1.8T Lack of performance

[vdubnewbie]

I was wondering if anyone has experienced this type of symptom before.

I have a '98 1.8t Passat Sport, AEB manual with 72k on the clock. I have only just purchased it and on test noticed a slight lack of performance. I have replaced the AFM and have had the codes read with only a coolant temp sensor fault stored. Replacing both these parts has improved but not cured how I think it should be.( I used to work for Audi in the late 90's and even the A6 1.8t went better than this and its a bigger,heavier car.)

The fault is it that is reluctant to rev freely over 3000rpm and I have never bothered to wait to see what happens at the red line because its takes so long. Even in second and third gears which you would expect it to rev freely in. Joining motorways is the biggest indication and anything past 20% throttle pedal movement is a waste of time. It actually feels better at small throttle openings.

I am not expecting a huge surge of power but 3000+ rpm is poor and my 2.0 litre BMW 3 series, 2003 model, leaves this for dead. With quoted power and torque about the same, but a difference in deliver granted, it feels and revs substantially better. I just want a comparable ammount of performance back which I know its capable of.

Any ideas?

[Crasher]

Hello and welcome. Do you have the facilities to check the boost pressure both as seen by the ECU (VAG-COM) and using a gauge?

[bora(ing) nick]

Hi and welcome to the forum....

Nick

[vdubnewbie]

Thanks for the welcome guys. I do have a friend in the Audi network who may be able to assist but he's not local so may take some organising, he did check the codes quickly when I first got it and he highlighted the coolant temp sensor fault. Have spoken to a workmate who has just started. He has a modded Seat Diesel and may be able to get a boost gauge so I can at least make a start.

I take it the gauge needs to be t-pieced into the small hose on the inlet manifold to get an actual manifold pressure?

Any ideas where I can source the values for this check and also when I get to test on the VAG-COM?

[Crasher]

I can mail you the check procedure if you PM me your email address and chassis number.

[vdubnewbie]

It turns out I cant PM until i have 5 responses of submissions. Would love to take you on the offer of info but don't know how to get around this stumbling block. Any suggestions?

[Crasher]

Check your PM folder, I have sent you my email address.

[vdubnewbie]

Have checked the boost pressure and there is none. Checked the N75 for resistance which seems to be fine and checked wastegate actuator is moving when N75 is disconnected (as described in VW test procedure) and thats fine too. There is voltage to the N75 but obviously need to check on a vagcom for phasing when opening and closing. Can't seem to get the N75 out of my head as there is no fault codes for anything else and that has a direct bearing on controlling the wastegate and boost. Any one had a fault with the N75 directly causing lack of boost rather than a wiring fault?

[Crasher]

Yes, often. I put a new engine and turbo in my Octavia VRS when I bought it and it would not boost properly, the N75 was permanently open but I could prove that in VAG-COM output tests.

[vdubnewbie]

Replace the N75. Checked how free the turbo shaft was spinning whilst i was in there, that seems ok.
Car driving better now but still not as good as I think it should be. Car is more willing to rev but still doesn't spin up with any real pace. Light throttle seems to work better than a boot full and although there is no audible noise from the turbo there is a feeling of momentum building and more purposeful acceleration in third for example.

Reattached the boost gauge and at full throttle in third only 2 psi showing. (same in second and fourth) I am sure it was zero before and the improvement in driving feel suggests a small increase in boost has appeared. Had a look at the boost test and it says check the N75 if under boosting buts thats what I have changed.

Any ideas?

[Crasher]

Do you have VAG-COM?

[vdubnewbie]

Not readily available unfortunately. What is you opinion on DV failure. Its holding vac when tested using the line at the manifold but if its stuck would it cause a recirculation of pressure rather than boost. I know VAGCOM is the way forward but its very difficult as my associate works a way away.

[GavinT]

I could be way off as I don't know much about these engines, but here's a suggestion for you anyway. You may have already mentioned this I'm not sure but on the VRS Octavia they had trouble with coil packs. Apparently its a common fault and can cause lack of power amongst other symptoms. I know the two engines are very similar so it may be worth checking?

[Crasher]

With VAG-COM you can drive the N75 valve in Output Tests 10 (on registered versions anyway, not sure about the share ware versions) and this allows you to check the valve and its circuit. Plumbing to the valve could be a problem so all the lines need to be checked for splits and it would help if you had a hand pump, known generically as a "Mittyvac" so you can apply pressure to the turbo waste gate capsule to ensure it is operating when it should. I had a similar issue to you on my Octavia when I fitted the new engine and turbo (I bought it dead) in that it would not boost and I found, using VAG-COM Output Tests, that the N75 valve was stuck open so it was dumping all the exhaust gasses and killing the boost. That won’t be the issue with yours as the valve is new but at least you would know the system was working if it was tested in output tests.

[CherryB]

Hello, i have similar boost probs with my passat, it has good days and bad days but most of the time it feels as if it is not boosting and then letting it rip in about 300 rpm. it feels a bit like the front wheels have hit ice at about 2800 rpm and it hit a brick wall about 3100. Its the 2.5 v6 tdi, but quite often feels like a di.. would the petrol boost system be a lot dissimilar? I have had the codes read but nothing shows up. i have N75 and maf on the mind, and can see that neither have been replaced in its lifetime ( previous owners huge history) and that covers 155k. I don't have vag com, but will be getting a vag v-checker soon. Any thoughts would be appreciated.

[Crasher]

With the 2.5TDI V6, the most common failure is the vanes on the turbo becoming jammed with carbon.

