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Do i have a missfire or is it more serious???????
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Ants  
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PostPosted: Tue Jun 11, 2002 11:06 pm    Post subject: Reply with quote

Hey Guys
I'm currently sourcing a replacement wastegate, and i have discovered a problem that feels like a missfire.
I had the car serviced last month ('80 931) and all was well untill i tried to overtake somethin' dropped it into 3rd buried the throttle and at 4000rpm the car lurched with a sudden split second loss of power picked up again, (still with foot on the throttle) lurched again.
I can go up to 5000rpm in second no problem and i can pootle along happily at 3000 rpm in any gear but under load at between 3800-4200rpm in 3rd i get this powerloss lurch (kinda kangaroo thing). Needless to say its put me off overtakin stuff.

Could this be a missfire (how to diagnose) is it somethin to do with the turbo/waste gate.
Please help

Ants
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kevrl  
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PostPosted: Tue Jun 11, 2002 11:59 pm    Post subject: Reply with quote

Hi Ants,

Yeah I had the same thing when the overboost cutoff switch failed. It began to cut off the fuel supply whenever I put my foot to the floor and boost pressure rised. Then I had it again with the new cutoff switch when I tried to increase the boost too much - but that was a good thing, it stopped the engine going BOOM !!

Have you got a boost guage ?, maybe with a faulty wastegate you are producing too much boost, so operating the overboost cutoff ?. If the switch is faulty, a good way of testing is to connect the wire from it straight to a good ground point. BE WARNED, this removes all overboost protection !!

Then again, it could be nothing to do with the overboost switch

Kevin
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Ants  
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PostPosted: Wed Jun 12, 2002 12:48 am    Post subject: Reply with quote

Hi
No i don't have a boost guage just a restrictor somewhere in the wastegate as it wasn't sealing properly (hence sourcing a secondhand one to rebuild) I'd rather not remove the overboost protection
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Paul  
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PostPosted: Wed Jun 12, 2002 12:37 pm    Post subject: Reply with quote

I recently had the same problem, it was a loose connection on the over boost switch. (I had just installed a brass T to add a boost gauge and evidently damaged the electrical connector.) This connection provides a ground for the fuel pump relay. If its wiring is bad, the fuel pumps cut out.
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924 turbo  
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PostPosted: Wed Jun 12, 2002 8:22 pm    Post subject: Reply with quote

I would suspect the overboost switch as well, but check your distributor rotor for buildup, and check the ignition timing. In my experience, the timing setting is very picky, and will misfire if set incorrectly. Best of luck.
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Ants  
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PostPosted: Wed Jun 12, 2002 11:07 pm    Post subject: Reply with quote

Thanx guys

I'll check the distributor and the overboost switch.

Cheers

Ants
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Rick MacLaren  
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PostPosted: Thu Jun 13, 2002 12:43 am    Post subject: Reply with quote

The overboost switch, if it's old, will have kinks in the plastic sheath too...I'm replacing the wire as far back as I can find it.
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Smoothie  
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PostPosted: Thu Jun 13, 2002 1:21 am    Post subject: Reply with quote

If you check everything above and still have a problem, have a look at the throttle switch. They can give similar symptoms when they go bad. You might get away with just cleaning the contacts with sandpaper. I did that myself without breaking anything, but the problem I had turned out to be something else - a connector to the DITC box (only installed on later 931s).
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numbers  
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PostPosted: Thu Jun 13, 2002 3:46 am    Post subject: Reply with quote

Well, since your wastegate is not functioning properly, you are probably reaching max boost, and your overpressure switch is activating just as it should.
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Rick MacLaren  
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PostPosted: Thu Jun 13, 2002 4:49 am    Post subject: Reply with quote

Yeah Numbers is right...watch the overboost. Look, even if the switch catches it before it gets to say 17 or 21 PSI or whatever (some nerd with a spec book will have the exact number) this will not necessarily save your engine!

Even a boost over 11 PSI can be too lean for your engine, and the result, devastating.

