94 shutting off after starting (1 Viewer)

[catavinacruiser]

Experiencing a problem with my 80 – it shuts off right after starting. Key in, engine starts, RPMs rise, then drop within 2-3 seconds. Explored potential issues; leaning towards the Air Flow Meter. Checked EFI Main relay and fuel pump resistor – both seem to be good. Noticed the car stays running in idle when the Air Flow Meter harness is disconnected, but shuts off when connected. Any insights on whether it's the harness or the meter itself causing the trouble? Any help in getting my 80 back on track is much appreciated.

 

[jonheld]

The engine should NOT stay running with the AFM disconnected. The flap in the AFM provides the ground path for the secondary coil of the circuit opening relay. The primary coil is off the starter circuit. From the 1994 EWD:

 

[catavinacruiser]

The engine should NOT stay running with the AFM disconnected. The flap in the AFM provides the ground path for the secondary coil of the circuit opening relay. The primary coil is off the starter circuit. From the 1994 EWD:

View attachment 3488916

Thanks for the insight. I'm not great with electronics, so if you could break it down in simpler terms, that would be super helpful. Sorry for my lack of understanding.

So you're saying I should check something regarding the primary coil?

EDIT: Just went through the FSM now. All I could find about checking the primary coil. Going to go out there with the multimeter right now.

EDIT #2: Tested the primary coil with the multimeter and the resistance is off spec from FSM. Going to source another and see if it fixes the issue.

 

[ppc]

Jon is referring to the circuit opening relay it has two coils.

 

[catavinacruiser]

Jon is referring to the circuit opening relay it has two coils.

View attachment 3488964

Thanks! Going to check and replace the circuit opening relay if defective.

 

[ppc]

Your failure indicates a problem within the circuit with the fuel pump cutoff in the AFM. It could be the circuit opening relay or more likely the AFM. Did you mess with the two screws on the connector on the AFM. The connector and wiring to the AFM could have been damaged. Any maintenance done in the area prior to the problem occurring?

 

[catavinacruiser]

Your failure indicates a problem within the circuit with the fuel pump cutoff in the AFM. It could be the circuit opening relay or more likely the AFM. Did you mess with the two screws on the connector on the AFM. The connector and wiring to the AFM could have been damaged. Any maintenance done in the area prior to the problem occurring?

I haven’t done any kind of service around the AFM lately. The harness connecting to the AFM is kind of loose due to broken tabs. I’ve had someone hold it in place while cranking and it didn’t help.

 

[Marvelicious]

I mean... If I'm considering throwing parts at a problem, visibly damaged things are a pretty good place to start, speaking for myself.

 

[catavinacruiser]

I mean... If I'm considering throwing parts at a problem, visibly damaged things are a pretty good place to start, speaking for myself.

UPDATE: I don’t know what I even did but it’s started and running again with no issues jumping the diagnostic ports(planned on checking codes) with a paper clip. Also unplugged and plugged the COR back in(might’ve been loose or something, no idea)

About the connector, I planned on changing that out after the holiday.

 

[jonheld]

EDIT: Just went through the FSM now. All I could find about checking the primary coil. Going to go out there with the multimeter right now.

To get the nomenclature correct, that is called the ignition coil and has nothing to do with what I was mentioning.
The circuit opening relay (COR) has 2 coils that close the contact inside the relay. The primary coil (the one that activates first) is fired from the starter circuit while the starter is cranking the engine to supply the initial start up fuel pressure. The secondary coil (the one that activates second) is fired when the flap inside the AFM opens when the engine begins to suck air. This keeps the fuel pump running while the engine is running.

What's happening on your truck is the primary coil is firing from the starter circuit, and the engine stalls after 2 or 3 seconds after you release the key from the START position because the fuel pump is being shut down. That could be simply because the intake plenum is torn after the AFM allowing unmetered air to bypass the AFM, therefore not opening the flap in the AFM, which in turn shuts down the fuel pump.

I would remove the intake plenum and carefully look for cracks in the bellows sections. Below is an old picture of the plenum in my 91 FJ80 that would stall out in reverse due to the engine shifting on the mounts.

