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Pelican Technical Article:
Fuel System Pressure Tests

Steve Vernon

 
Time: 2 hours
Tab: $0
Talent:  
Tools:
19mm wrench, fuel pressure gauge, relay jumper, rags
Applicable Models:
Porsche 944 Turbo (1982-91)
Parts Required:
None
Hot Tip:
Use all precautions when working around fuel
Performance Gain:
Proper fuel pressures
Complementary Modification:
Check fuel flow rates
 
   

   
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     All internal engines need three things to run: air, fuel and spark. As engines have become more complicated, powerful and emission regulation dependent, keeping the fine balance of a proper running engine has meant tighter tolerances on the air, fuel mixture and timing of the spark.

     If your engine is not running properly or not at all, checking the fuel pressure is one of the first things to do to determine that the motor is getting fuel and the right amount. This article will walk you through how to check your fuel pressures for all Porsche 944 engines.

     You will be working around gasoline so make sure to take the proper precautions. Always wear proper gloves and eye protection. Work in a well-ventilated area and keep any open flame, spark or heat (shop lights) away from the area.

     There is going to be some gas spillage so be prepared to collect it in rags and dispose of the rags in accordance with your local regulations. NEVER store gas soaked rags as they become very combustible.

     If possible let the engine rest for at least a couple of hours before performing these tests to allow the system to depressurize.
Open the hood and locate the fuel rail that runs down the top of the engine.
Figure 1
Open the hood and locate the fuel rail that runs down the top of the engine. At the rear of the fuel rail you will see the fuel regulator (yellow arrow). Towards the front of the fuel rail you will see the fuel dampener (green arrow). At the very front of the fuel rail you will see a metal cap (red arrow). On the 16-valve cars it is located on the side of the fuel rail between intake runners #1 and #2.
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Use a 19mm wrench and remove the cap on the end of the fuel rail (red arrow).
Figure 2
Use a 19mm wrench and remove the cap on the end of the fuel rail (red arrow).
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There is a metal ball bearing in the end of the rail (red arrow).
Figure 3
There is a metal ball bearing in the end of the rail (red arrow). Do not lose this as you remove the cap; it is absolutely necessary when your reinstall the cap to seal the rail. Some gas will escape when you remove the cap so be prepared for it with a shop towel and/or a small plastic container to catch the fuel spilled.
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Attach your fuel pressure gauge to the end of the rail (red arrow).
Figure 4
Attach your fuel pressure gauge to the end of the rail (red arrow).
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You are going to want to jump the relay so you can test the pressure without starting the motor.
Figure 5
You are going to want to jump the relay so you can test the pressure without starting the motor. If the engine is not running, you do not want to keep turning the motor over and pumping gasoline into the cylinders to perform this test. Remove the fuse/relay panel (red arrow). On older cars it is located inside the vehicle under the dash and on newer cars it is located in the rear left side of the engine compartment.
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Locate and remove the fuel pump relay (red arrow).
Figure 6
Locate and remove the fuel pump relay (red arrow). You are going to want to jump terminals 30 and 87b. The terminals should be clearly marked on the bottom of the relay.
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While you can directly jump the two terminals by running a direct-shielded wire between them, I like to make a jumper that includes a fuse (red arrow, to prevent damage to the electrical system) and a switch (green arrow).
Figure 7
While you can directly jump the two terminals by running a direct-shielded wire between them, I like to make a jumper that includes a fuse (red arrow, to prevent damage to the electrical system) and a switch (green arrow). The switch will give you more control over the testing. If you jump the terminals directly, the pump will start immediately and stay on.
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With the relay jumped the fuel pump should start.
Figure 8
With the relay jumped the fuel pump should start. With the engine stopped and relay jumped you should have 2.5 bar of pressure +/- 0.2 bar on all 8-valve 944 and 944 turbo motors (red arrow); 16-valve motors should have 3.8 bar +/- 0.2 bar (yellow arrow).
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Remove the relay jumper, reinsert the relay and start the motor.
Figure 9
Remove the relay jumper, reinsert the relay and start the motor. With the engine at idle you should have 2 bar of pressure +/- 0.2 bar on all 8-valve motors (red arrow) and 3.3 bar of pressure +/- 0.2 bar on all 16-valve motors (yellow arrow). Substantially higher pressure is usually a problem with the pressure regulator and substantially lower pressure is usually a problem with the fuel pump or filter.
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You also want to check your leak down pressure.
Figure 10
You also want to check your leak down pressure. Stop the motor and let it sit for 20 minutes. Check the static pressure after twenty minutes; you should have 2 bar of pressure +/- 0.2 bar on all 8-valve motors (red arrow) and 2 bar of pressure +/- 0.2 bar on all 16 valve motors (yellow arrow). Anything substantially less could be a problem with your check valve and result in hot restart and vapor lock problems. After testing make sure to properly reinstall the fuel rail end cap and ball and clean up any spilled fuel.
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  Applies to: Porsche 944 Turbo (1982-91)
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