Weird System Pressure
 

I posted on the 930 side, but I wanted to get some opinions from the CIS gurus over here.

History: Car died while driving. When trying to restart, could not hear rear fuel pump run. I could hear the front pump running. Jumped the rear fuel pump, ran very sporadically. Replaced rear FP, now I hear both FP's running when starting car, as always. Also, now when I jump the rear pump is runs steady.

Car will now fire, but will not run very long, and will not take any throttle. If I let it idle, it idles for a min then dies.

Checked System pressure, goes up to 3.5 bar, then I hear a "click" from the area of the fuel dist, then pressure immed falls to 2.5 bar. Once it hits 2.5, it then goes back up to 3.5, then click and back down again. This cycle process continues, takes about 20 seconds to cycle thru: 3.5 >> 2.5 >>3.5.

1986 930, no emissions, O2 disconnected.

So what's clicking? A relay or could it be a faulty FP Regulator? Also you doing this FP testing with car not running correct?

I suspect the regulator.

BTW - I'm not a 930 fella but I suspect the FP should be in the 3.5bar range with car not running and it should not fluctuate as you describe.

Car not running, both fuel pumps running. System pressure should be around 5-6 bar. The clicking is kind of more like a dull sound of a rod hitting a housing. I'm sure it's the plunger in the fuel dist that controls the system pressure, just not sure why it is shutting off at 3.5 bar.

Tim,

I have not worked on CIS in a while and I forgot that System Pressure in these cars is like 5-6bar (80lbs or more) if I recall correctly my old 75 CIS car had system pressure around 5bar but it also had control pressure (CP) that came from the Warm Up Regulator (WUR) and this CP varied a lot from cold to fully warm. I think CP was down in the 1bar range or even lower when cold (10c) and then came up to the 3.5bar range once at operating temperature.

What are you measuring? system pressure - or - Control Pressure via the WUR?

Measuring system pressure, control pressure is fine. System pressure is only reaching 3.5 bar then cycles between 2.5 and 3.5bar.

Cold control pressure is right on spec, approx 2.25 bar for my temp, now it will not reach warm control pressure which is approx 4bar, because the system pressure will not go above 3.4bar.

System and control pressures........
 

Tim,

Check the delivery pressure or volume (flow rate/min) for individual FP's. If both FP's delivery pressures are within spec proceed to measure the control and system pressures. WUR directly responsible for the control pressure, while the primary-pressure regulator in the FD maintains the pressure in the fuel system constant.

So if both FP's are good and no fuel flow restriction (like clogged fuel filter or finched return line) the primary-pressure regulator determines the system pressure. HTH.

Tony

Could you simply test System Pressure by momentarily clamping the fuel return line and keeping an eye on the gauge to see that it climbs to 6bar? Just don't let it go much over 6Bar. This should give you a good idea if the pumps can deliver enough pressure.

Then you could switch to looking at the bouncing CP the same way. While looking at control pressure and seeing it bouncing 2.5 - 3.5 bar you may be able to find the cause by clamping each component one at a time. Like clamp off the WUR line and see what effect it has on the CP then if you have any other components that regulate CP (like a Throttle Pos valve) you could clamp this as well. I suspect that if you simply clamp off the CP line at fuel distributor the CP should equal the system pressure. So if you clamp of the CP line leaving the Fuel Distributor and the CP still is bouncing I'd suspect the Distributor has an internal failure.

Going to start back at the FP's this weekend.

1) Run both pumps for 30sec, should be 1170cc.
Fails: Put pressure gauge on inlet of rear pump, run both pumps, 2-4bar
rear pump bad, below 2-4bar frt pump bad.
2) After I eliminate the pumps I'll start looking down from the pumps at each device and go from there. Fuel Filter, Acc, wur, FD etc

Tim,

I have CIS testing equipment too. If you want to test two locations simultaneously let me know and I will bring it to you. I'd be happy to help read the pressures too.

Thanks Craig, may take you up on that. Right now I just need to rule out the pumps.

I believe System Pressure is a function of fuel pumps ability to deliver pressure and the setting of pressure regulator built into the fuel head at the point where the excess fuel is returned to the tank. The pinch test noted above might be a test of the regulator.

I assume you are testing this on the supply side? Testing on the return would be of no benefit.

I am not sure but I think there is a timer built in that if the car dose not start within a given amount of time, the fuel pump is turned off. This must be bypassed to do a test with out the motor running. Sounds like you have figured this part out. Note that fuel will be delivered through the injectors with this test running or not.

I suspect, until you get your SP back, there is no reason to worry about CP.

When a car has issues relating to fuel delivery the most overlooked consideration is a blocked or restricted fuel filter.

At this point, I am not worried about control pressure.

Going to test the pumps, make sure both pumps are working to spec, then start going down stream of the pumps.

Fuel is only delivered to injectors when the plate goes down, I am not touching the plate when running the pumps, so no fuel should be going to injectors during my testing.

Right on the injectors. What about the fuel filter?

Going to replace that while I'm testing the pumps just to rule it out.

Found the problem, both pumps running should produce 1170cc or 4.945 cups (1170/236.588) fuel in 30 seconds, only producing less than a cup.

Replaced the fuel filter, now checking the pumps.