Hi,

Got a problem, hopefully someone can help.

Bear with me on this.

Car was this spec before problem occured.

MY02 STI Type UK, 20G turbo, GT Spec II headers & up-pipe, 3" exhaust, fuel pump, APS TMIC. TEK3. RCM induction.

Then I had an APS 725DR FMIC fitted.

After driving the car away I noticed that when in 5th or 6th gear and applying say half a bar of boost of more and lift off the throttle slowly the OEM DV would go 'cha cha cha cha cha' and the car would also judder.

If I gave it half > full boost and lifted off the throttle quickly then you got the one noise.

Had the car remapped (as you do) and was thinking maybe the remap will sort this problem out.

It didn't. It was still there.

I was then advised that the OEM DV couldn't hold the boost well and to get a Forge recirc DV.

So I did.

No better.

Car sounds like a WRC car meets machine gun when lifting off slowly.

Was then advised to get the spring & shim kit for the Forge.

Got it, and tried the yellow/blue/red springs and shims and no different to the juddering/cha cha noises.

(note: Forge came with blue spring as standard, yellow & red spring didn't feel good, so have put blue back in)

So, I don't know what to do.

Car performs really well, pulls like a train.

However, I don't really want to be having to lift off the throttle in in motion after giving it some boost. I want to be able to ease off with no problems.

Where do I go from here ?

I see APS do a part recirc/part atmos DV ? should I try one of these ?

Any advice welcome.

THANKS,

Mark.

 

I have encountered the same problem. Fitted FMIC and got the above. (was on Forge VTA, so put the OE re-circ back on) Better, but car still does this..

V annoying when on motorway for example.. Nobody has ever given me a good explanation of why it does this, the best I had was "There is alot more air circulating in the FMIC pipework than with a TMIC, and it has to go somewhere"

Thought about running with no DV but that'd be worse I reckon. If you do get the APS DV Mark, please let me know if it cures it

Ta.

 

Try a larger diameter signal pipe to the DV.Thats the small dia pipe from the manifold to DV.
I had the same problem when fitting my FMIC.
Originally had a forge VTA DV and had the stuttering problem on part throttle, tried springs and shims to no avail. Then fitted a larger dia signal pipe ( only about 2/3mm larger ) and this almost cured it, Then fitted an o/e recirculating DV and is now pefect.

 

69WRX,

This pipe (arrow indicating) ???

Where do I get the pipe from ?

 

Yep thats the pipe.
I picked up some pipe from work. Any pipe a couple of mm larger dia will do to try, Then find something better ( samco etc ) if it works.

 

if its a bigger pipe, how do the ends attach to the manifold/dv

or does a couple of mm not make any difference ?

 

Jubilee clips

 

ok, heres the update so far.

Put the APS Twin Vent DV on and the stutter/judder/noise was still there, but left the APS on, as the in gear changes are well worth it. The turbo really stays in there when changing gear.

Today, I went to the local hardware store and bought some 8mm piping and I fitted it with jubliee clips.

Went out for a quick drive. First impressions are that the stutter/judder has gone, but the noises are still there.

The piping I used is crap as its really soft and kinks up. Visiting the motorsport garage I use tomorrow so will get them to look at it.

 

Refit the OEM DV with a good 8mm signal pipe and all should be OK.

Worked for me anyhow

 

69WRX
I also have a slight stuttering at the moment that I am trying to cure,
But I would say mine is quite a severe stuttering, on slight normal driving, say 10mph upto about 40mph, and the only way to get the car through it sometimes is to drop a gear and up the revs, but this is getting tiresome.
I also have a similar set up with a front mount/bigger turbo/ etc etc and a HKS dump valve.
I only have the usual sized bit of samco going from the dump valve onto the manifold probably around 5mm.....................I take it this is the bit you are reffering to?
If I get a bigger bit say 8mm as you recommend how does this improve things? And why am I getting stuttering off boost ????
Cheers Kev

 

The problem is compressor surge, which is made worse on the 20g because of the behaviour of the compressor. When you back off the throttle slightly, the air still being pushed out the turbo has nowhere to go, so pulses back out the compressor, through the MAF sensor really upsetting the ECU and so on.

Moving to a DV which opens MORE easily can help as on 69WRX's car, the idea is that as the throttle is partially closed the pressure build is more easily vented by the DV which will open with less vacuum. The forge with the softest springs and no shims at all (ie get it as weak as possible) is a possible solution, and again, a large bore vac hose to the control of the DV also helps.

Boost control can also play a part, if you have a Dawes valve or an ECU map which is trying to make boost at low throttle openings, it can make things worse. While you may only see atmospheric or a tiny amount of boost on your sensor connected to the inlet manifold, the pressure at the outlet of the turbo may be much higher, pushing the turbo close to surge.

Paul

 

ok, judder hasn't gone. yesterdays test drive was <60mph and all was well.

Today on the motorway >70mph the judder is still there, no better. Only thing that seems to have gone away is the 1psi judder problem, although I can still hear the problem.

Today I had the motorsport company I use, fit a secondary DV. Made no difference.

They also moved the location of the DV too, but again no difference.


