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elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

Hey Guys,

After a complete engine rebuild on my 81 S2 924 turbo and running the car it was idling rough and cutting out under acceleration.
Seemed lean so proceeded with fuel system pressure test and it was low at 4.5 Bar. Good range I believe is meant to be between 5.8-6.5 Bar.

Decided to replace the external fuel pump and in tank pump (as it was cooked) with a new 6.5 Bar external Bosch pump and strainer setup.
However with another test with the new setup still 4.5 Bar fuel system pressure. Ran a pressure test after the fuel filter and was getting 6.2 Bar, so up to the fuel distributor seems to be good.

Likely somethings up with the fuel distributor but not sure where to start, any ideas?
Would adjusting the pressure relief valve get me to 6 or so Bars from 4.5?

Thanks in advance
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1981 924 turbo (daily driver)

Cedric · Joined: 27 Aug 2004 Posts: 2814 Location: Sweden

Maybe its time for a rebuild of the system pressure regulator, it sits in the fuel distributor and have some orings than can dry out, ball, spring and shims. Its very easy to take out and rebuild.

Dont run lean, you might put a whole in a piston, so be careful to put boost into it until its all sorted

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1980 924 Turbo
www.instagram.com/garagecedric/

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

cheers for the quick reply cecdric.
cool will get it rebuilt and see if that fixes it.
no boosting atm for me just really rough and crashing idle atm lol
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1981 924 turbo (daily driver)

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

Removed and inspected the system pressure regulator. Looks pretty good, o rings seem fine and no wear on the control piston. Had two shims installed.
Guess the spring could have weakened over time which might be the problem.
Is the spring included in rebuild kits?

Put it back in for the moment with a new copper washer which was .2mm thicker which has further dropped system pressure around .3 bar.

Can you adjust the screw plug out so the regulator sits further in hence increasing pressure?

Is there anything else that could be attributing towards my low s.pressure?

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1981 924 turbo (daily driver)

Cedric · Joined: 27 Aug 2004 Posts: 2814 Location: Sweden

924RACR · Posted: Fri Apr 10, 2026 9:03 pm Post subject:

morghen · Joined: 21 Jan 2005 Posts: 9095 Location: Romania

If you csnt figure it out or just dont want to anymore, i sell an EFI conversion kit for these cars. The engine bay layout is kept and the parts are designed to look as close as possible to the originals. You even get a fuel distributor that looks very close to the original.
Check my website out: www.the924.com
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Supercharger and EFI kits
https://www.the924.com

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

Looks like a neat kit Morghen, if I cant get it running will hit you up.

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

New O-rings on the system pressure regulator but no increase in system pressure.
Also rebuilt fuel distributor and no change to system pressure either.
Adjusted fd plunger so in its starting position there was no fuel coming out of the ports.
If I put in an extra two 1mm shim can get it up to 5.2 bar and any addition after that barely does much.
Is anyone actually getting 6bar of pressure? saw a few videos and people were just above 5bar of pressure
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1981 924 turbo (daily driver)

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

Removed cold start injector to check for leaks and no problems there.
Ran a cold control pressure test (by opening the valve on the tester from a system pressure test) and the pressure would only drop from 5.2 to 5 Bar then slowly keep dropping down.
If I run a cold control pressure test from scratch no pressure builds up at all until i switch the pump off and 1.5Bar shows up.
Might look at the accumulator next only thing I haven't checked out, can a faulty one cause system pressure loss?
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1981 924 turbo (daily driver)

elitejaso · Joined: 24 Jul 2007 Posts: 146 Location: Melbourne

Cold control pressure should be between 2.4-2.8 Bar so thats not good. But incorrect system pressure could be affecting all my other pressures I guess.
Could also have a stuffed WUR? but that might just be another issue

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1981 924 turbo (daily driver)

924RACR · Posted: Fri Jun 05, 2026 8:53 pm Post subject:

That's a bunch of strange results...

When I had a bad WUR, it did indeed fail with the symptom of control and system pressure were the same; essentially, it was not bleeding off any pressure to the control circuit. If yours is slowly dropping, maybe it's really clogged yet not completely done for?

No idea what the other result means - trying to build control pressure with the valve open. That's quite curious.

The accumulator is in line between the pump and the fuel filter (thence the distributor)... usually they fail by leaking, as you probably already know... perhaps if it's providing a blockage as well, it could cut down system pressure?

I seem to recall they're relatively available new and not so expensive?
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Beartooth · Joined: 05 Apr 2022 Posts: 320 Location: Roberts, MT

I'd worry about system pressure before anything else. Yes, getting control pressure in line is important, but I think it's affected by system pressure being off. Make sure there are no restrictions in the fuel lines, and I'd cut open the old fuel filter to make sure you're not getting a bunch of crud plugging things up. I did have to clean a bunch of rusty sediment out of the sump on my car, but it was severely neglected.

The fuel pressure regulator is probably where I'd focus, but adjusting it can be a little wonky. I remember fuel pressure actually dropping if I put too many shims in it. Also, make sure that little plunger is able to move freely all the way to the end of the bore; maybe it's binding? It may be worth the expense of buying the new fuel pressure regulator. I do remember my fuel pressure being a little low until I monkeyed with it. I'm going off the range given by the Haynes manual, which is pretty wide - 5.4-6.0 bar. System pressure should be rock steady though: if you're seeing it drop off, that makes me think there's some restriction - like a bunch of sediment near the pickup that builds up on the tank filter when it's running, then falls away when it's off...

As I recall, I was only able to get to the low end of that range (maybe 5.5) and no more. Since my engine is clapped out and needing to be rebuilt, I just called it good, and it's run fine for around 9000 miles... As mentioned, it's very important to get it sorted, especially before you drive it hard. Once you get there, however, it should serve you well as long as you run good gas and don't neglect it. Obviously I can't pinpoint where your problem is, but hopefully there's something you can take from my experience.
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1980 931 diamond in the rough