Plummeting MPG in my cousin's A2

dan_b

dan_b

A2OC Donor
Hi team
Any ideas? My cousin's A2 TDi has suffered from a significant drop off in MPG in the last few months.
Last year I diagnosed they had a split intercooler and replacing that not only improved performance, but improved MPG! However, she is saying it has fallen off quite a lot recently and are getting less than 300 miles off a 34L tank of DERV.

I asked her to check if the temperature gauge gets up to 90 and it does - after about 20mins of driving (I was thinking the thermostat might be stuck). Do we think that's too slow in the current weather?

She's not been able to get it scanned for any error codes yet but before she does, what other things might be suspect do we think? Faulty MAF?
 
How many miles is that 20 minutes of driving? 20 minutes of steady 30mph driving sounds reasonable to me, but 20 minutes of faster driving would seem excessive.

The MAF & the thermostat are both candidates. Regarding the MAF, is she experiencing any drop-off of performance?

Binding brake calipers is a common issue which affects the MPG - if the wheel nuts are hot after a long drive then it is a strong suspect (the rears should be cold, and the fronts are normally warm but not hot). If the left & right are different temperatures then this is also a red flag. It would not show up in the scanned codes.

Low tyre pressures will also affect MPG.
 
Hi Mike - was hoping you'd reply!

I was thinking 20mins seemed a bit excessive but that was mostly urban plugging apparently, but it did hit 90 - I remember when my stat was on the blink it wouldn't ever hit 90.

Performance-wise, she has also said that "it feels as if the car wants to be in a lower gear than I would expect to be" - so for example she by experience would want to be in 4th, but she says the car feels like it struggles and has to go into 3rd. What obvious symptoms might a faulty MAF give off? Would it throw any codes?

I know it's very difficult to diagnose without myself having driven the car or having any codes - I'll see if I can get up to see the car myself but she's in Birmingham and I'm in London!
 
dan_b said:
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Performance-wise, she has also said that "it feels as if the car wants to be in a lower gear than I would expect to be" - so for example she by experience would want to be in 4th, but she says the car feels like it struggles and has to go into 3rd. What obvious symptoms might a faulty MAF give off? Would it throw any codes?
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When I was having MAF* problems (my A4, & my Vento) it felt as if the car lost power over the last 1/2 of the accelerator pedal. I would struggle to get up hills & going above 70. Turning the engine off & on again sometimes fixed it. But people report different effects from the MAF - I think if it overreads rather than underreads then it has no performance impact. I think that it will raise a code.

I would think that a split or disconnected vacuum line would probably feel similar, but I haven't experienced that myself. Judging from forum reports, the vacuum lines is a more common problem than the MAF.

(* The problem on the A4 was actually a cut in the air intake duct between the MAF & the engine. Because the air was bypassing the MAF, it acted the same as an underreading MAF).
 
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Dan
I've got a spare MAF lurking around in my garage
I thought mine was goosed, so bought another, only to find mine had previously been re-installed back to front.
It's done a few miles but still works

J
 
We did look at the vacuum lines last time when getting to the split intercooler, but agree they will need another look.
Are the MAFs the same for all models or did they change? I'll find out which engine code she has...
 
Hmm - perhaps the ECU sender part isn't sending, but the dash-part is, is that possible?
 
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Found out that the warm up time was 60+MPH on the A38 - to me that does sound like a very slow time to reach full operating temp in those conditions?
 
Hi Dan,

Just a thought.....what temperature is the climate set to?? I agree it seems like a long time to warm up, but if the temperature is set at 25 and on auto, or with the fan on mid-to-high speed, then the heater will be robbing the engine of heat.....Also, if the econ button is not pressed, then the compressor will also be robbing the car of some bhp!

Obviously the above is assuming the car has climate...Also, if it has the Webasto, I wonder if that is working (although the recent ambient temperature has been a bit above the starting temp for the Webasto)..

Cheers

Jeff
 
I call on the wisdom of the forum...

