Sequential turbo problems

Here is a look at the control systems of the sequential turbo system and the problems that may occur. Hopefully it will help out with a common set of problems.

Actuators and VSVs:

There are four VSV/actuator assemblies that control the sequential turbo operation:

Intake Air Control Valve (IACV)

Exhaust Gas Control Valve (EGCV)

Exhaust Gas Bypass Valve (EGBV)

Wastegate

VSVs switch manifold pressure (boost) to and away from actuators. The actuators open/shut valve butterfly flaps.

IACV

When this switches, boost generated by #2 turbo is allowed to join the intake stream

When it isn't switched, there is a small flap in the IACV assembly that allows any boost generated by #2 turbo as it prespools to join the intake stream, preventing turbo stall.

EGCV

This allows exhaust gas to flow through #2 turbo, causing it to spin up. When it's shut, there is no gas flow through the impeller of #2 turbo and therefore it doesn't spin.

EGBV

This allows some exhaust gas to bypass the EGCV and join the output of the first turbo. It serves two purposes - one, it prespools the second turbo by allowing some exhaust gas flow through it. Two, it controls the amount of exhaust gas going through the first turbo in much the same way as a wastegate - therefore controlling the maximum boost pressure generated by #1 turbo.

Wastegate

This bypasses exhaust gas from the first turbo, lowering the overall amount of exhaust gas going through *both* turbos and controlling the maximum boost pressure generated by both turbos in parallel.

What it should be like:

You get 0.7bar of boost on turbo 1, a slight dip in power between 3500 and 4000rpm, and then 0.8bar of boost with both turbos online. This should be a smooth and linear power delivery.

When modified with a boost controller/decat, you'll see an increase in boost after 4000rpm when the second turbo comes online. You may also get 0.8bar of boost on the first turbo with a decat. You'll feel the same power dip at 3500rpm but then a noticeable surge in power at 4000rpm when the second turbo comes online and hits over 1 bar of boost. Full boost is acheived before 4500rpm.

When things go wrong:

Problem: I don't get a 2nd turbo rush, it just seems to sit at 0.8bar.

The second turbo isn't kicking in. This usually means your IACV isn't opening - the turbo is trying to spin up but can't flow any air, so #1 turbo does all the hard work.

By the time #1 turbo is producing 0.8bar at 5000rpm it's way beyond it's efficiency map and is overheating the air and overspeeding the turbine. Try not to do this too often.

Problem: I don't get a 2nd turbo rush, and as soon as I should it dumps all boost instead.

The IACV is opening but the second turbo isn't spinning. All the air boosted by #1 turbo now has an easy path back to the non-pressurised side of the turbos via a non-spinning #2 turbo. In effect, it all goes 'backwards' through #2 instead of going into the engine. You'll maybe see .2 or .3 of a bar of boost but that's about it.

This usually means the EGCV isn't opening. The turbo can't spin as it has no exhaust gas flow path. Although annoying, this won't do long-term damage as #1 turbo won't generate much boost because the engine loses a lot of power, so no overspeeding or overheating.

Problem: The second turbo comes in late.

It seems like, instead of 4000rpm, your second turbo cuts in at 5000rpm or later. Usually this is caused by no prespool taking place - your IACV opens, your EGCV opens, but the second turbo is sat on it's ass doing nothing. So, not only has it got to suddenly get up to speed, but it's fighting against #1's output trying to go backwards through it. Hence, the looong lag.

This is usually caused by the EGBV not opening. You may notice more than 0.8bar of boost being generated by #1 turbo because of this, as the EGBV also acts as #1's wastegate.

Troubleshooting:

The usual cause of these problems is hose related. Either a vacuum hose has popped off or it's split. These hoses can get hard and brittle with age as they are subjected to continued engine bay heat cycling. It's usually a slightly split hose if the problem is intermittent, and a popped off or completely holed/severed hose if it's permanent.

To troubleshoot your problem, look at the pressure map of the system and trace the hoses/pipes between the suspect components. Check they are on and intact - removal of the hoses may be necessary to fully check for splits.

