ADCW96

Hi everyone, I know there are countless posts about diagnosing fuel issues/ no start issues but I am still left with a few questions in my current situation. Any advice or insights would be greatly appreciated!

I have a 92 RS 5.0 TBI, it has been sitting for about 9 months and I do not recall any major issues when I parked it. It hasn't wanted to start, I replaced the spark plugs which were long over due, the distributor cap and rotor (fairly corroded), and I siphoned out the old gas and put in a fresh half tank. It turns over fine, and I managed to get it started using starting fluid a couple of times. Once running it would idle and run pretty smooth for about 15 minutes then the RPMs would bounce between 750 and close to 0 until eventually it stalls out. It would then restart a couple of times and do the same thing and then no more. Now it won't even start with starting fluid but there is an initial pop that I assume is the starting fluid detonating, so there is spark. I replaced the ICM since the car stopped restarting once it was warmed up, but no luck still no start.

I picked up a fuel gauge and put on a new fuel filter. I attached the fuel gauge to the outlet end of the fuel filter and performed the turn-key fuel pressure test. The gauge read about 8psi and would not hold pressure, dropping to 0 within a minute. When trying to start the car it jumps up to 10psi but still does not hold pressure and drops steadily. I took a break and came back to give it a few more tries, I repeated the same procedure, the fuel pump whirring sound is still audible but now the gauge reads 0 with no movement at all?!? I double-checked the fittings and even moved the gauge to the TBI inlet hose under the hood but same deal, no pressure. I checked the fuel pressure regulator vacuum line and it didnt appear to have any leaks or gassy smell although it did snap like a twig, i glued and taped it for now but the no pressure condition began before that.

I am thinking that it's a bad fuel pump, is there anything else I can check to be sure?

I'm still confused about relays.. I'm in the northeast and have a fair amount of rust so I know taking apart the rear end will be hell. Also, the car has a violent shudder when taking off in first ( when it used to drive), I was thinking its a hotspot on the fly wheel or the clutch, if I do replace the fuel pump/take apart the rear end does it save me any trouble to tackle the flywheel/clutch situation at the same time?

Thanks!

ShiftyCapone

iTrader: (7)

Sounds like a dead pump to me. It was slowly dying, allowing you to start, but it should hold 8 to 10psi while running.

ADCW96

Yeah, all things do seem to point to the pump. I was never able to start the car with the pressure gauge hooked up but i suppose that shouldn't change anything.

Thanks for your response!

Schurkey

Wild guess: Plugs are now fouled. They may dry-off, or they may need to come back out so they can have an appointment with a propane torch.

The pickup TBI system does not hold pressure. I'm not familiar with TBIs that have a fuel pressure regulator with a vacuum hose. Fairly sure the cars are the same way--don't retain pressure. Spec should be 9--13 psi. IF (big IF) your gauge is accurate, it seems the pump is weak since it can't achieve spec during prime even with half the vehicle fuel plumbing removed from the test procedure. OTOH, it should fire and run on 10 psi. The big question is whether it can maintain at least that much under load, when fuel demand is higher than at idle.

Many fuel gauges have a button on them, and a coil of clear tubing. Run the clear tubing into a drain pan, crank the engine with the button pushed. The pump should maintain pressure while the gauge assembly bypasses fuel into the drain pan, simulating high fuel demand.

Pressure gauge failed, plumbed wrong...or the pump died.

I won't drop the tank to replace a fuel pump unless I've verified that the pump has an adequate power supply. AT MINIMUM, connect a voltmeter in parallel to the rearmost plastic harness connector before the harness goes up 'n' over the gas tank. Verify that you've got battery voltage within about two volts WITH THE PUMP RUNNING. If system voltage is 11 volts when cranking, you'd better have AT LEAST 9 volts at the pump harness. If the engine runs and you have a charging alternator, system voltage should be 14+, so there'd better be 12+ at the rearmost connector. Then look for voltage on the ground wire for the pump. There should be no more than one volt. GM uses under-sized wire for the fuel pump circuit, so some voltage loss is inevitable.

Ideally, you'd be able to test for amperage draw of the pump, and in a perfect world you'd use a low-amperage probe connected to an oscilloscope so you can verify the amperage draw of each individual armature bar of the motor; and calculate motor RPM. Most folks don't have access to a probe or 'scope, but typical multimeters can provide an average amperage draw--typically is 6--7 amps.

Only if the car is jacked WAY up at all four corners, and you have the battery disconnected. That way there's room to drop the trans and slide it out. Otherwise, no, there's no labor savings for clutch work when doing a tank-drop except for perhaps moving/disconnecting the exhaust system.

CLEAN all the debris out of the gas tank, replace the filter sock on the pump, replace the hose that connects the pump to the hanger, use REAL clamps not those crappy worm-gear clamps sold at every parts-store in America, and replace the in-tank wire harness, along with the pump itself. The lil' hose probably comes with a new pump, but if not, keep in mind that it's "special" for SUBMERGED use in gasoline.

ADCW96

[QUOTE=Schurkey;6500903]Wild guess: Plugs are now fouled. They may dry-off, or they may need to come back out so they can have an appointment with a propane torch.


The pickup TBI system does not hold pressure. I'm not familiar with TBIs that have a fuel pressure regulator with a vacuum hose. Fairly sure the cars are the same way--don't retain pressure. Spec should be 9--13 psi. IF (big IF) your gauge is accurate, it seems the pump is weak since it can't achieve spec during prime even with half the vehicle fuel plumbing removed from the test procedure. OTOH, it should fire and run on 10 psi. The big question is whether it can maintain at least that much under load, when fuel demand is higher than at idle.