Earlier in the summer, I did some work on my cousin's A2 to try help solve their long warm-up/poor fuel economy issue. I swapped the coolant temp sender (sure enough, the sensor was heavily coated in chalky limescale-like deposits), the fuel temp sender (seemed ok but I had a spare), fully flushed and replaced the coolant charge (it didn't look like it actually had any antifreeze in it) - and swapped the old thermostat, which was in a similar state of deposition to the temp sender.

This work was seemingly successful - the coolant would now run up to 90 and more importantly, their MPG over an average tank was now calculated to be approx 60 - job done!

Unfortunately, they've come back from a 2-week holiday to France, where it was of course hot and sunny, to say that now the coolant indicator doesn't get to 90 at all - it lies just after the line before the 90 on the gauge if you know what I mean, and that MPG has returned to mid-40s.
I briefly checked the fan - that runs with air-con on, but goes off with air-con off, so that seems to be working ok.

So, my question is simple - what could be wrong now?!

My initial thought was perhaps it was an air-lock? But I squeezed the hoses and ran the system to 90 and it didn't seem to produce any air bubbles - plus the system is self-venting isn't it? And wouldn't that have manifested itself earlier?
My next thought was perhaps the new thermostat I fitted could actually be faulty? Is that likely? It was a new genuine VAG unit, although I bought it mail-order so I guess it could have got knocked in transit?
My other thought, could it be indicative of a failing water pump? It's a 2002 car, unlikely to have had the water pump changed - but then if the pump failed, wouldn't the car overheat instead of failing to reach 90?

Any suggestions? They live in Birmingham - I'm wondering whether to suggest they take it to a more expert mechanic than I am - any recommendations in the area?
 
I wouldn't overlook the obvious and question how the car is being driven. If the car's been to France on those uncongested toll roads it's easier to driver faster than normal.

Just a thought :) (not questioning the accuracy of your information)
 
Hi Dan,

My first port-of-call would be a scan (might be an opportunity here for you to invest in VCDS).
The scan will show up any fault codes, but more importantly you can plot the engine and gauge temperature readings. The engine may be hitting 90 but perhaps the gauge isn't.

Again, as I said before, are they running with the climate on 25-28 and the blower fan on high?
Did you use G12 Plus antifreeze.....has the coolant level dropped in the expansion tank.?
Over what distance and speed are they checking the gauge?
Does the climate blow out good constant hot air when asked?
This is obviously a TDi, so when did they last have a full scan and service. I'd assume it was a while ago if the water in the coolant circuit appeared to be just water without additive......and on that issue the question that begs answering is why only water......there must have been an issue at some point in the past for the coolant to have been filled with ordinary water…..Does it loose coolant?
Again, as mentioned before.....fuel type.....cheap or V-Power?
Over what driving conditions (speed/journey distance/road type) are the poor MPG figures obtained.

I would get a scan done to see what, if any, fault codes are present.
Next I would use VCDS to determine if the engine was reaching 90(ish) and if it is holding there. VCDS can display multiple “live” plots simultaneously, so from cold you could watch the engine and gauge temperature rise (through VCDS) and determine if the visible results seen on the gauge correspond to the VCDS output.
Also if you notice (through VCDS) the engine temp continue to rise to 90(ish) but the VCDS output shows the gauge reading to be 75(ish) you know the temp sender has gone faulty (again).
If the engine is struggling to achieve 90 on VCDS then you might have some debris preventing the thermostat closing. One of the primary reasons for using G12 Plus in our engines (even in warm countries) is the inhibitor that prevents build of limescale, other chalky deposits and rust through the water jackets of the block and head.

If there is evidence of coolant loss (expansion tank level dropping) I'd be getting a pressure test done if there aren't obvious signs of the loss.

Obviously the poor fuel economy might be nothing to do with the temp gauge misbehaving, so perhaps a good service is in order to bring back some economy.

Hope you get it resolved.

Cheers

Jeff
 
I don't know about the water pump history, but my hunch says it probably hasn't been replaced. Could it be indicative of a failing water pump?
 
dan_b said:
I don't know about the water pump history, but my hunch says it probably hasn't been replaced. Could it be indicative of a failing water pump?
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In theory the waterpump should be changed at the same time as the cambelt, it might be worth looking at those invoices to see if is mentioned.
 
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