924Board.org

[Rob the plumber]

Hi,

I am a new owner of a 1981 931. The car seems to run perfectly, except under full boost the car cuts out. I am guessing that the wastegate may not be operating properly, and the engine is "shutting off" somehow.
I have no idea how this is accomplished on a Porsche. I know other vehicles will shut down in overboost conditions when the MAP sensor goes over 5.0 volts, or mass air sensor voltage gets pegged.

I have no boost gauge yet, so I don't know at what psi this is happening. It just cuts out when the boost comes on hard.

Thanks,
Rob

[Rasta Monsta]

If you look at the charge tube which passes over the valve cover, you will see see the overboost switch threaded into it on the side facing forward, and a brown wire connected to the switch with a spade connector. Check the connector for tightness, and the wire for breaks.

The switch is a simple open/closed that opens when boost exceeds 20 (?) PSI and kills the fuel pump. Is the car insanely, you can't really believe it fast? If so, your wastegate diaphragm is ruptured and the car should be parked until the wastegate is removed/refeshed. You can also check the turbo shaft while you have the down pipe off.

Welcome! Your plumbing skills will come in handy when it comes time to R&R the turbocharger.




rasta
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[Rob the plumber]

Thanks for the advise. I guess I should install a boost gauge to figure out if the boost switch is acting up, or if the wastegate is stuck shut. I crawled under the car today and noticed that the turbo is externally gated. It looks like a boost reference line attached by a banjo fitting is how it gets it's signal.
I checked the line going to the wastegate with the car running for vacuum leaks, and I found none.
The boost comes on a lot harder than I think 10 psi of non-intercooled boost should, so my guess is that the wastegate needs attention. Is rebuilding a wastegate a hard job? It looks like it comes off easy enough.....


Thanks in advance!

[Rasta Monsta]

Easy, new hardware from Porsche (studs, nuts, gaskets), and a diaphragm from Pelican (or Lindsey Racing if you want to get fancy). Heat will be required to remove the studs. Also, the switch can be bench tested with compressed air and continuity tester.

BTW, the wastegate is a 930 part. . .sometimes makes searching for innards easier.
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Toofah King Bad

• WeiBe (1987 924S 2.5t) - 931 S3

[ideola]

Here's a handy wastegate rebuild list I pirated from another post here somewhere. I used this to get all the bits from EBS:

[tuurbo]

I've had wastegates where the spring has DUG into the sides of the aluminum and made for wonky sidewalls, with grooves in them, that 'can' cause the spring to stick. The inner aluminum can be fixed up if needed...you'll see when you crack it open.
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1980 924 turbo, MSD, Meth. Inj, otherwise stock.

[Rob the plumber]

I blew compressed air and a little WD-40 into the wastegate hose to try to free it up. The wastegate seems to be holding air, so maybe it is just stuck.

[Smoothie]

I wouldn't use a petroleum product in the wg diaphragm area. If yours has a vent line attached to the top, and you plan on using it only as a vent in the future, you could try sending a little silicone based lube down that line. (Petroleum isn't good for rubber, but silicone is a good preservative for it, but silicone is very bad for o2 sensors and catalytic converters. -So silicone on the upper surface of the rubber diaphragm will help prolong its' life and keep it out of both the intake and exhaust tracts.)

Another possible cause for your symptoms would be a clog/blockage in the wg control line - though I've never heard of this happening.
One more possibility would be if there's a wg control valve installed in the control line, and it's closed too much. (This would be something added aftermarket by a previous owner. If present, it could be in the engine compartment or under the dash - just follow the line and see if one is there.)


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"..it's made in Germany. You know the Germans always make good stuff."


'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

[Rob the plumber]

Thanks again for the help. I installed a boost gauge today, and it is hitting over 15psi when it cuts out. Not good. So at this point I can assume that the wastegate is not opening.

Thanks!

[ideola]

Yeah, stuck closed, or torn diaphragm maybe?
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erstwhile owner of just about every 924 variant ever made

[924guy]

[Rob the plumber]

It is definitely stuck closed. I applied compressed air to the line yesterday at work, and I heard it pop open. Unfortunately it does not open during normal use.
Is there a place that I might order a rebuild kit online? I generally work almost 2 shifts a day and have very little time to chase parts in person.

Thanks!

[Rob the plumber]

FIXED

The signal line to the wastegate was clogged. The banjo fitting at the wastegate had a check valve in it that was stuck shut.

Now the boost sits at 6 psi, and the car is pretty damn slow. I think I have a manual boost controller lying around the garage though.
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1981 931