Turbo - now you feel me, now you dont!

[K Simmonds]

No gauge in this RS2 (my other one has a boost gauge, but it's getting junked as I'm becoming an old fart who likes things to be standard).

That reminds me, have a vent put by for you John, totally forgot about that sorry.

Can bring it along to ATIP Scotland or pop it in the post if you'd like it sooner.

Andy's list above is how I'd go about diagnosing the fault.

Got a couple more folks who need invites, will drop you a PM

[LYT4X]

Yes. If you took the time to read through my boost leak post link that I provided, you will find several examples of boost hoses with splits that do not open up until a certain boost is reached

Reading this answer again, I wonder if it's me who hasn't been understood. For clarity, if I get in the car just now, I can go and do 300 miles with boost and woosh in every gear and at every level (including WOT in top gear from low revs where max boost pressure might be reasonably expected) without any issue.

However, across that mileage, I may get two or three random instances where the turbo just isn't there. This can be in any gear and not necesarilly at high boost levels. I may be able to go through the gears with no boost acheived in first or second, but then acheive boost (from low down) in third to sixth. I've done enough miles that I can feel the point (at a light pedal pressure) where I'd expect to get a gentle push, but it's just not happening.

I am reiterating this because I believe (feel free to shoot me down in flames again) that a split hose will be a split hose and that it cannot rectify itself to acheive max boost 99 times out of a hundred.

[Error404]

Two likely reasons:
MFTS fault - the ECU is getting an overheat signal, likely to be caused by the connector or terminals being damaged or corroded)
Knock sensor fault - intermittent sensor dropping out or wiring fault

I would check the MFTS first by unplugging it. I've disabled the overheat boost cut function on my car by removing the pin from the T8 connector at the fire wall.if you are too dumb to notice the car is overheating you deserve a fried engine.

[Greg_S]

Also check the o ring on the inter cooler. Have a search for the 'mark h' inter cooler mod as the oring isn't up to the job and is a common source of boost leaks. When you have exhausted some of the other options listed also worth checking the diagpharm on the waste gate and its operation in case it is stuck open and bypassing the turbo too

[Rusty]

If you have a laptop and £10 you can get a vag com lite lead and read codes. If this problem is intermittent then codes are the only place to start. Spring boost can be triggered by a number of sensors and scenarios including failing cam sensor, MFTS, altitude sensor, knock sensors, N75...

You don't have to permanently install a boost gauge, just have one handy for when you need to fault find. I've done this a few times on other cars and it is invaluable.

[LYT4X]

That reminds me, have a vent put by for you John, totally forgot about that sorry.

Can bring it along to ATIPS or pop it in the post if you'd like it sooner.

Andy's list above is how I'd go about diagnosing the fault.

Got a couple more folks who need invites, will drop you a PM

Cheers Kit. No fuss about the vent - the blue car is currently a bare shell so it's not going to matter just now.

Great on the invites - PM (or e-mail) is good.

[LYT4X]

Mike, Andy, 404, Greg & Rusty........ Thank-You!

[LYT4X]

An update to this.

Charlie (Cardcrash) put the VAGcom on the car at the weekend and pulled the codes which showed nothing out of the ordinary.

We were actually looking for an new problem as the car was only firing on four cyclinders. Turned-out that during a week-long stint at the detailers, one of the (6) plugs on the bulkhead at the back of the cylinder head wasn't attached properly or had suffered some water ingress. So, the missing spark is now resolved.....

However, yesterday I took the car out and she was faultless. Then, just as she's been doing occasionally since just before this thread was started, I lost all traces of a turbo. Car drives just like a perfectly good normally aspirated machine.

So, VAGcom isn't showing anything, the car boosts perfectly well most of the time, but occasionally the turbo just isn't there.

One more thing which Kit, Charlie and I discussed over the weekend - I have a very enjoyable 'chirp' on gear change throttle-off. Very much 80's rally style of chirping. Whilst it sounds great, it shouldn't be happening. Related issue perhaps???

[rustybullethole]

Hi John

Only just found this thread and read all through it, although i dont have any answers for your lossof boost i can tell you that one of my RS2`s also "chirps" on the gear change but the other doesnt, so am waiting and watching this for a response from someone who knows.

