schwefel69

Got a 1991 Ford F150 with the 4.9 300I6, behind it a 4 speed manual and 4x4! ONLY GETTING MAYBE 9MPG!
I just bought it from a guy and he said it was sitting for 1.5 years.. But it fired right up!
I ran it out of gas on way home (gauges didn't work) and ever since that Its had a miss at low rpms and idle.. but if I throw pedal down she runs like a champ..
I already did full tune up (plugs,wires,cap,rotor,air filter) ALSO ran seafoam and 91 octane..

Had guys tell me timing could be off, fuel filter is clogged, intake or coolant sensors, o2 sensors....

All it has for exhaust is the 2 cats.. no pipe after, it has a manifold leak also..

What could cause this? Any opinions welcome!

memoniz

My truck sat for nearly 3 years when I decided it was time to get it running again. It fired up when the tow truck pulled up but ran like crap. Had it taken to a mechanic to do some work and he found the #6 injector was not working. Ran smoother then when it was running before I parked it. You can overcome a bad injector with high rpms. I'd check that.

Oh, and don't ask me how because I don't know.

cfodor

I would try the fuel filter first and see if it gets any better

schwefel69

Already changed fuel filter. No help. I cut off the cats and put glasspacks on. Helpped alot. Still has slight miss and 9mpg.. i unplugged egr and it runs way! Better. Havent checked mpg yet.

Forge-World-FORD

Um, basics first. Pull codes. Have you tested the EGR to see if it is functioning properly? If it runs better with it disconnected wouldn't that point towards issues with the EGR.

nightowlll

You mentuoned that you changed the rotor but did you actually change the distributor?
There is a pick up sensor in the distributor that sends a signal to the ignition control module. They go bad and often times will cause a no start or rough running truck with no engine code. May I ask how many miles are on your truck?

nightowlll

When my truck wasnt running and I changed the fuel filter and I also changed the fuel pump. It was still working but wasnt putting out enough pressure. Also, I disconnected the fuel line at the manifold and used compressed air and blew out the line back to the fuel filter (With the filter removed of course) and there was a bunch of dirty grit in the line. It ran much better afterwards but that wasnt the primary problem.
Think on it for a while and go over different systems that coud be the potential problem and trouble shoot it back. I would start with the least expensive solution first. Also, have you checked compression on each cylinder? What did your spark plugs look like? Just some ideas.....