Low boost - Suggestions required

[Chris P]

So, my car seems to only manage 0.5 to maybe 0.6 bar boost. Things i have done so far are:-

1) Plug into Vag Com = No errors
2) Pressure tested it = Went upto 1 bar with no hissing sound other than a slow hissing from inside the inlet pipe. It didn't hold the 1 bar pressure either but only slowly went down.
3) Waste gate frequency valve is new but no error on Vag com anyway.
4) Re-circ valve is new but did try another known working one and same boost.
5) Took all pipes off to check them and put all new clips on and tightend up. Same boost.
6) Ran car with waste gate unplugged and recorded 0.4 bar boost. Plugged back in and between 0.5 & 0.6 bar again.
7) Changed all vacuum pipes not to long ago.

So, based on the above, what would anyones next step be?

[AndyP]

Another quick question on this one, if the MFTS was faulty would it throw up a code on VAG-COM? The only fault code I can find relates to the G62 coolant temp sensor. If the MFTS is faulty I presume it pegs boost at mechanical, i.e. 0.4bar?
Other than that is a petrol can and a match, it'll never be as fast as mine

[Chris P]

Sorry, forgot to mention, it is a 1991 3B Coupe.

[AndyP]

Well after testing every sensor on the car it looks like this was down to a duff MFTS. Pulled the multiplug off and all the connectors were green with corrosion, tried the car again with it unplugged and got full boost. Chris phoned Audi yesterday for a price on a new one..........£103
I've just ordered a couple from 034, complete with a new plug and rubber boot, £40.

[RoadKill]

how di you get the car to drive with the maff unpluged , mine will not rev , pops bangs and farts , i still have boosting probs with mine , im just aboout to cut the maff plug off and put some push clips on the wires

[S2central.net]

Its the multifunc temp sensor (MFTS) he unplugged... Three signals of interest in there...

One goes to coolant temp guage.
Alarm-1 goes to Autocheck
Alarm-2 goes to ECU

Its the pin to the ECU that disables active boost when water temp is excessively high.

[giorgioS2]

Its the multifunc temp sensor (MFTS) he unplugged... Three signals of interest in there...

One goes to coolant temp guage.
Alarm-1 goes to Autocheck
Alarm-2 goes to ECU

Its the pin to the ECU that disables active boost when water temp is excessively high.

This doesn't show an error on Vag Com?

I'm having low boost problems and checked for leaks, n75, no errors on ECU, no bad fuses...

[S2Fan]

Hi, i'm currently having the same problem , 91 coupe quattro, 3B, will try the above and see if i can boost higher than o.5bar...
thanks.

[andy10v]

This doesn't show an error on Vag Com?

I'm having low boost problems and checked for leaks, n75, no errors on ECU, no bad fuses...

It didnt show on vag com on my 3b, new sensor/plug sorted it though.

[giorgioS2]

It didnt show on vag com on my 3b, new sensor/plug sorted it though.

unplugging the MTFS would temporary solve the problem? (to test)

[UrS4boy]

Hi, i'm currently having the same problem , 91 coupe quattro, 3B, will try the above and see if i can boost higher than 0.5bar... thanks.

Two things:

1. Moisture Trap - if the green vacuum/boost hose that runs to the moisture trap and then from the moisture trap to the ECU comes off the moisture trap, your engine becomes mostly naturally aspirated because you only get the boost based on the wastegate spring:

http://forums.quattroworld.com/s4s6/msgs/20401.phtml

2. Multifunction Temperature Switch (MFTS): On the ABY and ADU and probably the 3B, the temp signal goes to the ECU to cause the ECU to lower boost if the coolant temp goes over 119C. If that signal is absent, the ECU probably is programmed to assume the worst and therefore to lower the boost:

http://forums.quattroworld.com/s4s6/msgs/22584.phtml

Both of those links came from this Engine Bay Device Map with its live links (click on the label of choice to learn more about that device):

http://members.shaw.ca/speedtek/UrS%20Engine%20Bay%20Device%20Map.html