940 misfire, please help before I sell her

[oldskoolfool]

Right just done some looking and the dump valve is on the cbv on side of the turbo so surely it's doing exactly the same thing as it would standard but louder? Lol

[Steve940estate]

If it vents externally it will have an effect on how the car runs. Once the air goes through the air flow meter the ECU matches the fuel and ignition to it. if it leave the system before entering the engine itself it won't know and the mixture etc will then be wrong. While it does a similar job it creates an air leak.

A few pictures might help. You can either add them as attached file or by using photobucket. I think there is a how to about adding photos somewhere on here.

[oldskoolfool]

http://www.volvoforums.org.uk/attach...1&d=1420590748

I don't know whether this works?

[oldskoolfool]

Ahhhh right yeah I get what you mean,
Just posted a picture above :-)


Just trying to find one around my area I can test out

[TonyS9]

I think the dump valve just dumps out the air filter, its purpose is to dump manifold pressure so that the air flow through the maf sensor is representive of load and to stop too much air bypassing the throttle. Otherwise the engine can simply consume the air in the inlet without much flow at the maf.

A torn diaphragm causes the engine to stall after using boost and going into idle. It measures little air flow and doesn't inject enough fuel.

[Steve940estate]

Yes that is an external venting valve. People do use them but they aren't ideal and as Tony says if the diaphragm inside it is split or it isn't set up properly you will be getting less boost and a big air leak.

I have one I was going to fit onto my car but didn't bother in the end. New standard diaphragms were difficult to find when I wanted a new one so I bought one of these valves. I them got a new genuine diaphragm from Japan off ebay so didn't need it.

You could try taking your valve apart to see if the rubber has split but to make things a bit easier it might be worth getting a standard valve even if it's a used one, just to try.

If you get that sorted out keep a close eye on the boost pressure as what it 10psi now might be a lot higher if there was a leak.

[Dirty Rooster]

Well done that is a good clear photo.


Your plug leads : do you have a set of the big ugly original leads to swap over?

Are you comfortable with checking the fuel pressure regulator & it's vacc tube?

Check tightness of the little bolts ontop of your fuel rail which hold down a couple of electircal contacts there.

Tighten all nuts holding your intake manifold to the head.

[oldskoolfool]

Righto cool I'll take that dump valve off and check it, I'm trying to find someone local with one I can try on just to see,


I haven't got original leads to try mate unfortunately but these had only done about 100 miles when I bought the car,
I'll double check all the bolts etc
I've just recently put that fuel pressure regulator on and it did make a good difference to be fair

[crogthomas]

The ECU error you are getting is almost certainly due to the dump valve that is fitted which looks like it vents to atmosphere. It's also probably the cause of the rough running.

The ECU checks to see if the measurements it is getting from the various sensors are within certain bounds. In this case it is measuring a certain amount of air going into the engine, adding a defined amount of fuel and then expecting to see a lambda sensor reading within a certain range. It will then adjust the fuel to keep the lambda close to a particular value.

The dump valve is confusing the ECU by dumping that measured air and causing the lambda sensor to read a very rich mixture (less air, for the same amount of fuel), so it will try and compensate by reducing the amount of fuel injected. There is so much air being dumped that it cannot compensate enough, so flashes up a error code. Of course the dump valve only opens under certain circumstances, so sometimes the ECU is happy, and other times it is confused, which is why the error seems intermittent.

The standard Mitsubishi compressor bypass valve (CBV) returns pressurised air from the output of the turbo to the inlet of the turbo, all on the downstream side of the air meter and not to the atmosphere.

Put a standard CBV on and I suspect your problems will go away.

[oldskoolfool]

You suspect right my good man,
Managed to get a cbv off of a friend locally and guess what.........it's only ****ing sorted it!

I'm not holding my breath just yet though as this bugger has a habit of acting up when I think it's good haha,



Another thing though......I appear to be getting detonation (sounds like a loud growling / rumble)
It's running 12 psi which I've heard is very safe so wouldn't expect it :-/
Could it be to do with the exhaust blowing from around the downpipe? Possibly a cracked exhaust manifold I haven't checked yet