3f-e Intermittent throttle?/fuel? loss? (1 Viewer)

Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
I really need some help diagnosing an intermittent problem on my '92 FJ80.

It currently has 217k, and I've owed and maintained it for the past 7 years.

Early this year I started having a strange, very intermittent problem. While driving, RPM suddenly drop to almost nothing but the engine does not die. After a few seconds RPM comes back and the truck runs like normal again. It "feels" like it's lost fuel, then suddenly comes back. There does not appear to be any rhyme or reason to the issue, it has happened in cold weather, hot weather, freeway speeds and surface streets.

I've recently replaced the fuel sock, fuel pump(while I was in there) and fuel filter. I also tested the AFM and TPS per the FSM and they both checked out. None of these things fixed the problem.

Fast forward to Tuesday. After getting home from a 45 mile drive, I jumped back in to run a quick errand. Truck fired right up drove about 50 yards sputtered and died. Cranked it a few to and got no start. I pushed it to the side of the road cranked it again and it fired right up. At the time I talked myself into thinking this happed due to being low on fuel and being backed up on an incline (wishful thinking).

Last night, it happened again. This time in traffic on the freeway. I was able to coast it to the side of the road but this time it would not start. I started going through possible problems. Key on, CEL on solid, so EFI is good. Cracked the banjo on the fuel return line, cranked starter, no fuel. Jumpered the fuel pump, cranked starter, got fuel! I tightened up the banjo bolt and limped it a 1/4 mile to a parking lot at the next exit. There I pulled the jumper and tried to start it again and it fired right up. After starting and running it for a few minuets a few times, I drove home without issue. I went out first thing this morning and it started right up.

I have no idea how to diagnose this nor how to know when it's fixed. There are a few threads here that describe a similar issue but none of them describe a resolution. I'm hoping some of you guys will be able to help.
 
Joined
Dec 8, 2006
Messages
9,880
Location
New Jersey
If the fuel delivery issues can be eliminated by bypassing the fuel pump logic, then the issue must be with the fuel pump logic.
The fuel pump is fired through the circuit opening relay in the left kick relay panel. That relay has 2 coils. The primary is fired from the starter circuit and the secondary is fired from the flap in the AFM.
If both logic circuits are failing, then either the relay contacts are faulty or the +12 from the EFI relay that feeds the COR load side is faulty.

Before going further, I would be certain that your issue is in fact fuel delivery.
 
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
Thanks @jonheld for chiming in. I hoped you would. I agree that It would be best to verify the issue but I don’t know how to do that when the problem only occurs intermittently. I tested the EFI and COR relays per the fsm this morning and the both checked out. Not sure where to go from here.
 
Joined
Dec 8, 2006
Messages
9,880
Location
New Jersey
Thanks @jonheld for chiming in. I hoped you would. I agree that It would be best to verify the issue but I don’t know how to do that when the problem only occurs intermittently. I tested the EFI and COR relays per the fsm this morning and the both checked out. Not sure where to go from here.
If the system is working properly, then everything will always check out correctly. You have to diagnose when the problem occurs.
 
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
If the system is working properly, then everything will always check out correctly. You have to diagnose when the problem occurs.

The difficulty I’m having is that the problem is too short lived to get any diagnosis done. It just did it 3 more times but I don’t know anymore than I did yesterday.

Do relays fail intermittently?
 
Last edited:
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
You can force the pump on while the truck is sitting still.problem seems to be when it’s bouncing around. Mike
I cleaned all those connections when I replaced the pump and the sock. The problem started prior to that and has continued after.

As much as I hate throwing parts at problems, I don't know what else to do. I replaced the COR, and because it looks like the secondary coil on the COR is fed from the EFI relay, I replaced it too. After replacing the relays I drove it for 30 miles with no issues. But, since I haven't actually discovered the problem, I have no idea if it's fixed. It probably isn't. I'm still open to suggestions.

Gonna cross post this in the 60 section. Maybe the 62 guys will have some ideas.
 

red66toy

Supporting Vendor
GOLD Star
Joined
Feb 25, 2003
Messages
3,422
Location
Seattle, WA
Have you checked the wire harness at the notorious bend it has to take at the back intake brace? That is a known wire breaking area and could be causing weird issues.
 

red66toy

Supporting Vendor
GOLD Star
Joined
Feb 25, 2003
Messages
3,422
Location
Seattle, WA
Its the turn that the main engine harness takes just after the fuel pulsation damper as it goes behind the intake brace. When the engine is running, it may be worth jiggling the harness at this location to see if the engine runs weird. You can see the big crease in the harness in this pic. This is my harness while I was replacing the injectors.
49725850706_f882005672_b.jpg
 
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
Its the turn that the main engine harness takes just after the fuel pulsation damper as it goes behind the intake brace. When the engine is running, it may be worth jiggling the harness at this location to see if the engine runs weird. You can see the big crease in the harness in this pic. This is my harness while I was replacing the injectors.
49725850706_f882005672_b.jpg
Gave it a good jiggle with the engine running. It made no noticeable difference.
 

red66toy

Supporting Vendor
GOLD Star
Joined
Feb 25, 2003
Messages
3,422
Location
Seattle, WA
Gave it a good jiggle with the engine running. It made no noticeable difference.
Well thats a good thing :). It wouldn't be fun to fix. Another thing that can cause weird running issues is a bad EFI coolant temp sensor. Mine went bad and the cruiser died a couple times while driving.
 
Joined
Dec 8, 2006
Messages
9,880
Location
New Jersey
it looks like the secondary coil on the COR is fed from the EFI relay,
As previously stated, the secondary coil in the COR is energized from the flap in the AFM which completes the ground path and fires the relay.
The EFI relay feeds the +12 to the COR on the contact closure (load side) of the relay and the static +12 for the secondary logic side. But it is the closure of the switch in the AFM that completes the circuit to fire the secondary coil.

By jumping pins 1-8 on the check connector, you are bypassing all fuel pump logic and feeding +12 from the EFI relay directly to the fuel pump. In this condition, the fuel pump will continuously run when the EFI relay is energized (key in ON position).
 
Last edited:
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
As previously stated, the secondary coil in the COR is energized from the flap in the AFM which completes the ground path and fires the relay.
The EFI relay feeds the +12 to the COR on the contact closure (load side) of the relay and the static +12 for the secondary logic side. But it is the closure of the switch in the AFM that completes the circuit to fire the secondary coil.

By jumping pins 1-8 on the check connector, you are bypassing all fuel pump logic and feeding +12 from the EFI relay directly to the fuel pump. In this condition, the fuel pump will continuously run when the EFI relay is energized (key in ON position).
That was my understanding from the diagram on Fl-44 in the 3f-e manual. Thank you for verifying it for me.
My reasoning behind replacing the EFI relay was 1) It is involved in the circuit and 2) I had a spare anyway. I don’t think it was the problem but I have no idea what the problem is so I guess I can’t be sure.
FWIW there is no one who’s opinion I value more than yours when it come to this truck.
 
Joined
Apr 5, 2013
Messages
215
Location
Behind The Beard
Since replacing the relays I’ve driven it 2 times. Once about 30 miles and once about 40 miles. I did a mix of highway and surface streets. First trip was in the late afternoon when it was hot out, second was this morning before it got hot. Everything ran fine both times.
I’m gonna keep studying the manual to see if I’ve missed something.
 

Users who are viewing this thread

Top Bottom