I have an 88 F-250 with an EFI 302 manual 5 speed. 3 days ago I was driving down the road at about 55-60 MPH. The truck would start to hesitate like it was misfiring, then stopped having throttle response. I could shift down to 3 and the RPMs would rise but still no throttle response. My speed would drop down around 35 MPH and the problem fixed itself, sped up back to 55-60 and the truck ran like nothing was wrong with it. It had the same issues at different speeds that same day while driving. I checked the spark plugs that I had replaced a little under 2 months ago and they still looked bran new. Checked the O2 sensor in the exhaust and looked fine. Started it back up idled some what rough and 3-5 minutes later it died, cranked it back up and still the same problems. I replaced the TPS and it didn't fix the problem. Noticed the gasket behind the throttle valves is cracked planned on replacing today. I'm thinking it could be the Fuel Pressure Regulator or the Gas Cap causing vapor lock. I replaced the distributor cap, rotor and fuel filter a little under 2 months ago. Does anyone have any recommendations that I'm not thinking of or can confirm replacing the Fuel Pressure Regulator, Throttle Valve Gasket and/or Gas Cap would fix the problem. Thanks

 

Ford Fuel Injection How To Run a Self-Test

That is a link to a page that will show you how to pull diagnostic codes w/out a code reader. My advice tho, if you don't own a code reader, is to go buy one. A good cheap one will run you about $30 at O'Reilly, little more or less at other stores. Makes pulling the codes much easier.

From the sounds of it, the IAC valve could be bad. Here's more info on that- Ford Fuel Injection Idle Air Bypass (IAB)

Is the TPS on your truck adjustable? Whether it is or not, did you make sure that there was spring tension on it when you installed it? Sounds like a dumb question but that's an easy thing to overlook. It could be that the new one isn't set to the right voltage. Here's more info- Ford Fuel Injection Throttle Position Sensor (TPS)

Start with pulling codes and see what you come up with. Even if there's no check engine light, codes could still be stored.

 

The TPS is not adjustable. When I picked it up they only listed one TPS for an 88 which had the same part number as the one for the 89. They didn't have the one listed for the 88 in stock but had one from a different manufacturer listed under an 89 so I got that one. When I installed it the truck would immediately die and throw the check engine light on. I tested the old TPS while it was off the truck and seemed to be alright. I put the old one back on and the check engine light turned off but still had the same problems, ran for about 3 minutes then died. The spring tension is good on both. I'm going to pick up a code reader here in an hour or two. Thanks for the advice.