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930 Intermittent Turbo

Hi folks, I'm new to the forum here but a long time porsche owner. I just bought an all original 50K mile 1987 930. I live on the coast but bought the car in the mountains. Yesterday I rented a car and drove one way to pick up my dream car. It's about a 6.5 hour drive. The drive home was amazing! the car is almost perfect, tight as new. That said, in the final 30 minutes of the drive, the turbo started acting up.

Up until the end of the trip, the turbo/boost was functioning as it should. Around 2500 to 3000 rpm it would roll on and then bam... well if you have a turbo, you know what it does! When the problem started, 4 out of 5 accelerations would produce no boost at all. I thought maybe i was delirious after 13 hours of driving so i parked the car. Well, this morning i went out, started her up, warmed her up for a couple minutes and went for a drive to breakfast. Turbo worked on first acceleration, but second and subsequent attempts she failed. I went inside scratching my head ate a bagel, came out and drove home. On the drive home she functioned perfectly (and damn fast at that!).

Can anyone offer a diagnosis?

Background:
Outside of a B&B muffler, the car is comepletly stock. It did just receive a new distributer/wires and full service including plugs, wires and valve adj.

The only other thing I notice is some backfiring when I let my foot off the accelerator while she is still in gear (moving but not accelerating) and when i turn the car off she "runs on" for a moment.

Thanks guys!
Old 03-01-2013, 07:42 AM
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wastegate stuck open?
Old 03-01-2013, 08:52 AM
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Could be the over boost switch. It cuts off VERY abruptly. Overboost could be stuck or unplugged wastegate.
Old 03-01-2013, 09:58 AM
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wastegate sticky?

Thanks for the quick responses. Wastegate stuck open sounds like the likely suspect. Seeing as though the problem is not consistent - is it possible that the waste gate is "sticky"? Is the wastegate serviceable?
Old 03-01-2013, 10:33 AM
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Quote:
Originally Posted by kojo911 View Post
wastegate stuck open?
First place I would look. You can check the function of the WG with the engine running. Pull off the hose that goes down from the intercooler to the wastegate (and be sure to plug the nipple on the intercooler), start the car, attach a compressed air source to the hose, and pressurize to about 15psi. That should open the WG and you should hear the exhaust 'wasting' through the WG muffler (or straight pipe, whatever your car has on it). Or do the old 'potato in the exhaust pipe' trick (a wad of paper works fine) and plug the WG dump pipe/muffler, apply air pressure, and the exhaust should force the plug out.

All that will tell you is that the thing is opening up. If it doesn't close after removing the air pressure, then you've found the culprit. Could just be a sticky valve guide, and it's not all that big a deal to remove and disassemble the WG for inspection. Be beware that the bell on the WG holds a compressed spring beneath it, so you'll want to hold pressure on the bell while unscrewing the hold down screws so the damn thing doesn't become a missle. Could be the valve is simply carboned up from non-use (the 930 manual states that you run her up to full boost each time you drive the car, to clear out the WG of condensation, etc).
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Old 03-01-2013, 10:34 AM
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I think we are talking wastegate stuck closed, not open. Mark's test is good either way. Is the boost gage showing over 0.8?
Old 03-01-2013, 10:49 AM
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i believe that boost guage shows max of 0.8.
Old 03-01-2013, 11:29 AM
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BOW stuck open. No boost.


Jocke.
Old 03-01-2013, 11:30 AM
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Quote:
Originally Posted by 930turbobrian View Post
Up until the end of the trip, the turbo/boost was functioning as it should. Around 2500 to 3000 rpm it would roll on and then bam... well if you have a turbo, you know what it does! When the problem started, 4 out of 5 accelerations would produce no boost at all.
Let's go back to the original description. The BAM referred to I presume means what he's seeing when the turbo finally spools up and you get the instant HP increase, like a lightswitch especially noticeable in the stock 3LDZ turbos (but not so much in the K27 varieties).

