Recently noticed that my turbo takes longer time to spin up than it used before. The boost builds up much slower. Disconnected the recirculating pipe from the dump valve and found out that it's leaking air at idle. Is it supposed to be that way? Sometimes (especially on hot days) the boost won't go above 0.5 bar, but after some time it shoots up to normal 0.9. Is it a problem with the dump valve or should I check the wastegate?

P.S. forgot to mention that the above started to happend after installation of Samco hoses.

Thanks in advance,
Nick

 

As far as I know, there shouldn't be any air coming out of the recirculation hose at idle. The only time the dump valve should vent pressure is when there is a positive pressure difference between the intercooler (pre throttle) and the inlet manifold (post throttle).

Has the dump valve been connected up properly?

Are there any leaks in the connections? (Specifically the thinner hose going from the dump valve to the inlet manifold - a pressure drop in this may cause the dump valve to open under the wrong circumstances.)

Hope you get it sorted!

A.

P.S. You could do what the Cossie boys have been doing for ages and remove the dump valve altogether! I wouldn't like to speculate on what this would do for turbo life though!

 

Thanks for your help.
The recirculating pipe wasn't touched at all, as the Samco one didn't fit (it was a set for STI ver5). I am going to diconnect the dump valve (place a metal plate between the intecooler and the valve) to make sure it's the valve that doesn't work.


Laters,
Nick

 

Hi There,

I know that there was a problem with the MY97 dump valves (air bypass valve) leaking, check the FAQ. I had mine replaced under warantee (MY97) sounds like blowing over the neck of an empty bottle when leaking.

Not heard of MY99 problems as the valve was supposed to be an uprated item. Check your hoses, or put back to original spec. and visit your dealer.

Cheers
Chris

 

I'm aware of this problem on earlier models and was sure that it was solved on MY99, as it has a metal valve fitted right on the intercooler... Well, seems that it's not. The fact that the valve is leaking air at idle shows that there's definately something wrong with it. I have even taken it off and clened - no result. The spring looked pretty tight too. Another thing that worries me is that the valve starts going nuts when the outside temp. rises. Too bad it doesn't do a weather forecast for me

Yeah, another thing. There are no whistling or whoosing noises heard. It's all pretty quite as it was before.

Nick

 

Hi

The standard dump valve does leak air at idle.I discovered this after trying to convert the standard valve to vent externally.At tick-over the inlet manifold vacuum is strong enough to just pull the valve of its seat so air can pass (the turbo produces positive pressure even at tick over).

This is the reason nobody converts the standard valve to externally venting. It can be done but the internal spring needs to be stronger.

Mine is the older type,not mounted on the intercooler,but I imagine yours will work the same.

Hope this helps

 

It's the vacuum that opens the dump valve, not boost. At idle you have vacuum in the intake manifold so the valve will open. This is normal. The after market valves such as HKS, that can be externally vented, have a stiffer spring so they do not open at idle.

I have often wondered if the valve being open at idle will affect turbo response. I was thinking of putting a check valve in the line from the manifold to the valve to see if it improves turbo response, but then, the valve might not open at all.

 

Thanks for replys guys!

Another problem is that my car is not under any warranty (I live in Russia, if that explains it ), so I'll have to fix it myself.
The problem looks more like a valve leak than a hose leak, because you can see (I have a boost gauge fitted) the boost drop to 0.5 then after several seconds return to 0.9. As far as I know it can't be the wastegate as it's contrilled by ECU which is able to lower the boost, but unable to return it back to normal that quickly without a reset.
The qestion is: will I still have the same problems if I buy the same Subaru dump valve, or should I go for an aftermarket one?

[This message has been edited by Gevor (edited 16-05-2000).]

 

It wont be the same prob as the earlier cars, as you have noticed the valve is completely different, it sounds liek you may just be unlucky enough to have a duff valve, it should be a warranty job, although they may question the new hoses, depends on your dealer ?

 

Edited for being a pile of Sh*te

[This message has been edited by Diablo (edited 18-05-2000).]

 

Hi Gang,

Can anyone shed any light on Blitz induction kits and dump valves. Are they worth the money, or should I look at getting K&N.

I'm getting loads of work done next Friday while I'm in Vegas, trying to win me enough for a 22B. Would be nice, but I'm more likely loose enough to buy two.
Must remember to leave credit cards at home.LOL

 

Hi Diablo

The reason I did not say "I think, but I could be wrong" was because I am 100% certain that what I posted was correct.

It is not just the excess pressure when the throttle is closed that causes the dump valve to open.The dump valve has a main valve but also has a secondary piston that is acted on by the inlet manifold pressure(be it positive or negative).This is what the small pipe that connects to the top of most dump valves is for.

When you close the throttle two things happen firstly the pressure on the turbo/intercooler side of the throttle increases as you have rightly said.Secondly the pressure on the inlet manifold side of the throttle falls normally to quite a strong vacuum,this vacuum acts on the secondary piston causing it to open the main valve which then dumps the boost.So inlet manifold vacuum does open the dump valve.(partly assisted by the excess boost pressure)

You also can have positive turbine pressure and inlet manifold vacuum at the same time, this is the case when ever there is a vacuum in the inlet manifold, this is due to the restriction caused by the throttle butterfly.

Say for example you are cruising just under 3000 rpm.The partly open throttle does not allow the cylinders to fill properly so they pull a vacuum in the inlet manifold.

The exhaust gases are still driving the turbo,just not very fast but still fast enough to produce positive pressure between the discharge of the turbo and the partly open throttle.(The turbo does produce positive pressure even at tick-over,just not very much,I have measured it!)
You can prove this for yourself(just as I did) using two boost gauges,one connected to the inlet manifold and one connected to the discharge of the turbo.

I think there would probably be about 5 psi of boost upstream of the throttle and about 5 psi of vacuum downstream in the above example.
I think this unseen boost is the reason the UK cars seem to go better if you floor it at about 3000 revs,the turbo simply has less work to do to achieve max boost.Its already half way there.

Also I discovered after blanking off the valve that in normal operation it increases lag on gear changes etc it does not reduce it as people always say(you can try this as well).I think one of the main reasons it is fitted is to limit the maximum pressure the boost pipework/intercooler/turbo is exposed to,to improve their longterm reliability.

Is there anybody with a my99 that could see if their dump valve leaks at tick-over?.Its easy to do.Just take off the big recirc pipe from the dumpvalve, then cover the end of the pipe with your hand and get someone to start the engine and see if any air leaks from the valve.

Cheers

Andy

 

Hi Andy,

Agree with you totally

Just as well I caveated that post!

D

Prepared to admit being wrong

 

If you want to plug the hole permanently, you can use the cap off of a plastic bottle, maybe a soft drink bottle, or your car wash bottle or something. Use the hose clamp to hold it in. It not subjected to boost so it won't have to be very strong.

 

Andy. What did you use to blank off the dump-valve pipes? I'd like to try it ... not as a long term proposition, but just too see how it affects things.

A friend of mine has a 'mildly' tuned Cossie. His dump-valve recently went kaput, and since he had it removed he has reported an improvement. Not least in wastgate flutter on the overrun!

Cheer,

A.