Jetman150

okay gentlemen,
ive got a 91 z28 that just does not want to run. we replaced the fuel pump, the fuel pressure regulator diaphragm, bled the fuel system, intake temp sens, map sensor, and finally replaced the computer today. one of the transistors inside had burned up pretty bad.. so we figure thats it, thats got to be it..
we know its not mechanical, the engine will cold start on ether or carb cleaner (i know thats bad) , but it runs smooth with no knocking or rough idle. every plug is getting fire, so rotor and cap are good, timing is on the mark, we have gone through the car with a meter, and all the sensors check good and read spec, all plugs and connectors power/ground good. computer before and after read 1.2 codes, even with the bad computer.. same prom though..

we cannot figure it out. anyone out there had a similiar problem or know what to check next? any lucky aces?
any chance the prom was knocked out?

five7kid

iTrader: (14)

Are the injectors firing? What's your fuel pressure now?

Jetman150

Well guys, Ive got 45-47psi of fuel pressure when the key is turned to run, then after a few seconds, after the fuel relay clicks, the pressure bleeds down to about 40, then really slowly down to about 22-24. When cranking, the pressure stays at about 40-43. So I believe fuel pump/regulator, lines are good.
I pulled the iginition module last night, and as i did, the module grounded out and turned the fuel pump on. we did a check and it was bad, so we figured thats it! replaced it, but still not start.
I dont think we are getting injectors to fire off, but i checked the out put voltage of the blue/green signal line from the ecm, and its ranges from 4.3 to 9.8 volts, when cranking, and is period and repeating, so i believe the injectors should spray..
I dont think the Vats is a problem because im able to crank the car, and the security light goes off after 4-5 seconds, Also reading injector power..

I am completely baffled. we have spent a couple of hundred bucks replacing sensors, checking harnesses, computers...
The only two things we have not done so far is replace the tps, and the eprom.. The tps checks ok, as it reads a variable ohm load when operated, between two poles, and i do not get continuity between the outer pins..

Ive got a suspicion that the eprom has lost its flash, but have no way to check..
Im lost.. and broke..

TexasSilhouette

iTrader: (4)

Use a test light or NOID light across one of the injector connectors to see if there is a pulse.

If it runs when carb cleaner is sprayed into the plenum, good fuel pressure, then it sounds like the injectors are not firing.

How old is the gas in the tank? Any chance it has varnished?

Jetman150

Well, I believe for whatever reason the injectors are not firing, but i think its odd that I can read signal to them with a meter, and the signal 'oscillates' or goes up and down when we crank the engine..
I can only figure maybe two things that would cause the injectors not to fire, and that is signal from the ecm, from run loop, or from the VATS.. but im not finding any problem with the VATS, as we can crank the car, and the security light functions normally on the dash, we have replaced the ecm, kept the same EPROM..
I am wondering if we should try to bypass the VATS, or maybe try to reflash the EPROM(which the dealer wants bukoo $ for.. )
maybe im getting a big vaccum leak with the egr valve.. but you can operate the egr valve by hand, and it should stay closed during cranking right?

five7kid

iTrader: (14)

The injectors get power all the time. The ECM grounds the circuit to energize them.

That's why the noid light, as they call it, is the correct way to test them.

Jetman150

should be two wires to each injector, and one is from power and is hot when the ignition is on, and one from the ECM, which signals. I tested both by grounding out the meter.. I have power, and the signal wire is the one that shows a range of voltage depending on rotation of the engine or not..

it looks like according to the schematic, that the injectors are basically parrellel wired, and each bank would spray all injectors when signaled,
Im not sure how it would work if you fire only one injector per cylinder..

five7kid

iTrader: (14)

Correct, it's "batch fire", a bank at a time, not individual cylinders (as in sequential).

The "signal", as I said, is just the ECM grounding the circuit. If you are measuring from that wire to ground, it will fluctuate from full volts to zero (depending upon how quickly your measurement can respond).

Jetman150

Hmmmm, That may be a little strange that I can read anywhere from 4-9 volts, and its goes up and down when cranking the car.. My meter reads it slowly, but it does react and goes up to 9 volts, You had said it should read 0-12 volts??

Ive got two or three things more to try, and then I think I will take it in to have it put on the diagnostic computer..
replace the TPS, Bypass the VATS, Replace the EPROM..

If it doesnt start after that, then I am assuming its time for a rebuild

Jetman150

Checked the injectors with a noid light, and no pulse. so we moved on to the vats, and verified resistance from the key, and it functions correctly,
so we are trying to figure out why the injectors are not getting signal from the ecm.

CamaroRider

Something you said when speaking about the TPS:

''and I do not get continuity between the outer pins ..''

If your TPS is like the one in my 1989 Camaro (TBI) the diagram shows that the device should show SOME continuity between the extreme outer connectors. That should be maximum resistance of the device. The pin that goes to the 'wiper' should vary in resistance compared to any one of the end pins. All three pins are used in my Haynes diagram.

If you get NO continuity between the extreme ends of that device then that might be part of the problem.

Jetman150

we went ahead and replaced the tps just to eliminate it..
The injectors are definitely not getting the signal from the ecm. From what I can research, the ignition module and the vats module tell the ecm to fire the injectors. since we have replaced the ignition module, and it was bad, and the plugs are firing, i am thinking its either a bad vats module, or a bad prom.