Cayenne MPG after PCM upgrade - am i going mad...?

[williaa68]

I had the £150 PCM 3 upgrade done on my 2008 Cayenne S and am very happy with it. However, the MPG on the car, which was never great to start with, seems to have gone down! I used to get around 13-14 around town and about 21-23 on a run. I now get around 9 around town and 17-18 on a run. I am relying on the onboard computer for MPG and haven't yet done my own checks via recording fuel and mileage etc. Is this likely just to be a software problem or is there any chance that the PCM upgrade also did something (bad) to the ECU? All thoughts welcome. Car is under warranty and I have a durametric somewhere from when I had my 997 so could do some home diagnostics (although i am not a particularly skilled user!)

[Keggers]

Just a guess, but has it been incorrectly programmed to show US mpg rather than imperial mpg?

The difference seems about right (1 US mpg = 1 imperial mpg).

[williaa68]

thanks and good suggestion but it does say MPG(UK) on the trip computer so am guessing not. curiouser and curiouser....

[Keggers]

sorry, meant 1 = 1.2, but sounds like its not that anyway!

[onlynik]

Just a thought which is probably incorrect but are the correct tyre size selected in the tpm system. Don't think it would be connected to the mpg but just a thought.

[williaa68]

onlynik said:

Just a thought which is probably incorrect but are the correct tyre size selected in the tpm system. Don't think it would be connected to the mpg but just a thought.

That's an interesting point. Will check - thank you

[jackal]

Best to ignore it in any case as it wont be correct either way.

14mpg overall sounds right

[williaa68]

So, plugging in the durametric gives the following error code / message:

Siemens Digital Motor Electronics SDI4

Current Fault Codes

8121

Implausible signal

(picture attached).

A bit of googling suggests this may be a thermostat problem and expensive to fix but hopefully under warranty.

[thegoose]

I've noticed that on my (2006) S the actual mpg I calculate from fuel used/miles driven is at least 10% lower than the dash reports - so maybe you now have an accurate reading, not an artificially high one.

It amuses me that using the same end of stalk button that VW use, and which steps straight to mpg, on the Cayenne it takes between 10 and 12 "keystrokes" to get the same info. It's almost as if they didn't want you to find it, now I wonder why that would be?

[robrobc]

thegoose said:

I've noticed that on my (2006) S the actual mpg I calculate from fuel used/miles driven is at least 10% lower than the dash reports - so maybe you now have an accurate reading, not an artificially high one.

It amuses me that using the same end of stalk button that VW use, and which steps straight to mpg, on the Cayenne it takes between 10 and 12 "keystrokes" to get the same info. It's almost as if they didn't want you to find it, now I wonder why that would be?

Running a 2005 Turbo I find it a lot less stressful if I just put fuel in and then mutter "Hey Ho It is only money" and do no calculations