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Hello Ford Truck Enthusiasts! It’s been a long time since I’ve been on these forums. Today I come here looking for advise of what could be happening or what I should look at to repair a problem that is stated below. I’m looking at buying a 1986 F-150 XL with the 302 V8. The truck is in pristine condition. And it’s well worth the sellers asking price as it sits. But I also don’t want to buy someone else’s headache. And at this time in my life, I don’t really have time for a “project”. Quoted below is exactly what the seller has told me about the issue with the truck.
“Sometimes it’ll run just fine. Other times it’ll cut off while driving (usually when slowing down). Sometimes, when it cuts off, it’ll start right back up. Other times it takes a while for it to start again. Mechanic thinks it’s either the ECM or the fuel pump INSIDE the tank. We already replaced the fuel filter and one fuel pump.”
Being you don't have the truck it will be kinda hard to have you do some testing.
Also if you do get it fixed before you buy it see how fast the price goes up!
Who knows if it is fuel issue or not?
I would think if 1 pump was replaced and you were running off that pump, unless it is bad out of box, it would have to be something else.
Pump relay(s) going bad maybe?
IGN box going bad?
You & others need a little testing done to see what path to go down, fuel or spark?
Dave - - - -
Mechanic thinks it’s either the ECM or the fuel pump INSIDE the tank. We already replaced the fuel filter and one fuel pump.”
You don't mention if the vehicle has EFI or carb, one tank or two. I suspect it is EFI. If the pump inside a tank is bad, the vehicle can die under the condition you describe, slowing down or coming to a stop. if that's the case, in my own experience, it usually will not restart. However, I have two tanks. I just throw the switch and the fuel system primes right up. (the pump in my rear tank is weak but runs, the front tank pump works fine) Again in my own experience, when my 5.0L EFI truck dies under those conditions, it may be found to have a fouled throttle plate air bypass on one or both throttle bodies. (see pic) This condition is usually coupled with a stumbling idle before it actually dies. The fouled bypass is very easy to remedy. Just spray them clean with throttle body cleaner.
These EFI fueled engines are notorious for fouling Idle Air Bypass vales too. These valves are also pretty easy to thoroughly clean. Removal is required though.
Also, you mention one pump has been replaced. If the primary pump on the frame rail was replaced, they have replaced the wrong pump. As you mention too, the pump inside the tank would be the likely culprit.
I have been about 20 rounds with my 86 ranger. Same EFI system, just two less cylinders. When these trucks get this old, the wiring and connectors get old as well as everything else. The complexity of the EFI system makes it 10 times worse to figure out what the problem is. These older EFI systems were the first ones they made, and they are a little oddball. They made them better after a few years.
Just warning you, a intermittent problem like this may be hard to find. I am sure that is why he is selling a otherwise nice truck. It gets so frustrating you do not want to deal with it anymore. If you wanted to convert it to a carb, I would say go for it. If you have nerves of steel and want to mess with the efi system, I say go for it. If you are going to have to pay someone to work in it, avoid it.
I'd also suspect the TFI module too. Sometimes when they get old they will fail when the engine warms up from the circuit board inside delaminating, and then when the engine cools off they work again. The low pressure in-tank pump would be more likely to fail when the tank is nearly empty, again from overheating. It also could be that there is rusty crap in the tank clogging up the intake sock on the pump.
Assuming it is not a wiring issue, none of these would be a deal-killer if the price was right and the rest of the truck was in good shape. You just need to be systematic in chasing down the source of the problem.
You don't mention if the vehicle has EFI or carb, one tank or two. I suspect it is EFI. If the pump inside a tank is bad, the vehicle can die under the condition you describe, slowing down or coming to a stop. if that's the case, in my own experience, it usually will not restart. However, I have two tanks. I just throw the switch and the fuel system primes right up. (the pump in my rear tank is weak but runs, the front tank pump works fine) Again in my own experience, when my 5.0L EFI truck dies under those conditions, it may be found to have a fouled throttle plate air bypass on one or both throttle bodies. (see pic) This condition is usually coupled with a stumbling idle before it actually dies. The fouled bypass is very easy to remedy. Just spray them clean with throttle body cleaner.
These EFI fueled engines are notorious for fouling Idle Air Bypass vales too. These valves are also pretty easy to thoroughly clean. Removal is required though.
Also, you mention one pump has been replaced. If the primary pump on the frame rail was replaced, they have replaced the wrong pump. As you mention too, the pump inside the tank would be the likely culprit.
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