Thread: Breakdown - P0191 Fuel Rail Pressure Sensor

you talking about the HPFP belt at rear of the engine which is almost impossible to get at or is there another belt you are referring to?

Originally Posted by shamirj

you talking about the HPFP belt at rear of the engine which is almost impossible to get at or is there another belt you are referring to?

He is. It’s not impossible to get at. Take a look in the Good Oil; I think there’s a write up on changing it with body on that will help.

Correct amundo.

Originally Posted by BradC

…………………………………. but frankly I can run my TDV6 with the LPFP fuse removed, so it's not likely that is the issue.

These figures are all based on a 2.7L TDV6 in a D3, but there close enough to the D4 to provide diagnostic assistance.

Pray tell us more, as I actually bought a spare LP pump (they didn't seem to be an off the shelf item) as I could hear mine whine sometimes on cold mornings in the garage which worried me. I hate these single point of failure things. On

my D2 TD5 I had an external one fitted, like the ones used on Bowler's. I actually rang Drew Bowler for the details, he was very helpful, a nice guy, unfortunately he has passed away now. The external pump also became noisy, making a whine when I test ran it every so often, I discovered that I could just use it for priming, then switch it off, as the cam driven injectors would suck the fuel through the pump, had to keep the rev's up though, which I found was quite fun, as involved more driving skill.

Am I to understand that the HP/Injector pump will suck fuel from the tank through the LPFP, much like the above arrangement?

Cheers,
Randy

Originally Posted by RANDLOVER

Pray tell us more

....
Am I to understand that the HP/Injector pump will suck fuel from the tank through the LPFP, much like the above arrangement?

Yep. Not saying I recommend the practice, and it did set a fault code (although no visible indication of it) when I really planted the boot, but I drove it around the block a couple of times with the LPFP fuse out while monitoring the pressures at the schraeder (not surprisingly a fairly good vacuum when getting the right boot down).

I did it as part of my "fairly extensive and masochistic" diagnostic prior to replacing the HPFP to prove the fault wasn't caused by a blocked fuel filter or dicey LPFP. The reason I would not recommend the practice is it can cause cavitation in the feed pump and that could well lead to more extensive damage to the HPFP, but mine will certainly start and run at nearly full noise with the LPFP not functioning at all.

The HPFP is 2 pumps in one. It's a brass vane feed pump coupled to (and feeding) the triple piston high pressure pump. The VCV controls the volume of fuel going to the piston pump by bleeding fuel from between the two pumps back into the input of the feed pump.

The LPFP serves a few roles.
A) The pressure regulator in the tank bypasses a significant amount of its output which is used to operate the scavenging venturi in the tank to ensure a reliable fuel supply at most sane angles of operation and diesel levels.
It ensures positive pressures as the fuel filter progressively impedes flow
C) By keeping a positive pressure at the input of the HPFP it ensures the system is mostly self bleeding
D) By keeping a positive pressure at the input of the HPFP it inhibits cavitation in the feed pump

When properly primed up, the vane feed pump could comfortably suck a golf ball through a straw, so with insufficient pressure on its input it'll boil diesel nicely.

If all the stars are in perfect alignment, your tank is full, filter is clear and all the pipes are nicely open you should be able to pootle around town with no LPFP, with the caveat you might well be damaging the HPFP.

Put a gauge on it and make sure it's working. I see a vacuum on mine at full noise, up hill in third gear. Pretty much at all other times there is a positive pressure (albeit sometimes it's quite small).

All that is to say that provided you have fuel in the tank and a relatively clear filter, a non-functional LPFP should not be the cause of a no-start.

have u checked the fuel filter? when was the last time the fuel filter was replaced?

doubt its the fuel filter that was just done in last 1000kms. my bet is it could be the hpfp belt but unlikely even though its had 100,000kms on it without a timing belt change as yet. more likely to be one of the two fuels pumps possibly the one in the tank at a guess if my sensor is cleared of faults. just waiting on the delivery of the new sensor but have doubts even this is the problem. it was worth a shot replacing this part. will report back once new sensor fitted re outcome but guessing it wont cure my problem...

got the new sensor for the fuel pressure line and fitted which made no difference, still will not start. cranks but does not start.

thinking now it has to be either of the two fuel pumps, either the cheaper pump in the fuel tank, or the more expensive pump in rear of engine. the HPFP belt could also be snapped but unlikely to me.

upon ignition on, there is no fuel pump noise or any evidence the pump is actually pumping fuel through the system, my guess is that is the role of the LPFP in the tank? based on this i think it might be the in tank fuel pump that has died.

upon further investigation with my code reader, the fuel pressure in the rail was found to be 'high' which kinda doesn't make sense to me if the pump is dead. Could there be some other explanation as to how could the system have higher then normal readings for the pressure in the fuel lines. This was the code read from the hand held car reader. looks to be a tow to the mechanic to sort out as i have no idea.