No engine power..fuel issue ?
 

Hi,

yesterday I started my 928S (CIS Kjet engine) after 2 weeks since I renewed my wheels. Car run fine 2 weeks ago.
Now when car was started my idle dropped from 1000 rpm till 400 rpm and engine ran very rough.
I have mounted a permanent fuel gauge to my WUR so easy to check in case of issues : instead of fuel pressure raising from 1.2bar (17 PSI) to 3.5bar (51 PSI) when warming up the first minutes, it remained around 1.2bar !!
When driving, I could not drive harder than 55Mph !...at some point my max speed when pressing throttle all down was even 25Mph....I have had to keep throttle pressed to raise my idle in orde to keep engine running

So I suspect blocking fuel line. At WUR ? At Fuel pump ?
I would say WUR inlet screen would be blocked by crud...but wouldn't this rather raise my fuel pressure ?

Anyone experience with this kind of fuel issues ?

Is your system pressure OK? Read it at the WUR inlet - if its OK, that would sound like a problem internal to the WUR - allowing too much flow out to the tank. If system is low at WUR then supply out of distributor head is probably the problem. Or could be a total supply problem caused by pump and/or filter up the back?
jp 83 Euro S AT 57k

This sounds very similar to the issue I had when the pressure valve in the fuel distributor became stuck. I was getting very high pressure (145 psi) into the WUR but limited flow, so the car would go full lean under any load and not even rev beyond 2,500 RPMs.

The pressure valve is the 16mm head on the backside of the fuel distributor. If you remove the screw with the springs, you should also be able to remove the little valve with a small magnet - it should easily slide right out. If it's hung up or lodged in there that could be the issue.

As mentioned before, first thing you need to know is the system fuel pressure where it enders the WUR (the 10mm bolt). The 8mm bolt is the exit from the WUR to the fuel distributor.

Also, make sure you're getting 12 v at the wires on the plug into the WUR when the key is switched on (even without the engine running).

I have to check this, but 2 weeks ago, my system pressure was just fine ( about 5.8bar or 84 Psi )
Remember that my WUR is new since couple of months and I also replaced my pressure regulator O-rings at the same time together with a new
original Bosch fuel pump.
So I drove my 928S for 2 months without any issues...
I just had a minor issue 1 month ago when using injector cleaner ( add to fuel) and causing 1 injector to become clogged by loose debrie
But I unclogged it with some carburettor cleaner.

Okay I will check both control and fuel pressures. But what I already can say is that my control pressure does not raise gradually from 1.2bar to 3.5 bar
when cold engine is just started. It will be stuck around 1.5 bar , even after 2 minutes, so my control pressure is in fact too low.

I did the following tests :
- system pressure was 5,8bar (84 PSI) => OK
- WUR resistance was 22,4 Ohm => OK
- connector to WUR gave 12V when cranking => OK

So I am out of options now,
But I just received info from a friend of mine (Allan from France) that it could also
be the magnetic hot start valve that is not working properly and can lower my fuel pressure
He advised shunting the valve..

Low control pressure will make it run very rich as it warms up, and probably choke it when hot. If you have the hot start valve for lowering control when half hot (mine does have it), locate the temp sensor for it (on the heater outlet at rear of right side head) and check its resistance - should be open circuit except in the temp range specified (forget, but > 50C?) If you suspect this, disconnect its wires from the solenoid. This sensor& solenoid should only see power when cranking. The solenoid maybe leaking - IIRC there is something in the WSM on testing it.
jp 83 Euro S AT 57k

Indeed, I did not have this hot start valve on my other engine (4.5L euro). I will check it first. Since the fuel issues suddenly occurred when I started the car after 2 weeks ( 2 weeks ago it ran perfectly but it was very hot that day so perhaps temp sensor broke down due to high
temperatures ? )
I will recheck the procedure in the WSM for this hot start valve.

But disconnecting the wires from Solenoid won't make any difference if valve is stuck , not ?

If the solenoid is stuck open disconnecting wires will NOT help, correct. Do the test for leakage -P25-12b = detach and plug return line. Bridge pump relay socket . Valve/solenoid must no leak. Pic points at hard line connected to banjo bolt on top.
jp 83 Euro S AT 57k

Here are the results of my HOT START VALVE test :
- resistance of the solenoid/valve = 25 Ohm => OK
- solenoid/valve is not leaking when bridging pump with return line detached => OK
- resistance of the temp sensor for solenoid/valve (on the heater outlet at rear of right side head) = open circuit @ 20°C (68°F) => OK
...
Anything else except WUR that I can test or pinpoint to my problem ?

Could a block fuel pump (blocked inner filter at bottom of tank) also cause low fuel control pressure ???

If your system pressure is OK (was 84psi, Very good), there is no problem with pump or filter. Are you sure you are measuring control correctly? Low control as it warms sounds like there is a problem inside the WUR - bimetallic strip not moving, internal leak past the diaphragm or similar.
jp 83 Euro S AT 57k

Control pressure is measured via a permanent gauge that is mounted on the inlet fuel line of the WUR
WUR was rebuilt by a professional 2 months ago.


https://cimg3.ibsrv.net/gimg/rennlis...d8d314182d.jpg

Does the control rise if you jumper the pump to provide pressure, and leave 12V connected to the WUR, but dont run the engine? The 12V should heat the bimetallic strip and lift the control pressure. If it doesnt do that then the WUR is not working as it should....I cant think of anything else to look at.
jp 83 Euro S AT 57k

Well apparently the WUR will only do a small amount of fuel enrichment, let's say 0,5 bar rise, via the bimetallic strip powered by 12V
So in case the bimetallic strip would not work properly or not at all, fuel pressure at WUR should still rise till about 3 bar instead of 3,5
I have the feeling that my problem is much larger ,,,Because when I start my car with cold engine ( fuel pressure at WUR is low) and I revv
the engine, it should do it smoothly, even with low fuel pressure at WUR This is not the case !
Stupid question perhaps : could it be that I do not get sufficient fuel flow at idle?
My pressure could be OK, but fuel flow coming from pump is something else, not ?
I am telling this because the first time I have this issue, I drove my car for about 30 minutes, but when trying to accelerate
I would not go faster the 55Mph. If I was lacking power. I also needed to push throttle pedal half way to keep it idling.

Perhaps a good idea to check if the entrance of my fuel pump ( which is also renewed) is not clogged? what do you think?