I have an 86 F350 with a carbureted 7.5 liter 460. I bought the truck a few months ago and it had been sitting for a few years. I did a total tune up (plugs, wires, cap, rotor, filters, etc). It runs great, except for one significant issue.

If I drive it more than 30 minutes, the truck suddently loses about 50% power. It doesn't try to stall, so I don't think it's fuel. After the truck is off for a few hours, it runs great, and I never see this on short runs.

I'm guessing that I'm having some sort of voltage drop problem. Maybe the alternater or ignition module or ignition switch, but I'm not where to start or how to test. Describing me as a shadetree mechanic would be generous

Here's some other symptoms...not sure if they are related.

1. Occasionally, it turns over really slow (as if the battery is low, but the battery is not low) after driving it a shutting it off. If I let it sit awhile, this goes away.

2. Occasionally, it doesn't do anything when I turn the key. If I shake the wiring harness a few times, it will start. I think there may be a loose wire somewhere.

3. Occasionally, the headlights or tail lights stop working for a second, then come back on.

One more detail...the original tank fuel pumps and fuel tank selected were disconnected and an electric fuel pump was added to the frame with a manual fuel flow switch to switch between tanks. The previous owner also pulled off the fuel tank relay and hot wired the new fuel pump to that wiring connection so it comes on when the key is in the "on" position.

Any suggestions would be appreciated.

 

Check your grounds. Main ground from battery to frame and engine, ground strap to firewall etc. it sounds like a ground wire problem since it is both ignition and lights etc.

If it still does it, I would guess it is a coil problem. When you lose ignition only when hot, it is usually the coil or module. Coils do it more often and are cheaper than modules.

It could be a harness issue but I doubt it would spread so far since the circuits on the hot side (positive) are separated so much by design. They split off in multiple fuseable links from the solenoid although you may have one or two bad wires that are partially broken so that you have good voltage but there aren't enough wires to carry enough current, especially once they have heated up and resistance rises due to that heat.

Hopefully it is just a ground wire issue. I would just pull the grounds, wire brush any corrosion/rust or paint that is hurting contact and then reattach them tight. You may find a corroded cable once you pull the ground apart also.

I find many more bad ground cables than I do hot leads. If you have the stock ground cable, it is famous for insulation cracking and corroding to green flakes between the frame connection and the block connection where it makes that little bend and is exposed to all that engine heat.

Good Luck,

Lee

 

Where are you wiggling the harness? Over by the starter relay? I would check it over throughly. Sometimes when a truck sits, things get corroded. Often you can just go around and unplug and then plug stuff back in, and that's enough to make a better connection.

 

My '83 will occasionally give me the starting problems that you mention when the motor is warm. Mine is due to carbon build-up on the heads. Its not as bad as it used to be, though, since it only happens now after extended high-speed driving.

And no, my timing is not the problem.

As Lee said, bad grounds cause some wild problems. Mine had inside lights, gauges, and heater fan running strangly until I discovered that the cab to ground wire attached to intake manifold was actually attached to the valve cover. To make it worse, it was connected to a relay bracket tack-welded to the valve cover itself. The thing has never acted up again.

Check all of the grounds.

I'll be willing to bet that when your problem happens, your electric pump loses power and leans out that big engine.

 

Thanks guys for the advice.

Dave F, I'm wiggling the harness that comes out of the driver's side firewall.

My plan will be to check the ground cables, and pull apart the connecters in the wiring harness and clean everything and put back together, then see what happens and go from there.

 

I thought I would post the solution in case it's helpful to anyone else.

The issue with it turning over slow (like a low battery) was in fact a ground issue, specifically the ground cable from the battery to the solenoid was very weak.

The loss of power issue was caused by vapor lock. The original in tank fuel pumps had been replaced by an external fuel pump and the tanks were not vented. So after X number of gallons were used, there was no way the gas could flow properly without venting.

On the wiring harness, I just took every plug apart and cleaned it with electrical cleaner, and that problem went away.

Oh, and the headlight switch was shot for the headlight issue.

 

We all have the same problems with our Fords, dont we?

 

i have a 1980 ford f-150 with a 302 motor and i am having a problem with the instrument gauges. When i have the key on the gauge for the oil pressure goes all the way to the H side of the gauge but when i start the truck the needle goes over to the L side an stays there. But the other problem i am having with the gauges is the fuel n temp. gauses r not working at all. The fuel gauge stays on the E mark and the temp. gauge stays on the C side of the gauge. Does anyone have any ideas on what the problem could b n how to fix it?

 

Bugs,

Have you checked behind the instrument panel to make sure the wiring harness plug is plugged in? That's the only way I know of that all the gauges could be out at once.

 

It's just a few screws