L jet and LH 2.2 928s should be 2.5bar/36psi without vacuum reference. It should be 28-31 psi with vacuum (idle).

In an LH (and probably L-jet) 928, the fuel pump relay only closes if there is crank signal. If you pump turns on with the key, somebody has done some rewiring.

 

Instead of starting a new thread about the same thing, I pigged onto this thread.
My 86.5 has been having serious operational issues. I just checked full pressure with engine off by 1) removing the ball and nut from the fuel rail, 2) attaching a fuel gauge to the end of the fuel rail, 3) bridging the fuel pump relay, and 4) grounding the battery neg post to its grounding point. I read 1.9 bar at the gauge. This is low, by way of the spec. Have I missed anything in the process of checking fuel pressure ? Thanks.....

Mike
1986.5 USA 928

 

The spec is "approx 2 bar." Your measured value is approximately 2 bar. I'm not sure how much I would worry about that. It all depends on what you are doing and why.

That and I would bet your gauge is not more accurate than +/- 0.1 bar?

 

Dave; that reading was with the engine not running. With the gauge attached directly to the rail (very little air), I read 2.06 bar. Isn't that low for reading with the engine not running?

 

Ah. Sorry. My bad. Yes, you are correct. 2 bar with engine not running and the pump pushing fuel through the rails via a bypassed relay is too low.

If the pressure with the engine running is also 2 bar then one or more of the rubber diaphragms in your regulator or pressure dampeners has failed. Pull the "rubber elbow" off of them and re-do your test. If fuel shoots out of the regulator or dampeners then you've confirmed the problem.

Obviously, that is NOT the way you should do that test. Pull the vacuum elbows off immediately after doing your engine-off test and see if you get fuel dripping out of them and/or do a good sniff test.

Note also, that if the above is a problem, your engine off test will be filling your intake's air guide with fuel along with anything else connected to the vacuum system of which that line (or lines) are part of. That is hazardous to say the least.

If you find that with the engine running the pressure is ~1/4 bar lower than that of you static pressure test, then you have an issue with pressure delivery from the pump. Weak pump? Seriously clogged fuel filter? Blocked tank strainer? The next step would be to do a fuel flow test as outline in JPTL's thread: https://rennlist.com/forums/928-foru...ed-advice.html

HTH.

 

The pump in my '83 Euro runs for about 3 seconds when I twist the key, so if it has sat for a while, I say the 'Shark Driver's Prayer'--"Some sharks do 160, some do 159, If I can make mine run at all I think I'm doing fine." as I twist the key a few times, then it fires right up. My fuel pump runs at 5 BAR, but I have K-Jet. ( No Computer)

 

Early cars with CIS use a different fuel pump relay. The CIS relays do charge the fuel rail on key-on, then look for directly tach pulses to keep the relay closed. With the later cars, including the subject '85-86 US cars, the fuel pump relay is triggered indirectly by the Crank Position Sensor (CPS) via the EZF ignition module and ultimately the LH injection controller, with no fuel system pre-chanrging. The CPS actually needs to see the crank spinning before the fuel pump relay will close.

For the 928 cars as a group, if your original fuel pump relay includes a 31a pin, pulled low by tach pulses, you'll see the relay close for a couple seconds immediately on key-on. If your car uses a standard 53B relay for the fuel pump, the relay doesn't close until the crank actually moves.

Note that there's a (very bad) habit that some folks acquire, where a 53B relay is modified to serve in early cars when the relatively-expensive original fuel pump relay fails. The real purpose of these RPM-sensing systems is to shut off fuel in the event of an accident or failure where the engine stops turning but the ignition key is still in the 'run' position. Should that accident include damage to a fuel system component, the fuel pump would maintain flow until the tank was empty, or the fire consumed the wiring that supplies it. That wiring and the fire would be in the cabin with the driver and passengers.