5.8L 91 F-150 number 8 cylnder fills with fuel when trying to start, have unhooked electrical form injector to see if it was electrical problem, changed injector out, unhooked return line to see if that was plugged causing fuel to fill cylinder, checked compression to see if exhaust valve was opening compression is at 150 same as the other cylnders. Any ideas???

 

I have heard this one before, I think it was a bad Fuel Pressure regulator.
Pull the hose off with it running and see if it has fuel coming out of the regulator.

 

If it won't start just cycle the key with the vacuum hose unhooked. If the regulator is bad fuel will shoot out the nipple.

 

I just had the same problem and I will tell you what it was. I replaced the valve cover gaskets and had to remove the coil bracket. When I put the bracket back on I pinched the #8 fuel injector wires underneath the bracket when I tightened it. Fuel was pouring into the #8 cylinder. Took me a while to figure that one out!

 

NO NO NO

DO NOT diagnose fuel delivery issues on a hot running engine. We all like to get and give advice in here, but the best advice is BE SAFE. Checking the FPR with the engine running is NOT safe. Neither for the engine, or much more importantly for you. If that engine is hot, the fuel will hit it and instantly vaporize. If the hot exhaust manifolds don't ignite it, ANY stray spark will (do you trust your plug wires with 40,000+ volts on them?). You don't have to stick a match in a puddle of gasoline to have a major fire or explosion. Gasoline in liquid form is actually much safer than the vapors. You can see the liquid, but you can't see the vapors.

To test the FPR, use the fuel pump test connection on the diagnostic link with the engine cold. Disconnect the vacuum line from the FPR, turn on the ignition to the run position, and jumper the Fuel Pump test on the link. You can see the FPR when you do this, and minimize the amount of fuel that will certainly come squirting out of the nipple if the FPR is failed. This also allows you to run the fuel pump as long as you want without a hot or running engine. It will probably generate a code, but I'd rather reset an error code in my ECM than pay a doctor to relocate my skin on my body. One tiny spark with fuel vapors and a hot engine, and raw fuel in #8 will be the least of your worries. Your repair bill won't be on the truck. It'll be on your body for spending the next 6 months in a hospital burn unit.

 

Old_Paint you are very correct and I have put out the same advice many times, I was in a hurry that day and did not clarify "IT", sorry.
"IT" was the fuel pump(s) not the engine. Grounding pin #6 of the DLC (test connector) with key on a cold engine is the only way to test the Fuel System.

 

Wasn't trying to be 'right', just trying to help someone else be safe. Last thing I wanna read about in here is where free advice erases someone's face. I've had carb fires on my VW, and they ain't easy to put out. I can't imagine trying to put one out with 60PSI plus fuel spraying everywhere.

But yes, my primary point there was NEVER diagnose fuel delivery problems on a hot engine. WAY too risky.