91zswhitt

My dad who is good mechanic, and myself, who is a clueless mechanic have a tough one.....

I have a 1991 z28 5.7.......

New plugs/wires/fuel pump/fuel filter/air filters/starter/solenoid...has been looked over twice...

Problem: Something is shutting down the fuel system.....car starts then dies, after it dies it won't start again until it cools down some....engine is good and strong, compression is great......checked and it is getting fire...so we have spark

Odd: I started the car this morning and it idled smooth and re-fired 3 other times...but hadn't been touched for 24 hours...



I guessed injectors but you wouldn't lose 8 at the same time???

Has anyone had this problem, and what could it be? Also, I have a fuel pressure control/regulator ordered that I am going to try.....

What could be causing this..computer? sensor? HELP!!!

antares57

iTrader: (8)

Have you had the ignition control module (icm) checked? Heat will do that.

91zswhitt

Could it still be the ICM even if I have spark?

Fast355

iTrader: (2)

YES IT CAN BE!!!! Put a noid light on the injectors and make sure you are still getting injector pulses. The ICM can fail to provide a distributor reference pulse to the ECM and the ECM will not know the engine is spinning over and stop delivering fuel.

Usually a fuel pump just DIES and it will never start again.

Ozz1967

iTrader: (4)

Or the pump is weak/bad but not dead, which if primed enough can provide enough fuel for a short start/run but not enough to keep it running.

RBob

iTrader: (1)

Time to replace the injectors... If they are still the stock Multecs replace them. As the coils short out the one driver in the ECM can't fire them. So all eight are lost.

If you want can take the time to ohm them out. The ones that are under 12 ohms are bad, the ones under 16 ohms are on the way.

RBob.

THCARGUY

Sounds similar to what I just went through with my 91 TA 350 TPI. For the past year and a half I had replaced just about every sensor, complete tune-up, etc... and more recently, the fuel pump. It got so bad a few weeks ago that it died and I had to be towed 30 miles. When it was cold it ran great, but when the engine warmed up it would stumble off the line and "chuggle" while cruising. Got bad enough where it would idle but I couldn't give it any gas or it would die. Finally got an accurate diagnosis; leaky fuel pressure regulator and bad injectors. I ohmed the injectors and found two bad ones. Since they were the original multec injectors I ordered a set of remanufactured bosche 3s from Southbay. Replaced them and the fpr and I now have a new car again!

91zswhitt

I have the fuel pressure reg ordered, along with bosch injectors (ironic right), and the ICM....

Going to start here...thanks and I'll keep you updated.

IROCThe5.7L

You have a bunch of good suggestions. When it doesn't start. Does it just crank and crank? If so, next time it does this. Try starting the car with the gas all the way to the floor. If it fires your injectors are leaking when your car is off. Can't hurt to try this. Won't cost any money.

Abubaca

iTrader: (3)

I had a very similar problem, and I didn't think injectors for the exact reason you stated. Can't imagine all 8 would go bad. Replaced injectors. No more problems.

91zswhitt

Replaced ICM....problem not fixed....still wants to chug and won't start....fuel pump is good too....replacing injectors at the beginning of the week as well as the fuel pressure regulator.....

any more thoughts?

IROCinBlack

Oil pressure sensor? Just a guess. How is your oil level? Low oil pressure can cause fuel cut. Only other thing I can think if would be a coil issue, as heat can affect them.

Rolling Thunder

I have to recommend taking a step back and doing a little troubleshooting first. I mean rather than replace the fuel pressure regulator why not measure the fuel pressure? Same for injectors. Because the TPI motors have a batch fire set up 1 bad injector could take out that whole bank. However, rather than just replacing them, unplugging the bad injector(s) will allow the car to run again (although maybe not well obviously) and prove it. To sum it up, if you've proven 100% that no fuel is your issue, all you need to do now is check the fuel pressure and check to see if your injectors are firing.

ASE doc

I am with Rolling Thunder on this. From my perspective, I have to verify the actual cause of a fault before I start replacing parts. That's me job description. I would start with a fuel pressure gauge attached, prefferably when the car dies. Did fuel pressure drop? If so, then I would start at the fuel pump relay, testing for power to the pump. Don't forget the pump needs a good ground too. It's ground is located close to the tank, under the car. I start every diagnosis by familiarizing myself with the circuit in question. Study up on the fuel pump control circuit to fully understand how it works and where things are. The early GMs generally have a redundant system with power supplied both through the relay and the oil pressure switch. I've seen many fuel pumps that would shut down intermittently, then work fine again for a while. That's why I do a current ramp test to check the windings. To do this, you need a lab scope and a low current probe. A lot of crud in the bottom of the tank, plugging up the pick up sock after the pump runs a while, would create this same effect.

A few people have already touched on the effect of even a single injector coil going low resistance. This can pull down the injector drive signal. I use a lab scope to view injector drive voltage waveform as well as current. You can use the test of unplugging one injector at a time and trying to start the engine, you can do a resistance test cold and hot, since the injectors are wired in two separate banks, 4 injectors each in parallel, you can disconnect at the ECM and test each bank as a set. Each bank should read about 3-4 ohms, closer to 4. If a bank reads below 3 ohms, that bank has atleast one bad injector in it. Im with Rbob on the injectors anyway. If the OE Multecs are still in there, they have long outlasted their useful service life. A new set of Bosch IIIs will provide much better fuel atomization and be infinitely more reliable.

Don't forget to verify good power and ground to the ECM. The ECM grounds are at the rear of each cylinder head, tough to get at, and they may be making poor contact and creating high resistance in the circuit, which will cause lots of problems. Check the fuses in the underdash panel for good contact. Especially the INJ1 and INJ2 fuses.