924Board.org

[ideola]

From the Probst book:

CONTROL PRESSURE
COLD
Control pressure is related to temperature. If the regulator is cool enough for you hand to touch, it's cool enough to test. You can't fool K-basic and K-lambda by disconnecting the coolant temperature sensor because they don't have one.

Control pressure goes up when temperature goes up. Higher control pressure reduces slit size and reduces enrichment. If you study a typical control pressure specification graph (such as the one linked to in the O.P.), you can see that when the control pressure regulator is at ambient (or surrounding) temperature in the shop, probably about 70°F (20°C), cold control pressure for the car being checked should be between 1.4 and 1.8 bar (20-26 psi). [editor's note: compare against the cold control pressure cited from the tech specs booklet posted in the O.P.]

With the fuel pump running, open the pressure gauge valve so that fuel no flows through the control pressure regulator. The gauge should show a drop from system pressure to control pressure. Typical cold control pressure is about 1.5 bar (22 psi). The control pressure regulator is reducing the pressure; that will give the engine the richer mixture needed for cold operation.

WARM
For the warm control pressure test, reconnect the wiring connector to heat up the regulator [editor's note: the Probst book neglects to mention in the "cold" section that the electrical connectors to BOTH the WUR and the AAV should be disconnected for the cold test]. With the pump running, the pressure should slowly rise to a typical specification of between 3.4 and 3.8 bar (49-55 psi). [editor's note: as above, compare against the warm control pressure cited from the tech specs booklet posted in the O.P.] Starting from ordinary shop temperatures, it should take one to two minutes for the pressure to rise from cold control to warm control, so don't get impatient.

If control pressure did not rise properly, check for restriction in the return line. Also measure the supply voltage to the heater of the control pressure regulator (a.k.a. WUR). It should be at least 11.5 volts power supply. If return line is clear and the voltage checks out OK, replace the control pressure regulator (a.k.a. WUR).
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erstwhile owner of just about every 924 variant ever made

[ideola]

From the Probst book:

RESIDUAL PRESSURE
The residual pressure test measures the amount of system pressure leakage. When you turn off the pump after your warm control pressure test, system leakage should be small enough so that pressure stays at a specified pressure for a specified time: usually 3 bar (43.5 psi) for 10-20 minutes.

If residual pressure falls below the minimums with the gauge valve open, the leak is probably in the control pressure circuit, at the push valve o-rings. If it is an older system without a push valve, the leak is probably at the control pressure regulator.

Rerun the leakage test with the valve closed to shut off any possible leakage through the control pressure regulator. If you still observe low pressure on the gauge, the regulator is OK, but there is system pressure leakage.

In the system pressure circuit, the cold start injector or port injectors could be leaking, or it could be the system pressure regulator o-ring (i.e within the fuel distributor for the 924/931 units), or it could be the fuel pump check valve, or ti could be other components and connections (i.e. other components could include the accumulator(s)). Check the check valve as described below. Run the pump to build pressure in the system and look closely for fuel leaks at fuel line unions. To quick-check the fuel accumulator, remove the screw from the end of the accumulator. If fuel leaks out, the accumulator diaphragm is faulty and the accumulator should be replaced.

FUEL PUMP CHECK VALVE
The fuel pump check valve is part of the outlet fitting on the pump, but early models may vary, so check your shop manual. To check the valve, disconnect the fuel supply line to the fuel distributor and attach the pressure gauge. Make sure that the gauge valve is closed. Run the fuel pump until pressure reaches 5 bar (78 psi) and shut off the pump. If residual pressure falls below specification, replace the pump check valve. Before you remove the fitting remove the gas tank cap, clamp the pump inlet hose so no fuel will escape, and clean the fittings. Install a new seal and check valve and torque to specification. Check all connections with the pump running and redo the leakage test.

Remove the gauge and reconnect the fuel lines when fisnished. With the gauge, you've checked K-basic and K-lambda system pressure, and warm and cold control pressure, as well as residual pressure
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erstwhile owner of just about every 924 variant ever made

[ideola]

My notes:

• In order to run these test, you need to have a fuel pump relay jumper. This is a short piece of wire with a male spade terminal on each end. Remove the fuel pump relay and install the jumper on the 30 and 87 pins on the fuse block. The fuel pump should run.
  
• I strongly recommend removing the injectors from the head and inserting them into glass jars or bottles while doing these tests. If you have a bad injector, or if you have one or more bad ports on the fuel distributor, you will flood those cylinders with fuel, and it WILL cause hydrolock on the engine, as well as contaminating your engine oil. Do yourself a favor and go the extra step to pull the injectors. You can test their spray pattern while you're at it.
  
