running ruff and backfiring

[coreyg]

Ok where to start.
I've been working on my 71 scamp for almost 2 years now, just updating things in hopes to make it some what reliable. So here's the break down, its got a 1980 318 with 360 heads not sure on cam probably a stock 360 but I'm not sure I bought it this way and the guy didn't know what it had in it. So far I've switch to fitech efi, msd box, msd dis., msd coil, msd wires, and a 76mm turbo. I got the car far enough along to start driving and find any other issues I might find and fix. There have been a few. So all is going well, the turbo system is working really well and so is the fitech at controlling afr. I was having some decel backfiring and wanted to do some adjustment in the fitech to get that to go away. So took it out a few Saturday's ago and doing some pulls and adjusting the decel in the fitech and it seemed to be helping with the backfiring when letting off the throttle after a pull. Then a few minutes later the car started to hesitate a little, then progressively get worse where it was backfiring with just a small touch of the gas. Now at idle if I rev the engine it will start backfiring. I pulled the timing cover just to make sure I didn't skip a tooth and all is well there. I did a cold compression test and all cylinders where fairly close in numbers to each other. My fitech was giving a O2 sensor fault, the first time I drove it my O2 wire fell down and the connector layed on the exhaust and melted a little but it didn't seem to effect how it ran and the fitech's ability to adjust the afr, but I went ahead and ordered a new O2 and connector and redid that with no effect. And the crappy part is it was running strong for the first few week, my brother was coming up for work training and I wanted to give him a ride cause he is always talking trash about his c4 vette being faster and the weekend before he was it is when it started running bad. Any help would be great.

 

[dano]

I always thought backfire thought the exhaust was from ignition. Have you gone over it? Check plugs, wires, cap, rotor, timing static and dynamic, wiring etc? I'd pull the plugs just to see what is going on. Vacuum gauge hooked up? Hopefully you have a boost/vacuum gauge. Where is the o2, post or pre-turbo?

 

[coreyg]

I always thought backfire thought the exhaust was from ignition. Have you gone over it? Check plugs, wires, cap, rotor, timing static and dynamic, wiring etc? I'd pull the plugs just to see what is going on. Vacuum gauge hooked up? Hopefully you have a boost/vacuum gauge. Where is the o2, post or pre-turbo?

I've checked it all, the plugs are all a nice almond color except numbers 2 and 8 which are more darker. The distributors metal electrode on the rotor had some black marks on it, sanded them off and sanded the button. Going to see if that does any thing tomorrow. The O2 is post turbo around 18" down stream. my fitech is reading around 12 hg at idle.

 

[TrailBeast]

I've checked it all, the plugs are all a nice almond color except numbers 2 and 8 which are more darker. The distributors metal electrode on the rotor had some black marks on it, sanded them off and sanded the button. Going to see if that does any thing tomorrow. The O2 is post turbo around 18" down stream. my fitech is reading around 12 hg at idle.

That "got progressively worse" comment gives me the impression your spark is going somewhere it shouldn't.
I have seen coil wires burn the center out of them and gradually the spark has to jump so far it won't any longer as cylinder pressure rises.
As the cylinder pressure rises the spark chooses another path (like out of the coil tower to ground or something like that.)
As the gap burns out longer inside the wire it does it sooner and sooner in the throttle because it takes less and less pressure to let the spark go somewhere easier than jumping that gap.

Point is, a burning down the center coil wire acts exactly like what you describe.
It definitely sounds like an ignition issue.
One big clue is that an engine with this problem will backfire through the exhaust AS SOON as the throttle backs off and the cylinder pressure goes down allowing the spark to jump across again igniting the fuel in the exhaust system.

Just a thought because I have seen it a few times before.

 

[SuperStock]

Ok where to start.
I've been working on my 71 scamp for almost 2 years now, just updating things in hopes to make it some what reliable. So here's the break down, its got a 1980 318 with 360 heads not sure on cam probably a stock 360 but I'm not sure I bought it this way and the guy didn't know what it had in it. So far I've switch to fitech efi, msd box, msd dis., msd coil, msd wires, and a 76mm turbo. I got the car far enough along to start driving and find any other issues I might find and fix. There have been a few. So all is going well, the turbo system is working really well and so is the fitech at controlling afr. I was having some decel backfiring and wanted to do some adjustment in the fitech to get that to go away. So took it out a few Saturday's ago and doing some pulls and adjusting the decel in the fitech and it seemed to be helping with the backfiring when letting off the throttle after a pull. Then a few minutes later the car started to hesitate a little, then progressively get worse where it was backfiring with just a small touch of the gas. Now at idle if I rev the engine it will start backfiring. I pulled the timing cover just to make sure I didn't skip a tooth and all is well there. I did a cold compression test and all cylinders where fairly close in numbers to each other. My fitech was giving a O2 sensor fault, the first time I drove it my O2 wire fell down and the connector layed on the exhaust and melted a little but it didn't seem to effect how it ran and the fitech's ability to adjust the afr, but I went ahead and ordered a new O2 and connector and redid that with no effect. And the crappy part is it was running strong for the first few week, my brother was coming up for work training and I wanted to give him a ride cause he is always talking trash about his c4 vette being faster and the weekend before he was it is when it started running bad. Any help would be great.

Im not familiar with some terminology in your post, but for engine without turbo, backfire often cause by ignition timing. I would try to adjust timing first.

 

[coreyg]

That "got progressively worse" comment gives me the impression your spark is going somewhere it shouldn't.
I have seen coil wires burn the center out of them and gradually the spark has to jump so far it won't any longer as cylinder pressure rises.
As the cylinder pressure rises the spark chooses another path (like out of the coil tower to ground or something like that.)
As the gap burns out longer inside the wire it does it sooner and sooner in the throttle because it takes less and less pressure to let the spark go somewhere easier than jumping that gap.

Point is, a burning down the center coil wire acts exactly like what you describe.
It definitely sounds like an ignition issue.
One big clue is that an engine with this problem will backfire through the exhaust AS SOON as the throttle backs off and the cylinder pressure goes down allowing the spark to jump across again igniting the fuel in the exhaust system.

Just a thought because I have seen it a few times before.

So how do I track something like this down?

 

[TrailBeast]

So how do I track something like this down?

Ohm test the wires.
Look carefully at the ends where the core makes contact with the metal ends and see if it still there.
It will usually start burning at an end where the core is supposed to come in contact with the metal.

 

[dano]

I've had newer wires go bad and crossfire. I found it out the hard way while diagnosing a miss.

I'd ohm the two that had the off colored plugs first.

 

[coreyg]

Heres the results of the test. They are 8mm MSD wires
coil 051
1 99
2 110
3 103
4100
5 72
6 106
7 7
8 80