B5.5 Passat Sudden Rough Running

Hi everyone,

I'm new to this forum, just signed up to ask for a bit of help with my Passat, which has recently developed a running issue. My sincere apologies if this problem has already been clearly diagnosed elsewhere, I'm not very forum savvy.

I have an old 2001 B5.5 Passat TDI that was running like an absolute gem. A week or two ago, I noticed that on any given day, when I first got onto the motorway and put my foot down, I would get a few little coughs, reminiscent of fuel starvation on an old carb engine. These coughs would cease if I eased off the pedal, and after a few minutes of taking it easy, I can put the foot down again, and it is fine for the rest of the day.

Then, this week, as I was driving around the oxford ringroad, it did it much worse, to the extent it felt like I was driving on three cylinders (eg: if an old petrol car did this, I would have checked the HT leads), with lots of vibration, and obvious miss, and lump idle.

The next day it was fine again, but since then it has come back a few times while driving, usually starting with a few coughs. At slow speeds it feels like running on 3. At fast, it feels like I'm driving on very aggressive rumble markings.

I've also now discovered that stopping, turning the car off, waiting a few minutes, then starting again temporarily solves the problem.

I'm waiting on a friend to come round with a scanner in a few days. In the meantime, does anyone recognise my issues?

Many thanks for taking the time,
Matt

I'm not very familiar with the 01 b5.5, since here we only got the 2004-5 (afaik) which is a PD engine. I believe that yours would be a ve pump engine, like the ALH here. Does the check engine light illuminate? Have you changed your fuel filter recently? Restriction in the fuel line can cause similar issues, there's also the issue that was very common in the brm (05-06 Jetta here) where the turbo vanes would travel too far and hang up a bit, causing a delay and shudder, but normally that's temporary.

Does the car smoke any more than normally?
Another thing could be mechanical failure in the engine, as in rings or valves causing lower compression. If you open the oil cap when it's running, does it have a lot of blow by?





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PD engines sometimes start to stumble like what you describe when the fuel filter is plugged. They are a little different from the old rotary pump motors that just get slow when the filter is restricted. And the fact that shutting it off can temporarily alleviate the issue is also consistent with a fuel system restriction, since it relieves fuel demand temporarily.

The vibration and apparent misfire could also be cam/lifter wear, though it's unusual for symptoms from that to come and go as yours do; typically that will result in a constant misfire. Still a possibility though, as changes in operating conditions could change the effect if it's right on the margin.

I think by '01 you had the earliest PD engines in your market available in the B5.5 cars. If it's not a PD, then my suggestions above are both moot. AHU/AHH type engines might have still been around at that time. Are you able to confirm whether your car has a PD engine or not? PD will not have a visible external fuel injection pump on the driver's side front of the engine, while the rotary pump style engines would.

Hello! Thank you both for your responses, and sorry for taking so long to write back - it's been a very unusual few days here with the COVID lockdowns coming in, and fixing the car temporally drifted a bit down the to-do list.

Mozambiquer - My engine light does not illuminate, but in I also note that it doesn't light when I switch on the car, so possibly it doesn't work (or has been deactivated by a past owner).

I have not changed the fuel filter, but intend to asap.

The car doesn't smoke much at all, considering 186,000 miles on the clock.

I'll check for blow by tomorrow.

D24TDI - As you also mention the filter, I really will change this as soon as I can.

Regards engine type - I was looking under the bonnet today, and I did note what believe to be the pump in the top corner of the engine compartment on the passenger side (UK RHD), so the PD-type guess seems likely?

I borrowed a bluetooth ELM 327 gizmo today from a friend, but couldn't get it to connect. The same friend suggested that in Europe diesels weren't forced to adhere to this standard until 2003, so it's possible that this port is just decorative?

I also took the car out today on a few short runs - The first run to the supermarket was hassle free. The second run was delivering food to folks who are under isolation, and the car started all it's troubles. But after being parked for a few minutes while I was delivering the food, it ran faultlessly again home.

Thank you both, again, for your kind thoughts. Any further light you can shed would be greatly appreciated.

Matt

Do you hear the intank lift pump prime? or when you change the filter see if it will prime.

Hi 1.9glstdi - I'm not sure what I'd be listening out for in all honesty!

Hi QuickTD - According to the registration papers, my engine code is AVB.

Many thanks again,
Matt

AVB would make it an early PD100 engine according to what I can see.

I think fuel filter restriction is still your primary candidate. Checking the in-tank lift pump's operation as 1.9glstdi suggested would be a good idea too, on general principles, but PD engines generally tend to run OK even when those are dead. To my recollection some of the early European PD cars didn't even have lift pumps in the tank anyway, which might include yours. Someone more familar with the overseas cars might be able to confirm.

Fuel filter is a cheap and easy step and a good go-to anytime there's a driveability-under-load or power concern, particularly if it has not been changed in a while. And it's routine maintenance anyway so if it's due for it, you lose nothing by putting one on and seeing if it makes a difference. I'd try that first of all. Make sure you prime it with a hand suction pump (or with the electric lift pump if you have one and it works, and read the procedure for doing it), otherwise you'll send air downstream into the fuel system that may take a little effort to clear out.

If a new filter and the fuel system checks don't get you any improvement, you probably want to try to get a working scan tool on it and see if there are any codes stored for what it's doing, and try to watch a few measuring blocks when it acts up, as your next step before throwing anything else at it or letting us speculate further. VCDS is better than the general OBD tools but even a basic tool should get you some information about fault codes if you can get it to connect. If the car has an aftermarket radio installed, that can sometimes be the culprit for communication issues depending on how it was installed, and if so you might need to pull the radio fuse or remove the radio for it to start talking to the tool.

Hello again, thanks again for coming back with more thoughts!

I ordered a new fuel filter, which has just arrived. Yes, agreed, it's definable about time all the service items were done.

D24TDI - there is an aftermarket radio fitted, so I'll pull the fuse from that to see if I have any more luck with the gadgets. If not, I'll try and get hold of a VCDS system.

QuickTD - A disabled injector, which gets reset when the car restarts, does fit the feel like it would fit symptoms I'm having very well. I'll do the filter first because this needs doing anyway, then look into this & code reading gagets.

I'm going to see if the lift pump appears to be doing anything. If not, is there any good advice lying anyone can recommend for priming with a hand pump?

I'm rather new to diesel ownership and there's lots to learn...

Thanks again,
Matt