• Haynes has some additional pointers that the Probst book doesn't mention. One is to be sure to hang the gauge so that it is always at the highest point while doing tests, as well as opening and closing the valve several times when you first connect it to allow system pressure to properly build...otherwise, you may not be getting accurate readings. Not going to copy all of the Haynes text...get your own copy and refer to it...

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erstwhile owner of just about every 924 variant ever made

[leadfoot]

there's a bleed valve on the side, again the manual doesn't say it pisses out fuel from this when bleeding so use a rag here too...
thanks Dan good writeup
Stu
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1981 ROW 924 Turbo -
carbon fiber GT mish mash
LS1 conversion in progress...

[ideola]

[tuf-924]

Hey are you able to upload pictures of teater plugged in the vehicle ready to be used as cannot figure out how it has to be connected, I have the official Porsche part VAG 1318 I have no idea where to hook it in to
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Vass...

[dpw928]

Ideola,

Great posting, but just one question about jumping the fuel pump relay. On my 931 the 15 feed wire is a 16 or 18 gauge whereas the feed wire on my 928 is a larger 12 gauge. I have always thought that bypassing the 924/931 at the 15 connection could overheat the feed wire. I would love to hear that I'm wrong in my thinking. Have you or anyone else actually used the 15 connection and did it work out OK?

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[ideola]

The 15-to-87 connection is from memory...pretty sure it's correct, but should verify against FSM/Haynes.

When I've used the jumper, I've never had an issue with the wiring...but I don't like leaving the pump running for any longer than absolutely necessary...maybe a few minutes at most while checking the various settings on the tester. I certainly wouldn't operate the car this way...!
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erstwhile owner of just about every 924 variant ever made

[dpw928]

Thanks. I know it works OK on the 928 as my 78 5 speed was bypassed with a fused splice when I bought it. I didn't notice for 2 months. Never tried it on the 924/931 as the 15 lead appears to be much smaller. Think the WSM says to jump the 30 and 87 on both.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[ideola]

You are correct, FSM says to jumper pins 30 & 87. Will edit above.

I'm guessing the difference is that 30-to-87 cause the pump to run without the key in the on position, while 15-to-87 requires the key...I've gotten into the habit of doing the latter so I can switch the key on and off to turn the pump on and off...maybe it's not a good idea due to the wire gauge issue that you mentioned...
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erstwhile owner of just about every 924 variant ever made

[dpw928]

I also like the option to use the key (15 and 87) but that is because I'm an old f@rt. Just wondering if anyone has used this bypass on a 924/931 for any lenght of time.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[TheBib]

I jumpered 30 and 87 and yes indeed the pump came on with the key in the off position. Haynes manual says that you need to turn on the ignition to get the pump to operate, i think they are incorrect. I thought i had an issue, sounds like not.

my first question: Specifically regarding the warm control pressure, I am assuming that the car is not running, and that the reason the WUR, warms up, is because by jumpering the fuel pump, this also provides power to the WUR circut.

my second question: you mention about dropping the piston in the process of changing the shims. You mean like physically dropping it and damaging it when it hits the ground or another object correct?
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TheBib

[dpw928]

The 30 circuit is direct to the battery and the 87 is direct to the fuel pump, so connecting them will start the pump. The WUR heater works off the 15 (ignition on) circuit, so jumping the 30 and 87 will not heat the WUR. The control pressure can be tested without heating the WUR. If the engine is cold, just use the ambient temp to compare your test results. You test the heater by taking an ohm reading between the two connects and testing each connect for a fault to ground.

The haynes manual was probably referring to the pump running for a few seconds when the key is turned to on and the relay installed. This primes the fuel system.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black

[TheBib]

Understand about testing the heater, but there in lies an assumption that if the hearter works the warm control pressure will be on spec.

SO my question is if i do in fact wish to test the warm control pressure how do i accomplish this.

also Idola you mention about pulling the injectors. I know all of mine are fine, do i really need to pull them. Since the fuel pressure tests are performed with the engine off, the air sensor plate and arm should be down and therefore no fuel being injected into the intake. And i know none of mine leak when the sensor plate is at rest.
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TheBib

[dpw928]

Yes, if the cold control pressure is correct and the heating element is working correctly, the warm control pressure should be within spec also. If you want to test it warm, run the engine, measure the temp of the body of the WUR and compare the control pressure to the chart at that temp.

You would only want to pull the injectors if the system failed the leakdown test.

Dennis
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81 931 5 sp
78 928 5 sp Silver
78 928 AT Euro Black