 

[catavinacruiser]

To get the nomenclature correct, that is called the ignition coil and has nothing to do with what I was mentioning.
The circuit opening relay (COR) has 2 coils that close the contact inside the relay. The primary coil (the one that activates first) is fired from the starter circuit while the starter is cranking the engine to supply the initial start up fuel pressure. The secondary coil (the one that activates second) is fired when the flap inside the AFM opens when the engine begins to suck air. This keeps the fuel pump running while the engine is running.

What's happening on your truck is the primary coil is firing from the starter circuit, and the engine stalls after 2 or 3 seconds after you release the key from the START position because the fuel pump is being shut down. That could be simply because the intake plenum is torn after the AFM allowing unmetered air to bypass the AFM, therefore not opening the flap in the AFM, which in turn shuts down the fuel pump.

I would remove the intake plenum and carefully look for cracks in the bellows sections. Below is an old picture of the plenum in my 91 FJ80 that would stall out in reverse due to the engine shifting on the mounts.

View attachment 3489007

View attachment 3489011

Thank you for the extended explanation. If I experience any further issues I’m going to definitely refer back to your post. As for the intake hose, I replaced it with a new OEM one within the last year. Inspected it initially and didn’t notice any cracks.

Again, thank you and everyone else who chimed in for all of your help. Truck is starting up for now. Happy Thanksgiving!

 

[jonheld]

UPDATE: I don’t know what I even did but it’s started and running again with no issues jumping the diagnostic ports(planned on checking codes) with a paper clip. Also unplugged and plugged the COR back in(might’ve been loose or something, no idea)

About the connector, I planned on changing that out after the holiday.

What terminals did you jump in the diagnostic connector?
From the 1994 FSM:

 

[catavinacruiser]

I jumped TE1 and E1 cause I figured why not see if any codes were thrown. Turned the key to full on/start and the truck fired right up and stayed on.

I also unplugged and reinserted the relay before doing that. So it could’ve been loose or something.

 

[jonheld]

I jumped TE1 and E1 cause I figured why not see if any codes were thrown. Turned the key to full on/start and the truck fired right up and stayed on.

I also unplugged and reinserted the relay before doing that. So it could’ve been loose or something.

Did you remove the jumper in the diag connector?

 

[catavinacruiser]

Did you remove the jumper in the diag connector?

Yeah, I removed it.

Tried to start up again this morning with no luck. Got code 24. Going to remove the plastic intake hose and inspect it under the light, also going to take the cover off the air flow meter to see if the points have a weak solder or are disconnected.

How many pins should the AFM harness connector have? I’m counting 6 with 1 empty spot with no pin.

 

[catavinacruiser]

I took out the plastic intake hose, gave it a thorough inspection – all good, still in mint condition with no cracks.

Next, I removed the AFM and soldered the points. Put everything back in the vehicle, but now I'm facing a crank-no-start situation. Not sure what I did to make it worse?

Happy Thanksgiving!

 

[jonheld]

I took out the plastic intake hose, gave it a thorough inspection – all good, still in mint condition with no cracks.

Next, I removed the AFM and soldered the points. Put everything back in the vehicle, but now I'm facing a crank-no-start situation. Not sure what I did to make it worse?

Happy Thanksgiving!

Is the check engine light (CEL) illuminated with the key in the ON/RUN position before cranking the starter? No CEL=no start.
Yes, the AFM connector is a 7 pin connector with pin 3 empty. All this info is in the 94 EWD which you can download from the resources section.
From the EWD:

 

[catavinacruiser]

Is the check engine light (CEL) illuminated with the key in the ON/RUN position before cranking the starter? No CEL=no start.
Yes, the AFM connector is a 7 pin connector with pin 3 empty. All this info is in the 94 EWD which you can download from the resources section.
From the EWD:

View attachment 3490060

NO CEL in on position.

Thanks for the tip! I’ve been using the 96 manual this whole time because that’s all I’ve had access to. I didn’t know 94 was reuploaded to resources.

 

[catavinacruiser]

Quick update: Unwrapped some extra heat tape from the AFM harness and found a cut 7-wire (green/yellow) plus a barely hanging on 6-wire (green). Checking things out further along the harness. Patched up the cut cables for now.

Swapped in a junkyard AFM, no more diagnostic code 24. Truck fires up, but it's idling a bit rough, playing with RPMs and dropping to a stall.

At least it's moving now! Gotta keep digging into it.

 

[TheGrrrrr]

Any updates?