The sequential system

[Ian C]

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.

 

There are a series of attached pictures which show where all the components of the sequential system are located on the engine. They are indicated by a number - look at the pressure map for a key to these numbers and their logical position in the system.

 

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 1: 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 2: 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 3: 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.

 

21-9-2005 Update - this may also be an issue with the pressure tank. Investigations by Heckler and Soop Dogg are ongoing

28-6-2006 Update - Soop Dogg has found that a dodgy fotwell connection can cause it. Don't ask which one yet though. See FAQ on refurbing electrical connectors

 

Problem 4: I can't get any boost on the first turbo, it oscillates around a low boost pressure.

A rare one this, your IACV is stuck open, and every time turbo #1 makes boost it takes pretty much the easiest way out - via the non-spinning 2nd turbo and back out to the air filter. It's like Problem 2 but happens all the time, not just at the transition point.

 

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.

 

-Ian

[mickeyblade]

hi is there any advantages to running turbos in tandem father than sequential? if so how is this done?

[heckler]

No real advantages, car will sound louder too. Just a different type of power delivery.

 

Click here to see how its done

[mickeyblade]

HI MATE THANKS FOR THAT I FOUND A ELECTRICAL WAY TO SWITCH BETWEEN BOTH USING A TWO WAY ROCKER SWITCH OFF MK IV.COM IT LOOKS WAY EASIER http://www.internetwork.org.uk/ettc.htm LOOKS LIKE THE SAME PLACE WILL GIVE IT A TRY

[4packet]

Hi, I have a question regarding the effect of running with an EBC on the 1st turbo boost level. My observations show that running with the EBC off with a 1bar restrictor ring I will see roughly 0.55bar boost from the 1st turbo. With EBC on I will see roughly 0.8bar from the 1st turbo and the car is noticeably quicker.

 

I understand that in the single turbo phase the boost level is controlled by the exhaust bypass valve, so is the EBC inhibiting its function and hence pre spool is non existent?

 

Thanks

 

Martin

[colsoop]

Hi, I have a question regarding the effect of running with an EBC on the 1st turbo boost level. My observations show that running with the EBC off with a 1bar restrictor ring I will see roughly 0.55bar boost from the 1st turbo. With EBC on I will see roughly 0.8bar from the 1st turbo and the car is noticeably quicker.

 

I understand that in the single turbo phase the boost level is controlled by the exhaust bypass valve, so is the EBC inhibiting its function and hence pre spool is non existent?

 

Thanks

 

Martin

 

You are better off posting this in technical fella.

[x2k.dk]

Hi Ian

 

hope you can help.

im having problems with turbo nr 1.no boost at all.and i think it might be caused by problem nr 4. iacv not working as it should. but in the other hand. if i run TTC mode wouldnt the IACV actuator stay closed all the time? and is the IACV closed when its pushed down or when its pulled up by the actuator?

 

Tanx

 

regards Neti

[markssupra]

Better off posting in technical Neti.

 

When I had no boost at all it was a vacuum pipe had come off the pressure tank VSV (under inlet manifold and easier to spot from underneath car)

 

Hi Ian

 

hope you can help.

im having problems with turbo nr 1.no boost at all.and i think it might be caused by problem nr 4. iacv not working as it should. but in the other hand. if i run TTC mode wouldnt the IACV actuator stay closed all the time? and is the IACV closed when its pushed down or when its pulled up by the actuator?

 

Tanx

 

regards Neti

[Guest Guido492]

My #1 has been doing all the work here lately. Only getting about 7 pounds. Couldn't figure out what was wrong. I found this thread, traced all my hoses and the rear hose in to the IACV had popped off. I re-connected it, went for a test drive and the #2 rush was back with a vengeance. Thanks for this post Ian. It was a great help.

[mellonman]

I think this might help being in here

http://www.max-boost.co.uk/max-boost/supra/boosting_the_beast.htm

[Guigsy]

Can the valves etc be freed up in any way. Been told they get sticky over time if not extercised. Or is that bs and a good last will get them moving?

[mellonman]

nothing wrong with abit of wd40 to loosen them up, but they are very stiff to move anyway other wise they would creap open

[JasonR24]

hi the links explaining how tandem works and how its done dont work, can anyone explain it to me or point me in the right direction. cheers

[mellonman]

google TTC or true twin conversion

[Jellybean]

Hey

 

Was at my first track day today, lost all power on the straight in the afternoon session , over fueling (black smoke) big thanks to colin in ssi for the help over the phone; knew what the issue was straight away!(Vacuum Pipe on Fuel Regulator)

 

I have two vacuum pipes going from the map sensor, one of the pipes is easy accessible on the fuel regulator on top of the intake but the second goes between the intake and cam cover , loops down into a metal pipe which splits into two vacuum pipes near the rear firewall

 

Just the fuel regulator vacuum pipe to MAP sensor is connected , the 2nd Vacuum pipe that disappears under the intake is disconnected at the moment so I could get home

 

Not too sure what they are for as she seems to run ok without it connected ?

 

can touch them with my fingers but cannot feel any tears or loose

 

Is it an intake off to get at the vacuum pipes?

 

Is it safe to drive the car with just the fuel regulator vacuum pipe to MAP sensor connected ?

[Guest mitch2jz]

Hi mate, first great write up. Recently my top two vsv units became dead as in had no current running through them. Upon looking around I found some off the vsv units which sit down below near the turbo mounted sided by side.

 

My questions is am I able to separate the two mounted together and use as single replacements for the top broken vsv units?

 

As pictured below, top units together in photo below are the ones near the turbo. The single units in photos are the ones I need to replace and thought maybe I can exchange them for the top two? they look identical but can I just exchange them?

 

Thanks, Mitch

 

[Guest AVI]

Hey , I am new to this site . this is the first time posting. I am having issue #2 on my engine . as soon as it hits 4k rpm the boost dies out . it was working just fine before I took the car to have a fresh new set of twin turbos replaced.the car ran like crap and struggled to make 10psi on boost. on a personal inspection , I did realise that the front or first turbo was definitely connected up all wrong , ie the vacuum system. I personally did research and did over the front turbo and now only the first turbo works well but then boost dies out at 4000 rpm. I have had the 4 main ICV sensors replaced with brand new ones and still the problem persist . I need some help here terribly.