Passat b5 loss of power at exactly 72 mph

[cluvie]

hi..at exactly 72 mph i loose power its like as if the turbo cuts out and if i clutch turn on and off the engine everything is fine again until i hit 72mph. Ive tried 2 different airflow meters both new albeit aftermarket and still problem persists..ive had it on a diagnostic but no faults appearing..one mechanic reckoned turbo is nackered. i find this hard to beleive as the car pulls like a train till i hit the dreaded 72mph???? The last maf sensor i bought cost me 125euro so it is a good one and bought from a reliable source..very annoying problem as i find it hard to trust the car whilst overtaking..any help or suggestions greatly appreciated..im in southern ireland if anybody is available to do a vag reading for me??? Thanks guys love the site by the way

[zollaf]

which engine, a pd or rotary pump ?

[Doctle Odd]

What did he scan it with?

[cluvie]

we just scanned with a spurios scanner and the fault code was charge pressure control negative deviation p1156...i googled this and it came back as a faulty vacuum actuator on the turbo..i gave all day today cleaning and checking hoses and they all look good..im absolutely stumped as to what the problem is..

[zollaf]

look, i know its a diesel engine but it could be a pd or a rotary pump, now which is it ???

[Doctle Odd]

Ross-Tech: VCDS: DTC Comparison (http://www.ross-tech.com/vag-com/dtc-comparison.html)
we just scanned with a spurios scanner and the fault code was charge pressure control negative deviation p1156...i googled this and it came back as a faulty vacuum actuator on the turbo..i gave all day today cleaning and checking hoses and they all look good..im absolutely stumped as to what the problem is..

[cluvie]

i see your point doctle odd :) zollaf its the pd engine..thanks guys

[zollaf]

you possibly have a split or slightly perished vacuum pipe. try a new pipe from the take off point on the servo straight to the n75 then to the turbo, thus eliminating the anti shudder and egr valve and see if thats any better. thats what i would do to eliminate a leak.

[JustinM3]

My old P reg was the same, put it down to jammed vanes. Was ok until I fixed the split boost pipe which was robbing boost so avoiding this I guess.

[cluvie]

i get what your sayibg about running a pipe direct to the turbo from the servo via the N75 but whats this N75?? im seeing it in a lot of threads..also i would have to get something to bring the pipe from the servo down to size to fit vacuum on top of turbo..any ideas?

[zollaf]

on a pd there are 3 vacuum controlled things, the turbo, this needs a vacuum to give boost, so no vacuum means no boost. then the egr and anti shudder valve. there are 3 solonoid valves, one to control each item. these valves are electronically controlled from the ecu to either give a vacuum or not. the vacuum comes from the tandem pump on the end of the engine and there is a thick pipe that goes to the brake servo with a smaller pipe coming off to feed vacuum to the solonoid valves. the one that controls the turbo is the n75 valve. even the smallest split in ANY vacuum pipe can result in a loss of vacuum which will result in low boost pressure.
one check is this, watch the engine as someone else turns it off. does the anti shudder valve (thats the one thats part of the egr) snap shut until the engine stops, then open again ?

[Peter D]

Are you sure it is 72 mph not the revs you hit at 72. Check in 4th and 5th gear, I suspect it is the rpm. Regards Peter

[cluvie]

awh yes its at the front of the engine..mechanic was telling me that these particular units give a lot of trouble..peter its definitely the speed or it appears that way. i can drive her hard in lower gears at high revs without a problem..

[Doctle Odd]

N75 will show up in a scan

[JustinM3]

awh yes its at the front of the engine..mechanic was telling me that these particular units give a lot of trouble..peter its definitely the speed or it appears that way. i can drive her hard in lower gears at high revs without a problem..

The higher the gear and lower the revs, the more load and time for the turbo to overboost. Mine was always 68mph on the flat.

[cluvie]

hi guys just an update i got it sorted and it was the vacuum unit at the front of the engine..got one scrap for 30 euro and luckily worked a treat. thanks for all the helpful feedback much appreciated :)

[JustinM3]

Is that the green sphere?