This is a real mystery but am pretty sure its a component malfunction or wiring issue. I can say I have checked for codes and have none, replaced every sensor except the knock sensor which I understand is not available and is pretty reliable and not a possible problem. Replaced all fuel pumps and regulator. Pulled and tested all injectors then refurbished with new seals and filters. Thought maybe after all this it was a computer problem. Replaced the ECM with one from another 86. Same issue. Usually happens driving at cruise speeds, but has happened under slight load or sitting at a light, just shuts off. Press on the accelerator and it kicks back in if i'm rolling then does it again. This is at operating temp. Also if I leave it in the driveway and idling at operating temp it will just shut off. But every time I can simply start it back up. Runs fine for a bit then dies. Have tried three different TPS and three different MAP sensors. Im totally at a loss lol! Putting this out there and have scoured these forums for answers. Also have posted this on another site recommended by a friend. Usually find my answers here or at least get good clues when I was chasing earlier bugs. Thanks in advance. Here is the list more or less of what I have done to trace the problem...

Pulled codes engine on and off
All sensors have been swapped out except Knock (TPS and Map 3 times)
Distributor
Cap,Wires, Plugs
TFI and Coil
EGR and Sensor
Vacuum Hoses
Fuel system from Tanks to injectors 35PSI even after engine dies.
ECM with a used one, (exact same stalling issue)
Relays
Engine harness
Rechecked all ECM grounds and added extra grounding cable from battery to engine just in case.
Air pump was removed by the previous owner but all lines are plugged off. ECU throws a code for this as expected.
Cat is also long gone so not a plugged exhaust.

 

First I am be far an expert and others know more than I do but here is what I think.


The 2 things that jump out at me above the TFI could still be bad even new and I here heat kills them. Do a search for TFI relocate kit and get it out of that hot dist. Maybe use a new one again?


The other is you did a ground from batt to engine but how about batt to body or motor to body or both? Are you sure the ECM is grounded good?


The only other thing is to find why it dies. Is it fuel or spark the cause? Once you know, fuel or spark, you have a path to go down looking for a fix.

Good luck
Dave ----

 

oh I missed you have 35 psi to the injectors after it dies so thinking maybe not fuel?
Unless no power to injector? There has to be a way to test this. Dave ----

 

Hey Thanks for the reply. Yes my mind does keep going back to a TFI as well. I know there have been a lot of issues with those and heat, also just a lot of poorly built replacements out there too. Grounds are all good for the body, engine, chassis and ECU.
Neg cable connects directly to the engine and another connects engine to body. I added an extra one to the engine from the battery and from battery to body. Figured it couldn't hurt.
Located all ECU grounds cleaned and reconnected them. The one that connects to the engine in the back left side was cut when I got the truck but it ran no different when I found and reconnected it last year. At that time I was chasing a poor idle issue which turned out to be a junk replacement MAP sensor. I've even changed that out again since this problem started. Runs the same with either one so I have a spare LOL.

 

Yes thinking of tapping the positive circuit to the injectors, connecting a meter and driving to see if that is an issue. My thought there is if they are losing power all will quit at once. If it was an issue on the ECM feed to them it would seem they all would not cut off at once, just would lose one of the two banks. Unless this is an ECM failure and both ECMs I have here have failed in the same manner.

 

It sounds like you have a good plan to figure what path to follow.
If the fuel checks ok then the next place would be spark.


Any way to tap in a test light to coil or IGN box power to see if that fails when the motor dies?
Dave ----

 

Following...

 

You mention testing for codes. You also mention a code set because of the air pump delete. Are there any other codes?

The ECM and the injectors are powered off the ECM power relay. Check and maybe replace this relay.

The 85/86 trucks have a unique ground wire from the ECM harness to the RR intake manifold bolt. I believe it is also an unusual color, like orange. If this wire is not connected, as often happens during engine work, the truck may still start and run, but be prepared for odd symptoms. Mine would intermittently not turn on the fuel pump relay. Easy thing to check and it may not show up as a code.

All said, it does sound more like an ignition problem rather than fuel or the ECM.

