2.9L key on fuel prime 5min

[Skooby]

This is the second cardone reman PCM I've gotten.....as far as I can tell the fuel pump prime cycle is all controlled by the PCM.

The fuel pump runs constantly with the key on and truck will not start. BUT... after roughly 5 min the pump FINALLY will shut off and the truck will fire right up and run normally.

Is there a more reliable PCM out there than the Cardone's??

 

[Pix3L8]

i would also like to know, I’m looking for a good aftermarket PCM

 

[RonD]

Yes, in pre-1995 fuel injected Rangers the PCM(EEC-IV) pin 22 controlled the Ground for the Fuel Pump relay

Each Key on should see only 2 seconds of grounding from pin 22, fuel pump runs for 2 seconds
Then PCM will ground pin 22 full time once engine RPMs are above 400, cranking speed is 200RPMs

Having the full pump on with key on would NOT cause a No Start so something else is going on besides the Fuel Pump running, fuel pump running is a symptom not a cause, of the no start

The 2 second run time is a safety thing, i.e. if there was an accident and fuel line broke the engine would stall, once RPMs were under 400, PCM would shut off the relay so broken fuel line couldn't "feed a fire"
But fuel system may need to be Primed for start up when at 0RPMs, 2 seconds of run time gives about 10psi fuel pressure if it was at 0psi, so enough to start engine


As said fuel pump running over 2 seconds means something is wrong in the PCM most likely, but that in itself would not cause a no start
Most likely the fuel injectors are not being opened by the PCM

Try to start engine when fuel pump is running when it shouldn't, and confirm a No Start
Then spray fuel into the intake(50/50 test) and see if it starts
If it starts and dies that would confirm spark was working but no fuel injectors
In pre-1995 the PCM just runs the injectors, spark is done by another module, independent of PCM as far as startup spark

 

[Skooby]

Yes, in pre-1995 fuel injected Rangers the PCM(EEC-IV) pin 22 controlled the Ground for the Fuel Pump relay

Each Key on should see only 2 seconds of grounding from pin 22, fuel pump runs for 2 seconds
Then PCM will ground pin 22 full time once engine RPMs are above 400, cranking speed is 200RPMs

Having the full pump on with key on would NOT cause a No Start so something else is going on besides the Fuel Pump running, fuel pump running is a symptom not a cause, of the no start

The 2 second run time is a safety thing, i.e. if there was an accident and fuel line broke the engine would stall, once RPMs were under 400, PCM would shut off the relay so broken fuel line couldn't "feed a fire"
But fuel system may need to be Primed for start up when at 0RPMs, 2 seconds of run time gives about 10psi fuel pressure if it was at 0psi, so enough to start engine


As said fuel pump running over 2 seconds means something is wrong in the PCM most likely, but that in itself would not cause a no start
Most likely the fuel injectors are not being opened by the PCM

Try to start engine when fuel pump is running when it shouldn't, and confirm a No Start
Then spray fuel into the intake(50/50 test) and see if it starts
If it starts and dies that would confirm spark was working but no fuel injectors
In pre-1995 the PCM just runs the injectors, spark is done by another module, independent of PCM as far as startup spark

I'll try that this weekend.....this issue is VERY predictable. As soon as the pump shuts of, I can hear one of the relays in the engine bay (haven't isolated which one) click, then the truck fires right up! I thought I had read somewhere that the pcm would not allow the truck to start during the prime cycle....you just dont normally notice it because it's only 2 Seconds?

 

[Paulos]

I thought I had read somewhere that the pcm would not allow the truck to start during the prime cycle....you just dont normally notice it because it's only 2 Seconds?

My '87 with a 2.9 starts up immediately without any delay in turning on the ignition (no waiting for the prime cycle).
Sounds like a wiring issue, possibly with the relays. Any evidence of altered wiring in that area? Are you absolutely certain that you have the correct ECU/ECM?

 

[RonD]

You hear the Fuel pump relay "clicking" off when fuel pump goes off

In 1991 there are 3 relays UNDER the engine bay fuse box, picture here: https://www.explorerforum.com/forums/attachments/relay-jpg.35945/

Green is Fuel pump relay
Brown is EEC relay
Black is WOT relay, only on models with Factory AC

When you turn on the key, the EEC Relay will click closed, it sends 12volts to the EEC(computer), the fuel injectors, all the solenoids, IAC Valve and the fuel pump relay

On power up the EEC grounds the fuel pump relay(pin 22) so it clicks closed almost exactly when EEC relay does, EEC then looks at RPMs, if 0 then it unGrounds the fuel pump relay within 2 to 3 seconds, well it should, lol

I would look at these relays, unplug them and look for corrosion, its a longshot since the issue is predictable and corrosion issue is anything but predictable

As Paulos said there is no connection between 2 second Priming and startup, EEC either opens injectors when it detects RPM/timing or it doesn't, and yours is not opening injectors, and the 5min prime is a symptom that EEC(PCM) is not working like it should, but not the cause of the no start

My 1994 4.0l did that same Fuel pump priming longer than the 2 seconds and no start, if I cycled key off and on a few times priming would do the correct 2 second run and engine would start, but would run oddly at times, lol
Changed the capacitors and been running great ever since, 2 years now

 

[Skooby]

Ok. Picked up an OEM PCM from a junkyard. Opened and inspected all the caps and checked for burned traces on the PCB, all good! Installed and the fuel prime/starting issue is solved! Fires right up and drives every time! A secondary issue that I had not asked about still exists though.....above 3000 rpm the truck starts stumbling real bad, I had hopes that it was computer related too but no such luck. So onto the next diagnosis....wish me luck!

 

[Paulos]

Does it run better first thing in the morning when it's cold (open loop), and then worse when after it warms up (closed loop)?

 

[Skooby]

Does it run better first thing in the morning when it's cold (open loop), and then worse when after it warms up (closed loop)?

It's my kids truck....so getting accurate info is sketchy at best lol! The worst part is It's an intermittent issue...but aside from that it only seems to do it when warm and over 3000 rpms. Was trying to figure out if the 91s had the rev limiter on the neutral saftey/clutch switch but I haven't gotten very far.

 

[Paulos]

I asked because stumbling/bucking is often clogged injector related.

 

[Skooby]

I asked because stumbling/bucking is often clogged injector related.

He's ran techron through it a couple of times, seems like dirty injectors would be more consistent and predictable? Or am I off base? I was leaning toward testing the MAP and rechecking the TPS (new).

 

[Paulos]

Yeah, I forgot that you had said it was an intermittent issue.