Won't start when hot, starts fine cold

[ExploderBlues]

I have replaced IAC, starter, tps, temperture sensor, battery, only happens after driving for longer than 5 minutes, let it set for a couple of hours and it starts fine.

 

[Rhett]

I doubt its vapor lock...so...

Are you getting a CEL (check engine light)?

If I was to replace something next, it would be the O2 sensor. Your 91 has just 1 O2 sensor I think. IF the O2 is bad, the truck may not start when hot, or it may die while running. On the other hand, when the engine is cold (such as when you first start it up in the morning) it operates in open-loop mode, which means the computer IGNORES the inputs from the O2 sensor. When the engine heats up to normal operating temperature, that is when it starts using O2 sensor inputs...and thats when you seem to have the trouble.

 

[ExploderBlues]

Rhett, thanks for the input, where is the O2 sensor located? Is it hard to replace? My next step is to take it to a shope with a diagnostic machine, although I am not getting a "check engine" light. Once again thanks.

 

[dogfriend]

I had this problem with my Ford Ranger and it was caused by an intermittent problem with the coil. You might want to check the coil pack.

You can simulate this by warming the coil pack using a heat gun.

 

[410Fortune]

Check the 02 sensor first with an OHM meter before you replace it. Your 91 will have 1 02 sensor, you are lucky.

Do you get pressure in the gas tank ever, when you stop to get gas does air ever ruch out when you open the cap?
The canister purge solenoid can cause this.

I would also take a listen to the starter solenoid, both on the fender and the one built into your starter.

Does the fuel pump prime?

Also you need to check the + and - battery cables for corrossion.

 

[Rhett]

Ditto to what's been said. Engine needs spark and fuel to start and sometimes yours isn't getting it.

FYI the O2 sensor will be on one of the two exhaust manifold pipes leading from the engine back to the catalytic converter and muffler. I have a 94 and it has an O2 sensor on each pipe, but you have 1 sensor and I'm not sure which pipe it will be on. The O2 sensor looks like a little cylinder with some wires leading out of it and the sensor threads into the exhaust pipe. To locate it's best to be underneath the truck, and look on the pipes as they bend up toward the engine. To remove, you can use a special O2 sensor socket (you can buy these at Sears or AutoZone or places like that) or you might be able to use a regular wrench (can't remember the size offhand). There are plenty of threads here about how to change O2s so you can search for that.