Diesel turbos - do they have dump valves?

EddScott · 13 Nov 2009 at 10:18

This isn't some thread about putting an atmos DV onto a diesel engine.

My mother has an A4TDI and for a while now when you build up pressure from the turbo and come off the gas the engine makes a very similar sound to a car without a DV - like compressor surge.

The aux belt came off last night which needs replacing and theres also a thump every time you change gear coming on/off load so a mounts gone too. I'm writing a note to the spanner monkeys with a list of jobs to do but not sure who to explain the odd noise from the turbo.

nicRob · 13 Nov 2009 at 10:49

On our VW diesels (passat, transporter) it sounds like a recirc DV, no compressor surge noise - would've thought A4 should be the same.

Sp00n · 13 Nov 2009 at 11:14

My megane makes a little DV/WG sort of noise but its very feint.

gurusan · 13 Nov 2009 at 11:16

There's no point really as diesels in general don't have a butterfly plate for the throttle.

Felix Da Housecat · 13 Nov 2009 at 11:17

It will have some sort of dump valve as the presure has to go somewhere when you come off the gas.

EddScott · 13 Nov 2009 at 11:18

This is quite loud - noticeably so. My last Impreza's DV didn't open on low boost so the comp surge noise was there but sounded much quicker between "chirps" than what my mother's car is doing.

Last time I asked the garage about it they said it was the clutch so from that comment I'd rather not give them a push bike to work on but the owner is a mate of mothers and she feels oibliged to take it there :rolleyes:

if its not one thing, its your mother!

If the valve has failed I would have thought it would weaken and leak pressure rather stick shut??

gurusan · 13 Nov 2009 at 11:20

Felix Da Housecat said:
It will have some sort of dump valve as the presure has to go somewhere when you come off the gas.

I think those are a bit different and electronically controlled as there's no vacuum to operate it as on a spark ignition engine.

Matt82 · 13 Nov 2009 at 11:40

Felix Da Housecat said:
It will have some sort of dump valve as the presure has to go somewhere when you come off the gas.

there is no dv at all. it doenst have to go anywhere. there is not a throttle stopping it

EddScott · 13 Nov 2009 at 12:03

Matt82 said:
there is no dv at all. it doenst have to go anywhere. there is not a throttle stopping it

So if the car doesn't have a DV, I wonder what the noise is.

Its definately linked to load on the turbo and sounds exactly like comp surge.

[TW]Fox · 13 Nov 2009 at 12:05

I noticed the same thing on a 118d.

gurusan · 13 Nov 2009 at 12:06

It could just be the compressor speed changing so you hear the change in air velocity through the intake.

killerxx7 · 13 Nov 2009 at 12:10

I can hear the same thing on my Isuzu d-max 4x4 truck which is a 3.0 tdi,its rather loud not as loud as my imprezas but still loud.

Deathwish · 13 Nov 2009 at 12:49

What year is the A4 ? As most of the newer ones (last 5 years) will have a VNT turbo, don't know if that makes any difference.

EddScott · 13 Nov 2009 at 12:57

Its an 02 plate. If I'm completely honest, its been doing it for months but my mother won't take it to Audi for (hopefully) a proper look. I'm not confident with her dealer-friends mechanics but she won't listen.

Its the higher output version but she doesn't push it and it doesn't smoke.

It didn't always do it which makes me think something isn't right.

Hawk Tuah · 13 Nov 2009 at 14:24

I can feel a noticeable release when my car goes 'off turbo', but I can't hear anything.

NickK · 13 Nov 2009 at 14:30

Matt82 said:
there is no dv at all. it doenst have to go anywhere. there is not a throttle stopping it

However that would leave the engine running oxygen rich which probably isn't good for it either.

Are diesel turbos running at a lower boost? If so you may not hear it compared to a higher petrol boost.

Dogbreath · 13 Nov 2009 at 16:18

NickK said:
However that would leave the engine running oxygen rich which probably isn't good for it either.

Since they aren't throttled the air/fuel ratio on diesels varies with the amount of fuel injected, and they run very lean in any case.

They also tend to run much higher boost than most standard petrol engines.

Hawk Tuah · 13 Nov 2009 at 16:28

Dogbreath said:
They also tend to run much higher boost than most standard petrol engines.

Really. I'd always assumed they were quite low boost.

I have a real lack of knowledge when it comes to diesel engines. Probably for the best. Will prevent me tinkering with it.

danb21t · 13 Nov 2009 at 16:42

electronically controller blow off valve (BOV)

Dogbreath · 13 Nov 2009 at 18:40

Bug One said:
Really. I'd always assumed they were quite low boost.

Don't forget that diesels don't have the issue of detonation, and they need to run with plenty of excess air to to ensure as much fuel as possible is burned rather than shoved out of the exhaust.

"much higher" was a bit of an exaggeration tbh, but productions cars are typically over 1 bar to maybe 1.5 bar. The JCB dieselmax land speed car was running 6 bar boost on both of it's 10 liter engines