2.4 fuel efficiency

[wmccann]

Hi All,

I have a 2.4l A4 Cabriolet 2003 - BDV. When I got the car 2 years ago it was getting about 14mpg around the city.

I have since replaced the MAF (which was dea), changed the oil to Castrol Edge, de-coked the engine and most recently changed the coolant temp sensor (got intermittant error on vagcom). I am now getting about 20mpg, with slightly more (about 24-25mpg motorway drving). .

This seems low to me, anyone got any suggestions of what I should do next?

Note we only have RON95 fuel in Ireland so I have no opportunity to try better petrol

The car has approx 100,000 miles, the only thing I can think of is to change the lambda sensors, these are though expensive so don't want to do it if it is not necessary. Is there any easy way to check them via vagcom?

Any ideas/suggestions would be great,

Thanks,

W.

[RON MOODY]

I have the same engine but in the 2002 saloon. I've been getting 30-31 MPG. since new. I am a hard driver !! And am happy at that. Please let us all know if you improve your MPG.

[iwaters]

I have the same engine in an A6, I get about 35 MPG )37 if I try really hard) on the motorway, but only about 17 -20 town driving, I do have a heavy foot.

Putting anything higher than 95 RON in wont acheive anything as the engine is not tuned to run on anything higher.

I wouldn't start randomnly changing lamda sensors, there a 4 on that engine and the about £100 a pop. If they were faulty enough to cause that greater drop in MPG then I suspect they would be throwing DTCs.

How are you measuring the MPG, via the on board computer? It could be a fault with that possibly.

I assume you don't have any unresoved errors in VAG Com? Does the car feel ok to drive? Is it missfiring at all?

If it feels ok, I would start buy looking for fuel leaks. When was the fuel/air filter last changed?

[wmccann]

Hi,

Thanks for the responses,

1) No DTCs at all from the engine (apart from the coolant temp sensor which I changed). Car has twice refused to start - just turn over but waiting and trying again it worked (though I have since change the coolant temp sensor as there was a DTC on that).

2) Measured MPG via distance traveled/amount of fuel put in.

3) Lately it has been very sprightly, before I made the changes above it was sometimes great and sometimes a bit of a dog.

4) Air filter changed just over a year ago to a high performance long life one (was quite expensive but made a huge difference to performance).

5) I have never changed the fuel filter - how often should one do that?

6) Don't think it is misfiring and certainly have had no faults stating such.

7) note I said it had done 100k, actually it has done 116,000 km (approx 72,500 miles)

W.

I have the same engine in an A6, I get about 35 MPG )37 if I try really hard) on the motorway, but only about 17 -20 town driving, I do have a heavy foot.

Putting anything higher than 95 RON in wont acheive anything as the engine is not tuned to run on anything higher.

I wouldn't start randomnly changing lamda sensors, there a 4 on that engine and the about £100 a pop. If they were faulty enough to cause that greater drop in MPG then I suspect they would be throwing DTCs.

How are you measuring the MPG, via the on board computer? It could be a fault with that possibly.

I assume you don't have any unresoved errors in VAG Com? Does the car feel ok to drive? Is it missfiring at all?

If it feels ok, I would start buy looking for fuel leaks. When was the fuel/air filter last changed?

[iwaters]

The mileage wont be a problem, mine has done 116K miles.

Ok, as you have VAGCom lets check the lamda sensors to rule them out one way or another.

Open the engine controller (the engine must be running at idle for this), select basic settings, group 030.

The 4 colums related to the 4 probes. The values in columns 1 and 3 should be 111 (notethe first value may fluctuate between 1 and 0). Values in 2 and 4 should be 110.

If you don't get those values, go to address 033:

Values in columns 1 and 3 should be between -10.0 and 10.0% and should fluctuate by 2%. Columns 2 and 4 should be in range 0.1 - 1.0 and should fluctuate by 0.3 volts.

Also check group 032:

You should be getting between -10.0 and 10.0% in all columns, with slight fluctuation.

