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Intermittant Boost?

87 Turbo, stock, new K27. Recently fixed a bunch of leaks and replaced the over boost solenoid (yellow one in the engine compartment).

Now it runs great often and them it will not boost all of a sudden? No "click" from the waste gate. Then I while later it will start working.

I took the WG off and looked around, nothing obvious. I was thinking a new diaphragm but it seems like that would show up all the time.

Thanks
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84 lime green back date (LWB 911R with RS rear flares) hot rod - absolute riot to drive!
RSR look hot rod, based on 75' SOLD
73 911t 3.0SC Hot rod Gulf Blue - Sold.
Old 11-24-2009, 01:00 PM
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Originally Posted by Elombard View Post
87 Turbo, stock, new K27. Recently fixed a bunch of leaks and replaced the over boost solenoid (yellow one in the engine compartment).

Now it runs great often and them it will not boost all of a sudden? No "click" from the waste gate. Then I while later it will start working.

I took the WG off and looked around, nothing obvious. I was thinking a new diaphragm but it seems like that would show up all the time.

Thanks
Wastegate doesn't click. But BOV with worn gaskets does. I suggest disassembling BOV and checking for pieces of (probably worn) gaskets. It might intermittently jam the piston which keeps turbo "recirculating" and thus not boosting.
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Old 11-24-2009, 01:08 PM
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Ahh, I read your other posts on the BOV. SO the click is the piston sliding, and it should not be clicking because there should be a gasket that prevents it from hitting the end?

If that piston is hanging up that would do it...crud I had that intake piece off a couple of weeks ago.
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Erik L.
erik.lombard@gmail.com
84 lime green back date (LWB 911R with RS rear flares) hot rod - absolute riot to drive!
RSR look hot rod, based on 75' SOLD
73 911t 3.0SC Hot rod Gulf Blue - Sold.
Old 11-24-2009, 02:57 PM
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Ahh, I read your other posts on the BOV. SO the click is the piston sliding, and it should not be clicking because there should be a gasket that prevents it from hitting the end?
Yes. There are two rubber gaskets that piston should be hitting. Eventually, they rot away and get sucked into engine or jam the piston. If piston hangs in the bore then there is a short circuit in turbo plumbing.
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Old 11-25-2009, 12:13 AM
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Quote:
No "click" from the waste gate.
That short sentence tells the tale. No click from the failing BOV means it has likely jammed. If the waste gate stuck shut you would overboost and if it stuck open your car would be loud but would still boost slowly at full throttle.

I took an old BOV apart some time ago and couldn't believe the nasty crud that results when those gaskets break apart.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 11-25-2009, 05:03 AM
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Excellent news; interesting point this morning I drove it to work, it was boosting fine....no "click". I think there is hunk floating around in there and a piece got jammed at the end so it wont hit the end of the housing and make the click.

Took it for an errand a minute ago - no boost no click....runs great.

Looking at old posts is the dealer the only place to get the gaskets? I dont see them on Pelican.

One other thing, Can the the gaskets be changed leaving the manifold in the car? It almost seems like there is room with the IC off. I just hate to imagine trying to get that hose back on that is under the gold saucer...that is almost an engine out procedure :-)
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Erik L.
erik.lombard@gmail.com
84 lime green back date (LWB 911R with RS rear flares) hot rod - absolute riot to drive!
RSR look hot rod, based on 75' SOLD
73 911t 3.0SC Hot rod Gulf Blue - Sold.

Last edited by Elombard; 11-25-2009 at 06:01 AM..
Old 11-25-2009, 05:58 AM
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Goran wrote an excellent thread on changing out these gaskets. A search should pull it up. It is a couple of years old if memory serves. Gaskets are very inexpensive.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 11-25-2009, 06:18 AM
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I've heard of people replacing the gasket closest to the rear end of the car without removing it but it's alot easier to clean everything if you remove the whole thing first.
Hold the end on while removing the first one, there's a spring in there that's compressed and it fly out.
Be sure to get the piston, spring, and spring guide oriented the same way when sliding them back in.

That decel valve hose on the bottom looks like it will be the biggest pain when putting the manifold back on but the rubber in the hose is usually hardened from age and holds it's shape and slides back on the aluminum pipe as you move the manifold around getting it in place.
A little silicone on the inside of the hose will help it slide back on then you can reach under there with a screwdriver and tighten the hose clamp.
Someday you may switch to a longneck intercooler and C2 blowoff valve, then that whole manifold is gone.

I replaced those 2 gaskets on mine when I first got my car and they were $5 each. They seem like they're made of something similar to white urethane but they turn into something like candle wax after 20 years.
The gasket at the piston end of the cylinder dry rots and gets hammered out by the piston, then it doesn't cushion it anymore and you hear the click and it could eventually become a vacuum/boost leak.
Old 11-25-2009, 08:15 AM
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Thanks Mr. Fairman and Brian. I had read the other post and I was thinking some one had referenced doing it in car.

I ordered the gaskets from Pelican - they are called pressure relief valve gaskets, that is why I could not find them in the catalog.

hope that is the last of the problems for a while. If this was my Car I would definitely get one of Rarly8's mufflers, C2 Long neck with BOV and an aftermarket waste gate. I think that would shave 100 lbs off the car and clean up a lot of hoses, reduce lag etc.. etc.....and then EFI and then and then.....oh what is the use :-)
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Erik L.
erik.lombard@gmail.com
84 lime green back date (LWB 911R with RS rear flares) hot rod - absolute riot to drive!
RSR look hot rod, based on 75' SOLD
73 911t 3.0SC Hot rod Gulf Blue - Sold.
Old 11-25-2009, 09:24 AM
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Cheers, guys.

I think my car's suffering some related problems (not getting full boost pressure, not getting full retard/advance timing, and not sounding like it used to when under full boost).
Old 11-25-2009, 10:01 AM
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P.S. One shouldn't do WOT acceleration drive with jammed BOV piston as turbo can overspeed. This can damage both shaft (due to overspeed) and impeller (due to cavitation).
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Old 11-25-2009, 11:07 AM
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Uh oh...is there any other way to drive a Turbo :-)
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Erik L.
erik.lombard@gmail.com
84 lime green back date (LWB 911R with RS rear flares) hot rod - absolute riot to drive!
RSR look hot rod, based on 75' SOLD
73 911t 3.0SC Hot rod Gulf Blue - Sold.
Old 11-25-2009, 11:42 AM
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