Perverse dump valve on the Bravo

This is an odd thing discovered on the Bravo 1.6 eco, while Joe and I were trying to find out (a few weeks ago) why the engine is randomly so crap at low revs.

Most of the time it works, but every now and the thing just chug chug chugs until it decides it's all going to work... there's not a lot of power at the best of time and it's downright dangerous if it decides to do this when you're pulling out into traffic, for example.

Normally, a dump valve is controlled by a diaphragm which, when the pressure exceeds a limit, opens the valve and diverts excess pressure to either the atmosphere or the exhaust.

This thing is completely the opposite... the dump valve is spring biased to the bypass position. With no vacuum, there is no boost. Vacuum pulls the diverter to allow boost; the vacuum appears to have discrete levels controlled only by the engine revs (and possibly throttle position) - just four levels from none to 'full' boost. This shows up when accelerating as the boost is cut - and not in any gentle and discrete manner; it feels as if the engine just stops for a second.

There is nowhere obvious where the manifold boost pressure is even measured... just a mass air flow meter. I assume that the vacuum comes from the brake servo reservoir.

It also means that there isn't any obvious way of fooling the damn thing. The only thing I can come up with at the moment is perhaps a non-return valve with a slow bleed bypass...

This engine ain't a patch on the 1.9 I used to have. But that got left in a ditch in Scotland last year...

Meh.

A manual valve might help, first lets plumb in a temp gauge to one of the pierburg connections, at least we'll be able to see a physical reading.

Is this the dump valve or the waste gate actuator? Not familiar with the engine so interesting to read your experience. Normally boost pressure is controlled via the waste gate on the 'hot' side of the turbo. The dump valve is more about turbo response and is situated in the 'cold' side of the turbo

Yeah, I'm probably confusing the naming here. The whole thing is a bit confused... I need to take some covers off and see whether it's the exhaust or the inlet side that's being dumped.

Hi Barnacle,

I am guessing this is a 1.6 JTDM (multijet) diesel. Diesels do not need dump valves in the same sense as petrol as you do not have a throttle body that fully closes to control revs (they do have a butterfly on later diesel engines but this is for nice engine shutdown without judder and to partially limit intake for emissions on light throttle). This is probably why it works differently as you describe.
If this has a dump valve it will be used for when the butterfly starts to restrict flow when going from full throttle to partial load to help turbo life but I think only even the ECU thinks the pressure will be over a certain limit.

Have you checked the EGR valve in this instance as they are the biggest issue on every diesel and when they start to go they can be intermittent and normally start to be felt on low revs. When the revs rise and the turbo starts to overcome the recirculated exhaust gases you get performance back.

I know on the 1.9JTDM's (multijets not the earlier ones) there was a new modified gasket between EGR and throttle body to restrict the EGR flow due to similar issues of poor low performance. I did this mod on my 2008 Bravo 1.9JTDM and was a Fiat dealer modification. Its the same for all the Vauxhalls that use the Fiat engines.

If you haven't already checked the EGR I would have a good look, if it is heavily clogged (most likely if never been changed) then cleaning them out doesn't really work but worth a go but replacement is the best way.

There is more possible things that could limit low rev power, but depends if it has a VNT turbo, these can stick if the car doesn't have an Italian tune up every so often

Had a bit of a chinwag with Joe while we changed the brakes and disks and stuck the winter rubber on this weekend.

The valve is a wastegate - it's on the exhaust side, not the pressure/manifold side. The valve is held *closed* - with full boost pressure - when the vacuum is hardest. As the vacuum is released, exhaust is routed round the turbo and the boost pressure drops.

I haven't done anything with the EGR - to be honest, I thought it didn't have one as it has the DPF but there are no engine warnings.

Though we did find the reason for the drive-line shunt: the bottom engine mount on the gearbox is completely detached in the rubber department