As Paul mentions, maybe this is just linked to the 20G and big FMIC's, as the APS725DR FMIC and the APS turbo don't give this issue...

I need to find a VAC hose now that is stronger than the crappy flexy pipe I was using yesterday to test with....

 

Kev, what turbo you got ?

 

Great explanation Paul :

I think the main cause is the sheer volume of air in the system when a FMIC is fitted, Approx 3 times the volume than with the TMIC and the DV struggles with this.
A larger turbo would probably make this worse, but mine initially had a very bad judder and I still have the standard turbo fitted.

Think its just a case of finding the right DV/signal pipe diameter combination.

For me that was a 8mm signal pipe and OEM DV off a 93 model impreza.


Hope you guys can get this sorted.

 

I have a MD309 turbo good for 400bhp, Pace front mount intercooler, many other mods with a Gems ECU.

There are many other scoobs out there running 400 plus that don't have a hesitation problem
So why is mine

Having changed plugs/coilpacks/sensors etc etc I'm trying all sorts, today I REALLY earthed my ECU after advice from Bob Rawle, so I'm taking my car to work tonight to if thats made any difference !!!!!!!!!!! I guess next I'll get bigger samco between dump valve and manifold...............................then I'm going to take my car to a field and blow the f*cker up

Kev

 

The hesitation can be driven around quite easily too. By backing off the throttle completely you will dump the air through the DV in a very positive fashion. The problem with surge related issues is that many people don't understand them and do the opposite to what's required. Have you tried the Forge DV again with the weakest spring yet? Also the OE dump valve usually gives good results. How is the boost controlled? Can you please give us some more info?

Paul

 

HKS FMIC and VF29 and exactly the same problem - been looking for a solution so i'll keep my eyes peeled.

 

I've already offered 3 solutions, how much eye peeling do you want to do?

 

Paul
Can I just clarify something for the stupid ones (myself included), you think by either getting a weaker spring set OR in my case (HKS dump valve) loosening the nut at the back to its maximum (aswell as getting a bigger hose from manifold to DV) will help or even cure most hesitation problems that have been mentioned above?
Cheers for your input by the way
Kev

 

Totaly agree with this i had tryed a few different dv's and settings for them in the end the solution a stock dv from a my95!!!!

 

mine's doing this since fitting a a fmic at the weekend and a PFC. It seems to be making ridiculous amounts of boost with or without the PFC (1.8bar ) and stalling the compressor on lift off. What i have noticed is that i seem to get 1 bar easily at 3500rpm with about 1/10th throttle... Is this the problem?

 

Boost solenoid not plumbed in correctly (or at all) and/or some incorrect settings on the PFC.

Paul

 

Yes that's pretty much correct. Although I've not found an easy fix with the HKS dumpvalves, the right combination of spring and preload are a big help, and recircing dump valves nearly always behave better in the face of a surge problem on a MAF sensor equiped car.

 

Dawes is now controlling the boost - plumbed in between the solenoid and wastegate? Still chattering and misbehaving - sorry for the hijack I'm going to try a standard DV just after the turbo and a Blitz one as well on the hardpipes. Chav

 

The dawes will not be helping as it's not sensitive to the throttle. A good boost control map on an Ecutek will throttle back the boost solenoid on partial throttle for you, while your dawes is frantically trying to make boost at partial throttle, leading to increased surge. As an experiment, replace the dawes with a plain piece of hose, and I think you will find the part throttle surge is vastly reduced. From there you can see about using a good electronic controller such as an avcr (which has some throttle control but not loads) or speak to your mapper nicely about getting some work done in teh ECU for controlling boost.

Paul

 

I have just fitted and tried the 'bigger hose to DV vacuum' mod, and it hasn't helped mine at all (8mm Fuel hose used)

I get the 'chuff chuff' noise, and judder after partially lifting off throttle after small boost. I also get a kind of 'miss' where it feels like the engine has 'cut' for a split second!!

Running FMIC on Tek3 with standard re-circ DV on P1 and a Hybrid turbo.
Have learnt to just lift right off throttle to compensate, but it still annoys me when it happens!!

 

Pickle,

Just bought my 8mm Fuel hose today and will be fitting it at the weekend....

 

Your Dawes is plumbed in wrong..

Should be plumbed in between the nipple on the compressor outlet side of the turbo and the wastegate.
The solenoid is no longer used with a dawes.

 

pickle, im getting the same chuff chuff noise and judder but no stuttering, and i think that bit of blue vacuum pipe that comes with the hybrid im using is a smaller diameter than the o.e vacuum pipe. its certainly a lot tighter fit than standard

Andy, correct me if im wrong but i cant see how going to an 8mm pipe will help as you are just plugging it onto two spigots that remain the standard size (if you see what im getting at) at either end i.e 3 or 4 mm at the manifold and dump valve ferrules

 

Been getting this also and after advice, fitted the lowest pressure Forge spring (green). This has helped and made it more driveable on the road but can still be a pain on occasion.

I've suspected that the judder is from fuel cut, or a series of fuel cuts. Watching the MAF reading on the hand commander shows a -ive reading where the excess dumped air goes the wrong way across the MAF and is expelled through the air filter.