causes of machanical boost?

[s2tommy]

can anyone help?? my car will only run on machanical boost!! i have done everything known to man kind!! checked, Cleaned. done all boost leaks an vac leaks. And vag com is not puling up any codes at all! at a dead end and dont know what to do next anyone got any ideas? cheers tom

[dkw_dude]

N75, if it's a 3B it could be the big temp sender under the water manifold (3 or 4 pin)

[s2tommy]

Have changed n75 but will be tryin another newish one 2moz. And changed the mfts switch alittle while ago 4 a known good one 4 testing as the cheapish I can get for is just over £100 so would like 2 be sure that was the problem.

Not gettin any ecu fault codes

[UrS4boy]

can anyone help?? my car will only run on machanical boost!! ....what to do next anyone got any ideas? cheers tom

If you are only getting mechanical boost via the wastegate spring, it means that the ECU is not controlling the N75 Waste Gate Frequency Valve (WGFV) (boost pressure control solenoid). Reasons for this include:

1. Boost/vacuum pressure signal is not reaching the Manifold Absolute Pressure (MAP) Sender in the ECU. Confirm connection and that there are no leaks in the hard plastic hose between intake manifold and the MAP sender.
2. WGFV is not plugged in. Check connection. If NOT plugged in, plug it the F in and kick your ***** for not checking this first.
3. WGFV is plugged in but it is fubar'd. Test the ECU and the WGFV together using the procedure at SJM Autotechnik: (http://www.sjmautotechnik.com/troubl...f20v.html#out3)
Fubar'd = http://en.wikipedia.org/wiki/FUBAR
4. The MAP sender is fubar'd (dead, tot, gone to meet its maker, not pining for the bleeding fjords).
5. ECU is fubar'd (see above) Not likely but always a possibility.

Check those out and report back. There has to be a reason. It just might not be cheap and/or might involve borrowing a know to be good ECU if 1, 2, and 3 check out.

[s2tommy]

Tryed all the above 3 or more times thanks for the pointers an will keep trying cheers tom

[s2tommy]

Right where I got 2! If I put a piece of vac pipe from inlet manifold stright to ecu still the same! If I put a 12v feed to the n75 then it clicks an it boosts! On machanical! There is a 12v feed down the normal n75 wires!! But can't get the ecu to read boost! Ecu ok an map sensor ok an all works fine in other car!! Anyone got any ideas??

[Ross]

Here's my understanding:

-The ECU is seeing and understanding the boost pressure
-The N75 is receiving a duty signal from the WGFV (which is known working)
-It's a new turbo
-Wastegate has been swapped for a known working one
-Boost leak tested
-It is boosting mechanically

If the ECU / N75 / WG / turbo are all mechanically sound and the ECU is getting a good boost signal then it has to be one of 2 things:

The ECU not pulsing the N75 - what inputs can cause it to boost cut / limp mode?
or
The N75 is not pulsing correctly, i.e. not receiving the signal from the ECU. Is it possible to log the WGFV signal at the plug and see what he N75 is getting

Best of luck Tom, hope to see some positive news on this

[UrS4boy]

Right where I got 2! If I put a piece of vac pipe from inlet manifold stright to ecu still the same! If I put a 12v feed to the n75 then it clicks an it boosts! On machanical! There is a 12v feed down the normal n75 wires!! But can't get the ecu to read boost! Ecu ok an map sensor ok an all works fine in other car!! Anyone got any ideas??

For sure you did this?:

3. WGFV is plugged in but it is fubar'd. Test the ECU and the WGFV together using the procedure at SJM Autotechnik: (http://www.sjmautotechnik.com/troubl...f20v.html#out3)

You have to test the ECUs communication with the N75, not just apply your own voltage to it.

[s2tommy]

yes have done all this and still trying but very close to the end of the road now!! we can pluse test the n75 throught the ecu and works,i get 12v with engine off and 14.2 withe engine running down the n75 wires. but still the ecu cant read any boost.

[prj]

Here are a few facts about this car:
* No boost leaks or mechanical issues
* N75 works fine
* N75 output test works fine in ECU, it clicks.
* Bridging Pin 30 (GND) and Pin 33 (N75 Switched) in the ECU gives me full boost.

So the ECU is not giving boost, however three different ones have been tried.
I'd think some sort of input parameter is going wrong, but:
* TPS works accurately
* Coolant sensor works
* IAT sensor works

Anyone has any ideas, feel free to let me know

[prj]

P.S. Altitude sensor works fine too...

[UrS4boy]

Anyone has any ideas, feel free to let me know

If everything you say is working, the one thing that you haven't said anything about is the MAP sender in the ECU. IMO, it must be knackered because it is the device that *should* tell the ECU what is going on boost-wise. No MAP signal to the ECU (other than atmospheric pressure), no ECU control of the apparently working N75 WGFV.

But then you will say the ECU works in another car.

If that is true, then I have to go back to an issue with the boost/vacuum hose to the ECU.

[s2tommy]

Map sensor ok ecu works fine in 2 other cars with their chips in it but with same 4bar map sensor! We just hard wired it! It clicks into live wen u first turn engine on but that's it does not pulse at all again!!

[s2tommy]

And still no ecu error codes!

[Jamo]

What about the obvious the wastegate is knackered, split diaphram or is stuck open