Many fuel gauges have a button on them, and a coil of clear tubing. Run the clear tubing into a drain pan, crank the engine with the button pushed. The pump should maintain pressure while the gauge assembly bypasses fuel into the drain pan, simulating high fuel demand.


Pressure gauge failed, plumbed wrong...or the pump died.


I won't drop the tank to replace a fuel pump unless I've verified that the pump has an adequate power supply. AT MINIMUM, connect a voltmeter in parallel to the rearmost plastic harness connector before the harness goes up 'n' over the gas tank. Verify that you've got battery voltage within about two volts WITH THE PUMP RUNNING. If system voltage is 11 volts when cranking, you'd better have AT LEAST 9 volts at the pump harness. If the engine runs and you have a charging alternator, system voltage should be 14+, so there'd better be 12+ at the rearmost connector. Then look for voltage on the ground wire for the pump. There should be no more than one volt. GM uses under-sized wire for the fuel pump circuit, so some voltage loss is inevitable.

Thank you for this!

OK, I broke out the multimeter. The battery is reading 12.3V and the connector right before the gas tank is reading 8.7V. a difference of 3.6V. I measured with the key on not while cranking as I don't have a helper. Could that drop in voltage be the root of the problem causing weak fuel pressure/ not the pump itself? I followed the pigtail behind the backseat and it appears to be fine atleast until the carpet. Where would you go from here?

ADCW96

I won't drop the tank to replace a fuel pump unless I've verified that the pump has an adequate power supply. AT MINIMUM, connect a voltmeter in parallel to the rearmost plastic harness connector before the harness goes up 'n' over the gas tank. Verify that you've got battery voltage within about two volts WITH THE PUMP RUNNING. If system voltage is 11 volts when cranking, you'd better have AT LEAST 9 volts at the pump harness. If the engine runs and you have a charging alternator, system voltage should be 14+, so there'd better be 12+ at the rearmost connector. Then look for voltage on the ground wire for the pump. There should be no more than one volt. GM uses under-sized wire for the fuel pump circuit, so some voltage loss is inevitable.

Thank you for this!

OK, I broke out the multimeter. The battery is reading 12.3V and the connector right before the gas tank is reading 8.7V. a difference of 3.6V. I measured with the key on not while cranking as I don't have a helper. Could that drop in voltage be the root of the problem causing weak fuel pressure/ not the pump itself? I followed the pigtail behind the backseat and it appears to be fine atleast until the carpet. Where would you go from here?

ADCW96

Ok after checking the voltage at the pump connector I moved to the fuel pump relay. Ive got 12.1V at the large orange wire on the connector and 0 at the larger red wire that goes to the fuse by the battery (which is not blown). When I jump the orange to the grey with a paperclip the pump comes on continuously. I plugged the connector into the relay next to the fuel pump relay, they appear to be identical, then I checked the voltage at the pump again and it is still reading 8.7V. So it seems like the problem is between the Fuel pump relay and the pump? Or would a dying pump draw less voltage and the pump could still be the problem? Its a jumble of taped wires under the dash, im not sure what would have happened while it was sitting if the problem is in there..

Schurkey

1. CHARGE THE BATTERY. 12.3 volts is half-dead.

2. There's no point to testing voltage near the pump unless the pump is running.

3. A loss of 3.x volts is excessive. That's a problem you'll have to track down and solve.

4. Have you checked for voltage on the ground wire? Voltage on the ground side adds to the loss on the supply side. 3.x volt loss on the supply, plus 1+ volt on the ground, means the pump has 4.x+ volts of loss total. If the battery has 12.3 volts, and drops lower when cranking--perhaps to 10 volts--the pump is running on only 5-something volts. When the alternator brings system voltage up to ~14 volts, the pump is running on only 10-ish volts.

5. I think it's possible that the pump could be drawing so much current that the voltage sags. Thus the recommendation to check amperage draw. Your multimeter is PROBABLY equipped to do this. I think it's more likely that you have a poor connection somewhere between the relay and pump.

ADCW96

Thank you Schurkey! Although this has taken me much longer than it should have I believe I have finally figured it out and learned a few things in the process.

Since my last post, the pump stopped powering on entirely. I believe this happened after I jumped some wires at the relay. Tracked down the fuse by the battery, it was blown. I replaced it, but still no power at the pump. I switched the relay connector to the identical neighboring relay and the pump powers on, I must have roasted the FP relay when blowing the fuse because it checked out ok previously. With the power on I checked the connector right before the pump and it is getting 12.3v and the ground is 0.1.

It seems like the pump is the culprit.
I really appreciate all the help!

JeepYJv8

I've had a clogged in tank filter and had issues like that. It Kind of sounds like yours might have trash in the filters or in the pump. When you drained fuel changing the filters would've been ideal. Sometimes there's one in the tank and one outside the tank.

When you were testing the first time. Key on only runs the system for 5 or so seconds to prime up. Then it turns back off (my TBI does anyhow).
Drop the tank and you will probably find a pump with trash in it or just worn out. With trash in it will also increase current draw and as result the voltage drops.
but as someone else said I your checking for voltage with pump off, not really going to tell you anything. Voltage at the battery while cranking can get pretty low to start.
When I'm checking batteries. I set my meter to min/max. Crank the vehical by yourself and the meter will record the lowest voltage. If I can't hold 9vdc while cranking that's when I change batteries. Ecm's s need at least 9vdc to operate properly. Keep an eye on that battery. Charge it up and test it.