940 turbo dies at junctions
 

So the wagon is road worthy at last, however I've noticed when you dip the clutch it tends to die. The car no longer has a recirculation valve like the original Mitsubishi turbo's do as I've fitted a larger blower, could that be the issue? Obviously the extra air has nowhere to go now so seems to lean out when you throttle off.

I've checked for air leaks on the boost pipes and all the vac lines have been replaced to and from the inlet.

It's running a HY35, 630cc injectors with an 012 AMM and TLAO chips.

Yes that's the issue. There is too much air (under pressure) and not enough fuel. This is what happens when the dump valve stop working on the standard turbo. The throttle and idle valve can't control the air (o2 feedback too slow), it can't determine there is extra air and it then doesn't inject enough fuel.

If you don't close the throttle completely or disconnect the throttle closed switch it will stop stalling but then runs badly, may affect the O2 sensed trim values.

You can just keep it above idle on the throttle after boosting it will bleed it out after 5s or so.

Proper solution is to replicate the the original valve function and dump the pressure pre turbo (vacuum trigger valve that bypasses the turbo basically), but doesn't dump to atmostphere (need weird aftermarket temporary opening valves that don't work that well).

Thanks for the reply Tony, is there a reason why it can't be vented to atmosphere? I hate the noise they make but it'd save more plumbing.
And do you know if it matters whethe the valve is place before or after the intercooler?
Ben.

The stock ECU can only take into account the air that has gone past the AMM, any that has been vented to atmosphere isn't metered by the ECU in any way so it can't really make a decision based on something it doesn't know. The AMM has told the ECU that more air has gone past it hence the leaning out. MAP sensors are generally used by themselves or in combination with a MAF to overcome this issue (modern OEM ECU's), but then you'll be looking at going Standalone.

The CBV of the stock turbo vented this back into the turbine housing to help it stop stalling and as well as controlling the amount of air going passed the AMM. No extra air should be going passed the AMM when the CBV recirculates, therefore the ECU knows the amount of air in the system and can work out the fueling according to that.

Ideally, your BOV should be placed after the intercooler as that when the air is most dense.

As it's been mentioned before, ideally, the vented air needs to be recirculated to work with the stock ECU. However, there are members of this forum that have figured out how to 'overcome' this issue on the stock ECU, I'm not one of them!

Hope this clears things up...

I know alot of people think this, but IMO its mostly wrong.

The unwanted air gets dumped out the air filter (this is physics), the MAF is mostly (but not completely) gnored during idle. The problem is the TB and idle valve cannot control the high pressure air for idle, it has to be dumped. Air flow is 'assumed' during idle and the O2 +Rpm used for feedback.

The CBV just stays open during idle, so its good to connect after the MAF.

Same thing happened to me when the recirc hose perished on my 940 Turbo - luckily a simple fix for the standard turbo setup.

A bit more plumbing required or a complete rethink of the ECU/MAF gubbins I think :\

Cheers

It can be (and is) vented to the atmostphere, but if you mean before the MAF input then you need a temporary opening valve that needs to shut after the dump. Not really sure how they work. But I guess they have a tricky adjustment to get 'enough' air dumped.

Before or after intercooler should be fine, I guess before is slightly better to retain the heat loss when it rebuilds the pressure. After means losing more of the colder air.

If you dump metered (by the Air Mass Meter) air to atmosphere you will cause more problems as the ECU will make the mixture too rich as there will be less air in the system than expected.

The solution is to fit a Compressor Bypass Valve either side of the turbo and luckily Bosch makes one that was used by Saab and Porsche etc:

https://i.ebayimg.com/images/g/nFEAA...e~/s-l1600.jpg

It's vacuum operated so it should work off the same manifold connection as the old turbo's CBV.

Obviously you will have to modify the turbo inlet hose and turbo to intercooler hose to allow you to connect the CBV but you should return to smooth running without the noise of a BOV.

Just had a look and a used one is on sale here:https://www.ebay.co.uk/itm/Turbo-Byp...MAAOSwnDpaFrb4

That's exactly the one I'd seen, I found them new for just over £40 which I thought was pretty reasonable.

It won't cause any problems dumping metered air, the engine does not remember or know the air is pressurised. It just assumes the amount going into the engine is the amount going into the MAF.

In my understanding (which isn't perfect) I think the engine ignores the MAF during idle, but doesn't like it being disconnected. Therefore you could use a CBV (always open in idle) open to the atm rather than a temporary opening dump valve or connecting to pre-turbo pipe work. The valve will close when off idle and the air will be measured via the MAF as normal. I'm not sure though.

The MAF may take a little longer to react to the bigger step increase in ait flow when the valve closes for power.