[CherryB]

Thanks for getting back, Crasher, the turbo was replaced at 120k according to the servce records, i have checked the actuator to ensure smoothness and all seems well. When you pull off the hose to the actuator from the boost control unit, you can blow ond suck through it with no resistance. Is this normal? Also there is no real pattern to it wrking well or not. sometimes you can start and it runs well, then switch off only to restart it a short time later to a different car. There is no real amount of diesel smoke out the back than i would expect, not a fogbank like a total failure.

[vdubnewbie]

Having driven the car for a couple of days since fitting the N75 I have noticed it may be slightly better when cold and also at the end of an extended spell at high speed. When it was coldish I heard a build up noise from the turbo as it was coming on song in first and second gears. I think everything is in place for it to boost but something is telling it not too. When I tested boost it started to rise well on the gauge but just stopped and maintained at 2psi everytime. After getting closer to home once I have been doing 70mph + for 30mins it seems a bit more eager. The coolant temp sensor has been replaced, so thats fine but cant help thinking there may be a slightly interm thing going on.

Is the sensor in the inlet manifold for temperature or manifold pressure? i would imagine temperature but was wondering if there are resistance checks which can be done because although it may be in its operating range (like a MAF fault) it may be giving a reading thats false.That was the reason I changed the coolant sensor because that said it was constantly too high and things were being retarded for safety. If thats so is there any spec for resistance which is available? All i am trying to do is check as much as I can in case whilst the opportunity for a VAGCOM check isn't available.

[Crasher]

When you pull off the hose to the actuator from the boost control unit, you can blow ond suck through it with no resistance

Which hose and to where?

[vdubnewbie]

You had me confused there for a second, you are referring to Cherryb's response I take it? I thought I never said that!

[CherryB]

Sorry for the confusion vdubnewbie. Crasher, the hose/pipe is the one which goes to the turbo actuator housing, the gold flying saucery thing next to the turbo. If i pull it off from the turbo end, i can blow or suck with no resistance ( with the engine witched off), if i pull it off from the boost control solonoid end i can see the actuator rise and fall with resistance. If i start the engine with all pipes connected, the actuator pulles the arm on the turbo up to its stop. If i pull the pipe from boost solonoid to actator off whilst running, the arm on the turbo drops sharply to rest. Hope the description is not too confusing, cheers, Cherry.

[Crasher]

That sounds like the N75 is stuck open which would dump all your boost, I will double check that tomorrow against mine.

[trailking]

vdub , i would check your maf sensor, have you tried driving with it dissconected to see if its any worse? seeing as you dont have vag com. i have a similar problem to yours, buy my wastegate wont move with the n75 dissconnected, does this mean my wastegate it siezed? boosting 2 psi also :zx11:

[vdubnewbie]

I am pretty sure that with the N75 disconnected the reason why your wastegate actuator rod moves is because the exhaust gasses are blowing the wastegate open when you rev it and obviously this can be seen aswell at the rod. It could be the wastegate is being blown open and the end of the actuator has broken off and you can't see the rod moving because of that, or as you say the flap is siezed.

I have fitted a new MAF sensor already, it isnt a VAG supplied part but is identical in every way and marking except the VAG pt number.

I have a couple of things I want to check focusing on the vacuum supply to the N75 and wastegate actuator. I have got a VAGCOM arranged for a few weeks time but just want to satify some curiosities before that happens. You never know, I may just turn up a gem! I will let you know how it all turns out in case it helps.

Don't suppose you know where the ECU finds what the boost pressure is/where it senses the boost pressure for its calculations do you? I have checked everything on the net I can think of and even the system diagram I obtained from VW doesnt show one. Just another avenue I am thinking of.

If you PM your e-mail address i will have a go at forwarding the test procedure i have in my possesion if you would like them

[trailking]

hi vdub, managed to get the wategate to move last night with the n75 dissconnected, it seems fine springing back in to place every time, i also have a new maf and n75, but its still the same,
over 3000 rpm its very slow.
Thinking the catalytic converter may have collapsed i removed the lambda sensor before the cat and screwed in a compression tester, which read about 7psi when reved in neutral (not sure if thats normal or not) i managed to visually inspect the cat by removing the bolts securing it to the turbo and loosening the exhaust brackets, it also seemed ok, supprisingly the element was white, i would of expected it to be black :o
my email is George_hull@hotmail.com would appreciate those test procedures.

not sure where it gets the boost measurments from sorry,
could this be a throttle position sensor problem? hows your idle? mine fluxuates slightly, :confused:
hopefully we can get to the bottom of this

[vdubnewbie]

Hi

Everything apart from the lack of performance over 3krpm is fine and I am pleased with the car. If I could just get this last few PSI back I will be chuffed. The only thing I can think of with mine is a wastegate or actuator fault. I have tried some checks today and although not conclusive, still lead me to this area. I will be getting on a VAGCOM in the next couple of weeks and think it will show that the N75 is wide open indicating a boost leak,wastegate problem, or some other reason its not building up boost.
I will keep you informed of developments as they happen.

[trailking]

Hi vdub, i have resolved my problem. hopefully my soloution can also help you.
my catalytic converter had collapsed, chocking the engine. i noticed a rattle from the cat at idle using a large screwdriver as a listening device, which lead me to do the back pressure test at the o2 sensor.
i found around 7psi of back pressure at about 6 thousand revs in neutral. thinking this was a little high i removed the cat and found my problem.
I have heard of this happening before on the american passat forums.

I strongly recomend you investigate this as my symptoms sound identical to yours.
Its not easy removing the cat, you would need a couple of buds to help you i would think. if you need any instructions on removing it i will do my best, but it was no means hasstle free. i myself knocked the centre section of the catlyst out, you may want to replace the cat if you intend to pass an mot. :o
all the best, and the best of luck finding your problem.