DavidGR

Any help with the following much appreciated.

My 02 996TT was boosting normally (0.7/0.8bar on the guage) and I noticed one day it felt slow. For a couple of weeks it would make normal boost when started from cold but after it was parked up warm and re started it made only 0.4/0.5 bar. This continued for a couple of weeks until it became permanently stuck at 0.4 bar, occasionally 0.5.

No error codes recorded, no CEL.

So, I set about investigating and have:

Replaced 2 crusty actuators, set them up to 4mm movement at 7.5psi as per the factory manual.

Freed up the wastegates so they move freely by hand when the actuator is disconnected

Replaced the N75 valve

Checked all the check valves and the venturi

Pressure tested for leaks - there is a tiny hiss at the N75 to wastegates - now zip tied and airtight. There is still a very very slight leak but it takes over 60s to lose pressure from 10psi

Tried another MAF lent to me by a friend with a fully functioning car.

Whilst everything was out I changed the coil packs and spark plugs.

Turbos both look good from the inlet side, spin freely and equal and minimal play in the shaft.

Disconnected the battery to reset the DME.

Initially on putting everyting back together the car ran a steady 0.4bar.

When I tried my friends MAF from a cold start I got back up to 0.7bar - when I stopped the car and restarted it 0.4 bar was restored. Exactly the same behaviour with the original MAF back in...

Stumped.

Any ideas from the collective wisdom?

Thanks

Kevin

I recommend datalogging your fuel trim and request and actual boost, and injector duty cycle. Post the results.. Did you pressure test the entire intake system or partial.

Did you TEST your DV's >>pull vacuum on the control port.

DavidGR

Thanks.

I've only got a very basic code reader - fault codes only and very limited data logging. What are you thinking may be at fault? I'm getting close to taking the car to my local indy to let them tear their hair out over it... I can recommend they log this with their kit...

Yes, I tested both DVs off the car - both holding vacuum from the top port.

Pressure tested from the intake hose post MAF and also at the turbo inlet.

David.

RngTrtl

you can get an OBD-II BT or WIFI reader that plugs into the port for <30 bones of ebay/amazon. Then download the Torque app and you can log many things and see what the ECM is getting as far as data input.

TeCKis300

Do the wastegates move without binding once everything is connected?

Just throwing other ideas out there:
1) Intake plumbing pre-turbo collapsing under vacuum?
2) Cats plugged?

DavidGR

Thanks. I had a look at this and the Torque app seems to be Android only which is a pest for this iphone user - is there a desktop equivalent? I did find EOBD-Facile / Ultimate which looks similar with a BT or WiFi dongle. Any experience?

DavidGR

Wastegates are free and opening smoothly when 7.5psi applied to the lines.

SFAIK the intake pipes are all rigid tubes aside from a very short rubber coupler (both of which are new) - happy to stand corrected

Haven't checked the cats - the bolts holding turbos / exhausts on are seriously corroded so I'm trying to leave this until last when everything else has been checked. I suspect that because I can get occasional full boost they will be fine but I'm struggling to see why the fault would seem to be temperature dependent.

Thanks

docboy

Sorry for the hijack OP... I'm curious to know, for a stock 996tt manual tranny with no tune and no mods, what is the "normal" boost level? Is 0.7-0.8bar normal?

I have a UMW tune, running on 92 octane; is 0.7-0.8 bar boost also expected?

Kevinmacd

Mine is .6 - .7 for standard turbo stock.
Most tunes bring standard turbo to 1.0 bars.

TeCKis300

Sound checks that you have already made. I point at the wastegates, as I've seen situations where when cold, they open fine, but bind once warmed up. Not sure if your wastegates are stock (stocks are less finicky), but with some aftermarket ones, the rod angle along with how the rods are pre-loaded can cause binding.

RngTrtl

I despise all things Apple so I cant help you with an app, but I am sure that if you go to the iStore(or whatever its called) and do a search for obdII or something that there is some equivalent app for the iPhone.

NTY

I experienced the same symptom once. Generic scanner such as Torque app doesn't do it. My car did not throw an engine error light. I got it scanned at a workshop with proper a scan tool and ecu recorded a boost pressure over limit error. It sends the car into a limp mode and limits its boost to 0.5 bar. Erased that error and it never returned

TheDeckMan

Sounds like there is a small leak still, I found on mine the hard plastic tube under the Y-Pipe that when it got hot the rubber hose that connected it would weep under boost. When it was cold outside it would not put the car into limp. Check that hose and make sure to zip tie it.

32krazy!

check the boost line at fuel pressure regulator on the pass side. often pops off

docboy

This is a good thread. It would extremely helpful if someone could share a pic or link with pictures for the DIY new on places to check for boost leaks. Does any such pic exist?