94 won’t stay running

[dragginNCrds]

I have a 94 3.0 automatic. Will idle all day long but as soon as you go drive it it cuts off and takes a min to start back up. I’m at a loss as to what it could be. Checked for fuel pressure and fuel pressure is fine when it cuts off and I don’t think it’s a power issue because I don’t loose any interior power. I just don’t know where to go from here

 

[RonD]

1994 still has distributor with TFI module, and they can cut spark

On an automatic the computer should increase idle when you shift trans into Reverse or Drive, if idle was 750 it should go to 800-850 when in gear
It does this by opening IAC(idle air control) Valve a bit more when in gear.
You can tell if IAC Valve is working as it should by high cold engine idle that gets lower and lower as engine warms up

There is an NSS(neutral safety switch) on the A4LD transmission that tells computer when transmission is in gear.

 

[dragginNCrds]

Put a new Distributer in already. Verified it is in time as well. Replaced iac and it’s working properly. Like I said will run and idle fine. Will drive down the road and will cut off, shift into neutral and will turn over but takes acouple times to fire back up. We have replaced, distributer, fuel pump, iac, cap and rotor, plugs and wires. Cleaned the mass air, I’m stumped as to why it cuts off driving but will sit in the driveway and idle all day long. When I test fuel pressure while driving with koeo it’s like 40lbs and when koer is like 30-35 and when the truck cuts off while driving the fuel pressure dosent drop off stays at 35ish until you cycle the key to restart it. I just don’t know where to go next as to diagnosing the cut off issue

 

[RonD]

I would let it idle and start moving wires around, see if you can get it to stall.

There is the Inertia switch in passenger footwell, you could hook up a test light to it and then go for a drive to see if fuel pump power is being cut momentarily but that should trigger CEL(check engine light)

Fuel Pump Relay or PCM/EEC Relay could cause a stall if either opened momentarily

 

[dragginNCrds]

I’ll chdck. I just figured that with fuel pressure staying where it needs to be it wouldn’t be a fuel related issue

 

[RonD]

Yes, but your description of no start just after the stall would point more to fuel issue
Spark is pretty much on or off.

Only 3 reason an engine will stall
No spark
No fuel
No compression

No compression would not allow re-start until repaired, i.e. broken timing belt

So its No spark or no fuel

EDIT: no compression got me thinking about exhaust, partially clogged exhaust
Does the engine REV up just fine, can you hold it at say 2,500rpm for 30 seconds without issue?

 

[dragginNCrds]

Yea I understand that. I’ll check relays and inertia switch. And yea when it runs I can rev it up and hold no problem. Dosent pop backfire or anything.

 

[dragginNCrds]

So I checked the relays. Bypassed the inertia switch and it still continues to cut off when the truck is moving. It will idle all day long but as soon as you start to drive it just dies out. I even checked the ignition switch and that’s not the problem either. It has been replaced though

 

[pjtoledo]

put the rear axle on jack stands, go for a stationary test drive.
use brakes to simulate a load while driving.

have someone stand near the intake with a spray can of starting fluid, spray as it dies to isolate fuel or spark.

don't spray on the MAF, spray in a hose connection or whatever.

how's the air filter?

have a timing light to check for loss of spark?