View Full Version : yet another T4 power loss problem!!!

10-11-2007, 03:37 PM
Hi everyone,I'm having a small problem with my 98 transporter/caravelle.It's a 2.5 TDI with intercooler, engine type is ACV.I first noticed the problem after the engine had been rebuilt at the main dealers in Boulogne ( the water pump ----itself and took the cambelt off at 80 mph!!!!) It feels as if someone is flicking a switch and taking about 20 bhp away but it only happens after you've given it a fairly large portion.It felt as if it was losing boost so i bought a boost guage.I've just been out and under load the max it gets is about .9 bar which according to info on the forum is pretty good,but after that the old problem appeared and now there's only .5 bar.If you switch off and restart it's up to .9 again untill you give it a fistfull then it drops to .5 again.I've tried disconnecting the MAS sensor and this doesn't seem to make any difference,anyone got any ideas?

10-11-2007, 06:19 PM
what your describing gypsy sounds like your van is goin into limp mode due to a turbo pressure issue. I've had a similar problem, mine was caused by moving the n75 valve to accomodate a larger intercooler and lengthening one of the vacuum hoses. Problem fixed now. I'd be looking at the n75 valve vacuum hosing, making sure it is all top order, not kinked split or disturbed by the engine work that was done.etc. It could be the valve itself. When you say MAS sensor, do you mean the MAF or MAP sensor? Anyhow it seems like you knew about disconnecting it to check if it was the problem. You could get a VAGCOM
check, that should pinpoint your problem. :beerchug:

11-11-2007, 11:52 AM
Thanks for the prompt reply,I did mean MAF sensor. Just checked all the hoses and they seem to be OK,is there anyway I can check the valve you mentioned?

11-11-2007, 12:45 PM
No worries. |Check the vacuum hoses at the turbo end too. Theres a small 4mm ID hose that comes off a rubber hose on the intake side of the turbo that someone else found had split around the clip. That caused his boost loss problem. As far as I know there is no way to check the n75 valve is operating correctly without a vagcom check. I'd try and find a local independant vw garage to do this. The readout will solve the mystery I'm sure. A new n75 is around 80.00 I think. I would get a genuine vw part if thats what it turns out to be.

11-11-2007, 12:50 PM
Ta for that

13-11-2007, 01:31 PM
Good advice from Herman there !

22-11-2007, 06:21 PM
Hi again,took your advice and went to see my local dealer.The trouble is I'm living in France at the moment and my french isn't really up to deep discussions about sensors and fault codes............ Never mind the bloke was very helpfull even tho he doesn't speak the queens,we connected it up and he muttered and pointed to all the small bore hoses between the turbo and the valve behind the header tank and everywhere else come to that. He disapeared and came back with a roll of hose and sent me on my way! So I've replaced all the ones I can get at with the hose available.The ones I haven't replaced I have checked by sucking the bloody things and sticking my tongue over the end to make sure they hold the vacuum. Right ,up the road and you guessed it, no bleedin difference!!! Back to the dealers and he seemed to be trying to tell me the next step was trying a load of new bits. Not really what I want to do, can the diagnostic machine not pinpoint what part is faulty?

22-11-2007, 06:43 PM
A pain not being able communicate with the mechanic..:(. If you changed all the hoses and kept them all about the same length you can eliminate them as the problem. I'm trying to second guess from a computer!, but, I'm assuming the mechanic went to your n75 valve and hoses as the source of the problem because vagcom had given him a fault code. Sorry mate but I think you have to choose between gambling that changing the n75 valve will solve your problem or finding an intepreter to get a proper understanding of what vagcom check is revealing. I'd hate to see you chucking money in the wrong direction, been there, done it!!. Your probably getting limp mode because your losing boost control..my money is on the n75...but its just my guess!! your call. Best of luck.:beerchug:

23-11-2007, 01:00 PM
Hi Herman, The bloke gave me the printout in french which I've been try ing to translate without too much sucess!!! Perhaps the codes are universal?
00575 017 "pression turbulure d'admission,difference de regulation" which I recon means that the turbo pressure is not as it should be.
17978 P1570 035 "calculateur du moteur bloque" could be ECU fault?
00553 031 "debitmetre d'air massique-G70 coupure/court-circuit a la masse" which I think is when I disconnected the air flow meter to see if that made any difference.
I'm coming home for christmas so if we can't sort it with help via the forum could you recomend anyone to fix it and perhaps do a bit of a a power upgrade at the same time? I don't want to stress the old girl too much,perhaps another 20 or 25 extra horse would be OK,what do you think? cheers Steve:beerchug:

