I just bought a 1990 f150 xlt lariat 5.0 2 weeks ago.It was running great until recently it started to idle erratic.It does it when I first start it and its cold the rpms will drop then go back up.Then after a minute or two it stops sometimes it doesn't do it at all.When its doing this if I rev it up to about 3k and let off it will idle down so low it dies.I think its the tps.The previous owner has been having this problem hes replaced it twice with a bwd from oreillys.I figured maybe its just a junk brand and I can just throw a motorcraft on it.Im worried it could be the computer because it doesn't throw any codes.Anyone have any experience with this?thanks.

 

Since no one else seems to be responding, I'll give you my ideas. I have not had this issue but if I did I would check all the basics first. Check to see if distributor cap and rotor are clean and no cracks. Check to see if plugs are clean and gaped right. Check plug wire conditions. Check fuel filter and fuel pressure. Check vacuum lines for leaks. I don't know to much about the TPS but I've read other posts saying that it's critical to get that set right. You might want to check that. Also you could run Sea Foam through your system to clean it out. I would also check the timing.

Your computer throws no codes at all or give the system OK code?

 

This where or what I would start also. Also clean the tb and iac.