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Is my MAF or N75 knackered? Please take a look at graphs

3928 Views 2 Replies 3 Participants Last post by  Fluid
Hi all,

I'm hoping you may be able to help me.

I have an Audi A4 130 TDi (AWX) that also has poor mpg. Even when driving it like a granny I can only at best get 46mpg.

I have changed the MAF sensor last year, and recently the coolant temp sensor and thermostat. Car is recently serviced with all new filters.

It runs fine but just has poor mpg and occasionally takes a few cranks to start up (both with hot and cold engine).

I suspected an N75 fault due to the following log of block 011:

To check this I have run the car with basic settings group 011 running that cycles the N75 valve at 1407 rpm and moves the actuator rod up and down. The rod seems to move ok and I can hear the exhaust note change and can hear it boosting. Checking the actual boost values also confirm that the car comes onto boost when the N75 pulls the actuator rod.

I then logged block 011 again and now get better and less erratic actual boost and duty cycle readings as shown below:

Does this graph look ok to you? They were logged by accelerating at 100% throttle in 3rd gear and repeated twice, hence the 2 curves.

I also logged the actual and specified MAF values at the same time and these are shown below:

I have checked all the vacuum hoses and turbo hoses and all seems ok but to be on the safe side have replaced the 90 degree bend hose that goes into the inlet manifold as the sprung clip connection was a little loose and have also replaced the o-ring seal in the sprung clip connection on the pipe from the intercooler to the rigid bit of pipe the G31 is connected to.

When accelerating I notice a slight reduction in pull at around 3000rpm which from past experience may indicate a MAF problem but this was only replaced at the end of last year. When I replaced it I noticed an increase in responsiveness and accleration but no change in mpg.

Please can you take a look at the graphs and let me know if you think they look ok or are indicating a problem?

Many thanks

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1 - 3 of 3 Posts
The graphs look normal to me and 46 mpg isn't horrible. Is that imperial gallons instead of US gallons? The 18" wheels and more powerful engine won't help fuel economy. Automatic transmission will also reduce fuel economy. Do you have quattro? That will also reduce economy. Driving style is also a major factor.

The hard starting could be related to faulty or dirty injectors. AWX engines are pumpe duse (PD) so it's possible it has camshaft wear if it doesn't idle well. This is how you check: http://www.myturbodiesel.com/1000q_how_to/multi/camshaftInspection.htm (the AWX is single overhead camshaft, SOHC).

Also check camshaft torsion as well. Let me know if it makes a difference - I've never noticed much. Here is how

The N75 looks dead on. The first MAF run looks fine but the second does look like it dropped off early. However, if you stomped on the throttle in that run it could explain the intial spike. I wouldn't blame the MAF for the low mpg though. What MPG were you expecting?
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