24-11-2007, 06:19 PM
I think your assesment of the readout is about right. I dont know enough to give you the solution however!! Like I said I'd go for the n75 valve as the culprit. I belong the the vwt4 forum based in england and I've seen others fix similar probs by changing the n75 valve. I might be able to point you to a garage in the south of england if that helps. I recommend you join the t4 forum anyway, its very useful.

As regards power enhancement I have new .216 injectors in new bosch bodies, a new nissan top mount and solid flywheel waiting to go in my 2.5. I've spent about 1k so far and I'm not finished!!. I've decatted and removed my centre box and modded my airbox. I'm hoping to get around 160- 175hp with these mods. Other guys have achieved 155hp with these injectors and standard intercooler. It does require a custom remap though. (chipwizards) Oh and fuel consumption generally improves!!. If that doesnt wet your appetite an extra 30hp is easy with an aftermarket tuning box. I like the look of the german tuning company Koch as the source for more power from the standard setup.

Steve. (yes I'm steve too!)

24-11-2007, 06:22 PM
text was missing:confused: "nissan topmount intercooler and uprated to a vr6 size clutch and solid flywheel.."

25-11-2007, 08:32 PM
Herman, you were right! Had a brainwave this morning, lifted the bonnet of my golf and there was a similar looking valve lurking on the bulkhead. The part numbers were very much the same so I've tried it on the T4 and it's sorted it. Is there any point in fitting an uprated one or should I just go to the dealers and buy a standard one? Cheers mate and thanks for all your help. Steve:beerchug::beerchug::beerchug::beerchug::beer chug:

25-11-2007, 11:33 PM
Good news!:D There isnt an uprated n75 valve available, I'd just get a genuine vw part....funnily enough I finished fitting my big intercooler today and went out tonight. More power :firedevilbut its going into limp mode again :mad:!! . Not a boost issue as such coz it does it on a trailing throttle at around 2750 rpm. I can floor it as much as I like, no prob..apart from clutch is slipping!! I think its getting too much air coz of my airbox mods..best I get those bigger nozzles in asap!!


05-12-2007, 07:13 PM
Spoke too soon!!!! got a new N75 valve from the dealers "1HO906627" fitted it and it's the same as the old one! Took it off again,re-fitted the Golf one and this time it went into "limp mode" with that one as well, looks like it can't be that. Two questions, what is the valve on the bracket underneath the N75? and does anyone know if there is a diagram somewhere of where all the vacuum and boost pipes go( just for if the people who rebuilt the engine got it wrong)

05-12-2007, 09:31 PM
Ok mate, sorry to hear its not solved your prob. I think your "other valve" referring to the grey coloured vent solenoid....


This link should give you a diagram. I'm registered on vagcat. Its a great info source. I've typed in for a 2002 model t4 like mine. Yours should be similar. It shows the vacuum pipe arrangement.

Keep me posted,


06-12-2007, 05:49 PM
Hi Steve,Ta for the link to vagcat, mine is a 98 so it's a little different to yours.Checked the picture for the hose routing and the other valve (it's called a pressure convertor as far as I can tell) and it looks like I should have checked here first, the ******* that re-built the engine got the hoses all to ****!!!!! Fairly obvious when I worked it out, for instance the hose from the ecu instead of going to the intercooler went to the EGR valve!!! Anyway it now has a cleaned and blanked off EGR valve and the hoses in what I hope are the right places. Holds .9 bar of boost uphill and seems a little more responsive with the valve cleaned.Might just have a do at the exhaust next week.Thanks again Steve
PSjust bought 5 .216 nozzles from Canada for very sensible money so it looks like I've got a busy week in front of me!!!!:beerchug:

06-12-2007, 06:16 PM
No worries, it makes you wonder what goes on in some garages!!. Good luck with the nozzle swop, let me know how you get on....you should have around 140hp with those without a re-map!:beerchug: