2L BKD - P0234 Overboost + Limp Mode

[tdiblacky]

My bkd golf has started going into limp mode in 6th gear when accelerating on the motorway around 80mph and upwards. When I scan it I get code P0234 overboost.

From searching the forum here I see lots of threads about this and the general cause seems to be a sticking vanes vnt in the turbo. However, my car is used on a Mon - Fri daily 100 miles return trip of 90% motorway so could my turbo vanes still be carboned up with this kind of motorway driving? ie: very little short trips. The turbo is original I believe though and has 234k miles on it so maybe it's gummed up with just age?

I have a query regarding limp mode on this golf or VW's in general. Sometimes the car still has normal power and no EML on but yet when ideling it can't rev past 3000 rpm. This happened today actually, I pulled in and the car wouldn't rev over 3000 at idle, but it would rev over 3000 when driving through the gears, when I got home and parked, without switching the car off I could suddenly then rev it over 3000 when ideling. Very weird I thought!

Is there two types of limp mode with these cars? Sometimes with the EML on and sometimes with the EML off but complete lack of power and limited revs to 3000 sometimes? Can anyone clarify the versions of limp home modes for me please?

[Crasher]

Boost wise there are two limp modes, P0299 underboost and P0234 overboost, both give limp mode which resets after cycling the ignition. Underboost is almost always a leak in the vacuum system, sometimes a leak in the charge air system which gives black smoke with whooshing noises and overboost is almost always (99%) a carbon clogged VNT system and no, chemicals such as oven cleaner do not work.

[tdiblacky]

Cheers for the reply Crasher. In the 1% of cases where the over boost isn't the turbo's vnt clogged, what are the alternatives?

I put a Mityvac directly on the actuator and it has full easy travel. I disconnected the actuator then too and can move the lever easily by hand, they definitely aren't clogged. I would of been very very surprised if they wwre as I do 500 motorway miles a week and even on short trips I drive the car hard. And change the oil every 8000 miles and there's no Dpf by standard on this car so there's not even that to clog the system.

It only does it at 80-100mph, I checked all the turbo hoses and they seem secure, some oily leaks on the connections but I think that's normal enough to see at the lower intercooler hoses.

Could it be an electrical wire fault? My Boost sensor on top of the lower intercooler pipe had broken wires before but the fault cose was different and stated circuit high or something to that effect and was randon when fault would occur at any various speed. This overboost issue only seems to happen at specific accelerating from 80-100mph in 6th.

Are there any other things I could check? I have a genuine VCDS, I see some folks with same issue were checking group 011, what value should I expect to see in here?

[FrenchAudi]

It may be due to the new, secret, smart underperformance commanding kinetic signals (SUCKS) technology, which uses a powerful radar interference computer killing system (PRICKS) to modify the computer software of cars which habitually exceed the speed limit. Typically, this will cause random malfunctioning of the onboard computer and induce limp mode in the driver's vital organ.

[Crasher]

In rare cases I have know the Thurust (MAP/IAT) sensor over read and stick at 2880mBar. What I do is going out on a test drive with measuring blocks 11, requested and actual ticked, and shown in a graph. The scale has to be tweaked down to keep it on the screen and I slow the time base. Then I do a series of 3rd gear low rpm pulls and watch for overboost as below. Green is requested and yellow actual where you can see the yellow peak well over what it should do, this is classic VNT sticking. If the MAP sticks it goes up to 2880mBar and stays there.

https://hosting.photobucket.com/4493c99d-e8cc-46f0-b7b5-7857cc7d4998/cf105296-de83-4529-a30c-05586e013529.jpg?width=590&height=590&fit=bounds

[tdiblacky]

Thanks Crasher. for the alternative. I'll try check that via VCDS.
Is the MAP/IAT the MAF? I believe the air intake temp sensor is incorporated into my MAF sensor. When you say MAP do you mean Manifold absolute pressure or what does your reference of MAP stand for? Mass air pressure?

[Crasher]

The MAP sensor is a combined unit VW sometimes call the "Thrust sensor" in the boost pipe which also measures the charge air temp. VW call the MAF an Air MAss Meter and this also incorporates the inlet air temp sensor.

[tdiblacky]

Hi Crasher, Here are some pics I took from VCDS. I'm not sure how to log graphs, I just clicked into the graph but I need to read up on how to make / record an actual log.

Does the pic of the fault code with the freeze frame info at when it happened help us? Does the absolute pressure 1897.2 mbar and 2478.6 mbar at the bottom of freeze frame data tell us anything that might be amiss?

https://i.ibb.co/dpBwCvt/IMG-9595.jpg


MAF

https://i.ibb.co/k6xF0Rj/IMG-9596.jpg


Ideling

https://i.ibb.co/b3K7SmF/IMG-9598.jpg

Stationary but revs held at 2000

https://i.ibb.co/fF27p31/IMG-9599.jpg

Stationary but held at 3000 revs

https://i.ibb.co/FswdFfS/IMG-9600.jpg

[tdiblacky]

Ideling graph

https://i.ibb.co/zVjXSWg/IMG-9603.jpg

Random graph during driving

https://i.ibb.co/mTmrxvr/IMG-9605.jpg

Another randon graph during driving

https://i.ibb.co/8YWQj10/IMG-9606.jpg

I also checked the 001 in measuring blocks whilst driving and it doesn't stick at the 2880 mbar that you said does be seen when the Boost sensor sticks.

Can you see anything amiss in my pics, Crasher? I believe it's the green and yellow we're interested in, green requested and yellow actual. The yellow seems to be going higher in my pics so is it constantly overboosting? Note: it didn't hit limp mode during my drive when I took all these pics on the same drive.

[Crasher]

Classic BKD turbo overboost due to internal carbon build up, seen it hundreds of times.

[tdiblacky]

Have you seen it even where the actuator lever for the turbo's variable vanes geometry moves smoothly by hand, perhaps it's only starts to stick when hot and when pushing the car that extra bit further.

So turbo off then, might be a job for over the Christmas hols! :)

[Crasher]

I used to see that all the time (we don’t see many Golf 5 BKD’s now, it’s either classic stuff or modern techno nightmares), not just by hand but engine running in basic settings 011 and with a hand vacuum pump.