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chilibeerman · 05-19-2012, 06:55 PM

We are back to square 1 and we still can't get it to run. When you turn the key on the check engine light comes on then goes off as you are cranking. the injectors spray and the engine starts and within 2-3 seconds shuts down. We checked to see if we were getting spark to the coil, yes, spark to the plugs, yes. Here is what we have done so far as far as replacing things.

1. Fuel filter- fuel pump seems to be pumping plenty of fuel when we cranked it. I hooked a plastic hose to the one end of the line going into the fuel filter and it filled a cup rapidly
2. Replaced the ECM
3. Replaced the TPS
4. Replaced the Oil pressure sender
5. Had the distributor module checked and it tested OK
6. Replaced the idle air sensor
7. Rebuilt the throttle body with all new gaskets and seals
8. New Map sensor

I know you aren't suppose to randomly replace things without knowing what is wrong but we figured at 23 years old it wouldn't hurt. When it shuts down the injectors are spraying and there is spark showing on the tester to the plugs. If you pump the throttle it will run a little longer but ultimately shuts down.

HELP!

Eddie Foy · 05-19-2012, 08:27 PM

I would put a fuel press gauge on it and monitor it. Se if the dying is related to fuel press drop.

nepagearjamm · 05-19-2012, 09:02 PM

x2 on fuel pressure drop. It's loosing one of 3 basic things...fuel, air , correct timing= must run. Sounds like a loss of fuel pressure.. A noid light test will verify ECM is getting signal from distributor to fire injectors.

BA-in-Mich · 05-20-2012, 06:11 AM

Just for giggles look at the the back of the ignition switch. I have a 1990 and the back of the switch was melted. The switch was a problem on the old horses.

Dave78Chief · 05-20-2012, 07:02 AM

I know that you have gone through several iterations with this. Since it now runs for a short bit, have you checked to see if your now getting a fault code? That may point you to the problem area.

Unfortunatly scanners for the GM OBD1 system are not many. Usually you are limited to the pulse code on Check Engine Light (CEL). The actron CP9145 (with GM ALDL connectior option) does have a selection for 1990 P Series 7.4L configuration though. Reads and clears codes as well as let you see visual sensor data.

Dave

Dave78Chief · 05-20-2012, 08:11 AM

With this system the fuel pump relay is only used to start the engine. After the key is turned ON, the ECM will de-energize the relay after 2 seconds. When oil pressure builds up, the fuel pump is kept energized via the oil pressure cut off switch. This sounds like the relay is working (energizing the pump so fuel pressure builds to 10-13PSI) but the pump is not kept energized once the engine is running (loss of fuel pressure). I do not know if the cutoff is built into the oil pressure sender or if the oil pressure sender performs both functions on your application.

Connect test light to fuel pump test connector. Turn key ON and verify pump comes on for 2 seconds. Start engine and verify pump comes ON via oil pressure cutoff switch.

Dave

Dave78Chief · 05-20-2012, 09:32 AM

For reference here is the fuel pump circuit:

and a basic overall wiring for this type of ECM

Dave

Eddie Foy · 05-20-2012, 09:51 AM

Unless I read the schematic wrong it is just like all C4 corvettes. The oil press switch is not a cutoff switch. You can see it is in parallel to the relay. It is there to provide an alternate power source for the fuel pump in the event of relay failure. As soon as oil press builds to more than about 4 psi the switch closes. The fuel pump runs for about 2 secs when you turn the switch to On. If the engine is not started it then turns off. As soon as the ECM sees ignition pulses it again energizes the fuel pump relay. You can lose all oil press and the fuel pump will continue to run. The "oil press cutoff switch" is a common myth in the Corvette world.

Eddie Foy · 05-20-2012, 10:05 AM

Quote:

Originally Posted by Phobos

Just a thought here, but have you checked the ECM relay? They are usually located in or around the fusebox and are pretty generic.

Don't think it has a relay.

Dave78Chief · 05-20-2012, 03:34 PM

After re-reading the documentation I agree with what Eddie says. However, one thing to note is that fuel will be shut off if the ECM does not recieve the reference voltage pulses from the distributor ignition module. ECM uses the pulses to determine if engine is runnig and adjusts timing based on them.

Did you by chance set the timing and forget to reconnect the "Set Timing Connector"? The ECM uses the ref pulses from the Ign Module and sets the "Bypass signal" when RPM is above 400rpm. This tells the Ign Module to start using the "EST" signal for timing.

Dave

chilibeerman · 05-21-2012, 07:08 AM

We have not messed with the timing at all. Yesterday we hooked up a fuel pressure gauge to the inlet of the TBI. When we turned on the key the fuel pump ran for 2 seconds but the gauge showed no pressure. We then ran a 12 volt wire directly to the fuel pump hot lead. When we energized that the pump ran but we got no fuel pressure on the gauge. Since the direct wire to the fuel pump is spliced into the positive wire we put a test light on the this end and turned the key on. The engine started and ran for a couple of seconds. The test light stayed on for 2 seconds then went off. The fuel pump did not get power after the 2 seconds. At this point is points to a bad fuel pump but we want to try a different pressure gauge. Both of us feel that the gauge might be faulty. When we remove the fuel line from the inlet side of the TBI and run the pump it pumps plenty of fuel. Even a weak fuel pump should show some pressure.

Dave78Chief · 05-21-2012, 08:33 AM

Earlier you said:

Quote:

1. Fuel filter- fuel pump seems to be pumping plenty of fuel when we cranked it. I hooked a plastic hose to the one end of the line going into the fuel filter and it filled a cup rapidly.

OK, fuel at input, how about the output of the filter (i.e is the filter clogged?)

Just to make sure, is the filter located on the inboard right hand framerail in the area of the transmission? I am using pdf page 140 of http://www.bdub.net/manuals/P30/P30.pdf (20MB file size) as a reference. The actual filter for TBI is discussed on pdf page 128.

P30 TBI Fuel system description starts on pdf page 124 of that document.

Dave

Eddie Foy · 05-21-2012, 10:22 AM

Hot wire the fuel pump and see if it runs. You can do this by applying 12 V to terminal G of the ALDL. I think you need a new gauge or it is not attached properly. If you have no press the engine will not start even for 2 secs.

Eddie Foy · 05-21-2012, 10:41 AM

Quote:

Originally Posted by chilibeerman

The engine started and ran for a couple of seconds. The test light stayed on for 2 seconds then went off. The fuel pump did not get power after the 2 seconds. At this point is points to a bad fuel pump .

Sorry but this doesn't point to the fuel pump. If the engine started at all, the pump is probably OK. If the power to the pump went out and the engine died it is not the pump.

1. Fuel Pump Relay
2. ECM
3. Fuel Press Regulator.

You really must see what the fuel press is doing to diagnose properly.