Hello all. I have a "new" 1991 f 150 w/300-6. The truck starts just fine, idles great, and drives really well. BUT, after about 20 minutes of driving, the truck loses power, misfires, and just plain won't go. I learned that if I pull off and shut the truck down for just about 3-4 minutes, it will re-start and drive just fine for another 20 minutes, then the cycle repeats. Any ideas? I am thinking that there is some module that regulates air or fuel that has gone defective.

We got a code reader at Autozone, and the codes it throws are 551 and 172. The book identifies the code issue, but doesn't tell you what the fix is. Any help will be much appreciated.

Rich

 

172 indicates that the O2 sensor sees a lean condition in the exhaust stream, meaning not enough fuel is being supplied to the engine, or too much air, which could be vacuum leaks or exhaust leaks.
551 indicates a problem with the Idle Air Control Valve circuit. The idle Air Control valve lets air into the throttle body, when your foot is not on the accelerator.
I would start with a fuel pressure test. Rent a pressure gauge from a local parts store. Connect it at the Schrader valve on the fuel injector rail. Correct fuel pressure for a 4.9 engine ranges from 45 PSI, when you cycle the key to the run position, to 60 PSI when the engine is running and you pull the vacuum line off the fuel pressure regulator.
You may have to take the Schrader valve out of the valve stem in order to connect the fuel pressure tester, if the correct adapter isn't available.
Here's a thread on testing and cleaning the Idle Air Control Valve How to Clean and Test your IAC | Bronco Forum - Full Size Ford Bronco Forum https://www.fullsizebronco.com/threa...ur-iac.206960/
Don't replace parts until you've thoroughly tested them.
How are your vacuum lines? They're known to get brittle and cause vacuum leaks. They're pretty easy to replace with 5/32" ID vacuum hose.
Does the engine idle high? Vacuum leaks cause high idle.

 

Thanks Bean,

The truck has been in the shop for a week now, and they just started to do a little work on it yesterday. Evidently they saw the O2 code, but eliminated that as the problem. They did test the fuel pressure, and it checks out OK. Where is the Idle Air Control Valve located?

Thanks again for your help,

Rich

 

The Idle Air Control valve is bolted to the throttle body, and straddles the throttle body blades, so it can supply air to the intake when the throttle blades are closed, as shown in the link I posted.

 

A good car Buddy of mine suggested this today: there is crud in the tank, and after 15 minutes of driving, the crud blocks the pick-up tube in the tank, and starves the engine of fuel. This makes sense. I stop the car for 3 minutes and it starts right back up. OR, I flip the tank switch and the truck keeps running.

It makes sense that as the suction on the pick-up tube is switched off, the crud migrates away from the tube and gas can flow again. UNTIL the crud gets sucked back to the tube, repeating the cycle.

I think I've been over-thinking this thing. Fuel delivery is interrupted, and the tank needs flushed or replaced. thanks Bean for the help, I'll let you know how it turns out.

Cheers,

Richard

 

I would use some circuit cooler and spray it on the TFI ignition module the next time the engine is struggling and see what happens.