zoomie

Car is stock a 1987 930 except for 1 bar spring and Zork tube. Went to the track and under full throttle and some boost, car would miss and cut out. It was a hard cut out, but it would come right back on. It was only off for a fraction of a second, but it was almost undrivable. Also it had a strong odor of gasoline. After 15 minutes or so, the problem would go away somewhat, but then it seemed down on power.

Ideas? Thanks for your help, the car is new to me, has 50k on a engine / turbo rebuild.

sand_man

Overboost switch/relay engaging?

SGOGT4

fouled up spark plugs are not keeping up with fuel at high RPMs/Boost at least that was part of the problem I had.

AZ930

x2

InC

hi guys,

What would cause the over boost switch to cut out? Could it be the recirculating valve not holding the boost or air leak? Is there anyway to elininate the switch by bypassing the system? Thanks.

JoeMag

x2 on the plugs. ...change'm and see what happens.

nathanUK '81 930 G50

This overboost switch screws into the intercooler and has 1 wire going to it. It is the smaller device (the larger one with 2 wires is the sender for the boost gauge in the rev counter).

To bypass it just connect the single wire to a good earth. When testing make sure you don't overboost the car, watch the gauge and be careful with your right foot. I have an additional boost gauge in my ebc so I know how much boost my engine is getting after the intercooler. Overboosting your car will melt the engine so make sure you don't go over 1 bar of boost.

Jammed or not connected wastegate could cause overboost.

senna21

The overboost switch is a "fail safe" device. Most turbo cars (good turbo cars) have one. On my MR2 it cuts the fuel delivery, I'd think it does the same on the 930. This eliminates the fuel at high boost and therefore eliminates the chance of predetonation (knock). That sudden elimination of fuel causes the engine/drive train to buck as the CIS isn't the most graceful system. Effective, yes, but not as effective as EFI. Once the boost drops below the one bar point you get fuel delivery back.

Don't disconnect it! If you're over boosting you need to address the reason you're doing so, not the result- overboost switch engaging. Without a knock sensor this is one of the most important engine safety items on your car.

Chet 930

Check your fuel pressures in the CIS system and the CO output. Dont overlook the O2 sensor either. If all is good, I would look into the ignition system including spark plugs as mentioned too. It doesnt sound like an overboost switch although you can test it easily by grounding it out and seeing if the motor still cuts out. Just make sure you dont overboost consistently. Unground the switch and install a new one if it is malfunctioning.

InC

Thanks guys. Was trying to find out more about the car and symtoms when things go wrong. What about fuel? Would it have similar effect? I was told that the recirculating valve which comprises of two plastic diaphragms, some O rings and a spring. When the Diaphragm tears, the car will not boost correctly. Any idea?

zoomie

My mechanic says the igntion box is bad, he's going to replace it and the coil. We'll go from there. Thanks for all your inputs.

srf506

Ignition box ? I've rarely heard of one of those going bad that wasn't a hard failure...in other words the car wouldn't even start and run. Anything's possible I'm sure. I would believe it to be plug wires breaking down, especially if they are OEM or old and that, of course, could have fouled a plug. I'd also look at your distributor and rotor to see if there's any cracks or carbon tracking in there. Again, most of the time I find a coil goes to a hard failure. It won't run well at low or high-speeds. Good luck and let us know what you find.

Chet 930

Yeah it does happen. My 89 930 did the same thing until it eventually burnt out.Thats what I was talking about with checking the ignition system.

Mark Houghton

If it turns out to be related to the overboost cutout, when that happens (the cutout at full boost), all the presssure suddenly has nowhere to go, and usually finds it's way out through one of the orange o-rings on your air intake (intercooler). Once dislodged, it will play havvock on your mixture as the air is leaked out. Will cause rough running and poor performance.
Check them out while you're looking around the engine bay.

zoomie

Shop is installing a Permtune box. Are these quality units? They aren't cheap.