned delays). It's been more hectic than usual.

None of the local parts stores carry a TBI adapter for a fuel pressure tester. The Actron kit that's usually recommended (at least in my search results) is discontinued, and replaced by kit number CP7838, for about $57. It includes the TBI adapter...a shorter one than the thing that sells for $40 or so on its own, so it's easier to get installed. With that in hand, I could finally get to the portion of troubleshooting I didn't want to do.

The pump almost never did anything at key on/engine off. It's supposed to run for up to 2 seconds and build pressure. Once out of maybe a dozen times it crept up to 8 psi. Not even close to the 26-32 spec. When I got the engine started, it would fluctuate between 15 and 20 psi. At this point I knew that whether this was THE problem or not, it was definitely A problem that needed to be fixed. A lot of people recommend upgrading because the OE pump is basically maxed out already. I went with an EP381, which supplies the 96-97 Vortec motors with 60+ psi so I figured that gives me some wiggle room in the future.

Removing the skit plate and tank was easier than expected. I siphoned about 14 gallons of gas out...that's close to 90 lbs. Oof. Empty the tank was surprisingly light. The fasteners were in great shape: minimal surface rust, nothing was seized bad enough that pre-emptively spraying some PB Blaster didn't take care of it. Everything zipped out first try. I intended to replace the rubber hoses to the filler, but good luck finding them. The upper metal portion is sold everywhere, but I couldn't find a lower hose anywhere. The tank was in good shape, and the top of the pump was covered in a layer of some kind of yellow dumdum putty. I'm not sure what it was, but it did a GREAT job of preventing any corrosion at all. Scraping it off revealed a pristine lock ring and top of the pump assembly. The old pump housing was in fine condition so I had no concerns reusing it. The old filter sock was split wide open, not that I was going to reuse it. Everything went together as expected, the tank and plate were awkward to reinstall, but no real problems. Then came the moment of truth.

I turned to key on/engine off, and nothing happened on the pressure gauge. Tried it again...I figured it might take a couple turns to build pressure. Nothing. At this point I'm not happy. Then it hit me: The tank is empty, dumbass. 5 gallons and a turn of the key later, it budged a bit. Next turn, it shot to 28 psi and held for a couple of seconds before starting to dip. I let it drop down, then started the truck normally. Fired right up, pressure shot to 32 psi and held steady. Idle was rock steady, engine was noticeably more responsive to throttle input. The AC and shifting into gear have normal impacts on the idle, no more stalling or threatening to stall. After a few minutes I tried again, same great result. And again later that day. Went for a drive that night...the idle is spot on, the throttle response is definitely better than it was.

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Next repair: rear passenger door. The little plastic shark fin for the lock was broken, so the rod was disconnected. The door wasn't opening because it was stuck locked. Got a new handle assembly, but after poking around more I think the problem has to do with the power lock motor seeming to be hanging free inside the door. I need to find some pictures or diagrams of how it's supposed to be mounted inside, and how the rods are supposed to look. Mine was a twisted mess, and when I removed the 3 torx bits on the latch and it shifted, the lock motor fell off, hanging from its pigtail.
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Thanks for the help running this one down. I really didn't want it to be the fuel pump, but it wasn't too bad a job. Honestly I think dropping the tank on this truck was easier than on the Corvette, because everything was in better condition (20 or so more years of corrosion'll do that, I guess). I haven't put too many fresh miles on yet, but once the door is finished I'll be driving this as my daily for a couple of weeks to shake any other issues loose.

Glad to hear you got it running right, and thanks for the update.

Godholio said:

I set the IAC according to the factory service manual, but can't do the full calibration until I get the ALDL cable which is on the way. Also need to find which program will read my ECU...I'll be looking into that later today.

As far as fuel pressure, I've got the RV Morse adapter and gauge, but I have to say I'm extremely disappointed in it. Looks like I'd have to loosen a clamp somewhere down the fuel line to get it to flex far enough back from the throttle body to give the adapter room, but I think it would actually also have to be bent. Not happy about that. Not that it matters, because the gauge itself is too big to fit. Or the hole for the gauge to thread in is too close to the rim of the throttle body, however you want to look at it. I'd have to find another fitting so it could stand off further away from the adapter. So the jury's still on how I'm going to tackle that. After replacing the fuel filter I've had quite enough of laying on the pavement in the Las Vegas summer. As a 7.4, the fuel pressure should be 26-32 psi.

I can't find any more vacuum leaks, but I'll keep looking.

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Curious where you got the fuel psi spec. Most other manuals i see say it should be 9-13 psi. I have a 95 c3500 with 7.4 L. I rebuilt the tbi. Removed the factory set screw and installed my own screw. Turned on the key and got 22 psi, I then backed the screw out until it had no tension. I got 20 psi. I the went the other way and tightened it and got 30 psi. Truck starts up right away and have no check engine light. I am measuring after the fuel filter under the truck. Before I pull the tbi back apart i was wanting to know what the correct fuel psi should be for this year truck.

Some years of 7.4L TBI did have higher fuel pressure than the V-6 and small-blocks.

I don't know what year that started.

Fuel pressure spec will be in the service manual for your model year.

Schurkey said:

Some years of 7.4L TBI did have higher fuel pressure than the V-6 and small-blocks.

I don't know what year that started.

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Yeah, 94 & 95 had a 26 - 32 PSI (~30 is better) spring with smaller injectors (17104288).

I will have to look up the specs using my vin number. That must be the best way of knowing the specs for that engine.

CATman said:

I will have to look up the specs using my vin number. That must be the best way of knowing the specs for that engine.

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Look at the top of your injectors, they s/b color coded. if original. Yours s/b Pink/Purple.