i bought a '91 f150 from a buddy of mine about a month ago and it has been an ok pickup except for a few problems. first off sometimes when im driving and let off the gas it will die, i can start it right back up, the check engine light will come on and off especially when it gets to operating temp. another thing when im driving down the highway and come up to a stop sign and start to slow down the oil pressure gage will go all the way down...i have since replaced the IAC and fixed vaccum lines but no luck. any tips?

 

First off, it doesn't have an oil gauge. The "gauge" you see in the dash is suppose to read in the middle of the "gauge" if it has 12 lbs of pressure or else it reads zero- nothing else. You'll find that the "gauge" itself is going south and you can either replace it or I'd suggest an after market gauge that actually reads the pressure. You could try to replace the sending unit but I doubt it will help.
You need to find someone with an OBD-I reader. The current OBD-2 reader is of no value to you. I'd suggest looking on Ebay and finding one- yer gonna need it. Probabilities are high that the EGR is hanging open which is why it cuts off coming down from speed....but that's speculation- you need to have the codes read to know for sure.

 

someone who had it before took the cats off. would that have anything to do with why its running like ****?

 

taking the cats out of these older fords wont cause many issues, mainly just emmisions.
i suggest getting one of these- http://www.amazon.com/Equus-OBD-1-Fo...7757550&sr=1-8
they are about the only accurate way to get codes out of your computer. other than that theres always the check engine light method which is a real PITA.
if you have a gauge what rpm does the truck idle at? and is it steady? other than the cutting off how does it run? make sure your battery connections are clean and tight.

 

it runs great other than at idle, i had to unhook the IAC due to high revs at start up. what do u mean "theres always the check engine light method which is a real PITA"?

 

"you can do the check engine light method.
http://fordfuelinjection.com/?p=13 "
Compliments of a post "Matts Ford" made a little farther down the page.

 

sounds like a vacuum leak. check the intake manifold gaskets. and yea the cel method is a pain for me when trying to read the codes right.

 

IF you don't have any hose leaks or vacuum issues, consider this:

IF your 5.0L is like mine, you have one O2 sensor on the bottom of your manifold. IF that goes bad it can cause an eratic idle which will generally buck the truck a little and die at LOW rpms. It's a $45 part that is supposed to be changed every 30,000 miles. People don't and their engines start running rough. If money is tight try removing the O2 sensor and GENTLY cleaning it. Be careful not to strip the threads removing it: After letting the engine warm (a little) turn it off and use a solvent. Leave it be for 5 minutes and then carefully remove. If it's all grimey it can cause your problem.
The other thing is the MAP (Manifold Absolute Pressure sensor), it uses readings from the O2 sensor to figure out the correct air/fuel mixture (I believe). It's a $50 part. IF changing your O2 sensor doesn't fix it, try replacing the MAP. It's on the passenger side of the truck on the firewall (easy to find and change).

The symptoms you describe match some I had and replacing these sensors fixed it. Do the O2 first though. It is more suspect with exposure to water and road grime. Hope this helps.