Curious engine issue

[Chucaswatjfan]

Hello all!

First timer here but i think this forum will be just what i need. My name is Luke and i own a 1994 k1500 5.7L extended cab/short bed with 4l60e tranny. I've been experiencing a phantom issue over the past several months that results in intermittent misfiring, loss of power, hesitation and surging.

Anyway, those are the symptoms but not actually my question. The other day as i was poking around i discovered something peculiar. The engine was running (very roughly) at idle and it began to sputter and eventually stalled. After it had stalled i noticed that the fuel injectors continued firing fuel into the throttle body and the ignition coil continued sending spark to the distributor. I had the air cleaner off and watched as the injectors continued working as though the engine hadn't stopped running. I also pulled the spark wire from the ignition coil (i was prepared for the shock that indeed followed) and watch as the coil continued to arc to ground (a good inch and a half arc) as if it were still sending spark to the plugs, even though the distributor was no longer turning.

Maybe im missing something but shouldn't the injectors and coil be de-energized after the engine has stopped running? This behavior continued for several seconds (maybe even over 10 seconds) until i decided to turn the key off. I don't know if this is related to my original issue but, to me, this indicates something is malfunctioning and the ECU is somehow not aware of what the engine is actually doing. I did repeat this a second time to confirm that it wasn't just a fluke. The injectors and coil continued to operate after the engine had stalled the second time as well.

Does anyone have any ideas as to what could cause this (or maybe even confirm that for some reason this is normal behavior after an engine stall). Any suggestions are appreciated. Thanks.

Cheers,
Luke

 

[Schurkey]

That is NOT normal.

First Suspect = ignition module

Second Suspect = ECM

Third Suspect = wire harness connecting it all together.

Fourth Suspect = Radio-frequency Interference (RFI) somehow getting picked-up by the wire harness that the ignition module thinks is a pickup-coil signal. This seems VERY unlikely to me.

Might be nice to see the data stream on a scan tool when the engine stalls--RPM particularly. If the engine isn't running, but you have an RPM reading, clearly something is falsely triggering the ignition module, which produces sparks and also tricks the ECM into firing the injectors.

 

[Chucaswatjfan]

Ok, so it is the pickup coil that tells the ecm that the engine idea running? I want sure if the crank position sensor played a role in that or not. The truck seems to be running fine today but I'm back probing the ecm now anyway to record some data and check the harness continuity. I'll post back when i have some further info. Thanks for your reply.

Edit: i should note that the distributor and ignition module have been replaced in my attempt to solve this but i suppose that doesn't mean the new (cheap) part was any good.

Cheers,
Luke

 

[PlayingWithTBI]

I want sure if the crank position sensor played a role in that or not

The 94 TBI didn't have a crank position sensor, unless you have a Vortec transplant?

 

[Schurkey]

Ok, so it is the pickup coil that tells the ecm that the engine idea running?

Sort of. Pickup coil provides signal to ignition module. Ignition module provides signal to ECM. ECM provides signal back to ignition module, to injectors, to fuel pump, to...everything else.

 

[Larry-Cleveland]

I know my 89 has a sender by the distributor that is hooked into the oil pressure of the motor. I know this tells the fuel pump that the motor is running when there is pressure. I am not sure if this has anything else to do with the motor.

 

[PlayingWithTBI]

I know my 89 has a sender by the distributor that is hooked into the oil pressure of the motor. I know this tells the fuel pump that the motor is running when there is pressure.

Sort of, the oil pressure switch tells the ECM the engine has oil pressure then the ECM will tell the fuel pump to turn on and the injectors to fire. If you have a hot fuel module, when the engine is warm it will fire the injectors (IIRC). This schematic is for an 88 but I think your 94 is similar.

You must be registered for see images attach

I suppose, if the oil pressure switch is stuck closed the injectors might continue to fire but, the ECM needs to look at RPM and MAP to base its pulse width of the injectors?

 

[Schurkey]

Sort of, the oil pressure switch tells the ECM the engine has oil pressure then the ECM will tell the fuel pump to turn on and the injectors to fire. If you have a hot fuel module, when the engine is warm it will fire the injectors (IIRC). This schematic is for an 88 but I think your 94 is similar.

No. Looking at that schematic, the oil pressure sensor by itself tells the computer nothing. The oil pressure sensor is nothing more than a bypass--a redundency in case the fuel pump relay fails. The fuel pump can be powered by the oil switch even if the fuel pump relay, or the part of the ECM that controls the fuel pump relay, or the wire harness between them has failed entirely. A failure of that sort leads to longer-than-expected crank time, because the engine has to crank over long enough to build oil pressure that closes the pressure switch, which then supplies fuel to the injectors. The injectors would be firing, but with no fuel supply until the fuel pump activates.

There is an input to the ECM for fuel pump power, but it doesn't distinguish between power coming from the oil pressure sensor and power coming from the fuel pump relay. All it "knows" is that the pump is supposed to be powered (It can't tell if there's a break in the wiring after the splice that diverts signal to the ECM.)

I suppose, if the oil pressure switch is stuck closed the injectors might continue to fire but, the ECM needs to look at RPM and MAP to base its pulse width of the injectors?

Having fuel pressure via the oil pressure switch does not cause injector firing. The crucial element here is the RPM signal. No RPM, no injector firing. Some other sensors can be lost via "Limp-Home" mode, but RPM is essential.

 

[PlayingWithTBI]

No. Looking at that schematic, the oil pressure sensor by itself tells the computer nothing.

It sure looks to me that the oil pressure switch will bypass the fuel pump relay through wire #120 (tan/white) connected to the ECM. So, if the switch is stuck closed, the ECM's fuel pump input will see voltage.

The crucial element here is the RPM signal. No RPM, no injector firing.

And that's why I said...

the ECM needs to look at RPM and MAP to base its pulse width of the injectors?

I guess in other words, it must be PFM for the injectors to fire with the engine stalled or a direct short somewhere below the injectors since they see battery voltage on top when the key's on?

 

[Schurkey]

It sure looks to me that the oil pressure switch will bypass the fuel pump relay through wire #120 (tan/white) connected to the ECM. So, if the switch is stuck closed, the ECM's fuel pump input will see voltage.

The pressure switch DOES bypass (or suppliment) the fuel pump relay, and #120 does provide an input to the ECM. My point is that the computer has no way of determining from #120 if it's getting a voltage signal from the fuel pump relay, or from the oil pressure switch, or both.

I guess in other words, it must be PFM for the injectors to fire with the engine stalled or a direct short somewhere below the injectors since they see battery voltage on top when the key's on?

PFM? Is that some version of "Miraculous"?

There was a similar thread "somewhere else" fairly recently, where goofy firing of the injectors on a stalled engine was fixed by replacing a mostly-dead battery. The ECM didn't have enough voltage to think straight, apparently.