OT:fuel problem on tahoe

[70cst10_05]

I recently bought a 96 tahoe 4 door.....it has had previous fuel pump problems....yesterday it started acting up but there its this little wire that the guy rigged up so it would work...well today i get in it to go run around town and it goes about 4 blocks and dies and won't start.....i finally get it started and it runs for about 10 seconds and dies. This is all it will do. I figure i can leg it home so i do, and all the way home its fine and i get home and put it in park and it doesn't die so im thinkin its ok....i come inside and go back out later and it does the start and die thing......any idea what it could be??all help appreciated...
thanks,
kevin

[Piston]

Are you positive its a fuel problem?
By the description, its very possible there is a ignition module problem. It gets hot and shuts down, cools off and runs a bit more then dies. the longer you let it cool off, the longer it will run.

Verify whether your loosing spark or fuel then go from there.

[70cst10_05]

Piston, you might be right on it being that....im not sure at all what it is....it if is that then i don't think it will run for very long at all until tonight...because its a black vehicle and its about 85 degrees out no clouds.......how hard is it to replace that and how much would it cost??

thanks
kevin

[Piston]

They aren't too bad.Depending on if this is a true 96, it will be a vortec engine in which case the module is mounted in the same bracket the ignition coil is in on the right side of the engine. A 5.5mm nut driver or socket will handle the job. You may need to R&R the coil bracket with (2) 10mm bolts then seperate the coil bracket from the module.
They run about $70.00 or there abouts...

If it is the module, make sure you scrape the old grease off the heatsink and replace it with the grease supplied. Old dry grease will cause heat issues again and ruin the module.

The trick is, catch it while its doing it then find out if it has no/low fuel pressure or if its loosing spark.

[70cst10_05]

How would i go about seeing if it has no/low fuel pressure or if its losing spark.....i can't hear a fuel pump whine...but i figured if it was the fuel pump it wouldn't start.....this is just driving me insane....i was going to put a module in it but i couldn't find a parts store that had one....

[Jeepster376]

Diagnostics are in order to determine wether it's a fuel or ignition problem. A bad fuel pump is entirely capable of causing the symptoms you describe. also a plugged fuel filter could cause the symptoms. The fuel pump is controlled by a relay which could cause a problem. From your description I would guess it's the pump, which can run for a few minutes but overheats and jams. Either monitor your fuel pressure, or listen very carefully to your pump. You should hear if your pump quits just a second or two before the engine quits. If it does quit check for it for voltage, the computer should power the pump on for about 2 seconds when you first turn on the key, after that the pump will not be powered until the oil pressure comes up.

[Jeepster376]

I just re-read your initial post and realized what you described. From your description of starting and running for 10 seconds means your oil pressure switch is not activating the pump.

Each time you turn the key off and then on the computer will activate the pump for 2 seconds to build up starting pressure. Then the pump will shut off, after the engine starts, the oil pressure switch reacivates the pump to maintain running pressure. Sounds like your problem is in this circuit. Either a bad oil pressure switch, or relay, or wire.

[70cst10_05]

so it will die if that switch is not running the pump or what?? i understand what you say is wrong but i don't understand how it would make it die?? is it one of those things of thats just the way it is??
thanks
kevin

[Jeepster376]

Originally Posted by 70cst10_05

so it will die if that switch is not running the pump or what??

Absolutely the engine will die if that oil pressure circuit is not working. It is designed as a safety measure to kill the engine in the event of a real oil pressure failure.

[Jeepster376]

It is also a possibility that the PO jury rigged an ignition power wire to the pump, due to the same circuit failing before, now if his wire has gone bad you're back to the original problem. The engine will only run for 10 seconds per each time you precharge the system by turning the key on.

There is a pressure check fitting on your fuel rail that appears similar to a tire valve (but different) It takes a special guage, but if you start the engine and let it die, then depress the valve there will probably be no pressure. If you then turn the key off, then turn the key on but don't start it, then depress the valve and fuel will squirt out. If this is the case then that is your problem. There are check valves in the system to maintain pressure in the rail, there should just about always be pressure there wether the engine is running or not.

[70cst10_05]

ok i checked the valve.....i started it and let it die,(which only took about a second or two) depressed the valve and fuel squirted out....i then turned the key off..then turned the key back on..on pressure or fuel....started it and let it die then it squirted again.......what gives?

[Jeepster376]

Sort of an unscientific test, but it may blow my theory, without a guage it's hard to be sure though.

Here's another easy thing to try, after the engine dies, will it start right back up by just hitting the starter, or must you turn the key off, before it will restart. If you must turn the key off then my theory is correct. if it will restart by just hitting the starter then it is something else.

beyond that you need to do some real diagnostics, with a fuel pressure guage and a timing light and figure out if it's a fuel or spark problem.

[70cst10_05]

you have to turn the key off and back on...but it still starts hard......im givin up for the night....thanks for all of your help and i will be back tomorrow to try some more stuff....thanks again ill probly plug a computer into it on monday


kevin

[Piston]

Unfortunatly at this point its all guessing until some tests are done. When it dies, simply pull the coil wire off the dist and have someone crank while you watch for spark (position the wire near a ground). If you have spark and it doesnt start, move on... ignition is not the cause more than likely.

Also, on the fuel pressure delima, you almost need a fuel pressure guage because on this system if there is not exactly 55psi or higher, the injector unit will not work properly. So you could still have 35-40psi of fuel pressure (enough to spray out of the test port) but it will not run the engine.

I will forewarn you, these have a terrible history with fuel pumps.

A few questions for you though.

How does it run when its running?
when it dies is it just all of the sudden or does it gradually loose power then die?
when it starts back up is it ok or does it run badly?

[70cst10_05]

it runs great when it running, and it all of the sudden dies....i can hold it at 2500 rpms or higher and just dies....when it starts back up it usually takes a few crankings but other than that runs good.....the P.O. put 3 fuel pumps in it and it didn't change the prob. w/ it....it had a differetn prob. that he fixed w/ wires and a toggle switch taht he showed me how to use.....

[Piston]

Check for spark when it dies, lets rule that out real quick. I'd also be interested in getting that "wires and switch" thing fixed too. Obviously there are some fuel pump wiring issues that just may be causing the problem. But lets eliminate some easy checks first.

[70cst10_05]

alright guys i traced the problem....first of all the fuse box under the hood is loose...the clips that hold it are broke, there is a wire that goes to the fuel pump relay....well the relay is different, like an aftermarket compared to the other ones and my dad thinks the pegs or whatever are smaller than on the other relays because it slides in and out really easy. we figured out it'll run for a while sometimes but not others and this is because of a wire connection problem when you jiggle the wire it'll die......so i dunno if this is a question or a statement or some sort.....but thats what we figured out..

[Jeepster376]

So you found the problem, nows comes the easy part, to fix it.

Let us know how it works out.

[Piston]

Excellent! Glad you found the problem

If you need to replace the terminal ends, napa or orielly's sells the factory relay terminal ends that clip into the fuse block. Or you can just tighten the crimp on the existing connector.

Good job!