I have a 1990 F150 lariat xlt and it has been kinda jumpy on acceleration. It was bucking for a while until I replaced the tps. So I reran a koer test and got code 42 which is engine running rich. What would cause this?

 

Check your oxygen sensor (hego).

 

Ok where is my o2 sensor? And how do I tell if it is my problem?

 

It's located just before your cat on the exhaust. Looks kind of like a spark plug. Here is a good link for checking it. http://oldfuelinjection.com/?p=31 I had a lean condition with mine and changed it out and it solved the problem. Fairly inexpensive part. You may need some penetrating spray and get the truck warm to get it out. You can also get a special socket for it from most auto part stores.