How to tell you have a clogged fuel filter... via VCDS

Matt-98AHU

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I encountered a low power issue rather suddenly on an 05 Passat wagon that's in the family while it was in my possession yesterday for other work.

It included a little bit of a judder, similar feeling to a misfire under load.

As I was poking around some measured value data (including comparing Ross-Tech definitions vs. what I got from VW via ERWIN) I come across block 23.

VW calls it "unit injector solenoid valve operating times".

Under light loads, the numbers vary, average around 40. Under load, the numbers quickly switch negative. Usually further negative than -100.

Decided to compare those numbers with my good-running BRM (PD is a PD is a PD...). Numbers stayed very consistent mostly between 40-50 even upto WOT.

I replaced the fuel filter, power returned and the numbers now consistently stay in the 40-50 range just like the BRM.

So, what do the numbers mean?

This might be a clue:



Taken from the VW Self Study program which was still available for me to download when I was working at a VW dealer. Found in their Online Training.

So, I suspect that the numbers might be a variation of ECM expectations. When the numbers go negative, the solenoids might actually be reaching the closed status faster than usual because they're fighting less pressure internally due to fuel restriction or even air in the system.

So, while PDs don't have needle lift sensors to know precisely when injectors are popping open, it does still have a decent idea based on knowing exactly when the solenoids on the injectors close fully... that is until you get oddly low fuel pressure from something like a lift pump or weak tandem pump... or your bad mechanic deciding to let your 20k fuel filter interval stretch closer to 60k like I did. :rolleyes: In which case, since the injectors still pop mechanically based on fuel pressure, as the plunger is slowly depressed and the solenoid closes, it might take fractionally longer to build adequate pressure to pop the injectors than it would under normal circumstances, in which case actual SOI may lag when the computer actually thinks it's happening, unless it can compensate some for seeing the much lower time to get the valve fully closed.

Anyway, just thought it was an interesting bit of tech knowledge on PDs to share.

FWIW, there are a couple threads elsewhere on the interwebz that mention MVB 23, but still not very complete information out there. Hopefully this adds a little clarity, even though I'm not completely sure that my interpretation of the reading variations are correct.
 
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Tdijarhead

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That's an interesting bit of diesel sleuthing. I suspect as more guys come across this post and have occasion to check your conclusions they might be proven right. We may even eventually find out if the numbers are different for a "slightly" plugged filter vs say one that is so bad the car will barely start. Great post by the way.
 

oilhammer

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Excellent post! Since I know so well how PDs behave with neglected fuel filters, I've never thought about looking at data via scan tool. But this is some good bit of sleuthing for sure!

I will say that the PDs have very little tolerance for fuel filter neglect, and I think it is because it is "normal" for a bit of either engine oil or combustion soot, not sure which, to pass into the fuel system, and eventually get caught in the filter. Because I've seen PLENTY of ALHs go well beyond the 20k service interval for fuel filters, with seemingly no ill effect, but BEWs and BHWs, running the exact same fuel filter, often start to exhibit this chugging and power loss (feels like a bad plug wire on an old gasser) at in some cases 23-26k miles on the filter.

And with the BRMs, since they use a cartridge style, you can SEE the black gunk on the filter elements, whereas on CRs that use the same filter, you never ever see that, not even a little. And I have had BRMs start having similar problems after running the filters 2-5k "bonus miles" too.

Not to say one couldn't have a bad load of fuel gunking the filter up early, but from what I have seen if you change the filter at the specified 20k mile interval you are unlikely to ever have a problem.
 

Matt-98AHU

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I think why this BHW went much further on the filter than usual for a PD is because the injector seals have been replaced when I swapped in a set of good used units to fix a rough running issue caused by the previous owner's use of crappy California biodiesel.

Takes it a bit longer to get clogged!

Also, here's the measured value info in question I got for PDs from VW pertaining to this post.



Just the fact it references BIP (Beginning of Injection Period) correlates with what the self study program talked about.

Maybe I should retitle it "How to tell you have low fuel pressure in your PD via VCDS."

Very conceivable that a lazy or dead lift pump and/or tandem pump could also cause similar discrepancy in the MVB 23 BIP readings. It would at least help you quickly narrow down that it's a fuel system issue as opposed to something else without even hooking up pressure gauges.

Though once you've gotten that far, you'd still need to hook up pressure gauges to verify where the issue is if at first a filter replacement doesn't improve things.

I like finding new ways to easily verify that whatever work I've done has made a change/improvement. :)
 

Vince Waldon

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Agreed, excellent sleuthing.