In my opinion, do the following WITHOUT VARIATION:

1. Get a boost gauge installed.
2. Fix the wastegate if it's busted.
3. Ensure your overboost cutoff is installed correctly.
4. Trace any surges, resolve them, and don't drive it if it ain't right.

Doubt this advice? See my previous posts on a blown engine.

Good luck.

Rick

[ This Message was edited by: Rick MacLaren on 2002-06-13 03:51 ]
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Ants  
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PostPosted: Thu Jun 13, 2002 7:39 pm    Post subject: Reply with quote

Thanx
OK the story so far, i cleaned the rotor arm distributer cap, and as you all said i noticed the boost switch wire was a little frayed at the elbow of the spade connector. So i cut it off and reattached the spade and cleaned up the contact for the spade connector, result BUGGER ALL, it still does it.

I'm definately leaning towards the boost now cos it does it in 4th gear too at between 3500-4000 RPM that seems too much of a coincidence for it not to be turbo/boost related.
What i don't understand is why only when under load, i can smoothly/progressively get it past this point in any gear, but when i push the throttle harder (as if to overtake) it does it??

I emailed Andial for the bits to rebuild the waste gate, 1bar spring only $40 for those interested, but it may cost $100 to ship them $100 DOLLARS i could fly there from here in the UK and pick them up myself nearly
I'm going to contact the guy who replaced the turbo to see if he can get the parts reasonably.
Cheers

Ants
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Ants  
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PostPosted: Thu Jun 13, 2002 8:04 pm    Post subject: Reply with quote

ADDENDUM:

Ok so i have just ordered a secondhand wastegate for £85 + VAT + £10 shipping (approx $160), from a Porsche breakers here in the UK, with a 30 day guarantee.

So now if anyone can give me some tips on how to fit it i would be eternally greatful

Ants




_________________
1980 931 Euro spec, "you know the one with the power, without repeaters or those daft bumper extensions."

[ This Message was edited by: Ants on 2002-06-13 19:05 ]
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Smoothie  
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PostPosted: Fri Jun 14, 2002 1:00 am    Post subject: Reply with quote

Hope the diaphram in the new(used) wastegate is new or at least not too old. That's normally what needs replacing when they go bad. About $50USD. Hope it works out for you. Personally, I would have, and have just replaced the diaphram, but maybe your old one has a broken spring or some other problems.
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Rick MacLaren  
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PostPosted: Fri Jun 14, 2002 2:12 am    Post subject: Reply with quote

The wastegate may be baffed, but that does not necessarily explain the hesitation. After my recent rebuild, my wastegate was fine and the car did the same thing. It's probably no comfort, but it's either air, gas or electrics.

[ This Message was edited by: Rick MacLaren on 2002-06-14 02:48 ]
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Smoothie  
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PostPosted: Fri Jun 14, 2002 6:45 am    Post subject: Reply with quote

Quote:

On 2002-06-13 18:39, Ants wrote:

What i don't understand is why only when under load, i can smoothly/progressively get it past this point in any gear, but when i push the throttle harder (as if to overtake) it does it??



This would be confirmed with a boost gauge, but when you accelerate smoothly, your boost isn't reaching the pressure level that triggers the cutoff switch. When you press down for a harder acceleration, you do produce enough boost to trigger it if the wastegate isn't working. I saw this in my own car when the wastegate diaphram went bad. I already had a boost/vacuum gauge installed, so I could see the boost reaching unusually high levels, then if I didn't let up on the gas and let it continue to build more boost, the cutoff switch would do its' job and cause a sudden deceleration that felt like it could almost send you through the windshield. I'm 99% sure a good working wastegate will put you back in business. Also, when this happened with my car, I continued to drive it occasionally until the new diaphram arrived, but had to be very careful not to let the boost pressure get to high. The way I did this was just as you described - by accelerating "smoothly/progressively".

I think the key to telling whether it's your standard run-of-the-mill misfire or hesitation caused by fuel cutoff is in what it feels like when it happens. If it's a sudden violent deceleration like I described, you should find the problem is with the wastegate.

[ This Message was edited by: smoothie on 2002-06-14 06:37 ]
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