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924 Turbo no midrange power

 
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Pittspirate  



Joined: 17 Sep 2020
Posts: 4
Location: Ontario, Canada

PostPosted: Thu Sep 17, 2020 1:01 pm    Post subject: 924 Turbo no midrange power Reply with quote

Long time 924 owner. Iíve had my 79 over 30 years.

My son recently acquired an 81 turbo. It was a bit of a project, but itís on the road now.

Main problem is that it bogs under hard acceleration. As soon as the turbo spools up, the car dies, let off, itís fine. Under modest acceleration, half throttle, itís okay, but as turbo kicks in around around 3000, it dies.

Main pump pressure is ~80psi at regulator. Control pressure is same after warm up at idle. Suspecting WUR, any comments as to whether Iím on the right track? Anything more likely to be the culprit.

If, for some reason, the frequency valve wasnít operating, would these be the symptoms? F/V bench tests okay. Micro switches on throttle body okay.

Open to any suggestions

Thanks
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gerben1986  



Joined: 13 Mar 2008
Posts: 232
Location: The Netherlands

PostPosted: Thu Sep 17, 2020 7:53 pm    Post subject: Reply with quote

Fuelfilter blocked as soon as it tries to get more fuel?
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chuck21401  



Joined: 20 Feb 2005
Posts: 262
Location: Annapolis, MD

PostPosted: Thu Sep 17, 2020 9:55 pm    Post subject: Reply with quote

If you donít have one already, suggest connecting boost gauge, at least temporarily to see what kind of boost you have when it cuts out. Have you tried grounding the overboost switch? That cuts the fuel when there is too much boost.

Run some searches in this forum...could be any number of things.
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Pittspirate  



Joined: 17 Sep 2020
Posts: 4
Location: Ontario, Canada

PostPosted: Fri Sep 18, 2020 12:18 am    Post subject: Reply with quote

Drove with pressure gauge on, under load and when bogging, pressure remained constant, so I donít think itís a clogged filter.

Waste gate disassembled, inspected, re-assembled, tested okay

Have by-passed overboost switch, no difference

I havenít found a definitive answer to what the warm control system pressure should be on this car. Right now, when itís warm. Control system pressure when warm is equal to main system pressure. It doesnít change under load.

Although I understand K-Jetronic, The lambda system is new to me. Should the control system pressure drop as frequency valve begins to cycle? As I said if WUR numbers are normal, I think I need to look at emission.

Agreed, a boost gauge is a good idea, but engine bogs as soon as I hear the turbo spolling up.

I think I read about a dwell meter on the frequency valve. Would anyone concur that this is the next logical step?

Thanks
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jazz guy  



Joined: 26 Nov 2002
Posts: 411
Location: Colorado

PostPosted: Fri Sep 18, 2020 1:22 am    Post subject: Reply with quote

Boost leak? The o-ring seals on both ends of the charge pipe are known failure points. When I've run into this, the car will idle but immediately bogs as boost comes on. Though when it happened, I'm not sure mine ran well enough to drive. Limp home maybe.
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Last edited by jazz guy on Fri Sep 18, 2020 3:13 am; edited 1 time in total
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Mike9311  



Joined: 14 Dec 2004
Posts: 879
Location: Chicago-ish

PostPosted: Fri Sep 18, 2020 2:55 am    Post subject: Reply with quote

I second jazzguy

Look to see if the upper o-ring to the throttle body is sticking out at all. If you don't have the spacer between pipe and throttle body which a lot of 81-82's don't, it could be popping out under boost but sealing enough to seem fine

Happened to me too at one point
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Carrera RSR  



Joined: 08 Jan 2010
Posts: 2011
Location: Somerset, UK

PostPosted: Fri Sep 18, 2020 4:08 am    Post subject: Reply with quote

Mike9311 wrote:
I second jazzguy

Look to see if the upper o-ring to the throttle body is sticking out at all. If you don't have the spacer between pipe and throttle body which a lot of 81-82's don't, it could be popping out under boost but sealing enough to seem fine

Happened to me too at one point


I don't buy the black spacer/finisher ring allows the inner O ring to pop out on boost. I have run over 1 bar without that black ring. But for sure the o rings need to be checked to make sure no boost is leaking out.
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1980 931 - forged pistons, Piper cam, K27/26 3257 6.10 hybrid turbo, 951 FMIC, custom intake, Mittelmotor dizzy & cam pulley, H&S exhaust, GAZ Gold, Fuch'ed, Quaife
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Then www.924board.org/viewtopic.php?t=31252
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Mike9311  



Joined: 14 Dec 2004
Posts: 879
Location: Chicago-ish

PostPosted: Fri Sep 18, 2020 4:38 am    Post subject: Reply with quote

It happened

Middle of the night after wrenching I drove it down the street. Boosted it a bit and the car started running super crappy then died. Wouldn't hold an idle after that and I proceeded to kill the battery trying to get it restarted

Could not see the issue there and ended up pushing it back with one other person roughly a quarter mile. Initially to get under a street light but in the end all the way back.