Will add my voice to the chorus of "if your PD engine's a skipping or hesitating or a bit wimpy when passing...check the fuel system end-to-end".

I've seen it cured with just a new fuel filter (pulled the old rusty one apart... 75% jet black paper inside) and just recently fixed my daily by replacing the in-tank fuel pump motor. Heaven only knows how long it's been out...the car has always had the odd miss every once in a while which I initially took to be turbo lag... but discovered the dead pump when doing a diesel purge and a new pump motor took the skip away immediately.

My poor tandem pump must be breathing a sigh of relief... I wonder if I could have seen the poor fuel flow using the analysis above. :)
 

Matt-98AHU

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For those who are unaware, PD injectors still have mechanical pop points just like VE engines.

All the solenoid is doing is closing off the fuel's path to the return side of the fuel system, forcing the pressure to build inside the injector until it mechanically "pops".

So, while there is an electric component to PD injectors, they are not entirely fired by computer control like a commonrail.

I re-read my post and found I didn't make mention of this, I just referenced the lack of a needle lift sensor which was how the VE engines knew what the start of injection is since they also use mechanically popping injectors.

I'm fairly certain the early respondees to this thread were already aware, though. Plenty of smart folks here :D
 

oilhammer

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There are just too many to list....
Regarding lift pumps: I have had some quit running driving down the road, and get towed in, with a dead one. Then I have had others that I only discovered they were bad when I had to open the system up for something (usually a filter, but sometimes having to remove the tandem pump for whatever reason) and simply could not get the cars started again.

And recently I had one that seemed to me to be down on power (BEW manual NB), but could not really pin point the problem. MAF, boost, etc. all seemed fine, actual was matching requested. And it did NOT have the signature stutter and signs of a neglected fuel filter, of which I knew for sure was not the case as this car is a regular here. Just by dumb luck/curiosity I decided to listen for the lift pump to run during preglow... and heard nothing. Did the funnel in the fuel filler hole trick... heard nothing. Checked for power/ground at the lift pump connection, good. So, put a lift pump in... that car ran like it had all its 100hp again. Only one I have ever experienced exactly like that. So, I bet this dataset and informed analysis would have yielded something. I'll know for next time!

Anecdotal note: replaced lots of BEW lift pumps. A handful of BRM ones. But never once on a BHW, ever. They must be different/better?
 

Windex

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My BHW lift pump was dead for what I think was about 6 months due to a bad relay - only symptom was an extended crank cold when the fuel tank was below 1/8th full. Other than that you would not know it was non-functional.

Replaced the relay and it starts on half-a-crank ever since.
 

Vince Waldon

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Regarding lift pumps: I have had some quit running driving down the road, and get towed in, with a dead one. Then I have had others that I only discovered they were bad when I had to open the system up for something (usually a filter, but sometimes having to remove the tandem pump for whatever reason) and simply could not get the cars started again.
As I prepped to replace mine I learned there were two styles of BEW lift pump assemblies... an early version that will bypass when the pump is dead (so the car stays operational) and a later version that doesn't bypass .(leaving you by the side of the road :) ).

I also learned that you can no longer buy the first style... which seems sad to me... so opted to track down just the pump itself and spend a wonderful half-day disassembling and re-assembling carbon-saturated diesel fuel-covered brittle plastic parts.
 

Matt-98AHU

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Well, as luck would have it, a BEW showed up in my shop today for a couple things, one of which was hard starts when fuel level is low.

Check lift pump, sure enough, deader than a doornail. Doesn't make a sound. Hooked a container up to the fuel filter, ran basic setting 35 to run the pump for awhile, didn't move any fuel whatsoever. I get battery voltage when probing pins 1 and 4 at the pump with the relay switched on via basic setting 35.

Early style pump that allows the tandem pump to still pull enough fuel to work.

Here's what the BIP looks like at idle:



New pump is on the way, will check and see the difference the numbers make with the proper lift pump.
 
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imo000

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My '06 Jetta was down to half the power when I decided to replace the fuel filter and all the lost power came back. The filter was black with bits falling off when I removed it. It was well over due I know. On the other hand, my '00 I'm pretty sure had the original fuel filter when I sold it with 600K Kms and it ran fine. The filter housing looked old when I got the car at 200K kms and then never changed it because it ran fine.
 

Matt-98AHU

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The new pump didn't make the numbers as significantly better as I was expecting, but you definitely can see the improvement.

That is interesting to note, though, that the fuel filter causing high load performance issues, the BIP numbers still looked relatively normal at idle.

