Low boost below 2000-2500 rpm 1.9 dci FAP

[megowner2k]

Hi all, I've been struggling with a problem on my 06 Megane for quite a lot of time and I basically gave up looking for solutions, but hey, you never know . I'll try to explain as best as I can: the car has a severe lack of power below 2000-2500 rpm and an annoying hesitation (power hole) when shifting up or down for about 1-2 seconds. Since the mechanics around here only know to replace perfectly functional parts, I've so far replaced an EGR valve, MAF meter, and MAP sensor, lots of $ for nothing. So one day, the dpf light came on, did an active regeneration at the local garage and WHAM, the car was going like crazy, no lack of power, really responsive throttle and no hesitation. Only went well for about 100 miles, then started to gradually feel sluggish again. Decided to remove the DPF and remap the ECU but the car still doesn't take off. I went for a power buff since then at around 140hp but it's still pretty much dead below 2k. One thing that I dug up recently as I went on the road with the clip plugged in is that the actual (1300 mbar) turbo pressure is much lower than the requested pressure (2000 mbar) by the ECU below 2000 rpm at full throttle. Mechanic said it could be that the turbocharger's vacuum solenoid doesn't build enough vacuum. I've done some measurements and got 880 mbar at the vacuum pump and around 500 mbar for the solenoid. No error codes, no lights on dash. Sorry for the long post, I'm looking forward to debate this issue with you guys. Thanks.

TL;DR there's not enough boost below 2000-2500 rpm, ECU requests 2000 mbar and the turbo only puts up around 1300.

[megowner2k]

Cmon fellas, don't be shy

[megowner2k]

Quick update - replaced the boost pressure solenoid, no change whatsoever; still lacking power below 2k rpm.

[AlexB]

I don't know. Please feel free to have look at my OBD diagrams. It's F9Q-800, 1.9dci, 120hp, 8 valves, no FAP.
https://www.tboneandporcini.pwp.blueyond ... strun1.jpg

[megowner2k]

Thanks, I'll take a look. Meanwhile, I recorded the VNT actuator rod movement. It starts moving at around 5-10 mmHg vacuum and stops at around 400mmHg (533mbar). Which is weird, because the service manual says replace the boost pressure solenoid if it doesn't develop 700-800 mbar vac at idle. I believe this is the way a clean variable geometry should work. If anyone would have the possibility to test this on their F9Q FAP engine, that'd be great:)

https://www.youtube.com/watch?v=tXs2D-- ... e=youtu.be

[AlexB]

What manual are you talking about. The factory manual says the following (F9Q-804 engine):

- For a pressure value of 200 mbar , the rod movement
should be between 2 and 4 mm .
- for a pressure value of 550 mbar , the rod movement
should be between 8 and 11 mm.

It's in the sticky thread with manuals, above this one.

[megowner2k]

Solenoid valve operation check:
– Attach the pressure gauge to the solenoid valve outlet union,
– start the engine and stabilise it at idle speed,
– if the vacuum pressure does not reach 800 mbar ± 100, replace the solenoid valve.

This is what I found. In which manual exactly did you read that?

[AlexB]

MR364MEGANE1.pdf Engine and peripherals

It's in the sticky topic above this one. It has the word "manuals" in the subject line. See chapter "turbocharging", engine F9Q-804.

[megowner2k]

Well, I must be blind, cause I can't seem to find that part anywhere. I could only find preacutions as to not adjust the length of the control rod.

https://www.getflat.co.uk/phcdownload/up ... EGANE1.pdf this is the manual I'm reading.

[AlexB]

Yes, you are right, Ray has updated the collection with the 2009 edition of the files. Please look in the older 2003 document:
https://www.uas-bg.com/meganii/eng/MR364MEGANE1.pdf

Search it for "200 mbar".

[megowner2k]

Thanks, I don't know why they say you need to take the turbo off the car, probably to be able to measure the length. I don't know, does the rod look like it moves more than 10mm to you?
Anyway, I posted another video with the vnt rod and engine running, seems a bit erratic to me, but what do I know

https://www.youtube.com/watch?v=8Frt5dR ... e=youtu.be

[AlexB]

It seems to be moving fine. When you keep revs high at idle, the engine load I quite low. So, the ECU reduces the boost. The linkage moves only when the revs are sharply increased, which demands power to accelerate the flywheel. Then, the exhaust flow is increased, and VGT reduces the pitch of the vanes. Then, the speed becomes constant again, low exhaust flow, and the pitch is at max again.

The VGT actuator is included in the control loop. Hence, any small deviations in its sensitivity are compensated by the ECU. I think, the problem is somewhere else. When you move the actuator by hand, is it smooth? There also can be lack of vacuum at low revs... Just guessing.

[megowner2k]

Very nice explained, Alex B I tried superficially to pull it up and it didn't go anywhere, I'll try again tomorrow morning when the engine's cold. Meanwhile, I've seen our neighbors back at TDI forums having similar problems:

https://forums.tdiclub.com/showthread.php?t=361736

https://uk-mkivs.net/topic/218399-low-bo ... con-turbo/

They say they've been having problems after fitting reconditioned turbos on their cars. I bought this car almost new, at 20k kms. It's been like this since I bought it, but I never really knew how it was supposed to drive till after the first DPF regen. Who knows what the previous owner did to it...

[fredcamp2]

Turbo's are designed to give boost immediately which gives the engine low speed torque. Vacuum operated turbo on this type the actuator starts to move at 5inhg & is fully retracted at 18inhg. At start up the actuator rod fully retracts & stays that way. The ecu then controls the turbo boost solenoid valve & is also known as a dump valve because it has 3 ports. But on your 06 model theres a allied signal turbo fitted which works on a 0.5 bar turbo pressure (not vacuum) & needs to be removed to check properly Hth Regards fredcamp2

[AlexB]

fredcamp2, the turbo boost solenoid valve is not known as a dump valve, there is no dump valve in this engine, and dump valves are not given such a name due to having 3 ports...