That BAM could also be confused with the infamous 'face in dash' syndrome when excessive boost trips the overboost sensor and shuts the car down instantly. If that were the case, then we could be talking about a WG stuck closed.

If you're getting no boost at all, and your dash guage indicates same, but the car continues to run just fine albeit lacking the power it should have under boost, then we could be talking about a WG stuck open.
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Old 03-01-2013, 12:19 PM
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Originally Posted by 930turbobrian View Post
and when i turn the car off she "runs on" for a moment.
I believe this is fairly normal for 930s. Certainly my '77 3.0 does it.
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Old 03-01-2013, 01:02 PM
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I believe this is fairly normal for 930s. Certainly my '77 3.0 does it.
yes...ignition delay relay.

this relay keeps the CDI ignition powered up for a few seconds after shut-off, I suppose to burn off residual fuel in the cylinders and injectors/lines, etc. Side effect is that it almost sounds like the engine is dieseling.
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Old 03-01-2013, 01:18 PM
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What I'm having a hard time understanding here is how it is that you've received so much excellent diagnostic advice and yet nobody has chastised you for not paying your dues of at least one tantalizing picture of your 930! Come on....cough it up....
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Old 03-01-2013, 04:56 PM
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Quote:
Originally Posted by mark houghton View Post
What I'm having a hard time understanding here is how it is that you've received so much excellent diagnostic advice and yet nobody has chastised you for not paying your dues of at least one tantalizing picture of your 930! Come on....cough it up....
It's on his Advitar [need microscope]...
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Old 03-01-2013, 05:57 PM
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I once had something similar at an autoX. Turns out that when I pulled out the floor mats I must have pulled the plastic throttle pedal off its metal actuation rod. So the rod was sticking in the waffles on the back of the throttle pedal randomly. On some accelerations I would get boost, and on many others it was lethargic (I guess it would depend on which waffle the metal rod got stuck). Couldn't figure out what was wrong until I went to reinstall my floor mats and discovered the pedal flopping around.

OK, maybe the above is fairly low likelihood. But I was embarrassed when I finally figured it out.


Quote:
Originally Posted by 930turbobrian View Post
The only other thing I notice is some backfiring when I let my foot off the accelerator while she is still in gear (moving but not accelerating)
That's the turbo burble. Nicely antisocial. Sounds great when you're on decel, with the window open, next to a K-rail.
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Old 03-02-2013, 12:14 AM
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Quote:
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That's the turbo burble. Nicely antisocial. Sounds great when you're on decel, with the window open, next to a K-rail.
I always like downshifting and deacelerating right as I pass the motorcycle cop standing on the side of the road writing some poor smuk a ticket!

Advance the timing for better effect!!
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Old 03-02-2013, 04:56 AM
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I think the gate is stuck open too. But only when hot so the pressure test will be difficult. Just pop the gate off, take it apart and clean and lube as required.
Old 03-02-2013, 06:14 AM
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Here's a pic of my new baby. cheers!
Old 03-02-2013, 08:14 AM
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Ahh your the one who bought the car in Chatt, TN? nice purchase
Old 03-02-2013, 08:23 AM
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Thanks to everyone for posting. Conclusion is that the waste gate was stuck open. Can anyone recommend a shop i can send the WG off for rebuild?

This 930 is so much faster then my 79 SC and i'm in no rush for more HP. However, the power on the bottom is less than desirable (turbo lag, low compression). I've read a little regarding intercoolers and aftermarket turbo's reducing the lag and increasing HP.

Recommendations for next step? I assume everyone will say K27 and/or intercooler. I'd like to take one step at a time and remember that this car is bone stock (exception B&B muffler) and I like to keep my 911's original.
Old 03-02-2013, 08:31 AM
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My $.02, Anything you do to that car make sure that you can revert it to stock. The cars are becoming more desirable by the day and nice stock example get a premium.
Old 03-02-2013, 08:50 AM
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