fuel pump check valve, and what disables fuel injectors?

Post by **t5rrr** » 30 Oct 2011, 15:40

I recently had the fuel pump (insert only) replaced on my 94 855 NA after it suddenly died. Just this past week it died again, this time not even making out of our subdivision.

Key on, fuel primed so pump and fuel relay are fine. (I even jumpered the 15 and 87 pins of the fuel relay socket to double check, still no start).

Probed the fuel rail valve during priming and fuel sprayed out, but just a dribble of fuel after priming so my first hunch was a bad fuel pressure regulator (on the rail). Replaced that and still no start

After the FPR replacement, I probed the fuel rail valve again after priming sequence, not a dribble but hardly any pressure in the rail, the fuel sorta just "leaked" out. Does 93-95 850 NAs also have a check valve in the fuel pump assembly?? Cuz I replaced the pump insert only so could this be a separate failure of the check valve?

Another thing I did was to remove the spark plugs to see if the motor flooded with fuel, plugs were dry as a bone

Does lack of fuel pressure inhibit the injectors from firing or some other fault (e.g. Camshaft sensor) could prevent the injectors from firing as well?

Sorry for being long winded but this is the 2nd fuel related failure in the past month in my 855

waynej · Post by **waynej** » 30 Oct 2011, 17:32

The FPR acts as a check valve when the pump is off. Check valve has no effect when pump is on. Not sure what you mean by "priming sequence" but pump needs to be on when checking for pressure. Can you hear the pump running.

Post by **Ozark Lee** » 30 Oct 2011, 18:25

All of the 850s have a check valve but the bad news is that it is built into the housing on the fuel pump. Changing the insert won't fix a bad check valve. That said, a bad check valve will cause terribly hard starting but it won't keep the car from running and should never cause the car to suddenly stop. The check valve's job in life is to maintain pressure at the fuel rail after the car is turned off so that it will take less from the fuel pump to get the system pressure up to spec when you go to start the car again.

The injector triggering is controlled by the ECU and I believe the most important signal from the sensors that tells the injectors when to fire is the Cam Position sensor. The injectors are triggered by a pulse to ground from the ECU but the positive side of the injector circuit comes through the master fuel system relay. The ECU can trigger things to ground all day but without the positive battery voltage from the master fuel system relay nothing is going to happen.

The master fuel system relay supplies the positive battery voltage - full time - to each injector on the green wires on each injector. A quick check there to see if you have battery voltage on those wires with the key on will go a long way in narrowing down your problem.

...Lee

Post by **t5rrr** » 30 Oct 2011, 21:14

Thanks Lee, I went and tested the green leads on all 5 injectors against battery ground, with Key On, and they all came back with 12V.

So....Camshaft sensor? Will there be error codes expected if the cam sensor fell flat on its face?
Right now there's no error (1-1-1) on both A2 and A6 test ports. That being said, the battery was just unplugged for charging after my initial (failed) attempts to start the car, so maybe any codes registered may be temporarily cleared.

Post by **Ozark Lee** » 30 Oct 2011, 22:44

Posting a code for a bad cam sensor is a "Catch 22" thing on these cars. The ECU can set a cam sensor code if the signal get erratic but if it dies suddenly it will never post the code. What the ECU does is compare the cam sensor pulses with the RPM sensor pulses and makes sure they are in sync. The problem is that it won't even try to deal with it unless the car is running and the car will not run without the cam sensor - Catch 22.

...Lee

Post by **t5rrr** » 31 Oct 2011, 06:36

I guess it's time to hit the junkyards to gather up a bunch of cam sensors (and as many of other sensors I can find) for trial and error!

Crotext · Post by **Crotext** » 23 Nov 2011, 08:35

t5rr: Have you found the problem? Was it with the cam sensor? Too many posters fail to come back in the end and advise us what the cure was, if indeed it was ever found. I am having similar issues with my 850 so maybe you could cut some time and money off my experiments. It would certainly be appreciated. I am in the driveway and it is winter.