Fuel rest pressure

[Rustee21]

Losing more brain cells by the minute.

1981 NA model 320

The car will start cold but lags and stumbles until it fully heats up and then revs like a new born motor with no lag. I am testing my fuel pressure and when I turn the fuel system off the pressure drops almost instantly. I have checked the accumulator and it does not leak and also have replaced the fuel pump check valve. The cold injector is not leaking and neither are the fuel injectors. I have them out and jump the fuel pump relay and they stay dry. Fuel pressure still drops.. I have rebuilt the fuel distributor but not sure how that could relate to the loss in fuel pressure if the pressure regulating o ring is brand new and clean and in good shape.( Allen wrench on the side of the fuel distributor).

The car will not start if I let it sit for like 5 minutes until cold again. I took all fuel injectors out and cranked the engine and they are not spraying fuel unless I raise the plate by hand. It has a lot of pressure on it. But moves freely if the fuel relay is not jumped.

I am trying to solve the drop in pressure first but added all the additional info in case it correlates somehow and I’m sure it does.

System pressure- 72
Cold pressure-24 (I will re check tomorrow once cooled again)
Warm pressure 53
Ambient Temp 60F

Things I have tried:
Cleaned the WUR but did not adjust it
Flushed the fuel injectors
Replaced all O rings inside the fuel distributor I’m hoping the two on the middle rod did not fold. But I’m assuming I’d have issues with pressure if they did.
New in tank fuel pump and hoses in the back.

Could this all be caused by vacuum leak?

Thank you,

Mike

[epmedia]

Replaced all O rings inside the fuel distributor I’m hoping the two on the middle rod did not fold. But I’m assuming I’d have issues with pressure if they did.

Regarding those two middle rod O-rings being 'possibly' damaged, it would most likely be the lower of the two causing loss of rest pressure, accompanied by a very notable fuel smell in the airbox.

Check the accumulator again, by pinching the Small Diameter rubber fuel hose #4 Reference link.

[GDAus]

Losing more brain cells by the minute.

1981 NA model 320

The car will start cold but lags and stumbles until it fully heats up and then revs like a new born motor with no lag. I am testing my fuel pressure and when I turn the fuel system off the pressure drops almost instantly. I have checked the accumulator and it does not leak and also have replaced the fuel pump check valve. The cold injector is not leaking and neither are the fuel injectors. I have them out and jump the fuel pump relay and they stay dry. Fuel pressure still drops.. I have rebuilt the fuel distributor but not sure how that could relate to the loss in fuel pressure if the pressure regulating o ring is brand new and clean and in good shape.( Allen wrench on the side of the fuel distributor).

The car will not start if I let it sit for like 5 minutes until cold again.
I took all fuel injectors out and cranked the engine and they are not spraying fuel unless I raise the plate by hand. It has a lot of pressure on it. But moves freely if the fuel relay is not jumped.

I am trying to solve the drop in pressure first but added all the additional info in case it correlates somehow and I’m sure it does.

System pressure- 72
Cold pressure-24 (I will re check tomorrow once cooled again)
Warm pressure 53
Ambient Temp 60F

Things I have tried:
Cleaned the WUR but did not adjust it
Flushed the fuel injectors
Replaced all O rings inside the fuel distributor I’m hoping the two on the middle rod did not fold. But I’m assuming I’d have issues with pressure if they did.
New in tank fuel pump and hoses in the back.

Could this all be caused by vacuum leak?
Thank you,
Mike

Hi Mike ...

Looks like you might have a couple of problems here ....

First off your tests.

System pressure- 72 Good - it's acceptable to be 65~75 psi when tested ... if being set 68~71 psi
Cold pressure-24 Not Good - at 60^oF it should be 9.5~15.3 psi
Warm pressure 53 Good - at idle speed 49.5 ~ 55 psi
Ambient Temp 60F

So the stumbling during warm up is probably the high Cold CP. The higher pressure is stopping the metering plate in the Air Flow Meter to push the metering plunger in the Fuel Distributor high enough to give a richer mixture during warm-up.

I took all fuel injectors out and cranked the engine and they are not spraying fuel unless I raise the plate by hand. It has a lot of pressure on it. But moves freely if the fuel relay is not jumped.

Ok .. Perfectly normal ..... If you try and start the engine with the Fuel Injectors removed without anything plugging the injector holes ... you then have 4 vacuum leaks so there is no airflow through the AFM.

I am trying to solve the drop in pressure first but added all the additional info in case it correlates somehow and I’m sure it does. .

