Last930

Some years ago I had a miss under boost around the same 4500 rpm level. After checking just about everything - fuel head, pumps, over boost cutoff, ignition system, etc it turned out to be a spark plug breaking down under boost. It would run fine at all rpm with no boost and it idled as smooth as ever, but went bad under the pressure of boost. The plugs were new, with only 1,000 or so miles. I should have replaced them first and saved a bunch of money!

Mark Houghton

Ah...so your test would seem to support our theory that the air pump...or lack thereof...has little if any impact on the AFR's beyond idle. Interested to see what the adjustable WUR does for your on-boost AFR (you may need to adjust that portion of it as well, depending on your results).

For those following this thread and still running air pumps, how do you look at your AFR's? My guess would be that a person should set their AFR's across the broad spectrum without the pump injecting air. Then, hook it back up and live with a lean AFR on idle and/or adjust the mixture screw on the fuel head to get a decent idle reading and let the WUR control everything else. At least you would have the confidence that your fuel mixture is correct when things really matter.

Usmc1488

Did you ever solve the problem? Im having a similiar issue.

The car is a 1989 930. In any gear any throttle position the car cuts out hard as if the over boost is kicking in. Ive replaced everything: Over boost switch, distributor, plugs, wires, bov with .8 bar spring etc. Everything associated with the boost and fueling. Im at a lost. Could there be an issue with the CIS? Ideas?

Peter Carroll/Toronto

Had this on a couple of cars. In both cases it was a bad CDI Ignition box. Did exactly as you describe. It's easy to test if you can borrow one from a friend. Easy to swap too.

Usmc1488

Thanks, one more thing to check!!!!