79 f 100 backfire
79 f 100 backfire
Freshman User
Joined: Jul 2004
Posts: 41
Likes: 0
My Dad had this problem in his 76 with a 390 FE but the only time his truck would backfire is when he was pulling his 10,000 pound+ trailer when we pulled the engine to put the new 390 in we found that the head Gasket was blown and the distributor had a tooth out of it. So check the oil and see if there is any antifreeze in it.
Elder User
Joined: Jan 2003
Posts: 828
Likes: 3
From: Baltimore
Does your 79 have an air pump? If so, check the operation of the air checkvalve. If the check valve is bad, the pump is injecting too much air into the exhaust when you decelerate. That extra oxygen is mixing with the fuel rich exhaust and that causes a backfire in the exhaust. If I remember this correctly, the check valve is vacuum operated and it closes when you let off the throttle to prevent backfire.
Elder User
Joined: Jan 2003
Posts: 828
Likes: 3
From: Baltimore
Those checkvalves where out after a while. I think you can check the value by disconnecting the airpump output hose from the airpump. While the engine is running, if you feel exhaust being blown out of the hose, the checkvalve is bad. The checkvalve is in the hose between the airpump and the manifold.
Elder User
Joined: Jan 2003
Posts: 828
Likes: 3
From: Baltimore
I am glad I could help. I have an 1984 4x4 Crewcab that had that exact same problem. I couldn't fix it and so I put it in the shop twice to have them fix it. $400 later and it still wasn't fixed. I sat down with the Helm Manuals for that engine and after a bit of thinking about how things worked, I figured it was the valve. That fixed my problem too.
- Ron
- Ron
