I have a 91 f150 302, 5 speed manual. it misses at all rpms and speeds, when i got it the guy said it just needed a tune up, we he lied. i have change everything, the cap,rotor, plugs, wires, coil, distributer, map sensor, there isnt a cel, and the light does work. i can unplug the wires from the plug on cylinders 4,6,7, and nothing changes, and there is spark coming out of the end of the wire, i have warranted out everything at least once. but it continues to miss, i think it must be something either fuel or compression related, i havent ran a compression test yet, but it doesnt run like its a dead cylinder, it acutally runs rich and fouls out only those three, no other plugs foul. and it doesnt smoke, and theres no cat converter. so im stumped. but u can smell raw fuel in the exhaust. so im thinkin mabey an injector dumping to much fuel. it drivin me nuts and im gettin 6 mpg. help

 

i cant really help ya fix it but if it was me id take it to my brother in law aka a mechanic or sell the thing sorry

 

hahah thanks I think im gonna get a new motor

 

still need a compression check..

 

Some of the things I would check and in the order I would do them
1. compression check . 2. make sure firing order on plug wires are correct for standard 302 or HO motor. 3. fuel injectors to make sure the the ones that are fouling are not bad, swap one with one of the ones that are not to see if problem stays with the cylinder or moves to the other one.
Because (1) if it has good compression and (2)if its firing through your wires and firing at the right time and (3) getting the right amount of fuel and getting it at the right time, thats about all it takes for it to run.

 

Definately time for a compression check, 6 & 7 being next to each other physically on the engine could be a simple head gasket or cracked head. If I were you before I did a tear down for inspection, I would pull the engine and go from there. Its just easier to work on and if the engine is bad, well its already out of the truck.

Irregardless of what vintage your engine is, the miss is occuring in the 3rd, 5th, and 7th in the firing order, that is telling me its a motor problem versus an ignition or fuel delivery issue.

 

The comment a couple up about firing orders would be worth verifying. The 'regular' 5.0L is 1-5-4-2-6-3-7-8, while the 5.0L HO is 1-3-7-2-6-5-4-8. The '3' is switched with the '5', and the '4' is switched with the '7'.

As I hear tell, a 5.8L or 5.0L HO cam fits in a 5.0L - just have to correct the firing order. Of course, I could've heard wrong.

At any rate, a mixed-up firing order would account for 2 of your 3 problem cylinders - the '4' and the '7'.

 

Dumb question of the day... How do I tell whether or not I have a regular or HO engine?

 

This might not be the correct way to do it but I would check to see which firing order it was wired up on now and mark each wire at the dist cap and if the wires had enough slack rewire it to the other firing order to see if it helped, hurt or made no difference. If it helps great If not you just rewire it back.

That way you wont have to try to chase down what motor you have.

 

Also this might help