Hey ... the more info the better .. having to play 20 questions to find out what has or hasn't been done can get pretty frustrating at times.
But a couple of questions to clarify is good.

I have rebuilt the fuel distributor but not sure how that could relate to the loss in fuel pressure if the pressure regulating o ring is brand new and clean and in good shape.( Allen wrench on the side of the fuel distributor)

Replaced all O rings inside the fuel distributor I’m hoping the two on the middle rod did not fold. But I’m assuming I’d have issues with pressure if they did.

Do these 2 comments refer to the same thing?

Did you fully disassemble the fuel pressure regulator? or just remove the plug with the allen head in the end of the fuel pressure regulator?

If you didn't remove the regulating Plunger (3) and replace O ring (4) at the end of the plunger it may be deteriorate and leaking to allow the system pressure to dump when the engine stops. If you did replace (4), if it wasn't the ring size .. it may have rolled out of it's groove and again not sealing properly etc.

Cheers

Graham D

[Rustee21]

In reply to EPMEDIA:

i plug that hose and then i jumper the fuel pump relay and let it run for a bit. Once the initial fuel is dumped out of there i don't even get a trickle coming out of it. Fuel would keep dumping out of there if it was a bad accumulator correct? I don't want to take the distributor apart again haha putting those two middle o rings on keeps me up at night. I have a parts car so i will try to swap distributors and see if that solves the issue after i check the other recommended fixes. Thanks!

[Rustee21]

[QUOTE=GDAus;30597780]Hi Mike ...

Do these 2 comments refer to the same thing?
I am worried about the metering rod O rings. Those were a really tight fit and took some love to get the metering rod back in its place. I used lubricant as well.

Did you fully disassemble the fuel pressure regulator? or just remove the plug with the allen head in the end of the fuel pressure regulator?

If you didn't remove the regulating Plunger (3) and replace O ring (4) at the end of the plunger it may be deteriorate and leaking to allow the system pressure to dump when the engine stops. If you did replace (4), if it wasn't the ring size .. it may have rolled out of it's groove and again not sealing properly etc.

I replaced both of those O rings and checked them out again the other day and both seemed fine and clean but i will double check!

Thank you!

mike

[320iAman]

There is 4 items in Bosch CIS Testing - System Pressure, Control Pressure Cold, Control pressure Warm and Rest pressure.
~ 70-72 F
System Pressure 60- 72 psi
Control pressure Cold ~ 20 psi
Control pressure Warm ~ 50 psi
Rest Pressure 24 psi after 20 minutes----if it goes below this the WUR O-Ring on the Fuel Distributor Pressure Regulator is either not seating-wrong size or cracked or broken, the smaller O-Ring is involved with pressure in the Fuel Distributor that once exceeded sends excess fuel back to the passenger side gas tank. If the WUR o-ring is not good--this would cause warm start up problems, you checked the other fuel items.

What is your rest pressure ? Rest pressure shut off the fuel pumps( ignition key off) and watch the Fuel Pressure Gauge or check after 20 minutes .

My test results 2/5/20 and Temperature 52F

System pressure 71-72 psi
Control pressure Cold 16 psi , it would be ~ 20 psi at 70-72 F
Control pressure Warm 50
Rest Pressure 26 psi after 30 minutes

If the WUR O-ring on the Fuel Distributor Pressure Regulator is good and rest pressure is going down this points to the O-ring in the Warm up Regulator itself--I have changed these out before on more than one WUR , new they lower control pressure a couple of psi, they do get flat sometimes.

If both WUR O-rings are good Rest Pressure Good-then I suggest adjusting cold and warm control pressures. Cars Like rich startups cold or warm.

https://www.bimmerforums.com/forum/s...or-Adjustment&

Randy

[epmedia]

Randy, if the frequency valve was leaking internally, this can also cause lack of rest pressure, correct or no?

[320iAman]

No,, The Fuel Rest Pressure is trapped in between Big O-Ring(WUR O-Ring) on the Fuel Distributor Pressure Release Valve and the WUR O-Ring itself that is sandwiched in a grove in the Big Circular Disk underneath the Flex Plate in the Warm Up Regulator, if either one is too flat or damaged Rest Pressure will go down too fast and not be in specifications. The WUR O-ring on the FDPR Valve is the most common issue with Rest Pressure not being in specifications as fuel seeps by too much lowering psi too much in that line and effecting warm start ups. A new O-Ring in the WUR itself lowers control pressure a few psi yet will boost rest pressure a few psi from a flat one---two good o-rings rest pressure is in specifications and warm start up issues are not from there. Once rest pressure is determined then the next step can be done with the other CIS tests done as well.


