Hey,
My buddy has a 91 f150 that is back firing like crazy when under load. Idling its fine. From what he told me he changed the distributor cap and didn't label the wires. So he probably mixed them up when putting them back on. How does one go about figuring out which wire goes where? Thanks for the help!

 

first off... which engine?

Firing Orders for Ford Engines

 

What size motor?
Also, is the motor stock?

Let us know and we can give you the firing order.

 

Thanks for posting the link Elderstarr. I'll have to bookmark that!

 

Its a stock 302 out of a 1992 truck

 

I just heard back from him and he got a diagram and hes pretty sure its all correct but it still backfires. Any ideas?

 

ok, just be sure he has the corect diagram for the '94 and earlier 302's. '95 and later used a different one.

Can you give any history on the truck from your buddy? like when did the backfiring begin? Is there a CEL on in the DASH? Hsa he pulled code? There maybe some codes stored and the CEL doesn't necessarily have to be on show signs of a problem(s).

If it's running fine @ idle and then begins backfiring, there could be any number of things wrong. Checkvalves for the Thermactor/Smog System. Fuel pump on the way out. Bad MAP sensor.

Checking fuel pressure would be a good idea.
What Plugs is he using? (Platinums, Bosch, and High dollar plugs will NOT do you any good and cause more problems than good!) Stick with regular motocraft or autolites!!
What plug gap is he running?
Does he have the plug wires for 7 and 8 separated? (These 2 cylinders cause Crossfire) Be sure and separate them.


Let us know and will chime back!

EDIT: After re-reading the statement" My buddy has a 91 f150 that is back firing like crazy when under load. Idling its fine." tells me the firing order is correct. If the firing order was wrong, it would run like crap from the start. He needs to check my lists of possible other problems. If he doesn't know how to pull codes, you can do it for free with a jumper wire and a meter. Go to www.fordfuelinjection.com. Have a read there.

 

Do you know if it stared before or after your friend changed the cap?
If it started exactly after he changed the distributor cap, I could see no other possibility why it would be backfiring... unless he got some water in it.

Other possibilities...
Timing chain skipped a tooth
camshaft has the H.O. firing order
Bent/unseated valve

does the engine shake heavily at idle?

 

It started happening after he changed the cap and mixed up the leads. Its a new distributor cap.

The engine shakes but I dont think its anything out of the ordinary

Ill let him know and see what he says. Thanks for the input guys

 

After checking spark plug wire routing, make sure that the base idle timing is also checked.