CP4.1 Hpfp

[notallbad]

Firstly, apologies if this is in the wrong category.

Very glad I stumbled across this forum, very knowledgeable individuals here who hopefully can assist.

My issue is low fuel rail pressure and highly suspect the pump to be at fault.

My understanding of how the fuel high pressure supply from the pumps head unit maybe somewhat simple but here goes.

The ecu controlled quantity control valve delivers the required amount of fuel to the delivery valve in the pump head where it is allowed to flow when the plunger is forced down by the plunger spring which causes a suction on the delivery valve hence opening the valve. When the plunger is driven back up the plunger compresses the fuel at the same time assisting the delivery valve spring to close the valve and thereby forcing the fuel out of the high pressure outlet by way of a one way valve.

Ok,so my questions are
1) possibly a dumb one but would you expect this to work at atmospheric air pressure? or are oil and oil pressures required? would I be correct in thinking the pumps outlet pressure should be greater than the pressure regulated in the rail? The outlet valve surely needs to be sealed closed so the suction can be achieved at a lower pressure otherwise it would be overcome by the higher rail pressure leakage. Suppose there would be greater pressure when the plunger is on the upward motion.
2) should the suction from the plunger be able to draw air/fuel from the high pressure outlet valve? I would think not but this is exactly whats happening which is why I believe the pump is causing the low pressure issue however is the fuel delivery valve pressure assisted from the low pressure supply side which is 5bar.
3) is the high pressure outlet valve replaceable - the only way I see possible to remove it is by drilling out a collar/sleeve which holds it in place, is there anoher way? I have done this with a spare pump head to gain access to the valve but not able to find one anywhere. Perhaps someone would know of a suitable replacement/solution. The sleeve/collar was a soft metal so relatively easy to drill out.

Many thanks in advance.

 

[BrianPetersen]

You are overcomplexifying your explanation.

At its heart it is a simple check-valve-in, check-valve-out plunger pump.

In the applications that I have seen, there is an upstream low-pressure pump that is supplying fuel into the case of the pump, which is also the "inlet manifold" for the pump plunger itself.

When the pump plunger extends outward (camshaft, spring pressure pushing it out) a check valve lets fuel pass from the pump case into the plunger cavity. "Inlet" stroke. There is a regulating valve in that passage to limit how much fuel is delivered (this is the "quantity control valve") but it basically just acts as a restriction to this "inlet" stroke so that less fuel can get into the plunger cavity.

When the cam pushes the plunger in, that closes the inlet check valve (it's spring-loaded closed to begin with), and then, when the pressure in the pump cavity exceeds the downstream pressure in the rail (plus the spring pressure to overcome the delivery valve spring, which is peanuts compared to the 2000 bar fuel rail pressure), the delivery valve (another check valve) allows it to pass.

It is no different conceptually from a bicycle pump ... just deals with enormously higher pressure.

As for the potential cause of low fuel rail pressure ...

Is the upstream low-pressure pump delivering fuel? On VW cars, this is the in-tank fuel pump. I would expect it to deliver 3 or 4 bar pressure. If this doesn't work, there is not enough pressure difference on the high-pressure pump's "inlet" stroke - the flow across that quantity-control valve - to let enough fuel through for the engine to run properly, if at all.

VW fuel sending units have a check valve in them which has been known to act up. Same symptom - low or no output pressure from the upstream low-pressure pump. Same outcome - low rail pressure and engine won't start.

Is your quantity-control valve working properly? Proper signal to it, proper response to the signal? I've rarely heard of this being an issue.

All the usual stuff ... clogged fuel filter ... air leaks ... etc.

By the way, these fuel pumps are unreliable garbage. Very, very common for them to blow up and contaminate the whole fuel system with trash. My initial reason for refusing to buy a newer VW diesel was the existence of that Bosch CP4 fuel pump. There are now other reasons, but that started it. And it isn't just VW that have had trouble with them. Usually the roller cam-follower stops rolling on the cam, and then mayhem follows shortly thereafter.

 

[BrianPetersen]

Here's a diagram that shows how it works.

https://www.drivingline.com/s3/driv...g?quality=70&mode=pad&copymetadata=true&w=800

 

[notallbad]

Thankyou for the quuck response.

Its a BMW with a no start.

Pre-supply pressure is 73psi/5bar on a gauge t'd in which drops slightly to around 4bar on cranking.