 

Yes, gives a code 44 which appears associated with the missing air pump. Get this each time I pull codes. New ECM relay and fuel pump relay. Old ones tested ok but swapped them just in case and they were original as best as I can tell. The rear engine ground was off a couple of years ago which I found when trying to solve an Idle issue I had. never changed the way it ran at the time when I reconnected it and eventually cured that issue with a new MAP sensor. I had really hoped I would find this wire had worked loose again when the current problem started happening but its secure still. Tomorrow I plan to head to the Part yard and see if there is another truck there with the engine harness intact. Went over the existing one carefully today as best I could with it installed and with the engine running to see if I could force a shutdown. Not one miss or stumble as I pulled and tugged on it and the connectors. Walked inside the house for a moment and it died as soon as I walked out the door. Started right back up as always. I think its haunted. Anyway will keep at it. Seriously thinking a carburetor is in its future.

 

I personally question the ignition switch... get your meter and test for power at the ignition module as soon as it dies and before you touch the key cylinder (might get some reference voltages beforehand so you know what you should have and where with the key in RUN).

 

Can't remember what access is like, but can you get your hand on the switch body to check the temperature? I'm thinking a marginal switch could be heating up.

Or how about rigging up a remote start switch at the solenoid? Next time it dies, leave the key alone. Don't move it. Leave it in run. Try to start it using the remote starter. What if the mystery fault was a worn spot on the contacts? Whenever you cycle the key after the engine dies, you're changing the equation without realizing it. Cycling the switch may have cleaned the contacts just enough to run for a while again, but also frustrating your troubleshooting attempts because it made the fault temporarily go away.

 

It seems that the Ignition switch is a logical fail point and it could be heating up. Was hurt at work last month and have neck injury so was trying to avoid actually working under there even though I did reach under and check that the connector was not loose. Not sure its required but maybe easier to drop the column to get to it and just replace the switch. Its less than $20 and cant go wrong changing it out considering what I have spent over the years. As far as remote start, I'm an long time Ford fan lol. I think one of the first things I learned was the screwdriver start trick at the solenoid! So anyway yes when It has shut off in the driveway and the hood was open as it was yesterday I restarted it from the solenoid. Fires right up. That idea about a bad spot on the contacts went through my head the other day while I was driving and it quit. While it was coasting i tried jiggling the switch carefully between off and on. No results. Pressed down the accelerator and it started running as it will when it shuts off at cruising speed.
This is why I really focused on the TPS as the culprit. But 2 replacement ones, (the last was the OEM expensive one purchased at NAPA) had same problem. By the way, All three of the TPS sensors I have here test ok.But yes I seem to have the mystery truck. I know after 30 years and at least 3 POs, the last was a young North Georgia country guy who was really into hunting that I bought it from who I suspect had it in the water a lot at least up to the floor judging from the mud I found under the carpet. Its had a hard life but I plan to keep it. No rust worth mentioning.

 

Yeah, you have to contort yourself to reach up into there and it kinda sucks to do; I've heard of people being able to replace the switch from that lay-on-your-back position but dropping the column is lots easier IMO.

But don't think it can't hurt to go change it, you sometimes come across people who try that and suddenly they can't get their gearshift lever out of PARK because some of the linkage apparently got broken during that process.

But ya know, problems like this are also caused by bad electrical connections at critical points, that connector to the coil comes to mind... there was one guy here who, weeks later, told us he eventually found the problem and it was the spliced-in wiring of some sort of PO-added gizmo (the Ronco T1000 Spacewalk Immobilizer or somesuch) to disable the ignition & fuel as an anti-theft measure. So, be sure you look at all your basic wiring, computer grounds, do some wiggle tests of the wires, etc.

 

It really acts like an intermittent short. Will pick up an Ign switch today from NAPA. Going to take a run to the Pick a Part now and look for an engine/ecu harness. Shows on their website there's an 86 which arrived recently. My luck its probably been picked to bits or not a 5.0 or both. But at least they don't put the Bull Noses in the regular Ford section anymore, instead they go into the "Classic" area at the rear of the yard. Seems most don't venture there as often. Appreciate the help!

By the way, when I first bought it, the truck had both a Spacewalk Immobilizer and Rocket Boot recharger installed by the Po. Wiring was a mess in the engine compartment so replaced the engine harness 5 years ago.

 

I have a bending issue as well ...

I use a board that I did a cheezy upholster job on that lays on the floor out the door and sets on Milk crates or whatever is the right height.

Works awesome and I don't have to twist or support myself!

-Enjoy
fh : )_~