[LYT4X]

one of my RS2`s also "chirps" on the gear change but the other doesnt, so am waiting and watching this for a response from someone who knows.

The word at the weekend was that this was a fault with the dump valve which sits below the N75 valve. There's perhaps a fair chance that the two 'issues' are related, as apparently a chirping dump can cause turbo stall......

Bit of a shame really, as I adore the chirpy sound.

[andy10v]

Sounds like you have any issue with the DV, if you pressurize both sides of it when you do a boost leak test it will not show up. You can use a mightyvac on the vac line to check it holds vacuum or if the diaphragm is torn.

[S2central.net]

The word at the weekend was that this was a fault with the dump valve which sits below the N75 valve. There's perhaps a fair chance that the two 'issues' are related, as apparently a chirping dump can cause turbo stall......

Bit of a shame really, as I adore the chirpy sound.

The chirpy sound is turbo stall - caused by the compressor blades moving through a pressurised mass of air because the dump/recirc/bypass valve hasn't got rid of it all during the momentary vacuum between gears.

The issue you have smells like an intermittent electrical niggle or an occasional boost leak. The ECU doesn't show up faults as the wiring is usually good - it needs a long time to detect an open or short circuit condition on various sensors.

As already mentioned - check MFTS and MAF wiring. Also check the TPS and the WGFV. With VAGCOM running you can see the signals from TPS & MAF while you wiggle connectors etc

High temps or old age could cause intermittent behaviour of the WGFV (N75) even if the electrical connection to ECU is sound. Try unplugging the WGFV and take a short drive - if the car feels the same as when the fault crops up then you are on the right track.

Knock sensors have been mentioned - I doubt its those - the ECU doesn't shut off boost control unless it detects something major - like the MFTS alarm or multiple knock events - or the internal MAP sensor reading much higher than target boost.

Only way to see what & when the boost control is bad is with a boost gauge - just install one temporarily inside the car by T'ing into the boost hose behind glovebox at the ECU.

For boost leaks - the only way to be sure is with a leak test kit already mentioned - pay special attention to the TB hose which can cracked at rear/underside out of sight.

HTH

Paul

[macspring]

I know it is better to test the items systematically as Dave and others have suggested, but this is a now valuable RS2 that you own and as such should have a decent amount of love lavished on it and money invested in it.

On occasions with these cars you have to put value for money into second place to get the car 'right'.

As fault pulling and boost leak checking is a hassle for you, I would suggest that you 'invest' in a new N75, and new decent branded MFTS and a new MFTS brown connector and check and tidy / fettle the metal pins in the brown connector at the time of the change over. And also a new by pass valve. Even more absolutely so if any of these parts are original.

Stuff that is 20 years old which has had numerous heat cycles will not last forever, and a new part will be perfect and can only be 'better' than a 20 year old part.

Your RS2 is worth it, apart from having reliable boost for your own driving enjoyment, but also for peace of mind when overtaking.

Not the conventional answer to the problem, but it still make sense.

Let all of us know what happens / what the cure is.

Regards.

[UrS4boy]

One more thing which Kit, Charlie and I discussed over the weekend - I have a very enjoyable 'chirp' on gear change throttle-off. Very much 80's rally style of chirping. Whilst it sounds great, it shouldn't be happening. Related issue perhaps???

Chirping is literally for the birds (the feathered ones), *NOT* modern turbos (even if it is 20 years old).

You might want to do some investigative reading:

N75 info: http://forums.quattroworld.com/s4s6/msgs/20987.phtml

ByPass Valve info: http://forums.quattroworld.com/s4s6/msgs/20994.phtml

[LYT4X]

Firstly, thank-you all for your assistance / guidance.

should have a decent amount of love lavished on it and money invested in it.

Rest assured, this one has no concerns in that respect.

I would suggest that you 'invest' in a new N75, and new decent branded MFTS and a new MFTS brown connector and check and tidy / fettle the metal pins in the brown connector at the time of the change over. And also a new by pass valve. Even more absolutely so if any of these parts are original.

May not be 'the conventional answer to the problem', but as you say - it still makes sense and it's very much the way I tend to approach ageing items. The car has just had everything replaced underneath, as well as a very expensive, week-long professional detailing. The only issue is that she's not the only one in the stable, so she can be left for a while in preference to one of the others, hence the lack of urgency to get it fixed.