back with new problem on 90 camaro 3.1 v-6

[cocobolo95]

Have a fuel related problem with my 1990 camaro 3.1 v-6 auto trans car. Drove car to library last Monday. Came out and engine would start & run for about 4 seconds then die. Would repeat this behavior on repeated start attempts.

First thought was security system failure. But when key turned on security light cycles on then off like it should. Used scanner and security checks out with it also.

Ignition system is fine. Good spark. Pulled fuel rail, attached spare distributor which I hand turned with ignition on. All fuel injectors spray with a good spray pattern when dist is turned. Fuel pump also runs for key on prime and when dist is turned. Fuel pressure at key on prime is 43 psi.

Can run engine if I spray gas into throttle body. With scanner hooked up in data mode, all sensor readings are within normal limits except injector pulse width. Normal pulse wdith at idle usually runs about 1.7 to 1.9 . With engine not wanting to run unless gas sprayed into tb, pulse width is running at 1.1 to 1.2 .

Here's the crazy thing. Last Thursday, I came out started car and it ran normally, including inj pulse widths running in the normal 1.7 to 1.9 range at idle. Had not done anything to engine to account for this change. Engine ran and car drove fine.

Went out the next morning, started engine and it was back to dying after 4 seconds run time.

I've installed new spark plugs, swapped in known good iac, 2 other ecm, with no change. Checked power supply wires and ground wires and all is good. Engine still starts and runs for 4 seconds and dies.

I need some ideas on where to take my investigation next.

[1320_john]

What is the fuel pressure doing? Is the pump priming with key switched to on and the car is running on that initial prime? With many GM vehicles, the fuel pump runs off the oil pressure switch. When the key is first turned on, it primes the pump while the motor has zero oil pressure, then relies on the oil pressure switch to continue to run the fuel pump while there is oil pressure. My guess about the pulse width being so low might be that the pulse width is calculated from RPM and since it isn't running the RPM is lower than idle. Good luck! Let us know how it turns out!

[cocobolo95]

Fuel pressure is 43 psi @ key on prime. And when idling it is 36 psi, just as it should be. There's no problem with fuel pump running when engine is running. It's just that pulse width of injectors are too low to keep engine running without spraying gas into tb. I have the carpet rolled back from gas tank area, trap door for pump area is removed. I can actually hear the pump running with engine running.

[cocobolo95]

Have a question. Are the injector drivers in this ecm part of a quad driver circuit? And if yes, what other components are on this quad driver?

The injectors of 1990 camaro 3.1 are batch fire with 2 circuits, 1 for each side of engine.

[1320_john]

Is it possible to get a datalog of the car starting up and running, then a datalog of the car trying to start? Are the injectors batch fire on that engine? Maybe putting a scope probe on the ECM side of the injectors to make sure they have full voltage swing? Does the car have VATS?

[cocobolo95]

My scan tool reads sensor data but does not save it. I have read data when car is running normally. Have read data when assistant sprayed gas into tb. Big difference is pulse width of injectors. When normal pulse width is 1.7- 1.9 @ idle. When you open the throttle pulse width increases.

When car starts, runs, dies, pulse width is 1.1- 1.2 @ idle and stays there when throttle is opened. Injectors are batch fire.

Car has vats. It cycles normally when key turned to on. And scan tool reads vats as being good.

I asked in post # 4, if injector drivers were part of a quad driver circuit in ecm. And what other drivers share the same circuit.

Have scoped the injectors and wave form doesn't change when car is in start, run die period.

As stated in original post, last thursday, came out car started & ran normally. Drove car, off and on, all that day. Data on scan tool was normal. Went out the next morning and car did start, run, die again.

Car starts easily. One chug of starter and car runs. This is both when normal and problem occurs.

[steveo]

if it was a quad driver issue; you wouldn't have a predictable failure mode like 'starts for 4 seconds then dies'.

just a theory, but it's on prime cycle (for initial startup) in a primitive fueling mode based just on temperature/tps/rpm, then dies when it switches to speed density or maf or whatever. does it even have a maf? i don't know much about the 3.1...