can exhaust header leak cause engine misfire

[fredmeister]

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

[fredmeister]

Come on guys, 58 views

yet nobody had any ideas?

Need a little help with this one.

Thanks,
Fred

[henrymfisher]

I do not think so. My guess would be a vacuum leak

[Miguel Antonett]

"...The CIS is stock euro and the AF ratios are normally 12.8 to 1 at idle and 14.3 to 1 on cruise..."

I'd say that's a little rich; so if you have an injector a bit off/stuck, it could give you that symptom. I'd try some concentrated Techron in the gas tank and go for a spirited drive to see if it makes any difference. And take it from there, easy troubleshooting stuff first...

[wjfk32]

What is the gap on your plugs??? I Had a similar situation with the cold weather in the past.. As someone mentioned on the Fourm-- I think it was Mark H. stated my Plugs are gapped to much with my NGKs' Have a msd6al system & oil coil(slight leak)--car was breaking up--on and off..Drove me nuts. Went with Denso W24-straight out of the box gap-28 I believe... Some guys have no problem gapping over 35, but I had a problem with the msd setup...No 02 sensor in my hookup..Plus my relay-fuel pump was acting up also...

Walt

[T77911S]

i was thinking ignintion timing.

[snbush67]

Did you program a stock curve and advance? Select 6cyl? Which MSD?

I do not think a header leak could cause a misfire, but I do not know.

[timspu]

My GSF headers with heat are leaking internally and I don't jave a misfire. So I would not say a leaking exhaust would cause the problem, just a horrible ticking farty noise

[fredmeister]

Guys, thanks for the responses.

I started doing some digging and the front pump relay gets very hot while the rear fuel pump relay is just warm.

When I pull the rear pump relay it makes no difference to the way the engine is running. It still misfires the same at idle and will rev up and misfire as well. It really made no difference at all to the symptoms when I pulled the rear pump relay. BUT IT STILL RUNS WITHOUT THE REAR RELAY.

When I pull the front pump relay the car stalls.

So now I am thinking the rear pump is bad and the new front 044 pump is doing all the work which accounts for the high heating in the relay and the large voltage drop at the front pump terminals. The voltage is roughly 11.9 volts at the front pump and 12.1 volts at the rear pump terminals with the engine running and alternator putting out 13.2 volts.

Would the voltage on the rear pump terminals at 12.1 volts which is only a 1.1 volt drop make sense if the rear pump was faulty? I can feel it vibrating slightly but it makes no noise and seems ok.....I just thought the voltage drop would be greater if the pump were bad.

Still confused but would like to know for sure before dropping large $$$ dollars on another rear fuel pump.

Fred

[Norm K]

Originally Posted by fredmeister

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

In short, no, it can't. It's certainly possible that an exhaust leak might sound a bit like a miss, or if you had a miss already the additional noise level might make it considerably more noticeable, but a leak is pretty much nothing more than the exhaust pulses escaping by something different than the preferred route.