81 930 not running right.
 

My 1981 930 won’t run properly.
It gets to between 3 and 4 thousand revs and then starts to cut out/ miss. This is accompanied by a sort of a hissing noise and black smoke visible in the rear view mirror.
It’s worse in the higher gears, in 2nd it can sometimes be coaxed into the boost range and it behaves normally. But not in 3rd or 4th.
It’s also idling at 1500 rpm.
I’ve replaced the fuel filter and then the spark plugs.
The filter made no difference. The plugs fixed it, ran beautifully for a couple of short drives.
Now it’s misbehaving again. I used NGK B8EV plugs, and it would seem that they’re the problem? I just can’t see that plugs would fail so quickly.
Any thoughts? Stock turbo, intercooler and CIS , at least as far as I know.
I did see unclebilly's thread but seems 86 model might be different.
Thanks in advance.

Read through this link: http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/547032-sticky-common-answers-why-my-car-doesnt-start.html
This covers most running problems with these cars.
Fuel control pressures need to be checked and the results posted along with some history like how long you have owned the car and whether you have seen it run well. Have you or anyone else been working on it before the fuel filter and plugs.

Take a look at the below thread. Steve's suggestion is always a good place to start. There are few problem that have not already been described and diagnosed.

http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/575052-930-fuel-pump-fuse-overload-solution.html

I had similar cutout under load symptoms on my 1982 and this fix cured it. However, I didn't have high idle.

Thanks for the replies. I’ll get through those threads in the next couple of days and start investigating.
I’ve owned the car for 10 years and no one other than me has worked on it in that time.
That’s only been basic maintenance as it’s been a strong runner right up until this problem.

Hissing sound and black smoke? At 3 to 4k rpm you will be building boost, which sounds to me as though your air charge is escaping which will create an extremely rich condition. Check all o'ring seals at the intercooler and throttle body

+1 on the boost leak. If you have stock intake and your intercooler O rings are fine then I would check the slid valve.


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Yep - sounds like boost leak to me.
Alan

Thanks for the replies guys. Boost leak is a strong possibility. I haven’t been near the car since my first post - just haven’t had the time.
Rufrob, what do you mean by slid valve ?
Thanks again. I will update if I ever get a chance to take a look at the car!

Likely referring to this. It's an easy fix to replace the gaskets.

http://forums.pelicanparts.com/porsche-911-technical-forum/71370-changing-930-dump-gaskets-pictures-galore.html