Edit I deleted the images off my personal webspace so I've uploaded them direct to the thread. Pressure map first then all the component locations.

See also:

http://www.mkivsupra.net/resources/turbopipes/left.html

For images and descriptions of the pipework

-Ian

Ian, what a fantastic post. Well done.

Rich

Just something you knocked up in 5 mins eh Ian?

Fantastic info there:thumbs:

Top post mate, I have been looking for somthing like this for ages.

Well Done.

Nic

Gets my vote for best technical assistance post of the year, needs putting somewhere permanent. Congratulations Ian, well written and illustrated.

Excellent post Ian. This will be of use to many,many people on the BBS.

Good Man!!

(Have I seen that engine before somewhere....?)

great post ian

good to see close up pics with descriptions of what each part is.

on pipe marked up number 10 if you follow it round there is a part with a butterfly inside, what is that part called?

is it IAVC?

Like it a lot, well written Ian

Regards,

Gaz.

Originally posted by MONKEYmark

on pipe marked up number 10 if you follow it round there is a part with a butterfly inside, what is that part called?

is it IAVC?

If you mean the big diecast piping, yeah, that's the IACV

I'm glad this appears to have been worth the effort, thanks all

-Ian

Ian,

You are hereby nominated for post of the year:p

What a fantastic article, especially as I am currently suffering from 2nd Turbo cutting in late:)

I've re-done this post in the Articles section of the site, sadly I can't quite bring myself to move all the nice comments by everyone as that would be just too big headed

So go and post new ones hahahahaha

No really, off you go. :flame Dev

-Ian

Here is a look at the control systems of the sequential turbo system and the problems that may occur. Hopefully it will help out with a common set of problems.

Ian - The Man

This post - The Legend

Thanks dude!

I can't see the diagrams, are they still there????

Nice one ian

Erk - I mucked out my personal webspace and killed off the images. I'll sort this asap.

-Ian

Edit - sorted

Thanks

fantastic write up, is there a way of checking the vsv's eg an ohmic value or anything thanks

fantastic write up, is there a way of checking the vsv's eg an ohmic value or anything thanks

yes use a voltmeter and the readings should go up as you rev the car.

fantastic write up, is there a way of checking the vsv's eg an ohmic value or anything thanks

Whats the symptoms of the car.

Hi all wondering if anyone can help me

I feel the second turbo is not kicking in, however it works fine (most of the time) when i do the ttc mod and has the fault just intermitently

Sounds like the iacv vsv is dead mate. I have a good used one if you need it, removed from my car a couple of weeks ago

This is an interesting detailed thread where I am trying to troubleshoot an intermittent boost problem I have had for a while.

Whilst cruising at a constant speed for a while for example on motorway without using 2nd turbo and then I floor the throttle, the revs climbs up past 5k before the 2nd turbo decides to engage very gradually. If I back off the throttle and then repeat this again, the 2nd turbo comes on earlier as should. Seems issue is intermittent but frustrating when 2nd turbo does not engage when needed and revs climbs.

Can this be VSV associated? I tested the IACV and EGBV ones which both click with 12v power applied

This is an interesting detailed thread where I am trying to troubleshoot an intermittent boost problem I have had for a while.

 

Whilst cruising at a constant speed for a while for example on motorway without using 2nd turbo and then I floor the throttle, the revs climbs up past 5k before the 2nd turbo decides to engage very gradually. If I back off the throttle and then repeat this again, the 2nd turbo comes on earlier as should. Seems issue is intermittent but frustrating when 2nd turbo does not engage when needed and revs climbs.

 

Can this be VSV associated? I tested the IACV and EGBV ones which both click with 12v power applied

hose leak?

just had this on mine where the top mount at the back of the cam cover has split. cleaned it out and is ok with new hoses.

Thanks buddy. Which top mount you referring to behind cam cover?

I meant the IACV hoses.

much like everything old, these get very brittle and can leak. I ripped them all off and replaced with new hoses and found my turbo spool up problem stopped.

Just be very careful with the VSV. Where the hoses join it can also be brittle and they are easy to break.