I have a 1991 F150 Lariat with a 5.8 engine. Motor began misfiring and stalling and sputtering when accelerating. Mechanic replaced maf sensor and it improved for awhile. Problem has returned. Sometimes clears just by shutting off and restarting. Also having issues with new gas cap. Occasionally I need a wrench to unscrew it. Any info will be appreciated.

 

Check the fuel pressure off the shrader valve on the fuel rail. Specs are 30-45 psi. I would change the fuel filter if it is due. Check the spark plugs to see if they are worn out or fouled. If the problem started with the new gas cap, go for a test ride without the gas cap.

 

Plugs and wires were replaced recently. I replaced the fuel filter today. Havent test driven yet. I will have to learn how to test fuel pressure. Maybe need to replace pump.

 

You can probably obtain a loaner gauge from your parts store or buy a fuel pressure gauge. Every tool box should have one. Please test before you go through the hassle of changing a fuel pump. Other items can cause issues such as a relay or pump wiring. The pump(s) need battery power to deliver proper pressure to the system.
Even though you replaced plugs, you should remove each one to "read" it.

 

You can probably obtain or buy a fuel pressure gauge. Every tool box should have one. Please test before you go through the hassle of changing a fuel pump. Other items can cause issues such as a relay or pump wiring.
Don't forget the test drive with the cap removed.
Even though you replaced plugs, you should remove each one to "read" it.

 

Thank you raski for all good advice. New fuel filter seems to have helped. I need some free time to perform the tests that you recommended. I will report back with the results. Thanks again.

 

I had the fuel pressure checked, It was ok. I drove without the gas cap, no improvement. Still need to check the plugs. Others have suggested fuel vapor cannister or solenoid. Not sure how to check those. It occured to me that I never memtioned that my temperature guage hasn't worked since I have owned the truck. Not sure if that matters. Any advice will be appreciated. Thanks.