Engine not smooth on boost.

[popuptoaster]

Got a minor issue with the Turbot and i'm pretty sure its related to the MBC.

Car had a slight misfire at idle which i cured with a fresh set of plugs but what I'm noticing now (and had been putting down to the missfire) is the car surges slightly under boost, it feels like a missfire but I think its the other way round, lots of little surges of extra power accompanied by a pshh pshhh pshhh pshhhh noise which I assume is the MBC venting boost.

My question is do they all do it with these cheap MBCs on them or is it a curable issue, its not that bad, more annoying than anything and the car holds steady at 12psi the gauge isnt bouncing visibly.

This is what I have fitted.

[jrevillug]

Too much ignition advance to my mind - although TBF I have no experience of boosted engines.

[starionturbo]

Never liked these cheap MBC's, take it off and then apart, be careful if you don't know already, its basically a spring holding a ball bearing at a pre set tension to adjust the boost. Quite often, in fact very often, oil gets in and causes the bearing to stick to its seat, meaning it doesn't open smoothly causing boost spikes.

[gruss]

I've got the same mbc and have had zero problems apart from fitting it the wrong way round in the first place! What boost do these run as standard?

[popuptoaster]

Jun 15, 2012 23:48:17 GMT starionturbo said:

Never liked these cheap MBC's, take it off and then apart, be careful if you don't know already, its basically a spring holding a ball bearing at a pre set tension to adjust the boost. Quite often, in fact very often, oil gets in and causes the bearing to stick to its seat, meaning it doesn't open smoothly causing boost spikes.

Cheers I'll give it a clean, i know how it works, I was gonna DIY one from plumbing bits until I realised how cheap they are to buy.

Jun 16, 2012 6:23:16 GMT gruss said:

I've got the same mbc and have had zero problems apart from fitting it the wrong way round in the first place! What boost do these run as standard?

Mines a low pressure version so 3psi stock.

[gruss]

Could be the waste gate not holding the boost pressure? The noises your hearing could be the turbo stalling as the waste gate slightly opens.

[popuptoaster]

Maybe, I now curse word all about turbo cars (this is my first one) I know it has a recirculating one that feeds back into the intake, would it still be audible?

Actually I'm happy for it to be the stock wastegate as that's the best excuse to fit a noisy dump valve and then be able to say,


"Well I had to fit it, stock one wouldn't hold the boost"

[RobinJI]

Jun 16, 2012 14:29:26 GMT popuptoaster said:

Maybe, I now curse word all about turbo cars (this is my first one) I know it has a recirculating one that feeds back into the intake, would it still be audible?

Actually I'm happy for it to be the stock wastegate as that's the best excuse to fit a noisy dump valve and then be able to say,


"Well I had to fit it, stock one wouldn't hold the boost"

I think you're getting a bit confused between the blow-off valve and the wastegate. The recirculating BOV you're talking about vents excess intake pressure in order to prevent stalling the turbo with high pressures on the intake side of the compressor wheel, they either vent to atmosphere or back into the intake, and the management system tends to require that they stay with the original configuration. This should only open when you let off the throttle suddenly, causing the turbo to loose energy as the exhaust gasses suddenly drop, but the intake pressure's still high, so if the excess pressure isn't vented it can force the turbo compressor to stall. Hence making that little 'pstssss' noise when you let off the throttle to change gear (You know, that noise we've all made with our mouth when changing gear when we're on our own in the car .)

The wastegate is effectively a by-pass valve that allows some of the exhaust gasses to pass around the turbine wheel directly into the exhaust, causing the turbo to spin less and produce less pressure, it'll be built into the turbo, and it's what your manual boost controller controls through it's actuator. It's this that actually controls the boost pressure by opening and slowing the turbo when the actuator sees enough pressure to open it.

[popuptoaster]

Aah right, I get it now. Two different things.

[rwdrules]

sounds like a slow lambda sensor to me, how old is it, they only last around 30k if your lucky!!

[chairchild]

so.. you've gone from the factory 3psi,and you've upped it to 12psi??

Sounds more like the turbo going into surge condition. ie: it's trying to make far more boost than it can actually sustain, and you're introducing immense stress forces into the turbine shaft.



Turn the boost down, or expect the turbo to die rather soon. Reading compressor maps for turbos is relatively simple, and can quickly let you know what sort of psi a turbo can provide on your engine, without going either into "surge" or "choke" modes

What model turbo is yours? And what engine size is yours too? What rpm do you expect peak torque to occur at, and what is your rpm limit? These are generally all that's required to match a turbo map to your requirements

[gruss]

I'm running 16psi and don't have a dump valve, if you research pros and cons for dump valves there are some very intersting arguments.
If the engine mangaement system was only designed for 3psi then it may be struggling with 12psi, have you only just upped the boost and noticed this?

[Lewis]

I'd be inclined to dial the boost back a bit. Or a lot. Sounds quite optimistic, to say the least, on factory components and electronics!

[popuptoaster]

Nah, 12psi is fine on the 2.3 940, its been well tried and tested.

General opinion over on the Volvo forum appears to be that its the waste gate but the turbo is near its limit at 12psi anyway so rather than worry about it move up to a bigger turbo.

The ECU I have is fine at 12psi, over that it needs a mod to stop the fuel cut off at 15psi and runs a bit rich, but easily sorted with a chip set and some bigger injectors.

I bought this particular model specifically because it easy to turn the boost up, with very few mods it shoudl go from its stock 136bhp to 250bhp and still be bullet proof.

[speedy88]

I'd be interested to see what a pressure gauge on the MBC circuit says.

I had a similar problem with my 740 but it actually turned out to be a fuel pump seizing.

[30psi]

It's probably running quite lean and thats why it's holding back. Are you running standard plug gaps?

You need to stick a wideband on it and rule out fueling

[popuptoaster]

The ECU is capable of getting the fuelling correct right up to 15psi and over that it goes rich until the injectors max out.

[workinprogress]

the ecu is capable of going from LPT to FPT I.e 3 psi to 12 psi without a remap?

i'd say it isnt,,,never come across a fully ecu managed car that is yet,,,what turbo is it running?

it will be fuelling all over the place and the ignition timing will be wrong at full boost too,,,get it mapped properly and I would suggest your problem will go away.

oh and of course...if your getting boost surge this will damage the the turbo vanes,,,the worst ones ive seen have actually snapped the shaft clean in half.

[Brigsy]

Jun 17, 2012 9:05:47 GMT 30psi said:

It's probably running quite lean and thats why it's holding back. Are you running standard plug gaps?

You need to stick a wideband on it and rule out fueling

Id agree with this, the noise your hearing could be detonation....

[chairchild]

can you get a half-decent quality video of it?