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I've had this problem before, but never in a vehicle with a manual transmission...
89 F-250 460 and 5 speed manual, dual tanks
Today, after work I discovered that if I push the truck "to hard" in 1st or 2nd gear the engine will seem to cut in and out every second, almost as if the fuel pump is only running every other second. It'll happen in 3rd as well but I have to be pushing it pretty hard (3rd gear at 35-40 mph). If it were like that in 1st and 2nd gear I wouldn't mind, but the difference between "moving" and "to hard" almost doesn't exist in 1+2. The volt meter also goes nuts as soon as the stutter happens
It only happens under load, in neutral I can rev pretty high and fast and nothing out of the ordinary happens.
I will check for cracked spark plugs and other obvious signs as soon as I can. Could a worn/bad clutch be to blame?
The truck has a bad rear main seal, but plenty of oil right now, the catalytic converter was stolen years ago, and I disassembled and cleaned the IAC yesterday to solve a surging idle.
I would suspect fuel pressure. Here's a thread on testing that https://www.ford-trucks.com/forums/1...rd-trucks.html
My '88 would idle fine and drive around the yard fine, but about 4 miles from the house, it would start bucking like it was running out of fuel. In my case, it was the front, in tank fuel pump and a bunch of debris and corrosion in the Dual Function Reservoir (Fuel Tank Selector Valve).
Might also be worth it to look under the distributor cap for corrosion, although it seems the symptoms would be constant with that.
I would suspect fuel pressure. Here's a thread on testing that https://www.ford-trucks.com/forums/1...rd-trucks.html
My '88 would idle fine and drive around the yard fine, but about 4 miles from the house, it would start bucking like it was running out of fuel. In my case, it was the front, in tank fuel pump and a bunch of debris and corrosion in the Dual Function Reservoir (Fuel Tank Selector Valve).
Might also be worth it to look under the distributor cap for corrosion, although it seems the symptoms would be constant with that.
BRILLIANT! when I "rebuilt" the IAC I noticed the primed pressure seemed weak, the booster pump is only a few months old, and the front in-tank pump failed a month or so ago, but came back to life two weeks ago, I'll try driving on the front tank soon and see what happens, but replacing the fuel filter and back flushing the DFR seem like a must. Plugs, cap, wires, and rotor are only a month old.
I would be careful with the Dual Function Reservoir. They're getting hard to find quality replacements for.
If you test fuel pressure first, you can go from there.
The in tank pump output can be tested at the Dual Function Reservoir by removing the fuel line that's coming from the suspect in tank pump, attaching a hose over the fitting of the fuel line, and jumpering "A" and "E" at the diagnostic connector, so the selected fuel pump runs constantly when the key is in the run position. The in tank pumps should put out 6 ounces in 5 seconds.
I'm guessing you have a bad wire in your wiring harness and as the engine rocks in the engine bay, that wire is opening up and killing your engine. Start the engine and then shake the wiring harness near the master cylinder and see if the engine stumbles or dies.
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