Too much boost on 930... then the engine cuts out.

[john walker's workshop]

the two shims are supposed to be there. they are the same size as vw 1600 flywheel shims. you can actually put in a 1/4" stack of them to improve a weak spring somewhat, if you don't have a new spring.

[Emission]

Thanks for the info.

I had heard that there was another way, besides the stronger spring, to increase boost. It was with a shim. I had expected the shim to be at least .25" or so. The 'shims' in mine were nearly paper-thin, so I didn't think they were for increased boost.

[Emission]

Hmmm... this morning I took her out for another test drive. Once completely warmed up, I stepped hard on the gas in 2nd. The engine pulled strong and the boost began to rise. Once again, the needle pushed all the way off the scale and the engine cut out! Awrg...

I found records indicating the boost gauge was re-calibrated a few years ago by the previous owner. I don't expect it to be perfect, but I do believe I am overboosting.

I think I will buy a wastegate re-built kit (with a brand-new diaphram). Do you think that will work? A new 'overboost switch' will also be ordered (in case this one is defective).

[dean]

I think those big flat washers are supposed to be there.

[john walker's workshop]

if you're sure it's a 1 bar spring, you could ground the wire that normally goes to the overboost switch. then it won't cut out. generally, when you use a K27 and 1 bar spring, you do that anyway, because you're right on the edge. hotrod 930s generally don't use the cutoff switch as long as you know that the boost is at a safe level. you can hear the exhaust tone change as the engine starts to preignite under boost. if it stays with a healthy roar, it's probably ok. if the roar changes to a higher pitched, raspy sound, as the boost increases, that's preignition. you don't get the usual "tin can full of ball bearings" sound with a 930 under boost.

[dean]

Why don't you test your overboost sw.?
Connect the lead to it to ground and test drive. If you get no cut out then the sw. is bad

[Emission]

If I ground the switch, it is rendered useless. What will prevent the engine from overboosting (and 'blowing itself up')?

Aren't I just better off replacing the switch for <$20?

BTW - I am running high octance (racing fuel blend), so my pre-ignition problem has gone away.

[beepbeep]

Hmm...switch is there for a reason. Our cars don't like more than 1 bar boost. I suggest you to correct overboost problem first, and not to disconnect the switch. It would be pretty sad to discover that wastegate was boosting too much "the wrong way" ... by grenading the engine.

You can test if wastegate is operating properly by blowing in compressed air while monitoring air-pressure and listening to change of engine-sound when wastegate starts opening. This is the way they recomend in factory manual. They also state not to use more than 1.4 bar when testing (in order not to damage the membrane).

Good luck!

[930fan]

Can someone tell me where to get and how much a new wastegate membrane costs?

[beepbeep]

Last time i checked it cose 50$ in swedish currency. I can give you some numbers but they are in sweden.

[Jeff777]

I have a different type of turbo problem. A '79 930 which runs fine but doesn't seem to have a working turbocharger. The speed is linear, no evidence of boost except the boost indicator seems to read as one would expect starting around 3500 and increasing with revs. Even when I floor the pedal at 3500, there is no difference except rpm increasing steadily as it would on a non turbo. The engine was completely rebuilt last winter at great cost.
Could it be a waste gate valve stuck open? Any other ideas?
Thanks

[john walker's workshop]

Stuck or damaged turbo, open wastegate, damaged or disconnected sleeve at the turbo, o-ring blown out or missing on the boost side of the intake. For starters...

[faapgar]

If you measure them.They are normally .050mm shims.This is normal.