O-ring was popped and I wouldn't have believed it myself without witnessing personally so I get the pessimism

Edit: ohh... no ring in my case. Its why I Print them now. It wasn't the whole o-ring either. Just a little section of it popping out
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA M650 Herausnehmbares Dach
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car
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Pittspirate  



Joined: 17 Sep 2020
Posts: 4
Location: Ontario, Canada

PostPosted: Fri Sep 18, 2020 5:37 am    Post subject: Reply with quote

O Ring Ďlooksí okay, but I understand how this could be the problem. A boost gauge would tell the tale as previously mentioned, thanks

Again, any comments as to WUR pressures, possibly checking frequency valve dwell would also be appreciated.

Simple things first!
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2040
Location: MI

PostPosted: Fri Sep 18, 2020 6:49 am    Post subject: Reply with quote

The three pronged test plug coming out of the firewall to the outside side of the brake booster.. The black wire.. Is the test port to read the frequency valve duty cycle.. Easily read with a dwell meter..

The duty cycle of the frequency valve should adjust to get the o2 sensor to read/cross stioch..
At idle, adjust the AFR screw to where the dwell meter reads at average 45%, but it will be crossing back and forth +- a few %..
Make sure to plug the AFR screw access hole while reading and for running as it itself is a vacuum leak..

Otherwise, your symptoms sound like a boost leak where the upper charge tube meets the throttle body, or where the lower charge tube meets the turbocharger, to me..
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bureau13  



Joined: 07 Sep 2017
Posts: 311
Location: South Florida

PostPosted: Fri Sep 18, 2020 2:30 pm    Post subject: Reply with quote

I highly recommend you get or make a smoke tester. I found a bunch of vacuum leaks during my recent "troubles." None of them were the actual problem, but that's another subject

You can buy very expensive professional smoke testers, or you can buy the cheap ones from eBay (that's what I did) or you can make them yourself for even less. Super valuable for this task.
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2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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bureau13  



Joined: 07 Sep 2017
Posts: 311
Location: South Florida

PostPosted: Fri Sep 18, 2020 2:44 pm    Post subject: Reply with quote

Regarding the frequency valve, I'm no expert (AT ALL) but I had to do quite a bit of research into the lambda system while diagnosing what ended up being a bad coil (that's how much of an expert I am ) This site was invaluable:

https://jimsbasementworkshop.com/CIS/pages/descr_lambda.html

Yes, it's for a NA 911, so it doesn't show the big role the WUR plays in on-boost enrichment, but from what I gather, the lambda system works pretty much the same as for our cars.

In short, since you've confirmed that the frequency valve is doing something, I suspect it is NOT the source of your problem. I guess if you have a dwell meter it sounds like a pretty easy test. I don't have one so I only went as far as "is it buzzing when it should?"
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-----------------------------------------
1982 931
1986.5 Silver 928S
2007 Hummer H3 (Daily Driver)

Past lives (I miss them all):
2004 RX-8 (Wrecked)
1993 RX-7 (Sold)
1987 RX-7 Turbo II (Sold)
1985 RX-7 GSL (Stolen)
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924RACR  



Joined: 29 Jul 2001
Posts: 8334
Location: Royal Oak, MI, USA

PostPosted: Fri Sep 18, 2020 9:05 pm    Post subject: Reply with quote

Well, WUR sounds bad, common for those that've sat for a while. Control pressure shouldn't be the same as fuel delivery pressure. It varies with temp, but at 20C it should be between 2.4-2.8 bar = 35-41psi.

With the engine warmed, it should be 50-55psi, without boost. It should drop with boost to 2.7-3.1 bar/39-45psi.

So you probably need to have it cleaned/rebuilt. Along with fixing your boost issues.
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Pittspirate  



Joined: 17 Sep 2020
Posts: 4
Location: Ontario, Canada

PostPosted: Sat Sep 19, 2020 9:02 am    Post subject: Reply with quote

Re-ran. WUR test. Operating pressure ~74, cold control pressure ~74.

Going to solve this first. Will keep you all posted

Thanks
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