I've been battling this problem for some time. Here are the stats:
'91 Ranger 4x4, 4.0L V6

It never happens when I'm just driving around town. When I take an extended trip, after about two hours the engine just quits. No falter, no coughing, it just dies. Trying to restart it doesn't work UNTIL I turn the key to full OFF, then it starts right up. Two hours later, it dies again. It's really strange, because it always seems to die at about the same time, not random at all.

I took it to a shop, and they put it on computer analyzer. The only thing in memory was a bad mass airflow sensor. $200 later, I took another long trip, and sure enough, the engine died just like before.

My gut feeling is it's the computer itself, but I'd like to know for sure before spending more $$$.

Has anyone else experienced this problem?

 

never had it happen to me, but certainly sounds like a computer problem. Huh.... ask the Mod. (Ken), he's quite knowledgable with diagnosing.

 

Thanks. I know it's a strange problem. The engine runs like a top the rest of the time.

 

I have a hunch that an electrical component such as the coil, elect ignition module (if existant), or fuel pump are heating up and causing the problem. If it were mine, the next time it did it I would I would check for spark and fuel pressure to rule out one system or the other. At this point, I'm not convinced it's a problem with the computer module either.

 

I don't think it's a heat related problem. The system restarts when the key is put in the off position. That would kill the power to quite a few electrical systems, causing them to reset. It's definitely electrical, but I have no clue as to what.

 

I'm wondering if the ECU or fuel pump relay is kicking out on you. If you leave the key in the on position when it dies you can put a jumper from the t/lg wire (1989 wiring diagram) at the EEC-IV test connector and ground it and the pump should run and you will hear it, don't let it run for long or you'll burn the pump up. By grounding this wire you bypass the ECU in the electrical system for the fuel pump, it will tell you if the ECU & FP relays are closed and it might indicate a problem with the ECU. Just another opinion.

It would be helpful to know if you have spark and fuel pressure when this happens.

 

I agree that it would be nice to know if there is spark or fuel pressure at that time. The only thing I know for sure is, it seems to happen after two hours EVERY TIME, and last time I drove back from NJ to VA on RT 13, whenever I got close to the two hour mark and came to a stop light I'd turn off the engine, restart it, and the truck never died the entire 7 hour drive back. If it were a fuel problem, I would think the the engine would start to misfire before stopping completely. It always dies as if I grounded out the electrical system.

Thanks for your suggestions! I've been putting up with this problem for about a year now, but I hardly ever drive long enough to make it happen. As I said, it runs just fine commuting to work since that's only a half-hour drive.

 

The way you made it sounds was that it won't refire unless you turn the key to "off", if thats the case then you could crank it before you turn it to off and check for spark. I realize that you wouild need another person and find a spot were you could do this after 2 hours of driving.

 

Ok, I see what you mean. Thanks for the suggestion!

 

Ken, I was thinking along the same lines you were. Those relays are, on occasion, trouble. I hesitate to suggest that he just go buy new ones though since I'm a fan of diagnose and repair rather than throw parts at it and hope for the better.

Dusty1, here's an idea for ya (even though checking for spark or fuel delivery is best). If you're not in a position to test for spark/fuel pressure the next time this happens, leave the key on, go pop the hood and gain access to the power dist box, and with your finger snap one of the relays (fuel pump or EEC). Try to restart. If it starts, remember the relay you snapped with your finger and replace it. If it doesn't start, repeat with the other relay. Dont overlook the possibility that snapping one could also affect the other one. BTW, these relays are pretty cheap to buy.

 

CowboyBilly9Mile

Thanks for the suggestion. I agree that I'd like to be able to logically narrow the problem down too.

Any idea why it always dies after 2 hours? Wouldn't a bad relay be more intermittent a problem? Being able to reproduce the problem every 2 hours is really sticking in my craw.

Thanks again for the suggestions!

 

True, relays can produce intermittent probs, but that lil' coil in the relay generates some heat and relay contacts sometimes develop corrosion over time. There is a slight chance there is corrosion on the contacts in the power dist box too; I've been holding back on these and other thoughts until it's determined whether it's a spark or fuel related item. As for the "Any idea why it always dies after 2 hours?" thing, this causes me to suspect heat related, but for now something other than the computer. I've seen coils, electronic ignition modules, fuel pumps, IAC solenoids (yes, as odd as that seems), and other things fail once they were warm but so far not a computer in a car/truck.

 

I still don't think it's a heat related problem. Just turning the key to "off" and then restarting wouldn't give the relay (or whatever offending part) enough time to cool down. It has to be some sort of system that is reset when the power from it is killed.

 

I agree with maticuno, if it were heat related, turning off the key wouldnt let it cool down enough. It does sound electrical, if it was bad fuel pressure, it would sputter and cough before dying. How is the rest of the vehicle effected by this, does the radio turn off, or your guages, headlights, brake lights, etc.... Does the starter even crank? or is it like no power to anything in the engine compartment. I might look around at the wires to your maf and make sure they arent touching anything. Even if they were, i dont think it would kill your truck. But it never hurts to be sure, and maybe spray some contact cleaner on the maf connection for good measure.

 

Peter94

In answer to your question, nothing else is affected. The radio keeps playing, gauges keep working. In fact, the first time it happened the only indication I had was that the truck was starting to slow down and the tach was at zero. The radio was playing at the time, and I felt no surges or hestitation at all beforehand, so it was a complete surprise that the engine had quit. Yes, the starter will crank but the engine will not fire until I turn the ignition to OFF, then restart.

Again, it would be an incredible coincidence if a loose wire would cause the engine to die after two hours every time. That's why I thought there might be some diagnostic the computer runs every two hours, and when it gets to it, the engine dies. Unfortunately, I do not know enough about the computer to know if it runs a test every two hours. It would be really helpful to know so that I can rule out a computer problem.

If this problem were intermittent, I'd agree with the loose wire or perhaps a ground in the system. The fact that it happens every two hours of operation, I would think, would help me narrow down the problem, but it hasn't so far. Remember, too, that if I turn the engine off and restart BEFORE the two hours elapses, the engine NEVER dies.

Thanks for the suggestions. It's a real puzzler.