Have an 89 F150 with the fuel-injected 300 I6, 5spd O/D. Starts and runs great until it's fully warmed up.......usually 4-5 miles, and I can tell the check engine light will be coming on. If I try to keep the RPMs above 1500, it might take just a bit longer for this to happen.
My mechanic suggested disconnecting the O2 sensor for awhile to see if it made any difference, but nothing seemed to change.
When the engine is cold, it starts, runs, and smells fine. Once it gets up to operating temp, it acts like it's running too rich, and it smells bad! If I step on the gas while it's acting this way, it's happy to get up and go without stumbling and choking. The air cleaner has been changed, and there are no restrictions in the air intake hoses.
Just in case it could be related......the only other problem I have is with the gauge cluster. None of the gauges have ever worked, except the speedometer and the tachometer. All the lights and switches work. I had the instrument cluster out a couple times trying to locate a bad ground that I never did find. While I had it out, I cleaned every contact on every connector, but no dice. Every now and then I'll catch the fuel gauge "trying" to work.
Lots of posts on here with fuel-related problems with this year and engine, but haven't found one where it runs good ONLY when it's cold!
Any ideas will be appreciated!

 

If the CEL is coming on, read out the codes. If you disconnected the O2 and nothing changed, I'd bet the O2 is shot. Without an O2, it WILL run excessively rich when it tries to go into closed loop. When the O2 went out on my son in law's 89 Beretta, the gas mileage went down from 21 to 15 and it ran rich as hell.

 

I was thinking I should start with the O2 sensor, but wanted to see what ideas I could kick up in this forum. Wish I could put my code reader on it, but mine only does OBD2. Probably not worth trying to track down an OBD1 reader for one vehicle that's not gonna stay around for more than a couple years.

Thanks for the input. Appreciated!

 

There's a way to read out OBD1 codes with a paper clip bridging 2 terminals in the socket while counting flashes of the CEL.

I dunno about parts stores where you are, but one of the O'Reilly's here can read OBD1.

 

Finally managed to put in a new O2 sensor with the help of a lot of PB Blaster and words you didn't learn from your Mom! Threw in a new set of "stock" plugs, while I was at it, and it's running much better.

I was surprised to see the CEL still lighting up after only about 5 or 6 miles, though. Does something have to be re-set, now that the new O2 sensor is installed?

 

Disconnect the battery for about 20 minutes. If the CEL still comes on, read out the codes.