Hey guys,

I need some clarification as to a problem i am having with underboost.

Id like to grasp the concept first in a effort to making me understand more and hopefully resolving my issue!!!

So my question is : what is the relationship between turbo boost and speed??

I am under the impression through some logical explanation of mine, that a cars engine revs, makes boost etc the same in every gear and its the gearing ratios that determine how fast/how much power is created.

Im getting underboost from 3rd gear onwards but not in 1st or 2nd which is blowing my mind!!!!

Any help would be greatly appreciated

Thanks!!!!

There are veins in the turbo which can alter the speed of the air and thus how much boost is created so engine rpm and boost are not directly correlated.

Also, the faster you are going, the more power you need to increase your speed to overcome resistance

Tnx for your reply,

Yes i know bout the dreaded veins as i have just changed my turbo, they were stuck giving me an overboost code - now with new (reco) turbo im getting underboost.

Hence why i asked the original question to see weather it could be turbo related or something else

Does a p0299 code definitely mean turbo related??

Tnx

For 0299 I'd be looking for more of a boost leak or boost control sensor fault. Especially as you say you have replaced the Turbo

Try removing the diverted valve and ensuring that it's free to move and not bunked up or seized

Edited 10 July, 20169 yr by Mikek3111

I dont think i have a diverter valve.... thats a valve on the turbo right??

If so i dont have that...

Ok. Does your actuator hold vacuum?

I'd personally be looking toward a boost leak or split vacuum hose for under boost issues

Edited 10 July, 20169 yr by Mikek3111

Yep - its new and holds vacuum.... I've visually inspected boost pipes and they all seem fine. But ill do a proper boost leak test....checked all vacuum hoses and all are good.

One thing but, my solenoid valve block starts buzzing like a min after i turn the car on, i then squeeze one of the vac lines n the buzzing stops......normal???

Tnx for your replies mate - appreciated

Yeah solenoid valves will buzz a little on operation. Noise will change depending on the pressure inside the system

Damn - was hoping it was that, change the bloody thing and stop thinking bout it!!+

Thanks mike

ECU maps are related to engine load/pedal

 

so called Driver wish map...unless you have some 1.4-1.6TDi car...you rarely get full boost.

 

1400kg Octavia 2 with 103kW requires about 200Nm for highway cruise. It translates into about 2000mbar of boost. Full factory boost is 2400mbar (on BMM,BKD engines)

 

boost vs fuelling relations changes a lot...TDi usually runs with 30-25:1 air to fuel ratios...at full load/boost, you get around 18:1

 

 

What you say about gearing is so called gear limiter.

 

there is more limiters. smoke, ambient pressure, torque limiter...anything may limit your boost.

 

usually sticky vanes, boost leeks, vacuum hoses, rusted actuator, N75 valve gone and so on...I forgot...usual offender is MAF sensor, which screams as EGR problem ;-)

Edited 12 July, 20169 yr by sniper29a

Hi, tnx for your reply

 

So far ive changed : MAF, MAP, EGR cooler, EGR valve cleaned and vacuum checked, Turbo actuator new and tested, Vacuum lines hold vacuum. Ive visually checked the intake pipes and they seem ok, yet to do boost leak test.

 

ANOTHER strange this has happened - took it out for a spin tonight and its quite a cool night, again didnt boost like it did before but it DIDNT trip the computer either - so no limp mode.

 

What the hell is happening, does outside temp have anything to do with....anything??

 

DOIN MY HEAD IN!!!
 

Tnks guys
 

it is simple physics...colder the air, more oxygen it has and more fuel can you burn. Intercooler has poor performance...that's why most hardcore guys use water/meth injection into intake manifold. That's why you get "more" power over winter ;-) Alpine fresh air at -20deg C is like NOS :-D

 

SIMPLE LEAK TEST...take out intake pipe before airbox, which goes from front. With engine on, put hand over the pipe...engine must start to choke and stall in few seconds. If not, there is somewhere leak in intake manifold.

 

MAF needs about 400mg at idle if I'm not mistaken.

 

if everything seems to be ok...it looks like Turbo is dead

 

hard to say if I don't see ;-)

I get that re cold air but as mentioned its still didn't pull like it should but didn't go into limp mode either....

Ive done that and around my airbox lid i can hear air hissing as all the screws that bolt the air filter down are screwed BUT i would think that would not cause an issue as its before the MAF.

Iv also done the same thing (blocking off intake) after filter n before maf and it starts to choke and my intake that runs to turbo starts to bend etc... so i "think" thats ok and no major air leaks....

But again i have no friken idea..

Tnx for ur input

then it is turbo

 

 

how old is it?

 

if you know how to take care of your car...turbo usually lasts around 200-300k km

 

if you disconnect intake manifold after intercooler, do you have oil there? if yes (excessive amount, lots of toping up of engine oil), your turbo is gone and you need new one

Exactly what i think- but the place where i got the turbo reconditioned 2months ago state that their turbos are 100% tried and tested.

Hence why id like to do/check everything possible so i have a good argue points

.....its an 07 and this last turbo is its 3rd. Previous one was replaced by previous owner 2012, lasted 2 yrs hence why I had to replaced it. The vanes siezed up

....and there is some oil on the joints od the piping but no havent needed to top it up and have been told this is normal

Exactly what i think- but the place where i got the turbo reconditioned 2months ago state that their turbos are 100% tried and tested.

Hence why id like to do/check everything possible so i have a good argue points

People would tell you anything you want to hear ;-)

 

I would personally never use refurbished turbo. As designer and CNC machinist...I don't believe in miracles.

I agree with you but thought id see how it goes as these guys r the best here..

Anyways its looking like its the turbo so ill give them a call, plead my case and hope for the best...

Tnx for ur posts

I forgot about N75 valve, which controls vacuum. If this thing doesn't work, then vanes in turbo doesn't move. Vacuum hoses may be ok but tandem pump where vacuum starts may not work.

 

if you have VCDS or another diagnostic tool. You can run test. Your car has max. boost 2.4bar. At normal cruise speed, the engine needs around 2.0bars of boost.

I have a solenoid valve block so all the vac lines run through it.

Any way of testing this?? Its does click and make buzzing sounds but have been told this is normal...

I was looking at tandem pump aswell - i opened it up few days ago and from what i can see all looks ok. No parts missing, the walls of the pump do hav some scuff marks but that's normal due to wear and tear id say.... not positive but I'd say its thr original one and with 170k km ota bound to have some marks.

Ive got a scan done and the mechanics at vw said that it wasn't reaching 2000mbar, it would get to about 1800 n then pull up the p0299 code

N75 suppose to click...take off hose from tandem pump a there must be always vacuum based on revs

 

VCDS has got diagnostic of active modules. Even if N75 clicks, it doesn't mean there is air in ;-)

 

lack of boost...I had something similar on my Fabia a it was dead MAF. ECU maps have certain values in calibration maps and boost has got -400/+200mbar range I believe or vice versa. Then it kicks you into limp mode. For example map says 2500revs @ 70% pedal and 2000mbar but your turbo supplies only 1800mbar...bang ECU switches into limp mode.

 

If you have checked all...it looks like turbo

Im going tomorrow to a bosch specialist to check the tandem pump.

Then ill find a 2nd hand solenoid valve block and a 100% working MAF ( even though i bought a new one 3months ago) and see what happens.

Then last resort get onto the turbo guys and start the arguing process.

I cant think of anything else to try/change