I have a 91 F150 with the 300 I6 with EFI. It has 6 new injectors, 6 new spark plugs, 6 new wires, and a new distributor cap, annnd new fuel pump in the front tank. It misses at idle, runs smooth under light throttle up until 2k rpm and then starts to miss again. It misses when you let off the throttle no matter what. It runs much better despite the miss with the new parts. I drove it for about 5 years with the miss, or about 10,000 miles.

The mechanic thinks I might have a valve that's sticking and causing it to miss. They tried putting some water in the intake to knock the carbon off to no avail. I drove it to work Sunday and by the time I got down there it wasn't missing but it was stuck idling at 1900rpm. When I started it this morning it was Missing as usual. Vacuum leak has also crossed my mind, but don't know where to start.


Any ideas? My wife is going to kill me if I throw too much money at this thing after all this.

 

Could be worn out bushings in the distributor.

 

codes? no reader, count flashes, see below
Fuel Injection Technical Library » How To Run a Self-Test

(16) How to read a check engine light code on a Ford 1983-1995 EEC 4 system - YouTube

 

Do you have a check engine light?

 

OBD1 does not identify misfires like OBD2 does.

 

Even though yours is a 91, there's a TSB 95-15-11, that got republished, for 93-95 F150-350, Bronco, could help shed light on, if nothing else. Its too big for me to post

 

Easy Button: TSB 95-15-11

 

I have a check engine light on and I have a bad EGR valve, some codes for the speedometer (been driving off the tach for years). Thanks for the service bulletin.

My friend loaned me a test light and a compression gauge. Going to see if it's missing on the same cylinder consistently then put some atf in the cylinder to rule out valve vs ring. In some ways I hope it changes cylinders.

 

Consistently missing on cylinder 2. Going to put the compression tester on it this week.

 

The speedometer in 1991 was mechanical so I am confused as to how you are getting codes.

 

That's what I thought..maybe someone got confused. My check engine light no longer comes on though. Or at least not as often.

 

check again, for new codes

 

The VSS signal is generated by a discrete sensor located on the transmission output shaft or transfer case. It goes directly to the computer and speed control amplifier.

That is how/why you will get a VSS error code. Nothing to do with the PSOM or speedometer. PSOM did not exist prior to 1992.

 

Just in case anyone is wondering-

Misfire was isolated to cylinder 2 with a test light. It has become very consistent and rarely fires correctly.

According to wet/dry test it is a ring problem. 90psi dry, 120-125psi with 15cc of ATF in the cylinder. Looking at a rebuild because I'd like to keep the original motor that currently has 400+k on the clock.

 

Good job figuring it out. That’s a lot of miles and possibly a lot of wear. I hope the block isn’t too worn out to be rebuilt.