Randy

[epmedia]

Thanks Randy

[320iAman]

Sure Robert,,One other item,been busy at other endeavors, if pressure is bled off from the Control Line going to the Fuel Distributor,,this would not make a lean warm startup, this make a warm too rich startup and the car engine would flood, the metering piston in the fuel distributor would move higher than it should be with the correct warm rest pressure this would allow more fuel in than is necessary and thus flood the motor. 20/50 is very good all around cold/warm control pressure for the WUR.

As Robert says, Rustee21 test your Rest pressure and then report back

Cars do like rich startups cold-warm-hot--yet not too much or too little fuel. Its something like 10.5-11 to 1 and then when fully warmed up 15-16 to 1 Air Fuel ratio. They Start up Rich and then run leaner. All internal combustion motors are this way - no exceptions.

Randy

[lorkp]

In case it's useful to anyone, here's the spec chart for control pressure vs temperature. It says 77-79, but I'm pretty sure it's applicable to the 80-83 cars:
20200325_075403 (1).jpg

stdWUR.jpg

OP, looks like your control pressure is a smidge high (though only outside the range by 1 PSI). To make the cold start richer, tap the recessed post on the left side into the body. It will make the metallic arm apply more downward pressure on the spring that wants to put pressure on the diaphragm—more pressure on the diaphragm, the higher the control pressure.

Note that this may alter your warm control pressure—it's a dance. Warm pressure can be increased by tapping the circular protruding plug on the right into the WUR body—diaphragm is now closer to spring and therefore more pressure is created. Tap it outward, from inside, to decrease control pressure. I've done the former but not the latter. You'd have remove the diaphragm inside so that you're tapping on the metal only, but that adjustment would be outside my comfort zone.

[Rustee21]

Thank you guys!

Busy weekend and than thanksgiving hit but i am back at it.
I took apart the Fuel Distributor Pressure Release Valve and checked both of those O rings. Both seemed fine, but i replaced with new ones I had (Viton) and put it back in with o ring lube on it to make sure they dont get caught anywhere. I took apart the WUR again and had some crud in the inlet mesh, I know there is more to it than the naked eye can see so i am soaking it in carb cleaner over night after i agitated it.

Rest pressure:

When i go to test rest pressure it almost immediately goes to 0. Drops steadily down and then after 6 seconds it jumps back up on the gauge to 20psi and then falls again to 0. I am going to put the WUR back together again but i have a feeling that wont change the rest pressure falling. There was no fuel inside the unit and the O ring seems fine. Should i replace it anyway or troubleshoot other areas?

The immediate drop makes me thing it has to be the accumulator since its a lot of fuel just disappearing... I tested it by plugging the hose on the end and jumping the fuel relay. some fuel leaked out and then it was dry.

Should i put the WUR back together and test again or order new O ring and then put it back together? Not sure if i need to since it was dry inside, but i trust the pros.

Thank you everyone!!!

-mike

[320iAman]

Thats a good idea yes make it new, also adjust the WUR's cold/warm control pressure. Rest pressure the psi is trapped between two o-rings-one on the Fuel Distributor Pressure relief spring valve and one in the WUR itself either one flat or damaged will lose rest pressure too much, see the link I posted in Post #6-Adjusting cold/warm control pressure and in that link see WUR Servicing with Pictures Link.

Rest Pressure should go down slowly and stay steady at 25 psi or so for 20 minutes.

Randy

[Rustee21]

You think I could be losing pressure immediately from a poor O ring in the WUR?

[Rustee21]

I got the new O rings and replaced it and put the WUR back together. I am now reading about 18psi at 40*F so I am closer to the right pressure.

I am still losing PSI right after i turn the pump off immediately, It then has a little jump in pressure and then goes back down to 0PSI immediately.

New O rings on the Fuel Pressure Regulator and WUR and Still no pressure immediately after shutting fuel pump off.

Thank you!

[epmedia]

It's ok to pinch/kink that small rubber fuel hose on the accumulator while t-shooting rest-pressure issues.

Fyi 1): That small rubber fuel hose on the accumulator redirects fuel leakage (in the event of: internal accumulator fuel leakage) to the suction side of the external fuel pump. The only purpose of this hose is to prevent fuel leakage to the deck, in the event of accumulator failure.
Fyi 2): Euro model accumulators, instead of using a hose; had a screw plug with gasket for testing accumulator for failure (tell-tale plug).