Things that will kill boost on a 20VT I5 engine

[UrS4boy]

In general, the amount of boost in a 20VT (RR, 3B, AAN, ABY and/or ADU) engine is controlled by the Motronic ECU - based on a mechanical feed back loop from the intake manifold to the Manifold Absolute Pressure (MAP) sensor inside the ECU can and the output from the ECU to the N75 Wastegate Frequency Valve (WGFV). That said there are a number of issues that could decrease boost. These are listed below the diagram



Things that could limit boost on your 20VT (RR, 3B, AAN, ABY and/or ADU) engine:

1. Moisture Trap: Lack of connection between the intake manifold and the MAP sensor, e.g. hose off at the Moisture trap: http://forums.quattroworld.com/s4s6/msgs/20401.phtml

2. G71 MAP sensor: If the hose between the intake manifold and the G71 MAP sensor is connected and not split, there might be an issue with the G71 Manifold absolute pressure (MAP) sensor (but this is very rare). Nevertheless, G71 MAP info: http://forums.quattroworld.com/s4s6/msgs/20417.phtml

3. Disconnected or failed N75 WGFV - if the MAP sensor in the ECU is fine but the electrical connection between the ECU and the N75 is broken or disconnected or the N75 has a failed, you will only get mechanical boost based on wastegate spring pressure. N75 info: http://forums.quattroworld.com/s4s6/msgs/20987.phtml

4. Wastegate spring/membrane: The membrane might be torn (not very common) or the spring broken or the piston stuck. SJM Authotechnik has good info on this: http://www.sjmautotechnik.com/parts_...ts/wginst.html

5. G70 Mass Air Flow MAF sensor issues including the connector: will lead to the ECU going into limp mode

G70 MAF Info: http://forums.quattroworld.com/s4s6/msgs/20442.phtml

6. Boost Leak - the elephant in the room - you can not ignore this. You can not say that the hoses look good. You *HAVE TO* do the test and pressurize to around 1 Bar to find the splits in the hoses, etc. that normally look fine but open up under pressure.

RR, 3B, ABY and ADU boost leak testing: http://www.s2forum.com/forum/showthr...t+leak+testing

AAN boost leak testing: http://www.s2forum.com/forum/showthr...t+leak+testing

7. F76 MFTS: The F76 Multifunction temperature sender has different functions in the ABY and ADU than the AAN. For all three, the MFTS is the sender for the temperature gauge in the instrument cluster. On the 3B, RR, ABY/ADU, the MFTS is also connected to the ECU via pin 46 of the T55 ECU connector. Using this pin as a high temp alarm (> 119 C) - the ECU cuts off active boost control as a safety measure. However, for the AAN, pin 46 is not used. On the AAN, the MFTS F76/R pin connects to is the HVAC/climate system to shut off the compressor during such high temperatures. In addition, the MFTS is connected to the coolant temperature gauge.

If the MFTS fails on an RR, 3B, ABY or ADU it will affect the coolant gauge and potentially the boost via the ECU pin 46.

F76 MFTS info: http://forums.quattroworld.com/s4s6/msgs/22584.phtml

8. Knock and knock sensors: If you run crap gas/petrol, as the ECU increases the boost, there could be could be knock/pinging/pinking. The 20VTs have two knock sensors, G61 and G66, that when working detect this knock and signal to the ECU. The ECU in turn dials back the timing (and therefore boost). This could also happen if the G61 and/or G66 fail or are disconnected. Therefore something else to check:

G61 and G66 Knock Sensor info: http://forums.quattroworld.com/s4s6/msgs/21047.phtml

9. Ignition system - no brainer. Coils and plugs need to be in 100% shape. Any missing or hestitation that results from a less than perfect ignition system will impact boost.

One missing and hesitation diagnostic: http://forums.quattroworld.com/s4s6/msgs/22611.phtml

10. By-pass Valve - I debated about including this one. It is more important to maintaining boost between shifts and than the absolute boost that is made (assuming the case of the BPV isn't split, it's a closed system).

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

IF I missed something, let me know and I will add it. (I just did this off the top of my head, using links to info that I have compiled previously).

[newsh]

Bad MAF or MAF connector problems or corrosion will trigger fuel pump cut and limp mode.

[UrS4boy]

Bad MAF or MAF connector problems or corrosion will trigger fuel pump cut and limp mode.

MAF is item 1 in my Missing and Hesitation Diagnostic: http://forums.quattroworld.com/s4s6/msgs/22611.phtml linked above in Item 8.

But after lunch I will give it its own listing as an item.

[UrS4boy]

MAF is item 1 in my Missing and Hesitation Diagnostic: http://forums.quattroworld.com/s4s6/msgs/22611.phtml linked above in Item 8.
But after lunch I will give it its own listing as an item.

Done. Add the G70 MAF as item 5 and shuffled old items 5 to 9 to new items 6 to 10.

[Dynasty]

I am in the process of trying to figure out why my C4 S6 only produces 1.8(absolute) bar of boost pressure. Any tips on where to start looking?

[Error404]

Yes - How about all the links in the first post

[UrS4boy]

I am in the process of trying to figure out why my C4 S6 only produces 1.8(absolute) bar of boost pressure. Any tips on where to start looking?

Yes - How about all the links in the first post

Could not have said it better. Why else would I go to all that work.

That said, if the N75 WGFV isn't working, all you get is Wastegate spring pressure, which, on a stock engine is about 0.4 Bar relative (1.41 Bar absolute). This is less than the stated 1.8 bar absolute so either the WGFV isn't working and the WG has a 1.9 Bar spring inside *OR* there is a boost leak. I would start with disconnecting the WGFV then check to see if the boost drops to say 0.4 bar relative (1.41 bar absolute). If it does, then there is a boost leak. If it stays at 1.8 bar, the WGFV is the issue and the WG has a 1.9 bar spring inside.

[Dynasty]

Could not have said it better

Wrong. You said it better by reducing the 10 point list to a 2 point list. Thx for the tips.

[John.]

A lot of info there, thanks for posting. I'm going to be checking mine over soon.
Great work as ever!
Cheers Vancouver Chap!!

[Rikki Kitto]

Yes - How about all the links in the first post

[macspring]

Wrong. You said it better by reducing the 10 point list to a 2 point list.

Lovely, just lovely !

[macspring]

(I just did this off the top of my head, using links to info that I have compiled previously).

Thanks for persevering Dave.

[UrS4boy]

Thanks for persevering Dave.

No problemo. Now we have a resource to point people towards when they need ideas why there boost is low. Finding the true answer is a process. Check the easy (and free) stuff first and move up to the stuff that is more difficult and/or costs money.

[Balnas]

Nice thread ! Thanks for info, i think a lot of us will use this

[maiks]

Maybe I did not notice and someone has already written about that here.
I had problems with EM to turbo sealing, it leaked and turbo did not spool correctly. Pretty difficult to check.