Help!, I've got an 86' 5.0L 4x4 that has me scratching my head. problems started a few years ago with engine dying on me when it gets hot under load. I would let it sit for a half hour or more and it would start back up and run. This would happen frequently in the summer almost never during colder months. Some one suggested the ignition module. I replaced and had no further problems until lately (5 years later) when it did the same thing again, but now it has progressed where it will only idle roughly and die if I try to rev it.. Thinking the old axiom, it's either spark or fuel, I have done the following: Replaced the TFI ignition module & coil, and the entire fuel pump system, including the in-tank pump, canister fuel filter element, in-line high pressure pump, and the other can fuel filter. No effect on the problem. The cap and rotor, plugs and wires have been replaced previously and are low mileage. It cranks up and idles a few seconds then drops off almost dead in a continuous cycle. I have been able to check the timing in this mode to make sure it is somewhere in the ballpark of 10 BTDC. The truck is old but has pretty low mileage, 120K. I'm stumped!

 

Check the fuel pressure at the rail with a gauge. Can usually be rented/borrowed from a parts store. I believe it should be around 40 psi, should go up 5 to 10 when you pull the vacuum line from the regulator. There should be no fuel in the vacuum line when you pull it off with the engine off.

Did you drop the tank and clean it out? How is the sock filter on the pickup?

 

Spray some starter fluid into the intake and see if it runs better. Although, I suspect the ignition. That TFI may not have been a good one.

 

Yea, I dropped the tank and cleaned it, but it was surprisingly clean for being 34 yrs old. even the strainer on the pump looked relatively clean. I replaced the entire in-tank pump, as well as the in-line filters and high pressure pump. Thanks for the info. still skeptical it is fuel related having replaced all components but I guess a pressure measurement would be the definitive answer. Any type of vacuum feedback valve or sensor do something like this?

 

Hate to think I got a bad part off the shelf but not unheard of. Thanks for the info/quick response.

 

A bad MAP sensor can cause it to run like crap, but it shouldn't cause it to do that - disconnect it and see if it makes a difference.

 

Update on my problem. Changed out the TFI module (again) and situation improved somewhat; engine idle improved a bit but still idling rough, can also now rev the engine via accelerator pedal, but again very rough running. -also tried advancing/retarding timing while running; no improvement there other than it runs a bit better at correct timing (duh). I have not checked fuel pressure with gauge but depressing outlet on fuel rail gives a vigorous squirt of fuel. -and as stated, I replaced in-tank fuel pump, in-line high pressure pump and all fuel filters, and coil. checked rotor/cap. As per someones' suggestion, I tried squirting starter fluid in intake; that killed the engine. -still down.

 

Put two bottles of Chevron Techron in the gas to see if it helps by cleaning the fuel injectors.

 

I'd pull the plugs, look at them, do a compression test, inspect the cap and rotor carefully. Those and the wires may be "low mileage" but that doesn't mean they are still good. A compression test will tell you if you have internal engine issues causing your problem.

I remember years ago, I had a 1967 Ford beater with a 289 - it idled like total crap but ran "okay" - it had bad valves, compression readings were all over the place.

 

As suggested by previous posts, I replaced the TFI module again, and added injector cleaner. Now the problem turns more head scratching; I took off cap and rotor to double check for cracks and arching, pulling electrical connections and cleaned contacts.. Put it back together and fired it up. It seemed to be idling a little better, then as it sat idling, it continued to improve to near normal, and revving it sounded good. I took it for a 10 mile drive and it's running like a champ. Today I fired it up, seemed to be idling good, took it out and within a block it was back to rough running, bucking like a bronco. What the ??????