Help please.
I have done alot to this truck. It sat for about two years before I got it. I replaced the exhuast manifold, the old one was cracked. I reran the vacuum hoses, the old ones were dried and broken off. I replaced several gaskets and repaired the wiring crossing the engine from the drivers side firewall to the battery, etc. I replaced the fuel pump, the ICM, Rotor, cap, plugs, wires and replaced the ICM wiring harness back to the firewall. I adjusted the carb to as lean as possible. I adjusted the timing to specs I got here. I checked my vacuum pressure, good. The truck starts and runs smooth. However, when I turn it off it backfires. Almost everytime without fail, if I drive it anywhere. If it runs in the garage for an hour, I can shut it off with no backfire. Does anyone know what I missed? Where can I look? It is backfiring out the tail pipe.

 

I have seen this in 3 different Fords, an F150 4.9, a Mustang 2.3, and an Aerostar with a 2.8 engine. In all 3 cases they ran "OK". The problem was all three were Duraspark III ignition systems with part of the wiring and sensors missing/disconnected. This made the ECM go into "failsafe mode" or whatever Ford calls it. The vehicle will still run, but never optimally. The F150 was treated to a older carb and Duraspark II distributor and box. The ECM was removed from ther loop and the owner claims it runs better than ever.
The Aerostar was found to have an incorrect rebuilt carburetor, missing various feedback hookups. A correct carb was found and again the performance improvement was marked. Got better mileage and never backfired again.
The Mustang got a 5.0 transplant along with a Duraspark II setup and was VERY cured.
Previously all 3 vehicles backfired exactly as you describe. Although they seemed to drive well enough, how pitiful they really were wasn't evident until their problems were corrected.

 

Thanks. I'll try anything. I do know there is a severe elctrical problem in and under the dash. Could the wires going to the ECM be a culprit. I put off redoing this until I got it running, inspected and legal with plates. Also, I forgot to mention, I know this is not the orignal motor. Someone replaced it before I came along.

 

my 85 durasaprk 3 300 diesels after shutoff it's been doing it off and on since i bought the truck same situation some elect works some dosnt who knows whats missing . I have adapted by justing dumping the clutch when i stop.

 

Try reading the ECM error codes. I'd suspect some sensor or something missing, disconnected or malfunctioning. Checking the codes tell you where to look.
In the case of the Aerostar above, one of the many error codes was "TPS sensor out of range". Once I looked, this made sense. There WAS no sensor, the connector was just hanging there.