Fuel pressure dropping after shutting off

[SamW]

Owner of a 4.6is for a couple of months now and decided to ask for some tips here.

The initial issue was the car running lean, hesitation to start, lack of power and some intermittent stalling after start up. The cause was finally narrowed down to the fuel system. After shutting down the car the fuel pressure drops almost immediately. While compressing the fuel line the pressure stayed constant. I think it would narrow it down to the check valve leaking somewhere.

My question is related to the check valve's location, is it in the fuel pump? And if it is, should I get only the actual electronic pump replaced or the whole delivery unit? And what about the pressure regulator in the fuel filter, could it cause similar issues? Or is there any other potential valve's which could cause this kind of problem?

Thanks is advance!

[andrewwynn]

I had the same symptoms it ended up being the tiny o-ring between the FPR and the fuel filter.

I replaced the FPR/filter but when I did post mortem found the o-ring was completely flat not round anymore.

In a case like yours my MO would be buy the FPR/filter from a place that takes returns then also get the o-ring so you are prepared either way. Swap the o-ring and test before swapping the FPR.

Unfortunately I don't recall taking a size off the o-ring but I might have taken a photo with the caliper I'll try to find it.

[andrewwynn]

The outer o-ring wasn't leaking or I'd have had fuel leaking under the car.

The inner o-ring I think allows fuel to leak back to the tank through the return line.

Glad I found those photos I want to replace those o-rings on my car before I develop the same symptoms.

[SamW]

Appreciate the measurements!

I actually have the fuel filter already on hand and was about to install it some time ago but didn't have long enough extension arm back then for one of the cover bolts. I'm going to have a go on it probably this weekend again.

But now when looking at it I am not sure where the o-ring is located exactly? I mean there seems to be two kinds of filters if I'm not mistaken. The one I purchased looks like the one in the pictures below. Also identical to the one present in the Pelican Part's fuel filter replacement article for E53.

Then again some of the filters I have seen look like this:

Maybe it's different for M54? But Realoem still mentions "fuel filter with pressure regulator" for my car, but is it just built inside the filter?

[andrewwynn]

Filter and FPR are combined but usually they will come apart. For the price difference no point in buying just the FPR.

I don't even recall the mechanism that holds them together. It wasn't difficult that I recall.

If you already have the filter+FPR just do what I did and replace; confirm that pressure holds.

Take apart the assembly and I'm confident you will see an o-ring with a cross section that looks more like a D than an O. The outer edge was completely flattened in my case.

FYI I just see broken links not photos from your post.

[SamW]

Will do, I'll report back for the results. Thanks.

[andrewwynn]

Follow up; estimate the hours on your fuel pump.

Take the odometer reading and divide by the average mph example 130000/24=5417

Fuel pump should last about 5000-6000 hours.

If you are getting close it's time to replace.

When the pump gets old it will drop in performance but that will be masked by the FPR. Only if you tap into the fuel line going into the FPR you can actually get a sense of how worn it is.

It is possible both pump and FPR are worn out but it's almost assuredly the FPR o-ring causing the primary symptoms.

[SamW]

If the fuel pump has a service life of 5000-6000 hours it should be fine unless there has been some kind of premature failure since the current one should be at something around 2000 hours at the moment.

But I'll change the filter in the weekend and see if that has any effect.

Also added the pictures as an attachment to the original post, the left and the middle one look like the one I have, the picture in the right is the filter I'm referring as the M54 one in text.

[80stech]

Don't worry about the o-ring just change the filter/pressure regulator. It's just as likely the regulator itself is leaking and would probably be a good idea to change the filter anyway. The check valve in the fuel pump leaking has little to do with mileage or hours and still could be a problem.

[SamW]

Thanks for the tips guys. Changing the filter this weekend so atleast it can be ruled out if the issue persists.