overboosting?!?!

[Scoldy]

had the car in with the mechanic and on the computer and its overboosting! he said it could be the turbo vanes and something else needs cleaned and that could solve it! bt if that doesnt it could be the turbo but he reckons its more than likely the 1st thingy what do yous guys think? its a 2002 toledo 110 bhp!

 

[funkmonkey66]

Yeah sounds very posible. Scroll down the posts and u'll find lots of similar threads, particually 1 titled 'another ......... Boost problem' ( or some thing like that ). Have a read. The vanes act like a waste gate and control how much exhaust pressure is allowed to spin the turbo therefore controling the boost pressure, the vanes normally stick in a maximum boost position which causes the overboost once the engine revs have built up. Very common problem, which I for 1 am suffering with at the moment. You should find everything you need to no on this forum.

 

[Muttley]

There is a bit more to it:

funkmonkey66 wrote

The vanes act like a waste gate and control how much exhaust pressure is allowed to spin the turbo therefore controling the boost pressure

Noooo, the vanes restrict the inlet to the turbine section while keeping the gas flow smooth. At low revs there is less exhaust gas and the vanes close, which has the effect of speeding up the lower volume of gas, which spins the turbo faster and gives useful boost. The overall effect is to give usable turbo boost from lower revs than a wastegate turbo, and maintain full boost for more of the rev range. There is no wastegate on a VNT turbo, the vanes control the boost without the need for one.

the vanes normally stick in a maximum boost position which causes the overboost once the engine revs have built up.

The vanes default position is wide open i.e. the high revs position, not max boost. The vanes are manipulated to maintain max boost over a wide rev range. On starting the engine the vanes close down to minimum opening, to suit the small load that the idling engine requires.

In a high-mileage turbo there is frequently a build-up of soot in the turbine section, especially if the car has been used for short slow journeys (in-town commuting, shopping etc). This can cause the vanes to stick as load is applied. They don't open fully, the extra exhaust gas has to squeeze through a narrower opening and speeds up too much, spinning the turbo too fast and causing overboost.

 

[asthpsw]

So what is the best way to "Exercise" the vanes to help prevent carbon building up ? I assume "long journeys" will not help as the vanes would mainly sit in one place other than the odd hill or accelrating that might be done ?..............Hard acceleration can't help as it would be simple then for those that did short journeys or stop/start journeys to do the occasional hard acceleration (up a hill I assume?)

By what Muttleys says in that they default to being open & then close down when starting the engine is this not moving the vanes through its complete angle ?

So basically if you haven't got VAG.com what sort of driving will get the vanes to go through its maximum angle & back again ?

Paul

 

[oldman]

i would like to know how to do when one does have vag.com as well, just for completeness really

thanks.

 

[danderton]

i had a simular problem, it also seemed very noisy, turned out to be thrust sensor.

 

[sssstew]

So what is the best way to "Exercise" the vanes to help prevent carbon building up ? I assume "long journeys" will not help as the vanes would mainly sit in one place other than the odd hill or accelrating that might be done ?..............Hard acceleration can't help as it would be simple then for those that did short journeys or stop/start journeys to do the occasional hard acceleration (up a hill I assume?)

By what Muttleys says in that they default to being open & then close down when starting the engine is this not moving the vanes through its complete angle ?

So basically if you haven't got VAG.com what sort of driving will get the vanes to go through its maximum angle & back again ?

Paul

My understanding is that a nice long uphill section where you go up with the accelerator flat down will get them to open nicely, but i just used to combine some good hard driving along with monthly long trips where i used the full power frequently and never had a problem.

 

[Muttley]

The carbon buildup makes the vanes stickier, and although they can move easily when not loaded (at idle) under part boost they stick, the vacuum actuator can't put enough force on to move them throught the soot.

What you need to do is get rid of the soot.

One way to do this is by getting the turbo hot and keeping it hot for a few minutes. You're trying to burn the soot off, so part throttle is needed - at full chat there is no oxygen in the exhaust gas and you're on the threshold of producing more soot.

So you want to work the engine hard at medium revs. Climbing a long slope is the best way, if you have one (A-road or motorway) near you. Otherwise - well, maybe get four heavy buddies in the car and go for a 20-minute run down the motorway at the speed limit.

The two major soot generators in diesel cars are full-throttle motoring (specially if remapped) and the EGR system. Full chat is limited by the smoke map on the ecu, as adding more and more fuel will result in smaller and smaller power gains but massive increases in part-burned fuel = soot. Diesels are rev-limited by the combustion process, the fuel simply can't burn fast enough, and any attempt to up the revs results in soot.

The EGR system is there to reduce NOx formation at part-throttle high-load conditions (mostly). These are the conditions that generate maximum combustion temperatures, hot enough to burn the nitrogen in the inlet charge into nitrogen oxides, NOx. EGR reduces combustion temperatures by diluting the inlet air with an inert gas - exhaust gas. It is a foul concept, brought about (IMHO) by the requirements of the Californian market driven by left-coast greenies. I don't subscribe, the methodology is flawed.

BUT (and it's a big but) EGR increases soot formation. The pollution legislation recognises this, calling for a balance between NOx reduction and particulate increase. The other main drawback of EGR is intake fouling, which is the reason why my EGR is switched off.

 

[asthpsw]

Thanks Muttley............firstly, like you my ERG is off (mine's a 110 so easy to do !) & its been like that since 40,000, its now at 73,000.

As for shoot formation I always think back to a Graph I saw on "Freds" done by subscriber "Flybywire" & this showed that the least shoot formation was at 1900RPM (which also happenes to be when max torgue kicks in on mine)....so my short journeys I try to keep as close to this as possible.

When you say "turbo hot" do you mean "hotter" than normal running ? if so how do I get the temperature up to the ideal for burning soot off ? & bearing mind what I've said re 1900RPM would this be a good "part" throttle ?

Paul