My 1989 F250 4x4 460 EFI has been giving me headaces for weeks now. The story begains when the damn thing would not start, after five years on a brand new rebuilt motor and only 32K with not hardly a hickup.After changing the fuel filter and replacing the fuel pump relay, it started, how-ever there has been a missfire under load and sounds like shot gunns when you let off on the gas. The missfire is just like a fouled plug and the back fire is from raw gas getting into the tail pipes?I've spent way too much money already, trying allmost every thing. I'm lost.

 

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Have you checked your MAP sensor?

 

I found a defective pressure regulator on my F250 FI 302 1985. The way you tell is remove the vacuum line from the regulator and if fuel comes out that line it has a busted diaphram. When I replaced it all my loading up issues went away.

 

I forgot to mention to have someone turn the key on for a second to see if the fuel comes out.

 

Backfire is usually, but not always caused by a lean condition. Has any of the emission equip been checked out , if still on the vehicle?