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

[Emission]

('86 930 with K-27 and 1 bar spring)

I've got a problem with my car. Under full throttle, as the boost climbs up, it will exceed an indicated 1 bar if I hold the pedal to the floor... then the engine cuts out.

I can drive with a light foot and modulate the boost, but it gets annoying. What do you think the problem is? Is it a wastegate issue?

[930Chas]

Do you have the factory boost gauge? If so, you can not trust it, and will definitely need a reliable gauge if you are going to run the mechanical 1.0 bar spring. I really don't think your car should be cutting out because of overboost, instead that would be gradually be burning your motor up. Our cars are not recommened to run at over 1.0 bar, and if your car was or is it would eventually burn your motor up. You might post this question on www.911turbo.com, they can probably pinpoint that problem for you.

[Emission]

(It is already posted on the other board - thanks for the advice)

I have been told that the engine has a protection system to prevent overboost. It will retard the fuel - that is exactly what this feels like.

I am just wondering why the boost doesn't limit. To me, it sounds like the wastegate isn't blowing off excessive boost but rather passing it on.

[930Chas]

I am not aware of an overboost protection on the 930 motor. There are probably a lot of people that wish there was something like that on it. That is why you have to be so careful with the amount of boost you run. Now, maybe something was installed on your motor to help regulate boost, but again I don't know what that would be. Sorry, I am not very much help.

[RarlyL8]

There is most definitely over boost protection on the stock 930.
The sensor is located on the back side of the blow off valve. At just beyond 1.0bar the rear fuel pump will be cut off. Some models also have an ignition cut off as well.

The reason for your overboost is the concern. The wastegate should be the culprit. This is the only vent for excess boost. The spring may be bad, sticking, or it just cannot react quick enough to beat the cut off switch. You may need a 1.2 bar cutoff switch or a boost controller if the wastegate is OK.

[930Chas]

Oops, goes to show you what I know.

[dean]

To test the overboost switch ground out the lead. Get a test gauge put in. Compare the test gauge to your boost gauge. If you have a bad wastegate your boost will go very high so becareful. The same thing happens to my car, it's a weak overboost gauge. Mine cuts out at 12 psi. New one on order.

[beepbeep]

Hi there! I had the same problem. Of course, 930 has a overboost protection (all turbocharged gasoline cars i know of have some sort of overboost protection).

Your wastegate membrane is probably leaking. It's easy to check, just attach a short hose to venting-pipe on the top of the wastegate and try to blow/suck trough it (there is no other way for air to escape but trough ruptured membrane). If air is going trough, your membrane is shot, which will lead to uncontrolled boost. Membrane can be changed.

As always, i'm against 1-bar spring, as it's much better to control boost trough external boost-controller and be able to lower it if pinging occurs...but there are always "i did it this way for 20 yrs. so don't tell me what to do" guys who will still install those pesky devices.

[beepbeep]

And here is a picture of overboost-protection switch, installed on every produced 930:


It will typically cut out gas pump @ 1.1 bar...

[Emission]

Originally posted by beepbeep
"...As always, i'm against 1-bar spring, as it's much better to control boost trough external boost-controller and be able to lower it if pinging occurs...but there are always "i did it this way for 20 yrs. so don't tell me what to do" guys who will still install those pesky devices."

I bought the car three months ago with this stuff installed. I am taking apart the wastegate this weekend and checking the membrane. I will probably pull the spring and put a mechanical controller back in.

Thanks for the pic!

[beepbeep]

You don't need to detach wastegate in order to check if it's OK. Just try to blow trough it's venting-hose (on the top of the wastegate-bell)...(edited)

if leaking:
- your membrane is ruptured (common problem)

if it's air-tight then you have following options:
- Overboost switch is getting weak, cutting at too low boost (unlikely)
- Spring is too hard, boosting more than 1 bar (unlikely)
- Wastegate-piston is stuck

About the spring:

You always need spring in the wastegate...only difference between 0.7-bar OEM spring and 1.0-bar spring is that second one is harder.

So if you want to lower your boost, install softer original spring and use external boost controller.

[dean]

I thought I had a bad membrane because I can blow and suck on my wastegate line. It turned out to be alot of clearance at the valve guide. I heard the wastegate valve needs a fair amount of clearence so not to worry. My wastegate works prefectly.
Oh yea I am a firm believer in 1 bar springs if that is what you choose :-)

[beepbeep]

Ouch sorry, my bad.

To check wastegate membrane , you should do it on a venting hose (one on the top of the wastegate) and not on pressure side. Blowns post made me think. Trying to check from a pressure side will always result in leak, either trough valve guide or the holes/both. On the other hand, venting side has only membrane in the bottom, so air cannot escape trough anything else but a hole in the membrane...and when i check for a leak, i sure did it on a pressure side (had a new wastegate in my hands)

Thanx Blown.

[beepbeep]

Unbolt banjo-fitting on the picture, pull it out from the wastegate-bell. Pull out short piece of venting-pipe that is going to engine compartment (it's not going anywhere and it's pretty short). Put back banjo-fitting with venting pipe pointing towards you. Thread a piece of rubber hose over venting-pipe and try to blow trough.

I bet it will leak...

P.S. How do i know all this? Well i had to do all this troubleshooting myself half a year ago. It was a real b**ch to do...

[Emission]

Originally posted by beepbeep
"Unbolt banjo-fitting on the picture..."

Another priceless pic. Thanks.

After dinner (it's still early out here), I'll go blow my car. My wife will love hearing about this one...

One more thing: Do you have every square inch of your car photographed?

[Emission]

I 'blew' the car, and it held fine. No leak (just the slightest amount blew by).

Got the wastegate off and the bell off (damn, that spring is STRONG). I used a nylon strap to keep it together until I had all the nuts off. Once they were off, I slowly let it out...

Everything appears normal, and the diaphram looks fine. I am going to order the .7 bar spring and put in a mechanical boost controller. This is driving me nuts.

[Emission]

A hint when you re-assemble the wastegate to the exhaust system: It will not sit flush with the exhaust flange until it is tightened down. This is because the 'flap' is spring loaded as well (it is on the other end of the shaft). I threaded the bolts on either side, then cranked them tight as it pulled flush.

Here is a pic of the bolts. This is a pic of the 'flap' that is spring loaded

[beepbeep]

Hmm...so the diaphragm is OK you say...?

I recomend installing graded turbo-gauge (original one is not so reliable) and checking exactly what boost you are running. At least just temporarily.

You either have crack in pressure-hose going to wastegate, or your WG-spring is too hard. You can probably swap your 0.7-bar spring with someone...

Btw. your wastegate looks really fresh!


P.S. Removing that small muffler and just venting wastegate out in the open air sounds pretty good ;-)

[briankeithsmith]

BeepBeep:

I've been thinking about removing my wastegate muffler. What does it actually sound like?

To me , it would probably sound like of like a jet at the point at which it releases the excess?

What does it sound like?

Brian

[Emission]

I just searched the old records the previous owner supplied me and noticed back in December of 1997 they put 'shims' in the wastegate. When I pulled the wastegate apart, there were two very thin metal washers (one at the top of the spring, the other at the bottom). Were these the 'shims' they talk about? They were so thin, I thought they were there to protect the aluminum housing from the steel spring.

Can someone tell me if those washers are supposed to be there, or were they added to increase the spring pressure (and increase boost)?