can exhaust header leak cause engine misfire in turbo 930?
 

I have this post in the 930 forum as well but thought the experts here might help me too:

I have been tracing down a misfire in my 79 930 for some time now after changing over to an MSD programmable igintion. Have been blaming all the problems on the new ignition and made lots of changes in wiring and plugs to get it better.

However, I do have a crack in one of my exhaust header pipes that is causing an exhaust leak.

Can an exhaust leak in a header pipe cause an engine misfire?

The misfire is most severe at intial startup when th eengine is still cold and as soon as I put a little load on the engine to drive off. It idles fine but as soon as it comes off the cold start cycle it starts to pop and miss.

When I drive off it backfires and misses until I can get it above 3000rpm and develop some engine temperature.
With the colder weather outside the problem is getting worse.

The miss almost disappears when on heavy boost and high rpm and once the engine has fully warmed up. I am figuring the exhaust leak might be sealing slightly as the headers heat up and the increased flow under boost masks the misfire.

Can an exhaust leak cause an engine misfire with our cars being that there is no feedback system for the CIS from the O2 sensor? I mean the O2 sensor has no effect on the fuel injection for my CIS in my 79 930.

What is the mechanism from an exhaust leak that might cause a misfire? Would the cylinder run leaner than the others or would exhaust reversion be causing this? Again its only really prevalent at low rpm low load when the engine is relatively cold.

I am running 911SC cams in the car with 36mm intake ports and B&B headers with k27S turbo and stock euro muffler. The CIS is stock euro and the AF ratios are normally 12.8 to 1 at idle and 14.3 to 1 on cruise unless its missing then the AF gage goes haywire.

Thanks,
Fred

Anybody can shed some light or offer an opinion on this?

Come on guys, I need some advice.

thanks,
Fred

Fred I don't think it would, but I know why you might hope it would.
If the problem only started with the new MSD rig, swap out the coil or box with another one.
The green distributor wire can be an intermittent beyotch too, although I'm not sure if that remains with the MSD setup.
I'd also suggest leaving your CIS gauge on the car and observing what your fuel pressure is doing during the problem. The WUR can be flaky and inconsistent.
The other thing is to pull all the injectors and do the baby food jar test to make sure the head, lines, and injectors are all flowing within reasonable rates of each other.
Bad spark plug can do different things at different temps too.
PS- you know all of this but just want to buy new headers, don't you? ;)

No, a crack won't cause a misfire. If it's temperature dependent then I would check the cold start valve. Might be dripping and loading you up with fuel.