QCV is normally open, direct 12v applied and it closes. Reads around 2.5ohms. 2 wire fed, with 12v and 3 5v whilst disconnected, key in ignition on.

With qcv still on the car and connected:
unlock the car and get 12v on both sides, then the control side drops slightly to 11.7v. With key in and ignition on drops to around 11.3v and immediately rises to 11.5v then rises back up to 11.7v. Turn over and it rapidly jumps to 10v, 7v, 9v, 7v, 9v - seems to be when its nearly starting is when it goes up to 9v.

I realise this is a pwm signal but does show ecu control is present.

No leaks as far as I am aware and fuel filter changed and system bled for 10 mins by running the in-tank pump drectly fed 12v.

Yes have now become aware of the dreaded pump failures.

 

[TugboatEng]

9V is usually the threshold under which the ECM cuts out. What is the age of your starting battery?

 

[notallbad]

Just had a new battery fitted though cant remember if that was before or after taking these readings.
Also its possible the battery had drained from cranking.

 

[3DDave]

Also to look at - one can supply all the correct voltage in, but one has to check there is no voltage drop on the way out. A corroded or broken ground can leave enough of a drop that the 14.x battery voltage only produces 9 V across the ECM and 5 volts across the crappy ground. If you monitor across the battery while cranking you won't see the problem; one needs to monitor across the ECM.

 

[notallbad]

So to test that would I need to put a meter across the ecm power supply pin and ecm ground pin to monitor the voltage whilst cranking?

A good ecm and ground to it will maintain 12v whilst cranking, a suspect ecm unit or ground would be indicated by a substantial drop in voltage? What would be an acceptable voltage drop?

I realise that the ecm control signals rely on switching devices on/off by grounding the device which is the ecm ground so a bad ecm ground can affect device control.

 

[3DDave]

A good battery will maintain 14V during cranking. A weak battery will drop to 12. I would expect an ECM to fail in some aspects at 11 V.

 

[Compositepro]

A fully charged battery will maintain 12.7 volts. Only the alternator can create higher voltage for charging.

 

[3DDave]

Ah - right. So how much drop across the ECU, which was the original question?

 

[notallbad]

Shall take a look tomorrow

 

[BrianPetersen]

Is it tripping a rail pressure code? Is it setting any fault codes? What IS the reported rail pressure?

Is the engine trying to start, but not quite making it?

Do you have an OBD scanner with live data? It should be able to give you requested and monitored rail pressure and the duty cycle of that quantity adjuster.

 

[notallbad]

Yes.

4B90 - Low fuel rail pressure on start

Getting 4/5bar pre-supply

QCV around 33%

PCV around 35%

Rail pressure starts at 10bar, only rises to 15bar now but it was at one point building up to 180bar and trying to fire.

 

[BrianPetersen]

No leaks or stuck-open injectors?

If not - Sounds like it is knackered. Not sure if there are any official repair procedures outside of a Bosch workshop, but perhaps there are unofficial ones.

10-15 bar rail pressure is essentially nil.

 

[notallbad]

Yh that valve on the pumps high pressure outlet.

A leak test did not show any leaking injectors, at least not in that way. The cylinders do not appear to be locking up due to filling with fuel as it still quite happily cranks but not faster than you would expect i.e snapped chaindrive

Cant find a replacement. only a replacement head unit complete.

Something that has just crossed my mind is the possibility that the sleeve/collar/retainer just needs tapping in if it will tap in any further, Perhaps its been pushed out over time so the spring is no longer holding the valve against its 'seat' or the spring is tired and needs replacing.

Of course finely grinding the seat and polishing the valve head may also help but the only way I know to get it out is by drilling out the sleeve/collar and these are not off the shelf.

I dont generally work on cars other than simple things really like brakes, suspension, drop links and that sort of thing so this is certainly not.my forte working on fuel systems.

 

[notallbad]

So replaced the hpfp head and now getting the desired fuel rail pressure but still not starting, sounds like its almost trying.

What would be the next thing to do, leak off test now theres good pressure in the rail?

Any ideas please.

 

[TugboatEng]

Have you bled air from the system?

Use your diagnostic tool and check to see if the engine is getting a run command. Check to see if it's requesting fuel delivery. Verify the outputs or the speed/timing sensors.