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Car is now "bucking" on boost

 
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canyonblaster  



Joined: 20 Aug 2012
Posts: 373
Location: Denver, CO, USA

PostPosted: Fri Jun 02, 2017 10:09 am    Post subject: Car is now "bucking" on boost Reply with quote

Guys,

931 has been running great. Making 10 PSI consistently on the gauge. Pulling hard. It just today started to "buck" fairly violently as the boost comes on. I don't think it is the DITC. The car runs fine if I stay out of the boost/turbo, keep it under 3,000 RPM's.
It seems to have gotten worse quickly.

Any ideas? Thx
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canyonblaster  



Joined: 20 Aug 2012
Posts: 373
Location: Denver, CO, USA

PostPosted: Fri Jun 02, 2017 3:18 pm    Post subject: Reply with quote

I love this car. I will never stop fixing/developing it. The last couple weeks at increased boost (FAST), A/C works great, sick stereo w/SUB woofer in the left rear tire well. Such a joy to drive. Thousands spent. I get as many compliments all the time as my GT3, everywhere. ....but yet a new quirk. I will get it sorted but need my 931 brothers......
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ideola  



Joined: 01 Oct 2004
Posts: 15487
Location: Woodstock IL

PostPosted: Fri Jun 02, 2017 3:52 pm    Post subject: Reply with quote

Sounds to me like a massive boost leak such as what normally occurs when the lower charge tube bracket goes missing, and the boost cause the lower charge tube to lift off of the turbo outlet. However, with your setup, that's not the problem. I would check and double check all of the silicone couplers and t-bolt clamps to ensure nothing was missed on install. I would also check to make sure that you're not blowing the throttle body seal out on boost events. Do you have the secondary black seal with a squarish profile to it, where the intercooler outlet connects to the throttle body?
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15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
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morghen  



Joined: 21 Jan 2005
Posts: 7765
Location: Romania

PostPosted: Fri Jun 02, 2017 6:13 pm    Post subject: Reply with quote

Time to change plugs?
Spark quality ok?
AFR not too rich to blow out the candles?

What about if you only go 75% throttle ? does it still do that?
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1522
Location: Somerset, UK

PostPosted: Sat Jun 03, 2017 4:55 am    Post subject: Reply with quote

ideola wrote:
Do you have the secondary black seal with a squarish profile to it, where the intercooler outlet connects to the throttle body?


It's not a seal. Does nothing I can see to help seal boost.
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1522
Location: Somerset, UK

PostPosted: Sat Jun 03, 2017 4:57 am    Post subject: Reply with quote

Overboost switch kicking in? Overboost switch not earthing correctly? FPR cutting out?
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
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ideola  



Joined: 01 Oct 2004
Posts: 15487
Location: Woodstock IL

PostPosted: Sat Jun 03, 2017 7:37 am    Post subject: Reply with quote

Carrera RSR wrote:
ideola wrote:
Do you have the secondary black seal with a squarish profile to it, where the intercooler outlet connects to the throttle body?


It's not a seal. Does nothing I can see to help seal boost.


Yes, it does. It prevents the red inner seal from blowing out of the charge tube to throttle body interface. Trust me. I've seen it happen. Particularly on overboosted cars.
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15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
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924-76  



Joined: 05 Jun 2006
Posts: 1329
Location: Ontario, Canada

PostPosted: Sat Jun 03, 2017 7:45 am    Post subject: Reply with quote

Carrera RSR wrote:
Overboost switch kicking in? Overboost switch not earthing correctly? FPR cutting out?

+1
Ground the wire on the overboost switch and go for a test drive.
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 1522
Location: Somerset, UK

PostPosted: Sat Jun 03, 2017 9:32 am    Post subject: Reply with quote

ideola wrote:
Carrera RSR wrote:
ideola wrote:
Do you have the secondary black seal with a squarish profile to it, where the intercooler outlet connects to the throttle body?


It's not a seal. Does nothing I can see to help seal boost.


Yes, it does. It prevents the red inner seal from blowing out of the charge tube to throttle body interface. Trust me. I've seen it happen. Particularly on overboosted cars.


Had mine up to 1.4 bar and regularly up to 1.3 and no blow out. Even when I blocked up the recirc valve not a peep of red O ring movement. I see it as a finisher trim not a seal. If the red o ring moves, replace it for new and/or sort out the boost tube fitment.
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1980 931 - forged pistons, Piper cam, CGT turbo, 951 ducted FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife LSD
Now www.924board.org/viewtopic.php?t=34690
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ideola  



Joined: 01 Oct 2004
Posts: 15487
Location: Woodstock IL

PostPosted: Sat Jun 03, 2017 9:48 am    Post subject: Reply with quote

ideola wrote:
Particularly on overboosted cars with worn out inner red o-rings.


There is that. However, I assume since Porsche put it there, it has a purpose besides just looking good. You know, form follows function and all that
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15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
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morghen  



Joined: 21 Jan 2005
Posts: 7765
Location: Romania

PostPosted: Sat Jun 03, 2017 5:45 pm    Post subject: Reply with quote

Mine did blow out on a regular basis...even with that black cover/seal on.
Overboost switch sounds like a good idea to check...that connection should be tight and secure...the electrical connection I mean. If that's ok try a different sensor/switch.


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ideola  



Joined: 01 Oct 2004
Posts: 15487
Location: Woodstock IL

PostPosted: Sat Jun 03, 2017 10:51 pm    Post subject: Reply with quote

I've never seen an overboost switch fail in the manner being proposed here, especially at such a low boost level. They're usually either good or not.

Still, it is a good idea to eliminate it as a variable, and the best way to do so is to simply take the brown lead that connects to it and ground it to the engine block or head.

When doing so, be sure to check that wire carefully for any cuts or breaks. If there is a wiring fault, that wire lifts ground from the fuel pump circuit, which can cause a bucking behavior similar to what you're describing.
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15 981 GTS | 88 924S SE | 82 931 Holbert | 82 931 Rallye | 81 937 | 81 Euro 931 | 81 Weissach | 80 US 928 | 80 US 931 '941' | 80 US 931 | 80 931 GTR | 79 Sebring | 78 D-Prod Replica | 78 w/D-Prod kit | 78 Poli-Form | 78 Limited Edition | 77 Martini
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canyonblaster  



Joined: 20 Aug 2012
Posts: 373
Location: Denver, CO, USA

PostPosted: Thu Jun 08, 2017 7:47 am    Post subject: Reply with quote

Thanks very much guys. I have the car in the hands of an old school and savvy Porsche factory tech. I have shared all of these ideas.
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canyonblaster  



Joined: 20 Aug 2012
Posts: 373
Location: Denver, CO, USA

PostPosted: Sat Jun 10, 2017 9:40 am    Post subject: Reply with quote

OK, this is embarrassing, I should have checked this, but Porsche shop (local factory dealer whom I am very tight with service mgr), solved this recent Bucking issue in 2 mins. One plug wire at distributor wasn't seated well....

My AFR is still wonky, not accurate like it was prior to blow-by event, but car is running well again.
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BillA  



Joined: 27 Dec 2002
Posts: 159
Location: Syracuse, NY

PostPosted: Sun Jun 18, 2017 9:59 pm    Post subject: Finally Reply with quote

Ideola,
I think my over boost bucking issue is the result of not having installed lower charge tube bracket. I've gone through the entire twice and couldn't find anything wrong. Racking my brains over this one!

Anyway, does any one have the bracket they would be willing to part with? I have three dam engines and I can't find one of them!!!!

Bill
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