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Residual/ Rest Pressure Bad by Design?

 
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GuyS  



Joined: 06 Sep 2019
Posts: 3
Location: Richboro, PA

PostPosted: Thu Oct 31, 2019 6:23 am    Post subject: Residual/ Rest Pressure Bad by Design? Reply with quote

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: 173
Location: Sweden

PostPosted: Fri Nov 01, 2019 1:57 am    Post subject: Re: Residual/ Rest Pressure Bad by Design? Reply with quote

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: 9451
Location: Southeast Wisconsin

PostPosted: Fri Nov 01, 2019 4:50 am    Post subject: Reply with 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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GuyS  



Joined: 06 Sep 2019
Posts: 3
Location: Richboro, PA

PostPosted: Fri Nov 01, 2019 5:37 am    Post subject: Re: Residual/ Rest Pressure Bad by Design? Reply with quote

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.


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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1980 924
1979 928
1987 924S
2008 997
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 2055
Location: MI

PostPosted: Sun Nov 03, 2019 10:11 pm    Post subject: Reply with quote

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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safe  



Joined: 18 Mar 2017
Posts: 173
Location: Sweden

PostPosted: Mon Nov 04, 2019 4:03 am    Post subject: Reply with quote

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: 47
Location: Commack, NY

PostPosted: Fri Jul 24, 2020 12:18 pm    Post subject: Reply with quote

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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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: 47
Location: Commack, NY

PostPosted: Fri Jul 24, 2020 12:23 pm    Post subject: Reply with quote

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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