View Full Version : low boost

10-11-2010, 08:01 PM
I have 1997 A4 1.8T - bought used - turbo replaced, new N75 valve. Car has never had good acceleration- I measured boost at manifold and it peaked at 3psi. Put in a manual boost control- ran great- plenty of boost right up to the surge! I have vag-com just not sure how to troubleshoot wastegate setting, n75 duty etc. Check system for leaks using air compressor/tennis ball


10-11-2010, 08:11 PM
Do you have vag-com? That would be very helpful here.

3psi sounds like wastegate spring pressure. So either the N75's atmospheric input is blocked (unlikely), or it simply isn't getting a signal. Usually "limp mode" will give you about 5 psi, but we shouldn't rule that out either. So first thing to do is check for codes with vag-com (generic scan tool may not reveal everything here).

If no codes, the next thing I would do is an output test on the N75 to make sure it is working. That will test the power supply, the wiring, the valve itself, and the ECU ground signal all at once, so it cuts out a lot of messing around. It should click back and forth about once per second during the output test. During that time you can check it for flow by blowing in the pressure port. It should alternate between flow and no flow if it is working correctly.

That is pretty much all there is to checking the boost control system. You've already checked the turbo/exhaust/boost leak avenues by using an MBC and making sure the car can actually make boost. You could use vag-com to log the n75 duty cycle, but if it isn't giving a signal it should show a fault code for "why" in any case (ie: too lean, no maf signal, etc). Not a bad idea, though. I guess that would my 3rd step if the first 2 didn't find anything. In summary:

1 - check for codes
2 - check n75 valve and circuit using output test mode
3 - check ecu for boost request (n75 duty)

10-14-2010, 08:57 PM
cleared codes and drove with vagcom hooked up. 114 duty cycle went to 5.1 % and no higher. brand new N75 valve. I installed N75 per diagram --long port to intake, center to compressor output, shorter port to wastegate. not sure why the duty cycle called for is so low.

10-14-2010, 09:32 PM
Did any codes come back? I'd log the MAF next. Are you sure it isn't chipped and in some sort of "valet mode"? My brother has this option on his APR tune and it goes to minimum boost, peaking at 3 psi. Car is stupid slow, dangerously so. He has the security feature as well so he can put in valet then lock it up so you can't change the map until you "log in" with the special passcode via the cruise buttons.

10-15-2010, 01:20 PM
i bought car used so there is a chance it has been modified- i will pull out ecu and look-

no codes came up after clearing- had to clear because I put in a manual boost controller mentioned above-had misfires etc codes

10-15-2010, 10:18 PM
ecu stock-without codes ,I do not understand why the n75 is being commanded for low duty cycle. I will log maf- any other data I should log?

10-17-2010, 06:00 AM
ecu stock-without codes ,I do not understand why the n75 is being commanded for low duty cycle. I will log maf- any other data I should log?

I'd log timing pull on all four cylinders, Perhaps you have a bad knock sensor. That would command N75 into low duty cycle...and it may not set a hard code.

Other thought... Have a wifey or girlfriend? They didn't " accidentally " fill it with 87 Octane did they? [headbang]

Edit: These ECU's have a learning mode. If you did indeed get some bad gas it may take some time for the ECU to " re-learn ". Clearing codes will not do the trick. If you suspect a tank of bad gas run the tank till it's almost empty. Fill it with the highest Octane you can find ( What can you get in Colorado ? Shell V-Power 92/94 is pretty good in Canada. ), then disconnect the battery for 15 minutes.

Note!!!! If you have the stock radio, make sure that you have the radio code before disconnecting battery.

Then run a Throttle Body Adaptation on the car ( And yes, Drive By Cable cars do have an adaption feature. Check out Ross-Tech for procedure on DBC cars ). Then take car out and and wail on it.

10-17-2010, 06:21 AM
Procedure for TBA on DBC cars:


Throttle Body Adaptation:

Turn the key on but do not start the car.

[01 - Engine]
[Measuring Blocks - 08]
Group 060 (Group 98 in non-DBW engines)
[Switch to basic settings]

Once you do this you will see the top right display say ADP RUN. The TB
adaptation is being done as soon as you switch to basic settings. You will see the values change and hear the TB cycle for the first few seconds then it will stop. Leave it in basic mode for about 30 seconds, then switch back to normal settings and you're all set.

Be sure not to touch the accelerator and make sure the engine is NOT running when you do this!

10-26-2010, 11:46 AM
tried different ecu- 17967 error - tried adaptation per above- no luck as error in 4th field- clearing does not work- comes right back- found post on "fourtitude" saying pot on actuator shaft position slips- verified mine slipped. still could not adapt even moving pot. anyone know correct nominal voltage value for the second field so I can set and try before I bguy tb?

11-09-2010, 06:51 PM
Problem fixed-** bought used throttle body and it adapted without error - boost is normal-- in looking over my issue two things stand out-- the check engine light does not come on when car in the "safe" mode of wastegate 5%- I had a period of time where I had no fault codes and no boost-- maybe the n-75 was bad and the throttle body code showed up near the time n75 was replaced. I tried to fix the old throttle body by rotating the pot around shaft but it never would adapt.
Thanks all

12-23-2010, 09:45 PM
went thru bad coils and torn turbo inlet hose- what is proper N-75 duty cycle at full throttle- I have not seen over 50%.
Have vagcom but not sure what codes to look at other than 114-