2005 A8 D3 4.2 (BFM) - Low MPG

[MatthewH]

Originally Posted by Mechcanico Lee

The continuous running cooling fans are a clue here , this is a sign of the ecm not seeing the correct coolant temp reading and running fans as safety default

Coolant Temp sensors ..... is the ecm data block showing correct temp ?
Does it have two temp sensors ...one in engine block somewhere near cyl heads usually ,and maybe radiator outlet temp aswell ?

The lamda codes of lean mixture is strange like you say , are the pre cat lamdas 4 wire or 5 wire wide band with a red or pink wire on the 5 wire type ?


In data blocks for lamdas does it have a block called 'lamda equivalence ratio ' if it's five wire it will have , if it's 4 wire it will have a cycling voltage between 0 volts and 1 volt or 0 and 4 volts ...the cycling voltage is correct ( lamdas switching from rich to lean ) if lamda Volta sticks one way or the other it's over rich or to lean ....drive the car up the road at a steady pace and watch what these values .

Also have a look in the data that it says it's in closed loop mode , the running cooling fan default mode may keep it in a open loop situation
The running fans has to be your first port of call here , sometimes the fans control module can go faulty and put the fans on also ....you normally get codes for this though

But you donot always get codes for engine temps being massively out of range but ecm runs fans just because it does not know what the true temp is .

Thanks for the response.

The O2 sensors are 5 wire wideband sensors - the colours are: white/pink/yellow/grey/black.

I do believe the MPG dropped around the same time as the fans became faulty so I think you may be correct. I've just been unable to find any particular fault which would make the fans run constantly so far.

It does have 2 coolant temperature sensors. G62 near the cylinder head (I replaced this one recently) and G83 sensor somewhere near the radiator outlet (although I've never actually been able to find this). My understanding is the G62 is used for fuelling purposes and the G83 is just used for the gauges. If the G83 is also used for fuelling this may be contributing to the fault, though I've not had any fault codes relating to this sensor (assuming it would show up in the engine module).

I've watched the data blocks for the G62 temperature sensor and it appears to be working correctly. It climbs up to 90 degrees and stays there, which reflects what the gauge is showing. I've never seen the gauge go above half-way.

I've heard that a faulty air-con pressure sensor may also cause one or more fans to run constantly (not sure if this would also cause the main fan to spin). The fans spin constantly whether air-con is turned on or off so I'm not sure if this is the case.

I think it does go into closed-loop mode as expected but I will double check.

I assume the post-cat O2 sensors are irrelevant in terms of fuelling? I know on most cars they just check catalytic converter efficiency but apparently some also take them into account for fuel correction.

I will perform the tests you mentioned in the morning and post an update with my findings.

Thanks