Whereas with the dead lift pump, even at idle the numbers are worse, but the improvement in the numbers was not as noticeable as I thought it would be with the new pump.

Also probably worth noting that the injector o rings are likely pretty old on this car, whereas the o rings on both the BHW of this original post as well as my BRM are fairly new.

The old lift pump on this BEW was of course all kinds of blackened...

EDIT: Forgot to note that these numbers were after I had warmed up the car. The numbers from before the lift pump replacement, the car was still pretty warm then as well, so just out of curiosity, I watched the numbers the whole time during warm up. The deviation cold vs. engine fully warmed up (outlet temperature above 90 C, fuel temp above 70 C) was minimal. I'm sure that's because these are deviation from mapped numbers and it likely takes fuel temperature data into account in the mapping...
 
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Brian's96TDIPASSAT

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I have only felt a restricted fuel filter on our 05 Passat once, since then it gets one every 15K. I will keep this in mind when my cousin comes by with his 06 Jetta, he tend to push it to at least 20K.
 

peiphil

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found out on my dodge cummins the black in the fuel filter was from the carbon brushes wearing down in the fuel lift pump !
My bet is these vw pumps are the same.
 

Matt-98AHU

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I'm sure old injector o rings have something to do with it, too...



Not the first time I've seen that lower o ring compromised on a PD.
 

compu_85

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I just checked this on our V10 - we'd noticed some extra exhaust smoke, and some loss of power. Sure enough... looks like our fuel filters are overdue! Here's the injector timing data under part load:



(control unit 31 is a virtual control unit that lets you see both cylinder banks at once)
 

Matt-98AHU

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Well get to replacing those suckers and show us what the difference looks like when done! :D
 

compu_85

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I will - I need to check my parts hoard and see if I have a pair, otherwise I'll get some from the dealer. FWIW the filters on 04s are specific to 04s :)

-J
 

JETaah

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I have just such a problem with a BHW in the shop...low load jitters and less than typical power.
The numbers are hovering around +-0 at idle and dive down to as low as -45 at WOT.
 

Matt-98AHU

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I have just such a problem with a BHW in the shop...low load jitters and less than typical power.
The numbers are hovering around +-0 at idle and dive down to as low as -45 at WOT.
Change the filter and note any difference?
 

JETaah

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It's a brand new filter, lift pump shows consistent 7-8 PSI, tandem pump is 55PSI @ idle and 115 PSI @4000RPM. Hmmmm............ Thoug the needle bounces a lot at 4000 rpm. I don't have any restrictor in the gauge line.
Any other ideas?
 

Matt-98AHU

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The pressure sounds about right per specification.

I wonder if injector o rings could be leaking enough to not show a significant pressure drop at the tandem pump but cause pressure loss in the injectors, enough to at least cause the symptoms you're seeing, along with the BIP values to go negative.

Maybe also recheck the rocker arm adjustment to the injectors? Just a stab in the dark.
 

JETaah

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The pressure sounds about right per specification.

I wonder if injector o rings could be leaking enough to not show a significant pressure drop at the tandem pump but cause pressure loss in the injectors, enough to at least cause the symptoms you're seeing, along with the BIP values to go negative.

Maybe also recheck the rocker arm adjustment to the injectors? Just a stab in the dark.
I will have to dig into this.
Are you meaning that the fuel supply rail is leaking into the return rail because of faulty o-rings?

I did witness the group 023 numbers go south by pinching off the fuel hose to the tandem pump supply with pliers.
You are definitely on to something.
 

Matt-98AHU

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I will have to dig into this.
Are you meaning that the fuel supply rail is leaking into the return rail because of faulty o-rings?

I did witness the group 023 numbers go south by pinching off the fuel hose to the tandem pump supply with pliers.
You are definitely on to something.
It could be to return, it could be to combustion chamber (though it also has to get around the big copper washer, which will definitely slow it down a lot). I definitely have seen my share of lower o rings that seal the pressure/feed side from the combustion chamber/copper washer area degrade badly, but often times don't show very adverse symptoms.

I think the book says to clamp the return side if you have low pressure at the tandem pump, and if the pressure increases back to spec it's indicative of bad injector seals.

Similar principle when checking MVB 23 I would assume, but I suspect *any* car would see a jump positive in MVB 23 when you clamp the return line, so it may not necessarily be indicative of anything definitive for diagnostic purposes.
 

JETaah

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Thanks, Matt.

I will try the return fuel clamp and check the numbers. I don't want to needlessly remove $50 worth of one time use hardware to find out everything looks alright.

The source of the jitters wound up being an EGR problem.
 
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