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Discussion Starter #1
Hey guys,

So here’s the deal;

My GT TDI 130 has a severe lack of boost, you can hear the turbo spooling up, but accelleration is very poor. No signs of black smoke, engine runs really smoothly and does high 50’s MPG.

A friend of mine has VAGcom, which was plugged in and brought up the code “Mass Air Flow sensor open circuit/short to groundâ€￾. Assuming this was the problem, the component was replaced with a brand new one, and this has not fixed the fault, it keeps reproducing the same fault code. 
If I unplug the MAF sensor it doesn’t make any difference to the engine running.

The MAP sensor seems to give sensible readouts via VAGcom.
The VNT Actuator movement seems free.

Any idea what else it could be? If it’s a vacuum leak, would it still cause an error code? Would I still get no boost?


Thanks in advance guys!
 

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Normally that's a wiring problem - check for bent pins in the harness or rubbing wires. Inspect underneath the wires too because it can hide a rub through spot.

Also check EGR function and the VNT. Even if it looks like it's moving you really do have to put a vacuum gauge on it and see the range of motion and at what vacuum it stops moving. The EGR can cause problems because if the intake air is displaced by the EGR gasses the MAF can get screwy. Most likely is a vacuum line problem.
 

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Discussion Starter #3
Thanks! :) I'll check the wiring and vacuum hoses as my first port of call. I have been planning on checking the continuity between the MAF, N75 and EGR plugs back to the ECU anyway, but keep relying on the aweful british weather, which keeps letting me down :(
 

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yeah, it transpired that there was a break in the wiring inside the plug boot that plugs into the MAF. i soldered it back to the pin and instantly regained boost. :) i think the open circuit caused the ECU to go into limp mode.
 

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Discussion Starter #7
Very true, it took a long time searching and checking things.
The fault code (using VAGCOM) was “MAF sensor open circuitâ€￾, so I replaced that first of all. that made no difference, so I got a pin-out of the ECU and checked continuity from the ECU back to the pins in the MAF plug. One of them had no continuity, so I traced the wire, no breaks or snagging, so I opened the plastic boot on the back of the plug to find the wire had come away from the pin.
:nana2:
 

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According to the manual on my car pin 1 and 2 should have 12v with key on. Pin 2 and 3 5v with key on. That part tested fine. The N75 is supposed to have 25-45ohms mine showed 8ohms. Egr supposed to be 14-20ohms mine showed 10ohms. Maybe that's why swapping them didn't do me any good, both of them are bad. I'm going to have a guy with a VCDS diagnose for me soon.
 
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