Fuel pressure issue on XC90 V8

[Wonko]

Hello. Just joined so please be gentle. I couldn't find the info I'm looking for searching on here so here goes:

I have an intermittent engine hesitation / stuttering issue upon acceleration. I've determined that it's a fuel pressure issue. I have been able to log the pressure when the issue occurs and there's a definite problem.

Trouble is, I can't find out:

1. whether the fuel pressure is supposed to be constant (I suspect it is, other than under engine starting condition)
2. what it is supposed to be (Vida seems to suggest contradictory values of 380kPa and 480kPa).
3. Whether pressure regulation is completely governed by the ECM using the pressure sensor and PWM to the fuel pump or whether there is an additional fuel pressure regulator somewhere.

If anyone can answer those questions I'll be very grateful.

What I am seeing is when the problem occurs is:

1. I increase power demand with accelerator pedal
2. Revs increase and fuel pump duty cycle increases, pressure remains more or less constant
3. Fuel pressure suddenly drops to zero (this is when the hesitation occurs)
4. Simultaneously, duty cycle goes to 100% to try to bring fuel pressure back up.
5. Fuel pressure suddenly spikes (when the throttle closes) and quickly returns to "normal"..
6. This is weird: fuel pressure continues to rise in a linear fashion even though the throttle is closed and the revs are dropping after lifting off completely. Duty cycle is zero to try to get pressure back down.

Pictures speak louder than words:



Additional info: this only happens when engine is warm and gets progressively worse as the journey continues.

Any thoughts / observations welcomed. Thanks!

[SwissXC90]

I would suspect a blocked fuel filter

I had a car where the fuel pump/filter was clogged

It drove OK under light throttle, but when I needed power and opened the throttle, the fuel flow couldn't keep up with the demand and the pressure dropped, reulting in sudden loss of enginer power, stuttering, and even stalling.

On that car, it turned out to be crap in the fuel tank, and removing the tank, cleaning it out and fitting new pump and filter solved the problem for good.

So that's where I'd start:
1. New fuel filter
2. New fuel pump

And sorry, I have a D5 diesel so I cannot give you and fuel rail pressure figures....

[Wonko]

Thanks for that.

I am definitely going to replace the fuel filter first. I don't think that the problem is coming from there, but it won't hurt and I don't know when it was last done. I'm reluctant to buy a new pump until I'm sure the fault is there because of the high price.

I have found the answers to a couple of my questions by trawling through Vida:

1. I believe the pressure should be kept constant.
2. Both pressures (380 and 480) are valid. The former is relative to atmospheric pressure and the latter is absolute, so 380+100approx.
3. There is no other pressure regulation device, just the ECM with pressure sensor providing feedback for the control loop. However, there is a by-pass valve that opens when pressure is too great. Maybe this is contributing to the strange pressure graph I am seeing.

[Clan]

Originally Posted by Wonko

Thanks for that.

I am definitely going to replace the fuel filter first. I don't think that the problem is coming from there, but it won't hurt and I don't know when it was last done. I'm reluctant to buy a new pump until I'm sure the fault is there because of the high price.

I have found the answers to a couple of my questions by trawling through Vida:

1. I believe the pressure should be kept constant.
2. Both pressures (380 and 480) are valid. The former is relative to atmospheric pressure and the latter is absolute, so 380+100approx.
3. There is no other pressure regulation device, just the ECM with pressure sensor providing feedback for the control loop. However, there is a by-pass valve that opens when pressure is too great. Maybe this is contributing to the strange pressure graph I am seeing.

There was a problem with fuel pressure sensors in The R models , It might be the same sensor if on the end of the fuel rail ...

[Wonko]

Originally Posted by Clan

There was a problem with fuel pressure sensors in The R models , It might be the same sensor if on the end of the fuel rail ...

Yeah, I'm starting to wonder if there isn't a problem with the sensor also. And it does sound like the same sensor. The values it is reporting are all coherent, but that's not proof that it's not faulting in some way.

Having just been out to log some more, what I can't figure out is why the fuel pressure continues to rise when the car is coasting. Unless, like you say, the sensor is faulty.

In the following, the hesitation doesn't occur, but look at the fuel pressure as I lift off and let the car coast:



Duty cycle drops right off, but pressure continues to rise. Maybe this is normal behaviour? I noticed the duty cycle never seems to drop below 25%.

[SwissXC90]

You are doing all the right things posting your theories and evidence here. If I had a V8, I'd be out there in a flash with VIDA to log my data for you.

But I have a D5... sorry...

You WILL find the problem with the assistence of the community here. For sure.

[Wonko]

Thanks.

I believe that the rising pressure is normal. I had initially mistakenly thought that the duty cycle went to zero, but it drops to 25% so the pump is still pumping. When coasting, the injectors are closed to save fuel so the pressure rises constantly until you start using fuel again. If the pressure gets high enough then the by-pass valve opens and the fuel is returned to the tank.

I'm going to check the electrical connections on the fuel pump but what a faff to get at it! I've had to remove most of the trim from the boot and all the rear seats.

It's a pity it's so difficult to remove the rear seats, it seriously increases load space!

I'm also going to wire an LED in parallel to the pump to see if it loses power when the issue occurs (faulty module) or whether it still receives power in which case it's definitely the pump failing.

[Wonko]

To close out this thread, the issue was resolved with a new fuel pump.

New filter made no difference.

The LED showed that voltage was arriving at the pump when the pressure dropped so I deduced that the fuel pump was the likely culprit.

For anyone considering changing the pump, you will need a special tool to unscrew the pump and sender retaining rings. You can get one on amazon for around 10 quid.

Also, it's important to fill the new pump with fuel before you install it or it just won't pump anything. Ask me how I know...