I'm driving my truck (5.0, auto) about 50mph and all is fine. Suddenly the engine is bucking, no power and I can barely keep going. Then, as if you turned on a switch, it returned to normal for 4 or 5 seconds....then back to the loss of power. It sounds like a wire or something but where do I start? Any ideas?
Ken

 

year? id check the fuel pump and all wires going to it.

also check timing

 

Does the truck have a tachometer? If it does have one does the tach also fluctuate to "0". Could be the TFI module or the hall effect (PIP) in the distributor. Always a good idea to check for any stored codes in the computer before throwing money and parts at a problem.

 

It's a 1991. No tachometer. I have dual tanks and therefore, 2 fuel pumps. If I have a bad (or going bad) fuel pump, won't switching tanks tell me that one of the fuel pumps are bad? No engine light came on.

 

That would be a great way to determine if it is one of the pumps. If it happens on both, then it could be common wiring for the two pumps or ignition.

I have rigged up my fuel pressure gauge to watch the pressure while driving. If the pressure stays in range, then you know it's ignition.

 

OK. I have tried switching tanks and I still have the same problem. I checked for loose wiring and couldn't find anything. So what should I check next? By the way, it's 20 degrees outside. Last Saturday it was 60! Just my luck....

 

check the codes. I had 1 that had a lazy o2 senser that acted like that

 

BTW it's 15 here, it was 55 last Thursday If the cut out problem appears to happen only when the engine is warm it is usually something electronic. Most common cause is faulty TFI, PIP or a basic item is a bad ground. Cheap and easy thing to do is make sure all the grounds are clean and tight. You can pull the TFI, take it to a good auto parts store, they can test them. Make sure they do it 3-4 times to heat it up.

The toughest to diagnose without test equipment is the PIP. Usually it's the process of elimination that leads you to a PIP. It can be changed but you have to pull the distributor to do it. Most choose to get a rebuilt dizzy. If you want to swap in a TFI make sure you get the correct one. If yours is gray, replace it with a gray one. If it is black, do the same. Most auto parts store listings are incorrect. Go by the color of the one you have.

I had the same type of problem on my last '93 Lightning. It took months to finally trace down the problem to a bad PIP. At one point the truck lost power, I nursed it back home ~ 1mile, by the time I got there all 4 cats were cherry red and smoking. The CEL NEVER came on throughout this whole ordeal. After swapping coils, TFI and PCM from a twin Lightning I had it came down to the PIP.

 

I have a simalar problem. I've replaced the fuel pump in the tank, ignition coil, cap and rotor, plugs, wires, fuel regulator, map sensor. I've also checked all grounds and power sourses with little to no success. I also adjusted timing. The deal is that you can't even hold 3/4 open throttle without the engine sputtering on itself. anyone got an idea or 2 this ol soldier would appreciate it.

 

Sorry, but what's a PIP and a TFI? Where are they?
Today I was told to check the cat. If it's clogged, it could cause these symptoms. I'll check it out tomorrow.

 

maybe i missed it somewhere in previous posts but will it idle? does it only do this at 50?

 

Post #3: PIP is the Hall effect sensor in the distributor.

TFI is the Thick Film Ignition module. The PIP feeds through to the TFI to tell the coil to fire. Depending on the year of your truck the TFI is bolted to the distributor or it is remote mounted near the driver side hood hinge. Read here for further info: Ford Fuel Injection

 

The problem is throughout the rpm spectrum. It will idle only after it's warmed up, but just barely. BTW, it's a 1991.

 

sounds mabe like some vaccum leaks, take some carb cleaner, and spray it over the intake, and whereever your hoses aoe, if the idle rises, you have found your leak

 

I have my own opinions of the carb. cleaner method of finding vacuum leaks on an EFI engine. I have used it on three EFI Ford trucks, never found an issue using it but yet I still had issues such as high idle RPM, rough idle, etc. I suggest using a hand held vacuum pump/gauge to test each vacuum circuit and component. On my current '92 F350 w/5.8L engine it exhibited the high idle problem, but passed the carb. cleaner method multiple times. Only after using the vacuum pump/gauge method did I find many of the plastic vacuum lines less than solid and two rusted or cracked vacuum reservoirs. Even then I still had a less than textbook warm idle RPM after replacing all the vacuum lines and reservoirs. I finally replaced the gasket between the upper and lower intake, problem solved.

I never had a complete loss of power, i.e. cutting out at speed as the OP has stated. Probably have two issues going on here.