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GuyS
Joined: 06 Sep 2019
Posts: 3
Location: Richboro, PA
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 Posted: Thu Oct 31, 2019 6:23 am Post subject: Residual/ Rest Pressure Bad by Design? |
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Hello All,
Working on my 80 924 and I have a observation that maybe you guys can assist with.
I lose rest pressure very fast despite new injectors, accumulator and check valve. I also serviced the FD pressure relief valve.
I have worked extensively with CIS on the 928 platform. In that application, the WUR return line goes to the fuel distributor and therefore is controlled by the pressure relief valve. On my 924, the return for the WUR is directly plumbed to the main return line, after the valve.
So unless I am missing something, it will drain down the return line unfettered and therefore its impossible to control residual pressure? I noticed the earlier cars seem to have a different line configuration.
Thoughts?
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Guy
1980 924
1979 928
1987 924S
2008 997 |
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safe
Joined: 18 Mar 2017
Posts: 234
Location: Sweden
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| Posted: Fri Nov 01, 2019 1:57 am Post subject: Re: Residual/ Rest Pressure Bad by Design? |
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| GuyS wrote: | Hello All,
Working on my 80 924 and I have a observation that maybe you guys can assist with.
I lose rest pressure very fast despite new injectors, accumulator and check valve. I also serviced the FD pressure relief valve.
I have worked extensively with CIS on the 928 platform. In that application, the WUR return line goes to the fuel distributor and therefore is controlled by the pressure relief valve. On my 924, the return for the WUR is directly plumbed to the main return line, after the valve.
So unless I am missing something, it will drain down the return line unfettered and therefore its impossible to control residual pressure? I noticed the earlier cars seem to have a different line configuration.
Thoughts? |
You have a fuel accumulator near the tank, that is often broken. Also a check valve in the fuel pump (I think). |
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Paul
Joined: 02 Nov 2002
Posts: 9466
Location: Southeast Wisconsin
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| Posted: Fri Nov 01, 2019 4:50 am Post subject: |
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Where are you measuring the residual pressure? As I'm sure you know, the intent of the design is to maintain pressure in the injector fuel lines to reduce vapor lock.
I'd pull the new injectors and look for drips after shutdown.
_________________
White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy. |
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GuyS
Joined: 06 Sep 2019
Posts: 3
Location: Richboro, PA
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| Posted: Fri Nov 01, 2019 5:37 am Post subject: Re: Residual/ Rest Pressure Bad by Design? |
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| safe wrote: | | GuyS wrote: | Hello All,
Working on my 80 924 and I have a observation that maybe you guys can assist with.
I lose rest pressure very fast despite new injectors, accumulator and check valve. I also serviced the FD pressure relief valve.
I have worked extensively with CIS on the 928 platform. In that application, the WUR return line goes to the fuel distributor and therefore is controlled by the pressure relief valve. On my 924, the return for the WUR is directly plumbed to the main return line, after the valve.
So unless I am missing something, it will drain down the return line unfettered and therefore its impossible to control residual pressure? I noticed the earlier cars seem to have a different line configuration.
Thoughts? |
You have a fuel accumulator near the tank, that is often broken. Also a check valve in the fuel pump (I think). |
Correct, I replaced the accumulator, the old one was weeping. I have a new pump with new external check valve. Thank you!
| Quote: | Where are you measuring the residual pressure? As I'm sure you know, the intent of the design is to maintain pressure in the injector fuel lines to reduce vapor lock.
I'd pull the new injectors and look for drips after shutdown.
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I will double check them. I am measuring in the standard fashion Gauge set between WUR supply line. Thanks!
I realize this design has been around for 40 years, but I have never seen another CIS system plumbed in the same fashion as the 80-81 924. Is there anyone out there who has proper residual pressure on these years measured with the valve open on their CIS gauge? It does not seem possible to hold any pressure when the WUR is plumbed directly to the return line.
I appreciate the posts, thanks!
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Guy
1980 924
1979 928
1987 924S
2008 997 |
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2178
Location: MI
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| Posted: Sun Nov 03, 2019 10:11 pm Post subject: |
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I think the WUR should hold residual pressure too.. Maybe strip-clean it and see what their is to see in there..
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80 Turbo - Slightly Modified |
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safe
Joined: 18 Mar 2017
Posts: 234
Location: Sweden
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| Posted: Mon Nov 04, 2019 4:03 am Post subject: |
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| Fasteddie313 wrote: | | I think the WUR should hold residual pressure too.. Maybe strip-clean it and see what their is to see in there.. |
No, its just a somewhat variable "restriction" to create a pressure differential. |
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Sndtech
Joined: 23 Apr 2013
Posts: 49
Location: Commack, NY
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| Posted: Fri Jul 24, 2020 12:18 pm Post subject: |
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GuyS
I have a 1980 NA also and expereince the same thing with Residual Pressure loss (on the Control Side).
Have changed numerous components while restoring the CIS system.
Even the "Hot Start Valve" on the side of the FD is functioning ( that was included in systems from 79 on) to alleviate the hot start problem.
I am at a loss of why the pluming is that way to the tank return and my pressure falls within 3-5 minutes to 0.
Anyone any thoughts ?
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Manny-
1977 924 Martini |1979 924 N/A Dolomite Grey | 1980 924 N/A Minerva Blue |
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Sndtech
Joined: 23 Apr 2013
Posts: 49
Location: Commack, NY
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| Posted: Fri Jul 24, 2020 12:23 pm Post subject: |
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When I used the CIS gauge and took residual readings with the valve open the pressure fell in 3-5 min.
When I took the readings again with the valve closed the pressure stayed above 2B for 6 hours.
BTW.. WUR was rebuilt.
I am thinking of using a "fuel pump check valve" right on the dizzy feed going to the WUR. This will hold the residual (control) pressure.
Any thoughts anyone ?
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Manny-
1977 924 Martini |1979 924 N/A Dolomite Grey | 1980 924 N/A Minerva Blue |
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