If you don't get those values then you need to do a number of checks, let me know what you get and I can then give you the info on what to check next (don't want to type it all out for the sake of it!! )

[wmccann]

Hi iwaters,

Thanks for that, have scanned the car and here is what I get:

Group 030:
00111 1110 00111 1110
and sometimes
00111 1110 00011 1010

Group 32 - stable at:

-0.6% 3.1% -0.8% 1.6%

Group 033

-3.1% to 2.3%, 0.08v to 0.95v, -4.7% to 3.9%, 0.08v to 0.8v

Group 33 fluctuates A LOT as you can see the range is wider than you suggested.

Thanks for your help,

W.

The mileage wont be a problem, mine has done 116K miles.

Ok, as you have VAGCom lets check the lamda sensors to rule them out one way or another.

Open the engine controller (the engine must be running at idle for this), select basic settings, group 030.

The 4 colums related to the 4 probes. The values in columns 1 and 3 should be 111 (notethe first value may fluctuate between 1 and 0). Values in 2 and 4 should be 110.

If you don't get those values, go to address 033:

Values in columns 1 and 3 should be between -10.0 and 10.0% and should fluctuate by 2%. Columns 2 and 4 should be in range 0.1 - 1.0 and should fluctuate by 0.3 volts.

Also check group 032:

You should be getting between -10.0 and 10.0% in all columns, with slight fluctuation.

If you don't get those values then you need to do a number of checks, let me know what you get and I can then give you the info on what to check next (don't want to type it all out for the sake of it!! )

[iwaters]

The values for Group 033, columns 1 and 3 are interesting. They should be fluctuation by at least 2%, whereas yours a only fluctuating by about 0.8%.

So some things we need to check now.

Checking the heating element for Lambda probes.

Connect up VAG-Com again, Engine controller, Measuring Blocks. Group 041

Columns 1 & 3, they will be empty to beign with unless you have just driven the car and they are at operating temperature, increase engine speed will you get a value. When you have a value it must be between 0 and 0.9 kOhm.

If not go to group 042, check the values there.

If you don't get the specified values you need to check the voltage at the connection points, these are the black connectors that are attached to the firewall, one is under the coolant expansion tank, the other is behind the inlet pipe between the air mass meter and thottle body.

Start the engine and measure the voltgate between contacts 1 (+ve) and 2 (earth). If you are looking at the connector there are three conntacts grouped together and one on it's own, the one on it's own is No.4. Should be battery voltage, might fluctuate slightly.

If no voltage is present, switch off engine. move the earth probe of the multi meter to an earthing point on the car. Operate starter briefly, should be battery voltage again. If no voltage then you have a wiring fault between the fuel pump relay, fuse and connector 1, check for open circuts.

If you can't find a wiring fault then it might be time to replace the lambda probe.

If those values are ok, check the volatge between contacts 3 and 4, ignition on, shoul be between 0.4 and 0.5 V. If not you need an specilaist bit of kit to test the wiring.

If that all checks out ok, then you either have a leak in the air intake or a fuel pressure problem or the fule return pipe is clogged

Let me know how you get on.

[wmccann]

Hi,

Your help is much appreciated,

041, entries are 0.2 kOhm and 0.1 kOhm
042, entries are 0.1 kOhm and 0.1 kOhm

also I ran group 33 for 5 mins and captured the output, the results were:

item 1 item2 item 3 item 4
Min -3.9% 0.07v -3.9% 0.075v
Max 3.1% 0.715v 3.1% 0.725v

Group 32 was constant with no fluctuations what so ever

-0.6% 3.1% -0.8% 1.6%


Thanks,

Walter.

[iwaters]

Looking at those values I think we can rule out the lambda sensors as they all look fine.

So where to look next? I'm not really sure! Looking back at your early post I notice you say you haven't changed the fuel filter, I think this should be done every couple of years. I doubt a cloged filter would cause the decrease in MPG you have seen but it might be controibuting to it. It's and easy job and the filter is only about a tenner.

The lambda system is working as it should so I am wondering if the engine is running lean and that is then causing the ECU to open the injectors for longer to even out the mixture. You have already said the MAF has been changed so assuming that is not faulty then you should inspect all intake pipes for a leak.

Also check the fuel lines for leaks.

Does anyone else have any suggestions? I think I am nearing the end of my knowledge

[fourringsrus]

No, you just about said what I would