Poor MPG, TDi90

Cloth Ears

A2OC Donor
Hello all,

My TDi90 Sport normally returns around 50-54mpg in daily use (commute is a mix of motorway and stop-start heavy congestion). On the last tankful, this dropped to around 45mpg, which I put down to a load of really, really bad congestion (average speed for the tankful was down 10mph too) due to some major roadworks near where I work.

But the current tankful is showing around 44mpg, and if anything, traffic has been better than average lately. I do notice that in very cold weather (cold for Manchester, anyway) the average often drops to around 48-49 anyway, so the current figures are unusual. It may be just a blip and a combination of unusual traffic, but I'm loathe to just ignore this sort of anomaly.

I do recall something similar happening one winter about 8 years ago. No fault found by Audi, then about a month later, the turbo blew. May be just a coincidence, obviously, but when Awesome replaced the turbo, it transpired that the hose from the intercooler was leaking and I suspect this had caused an overspeed. I don't hear any obvious extraneous noises this time, and am alert to the possibility this time around. I'll monitor things for another tankful over the holidays, and see where we end up, but some thoughts from the assembled luminaries wouldn't go amiss.

Thanks.

Steve
 
HI Steve

On one of my TDI90s the rear pads were binding which clearly will have impacted on MPG. They were not locked on but did not release properly.

Another issue could be tyre pressures or even temperature sensors. A diagnostic scan would perhaps help.

I assume that the service history is all up to date and filters (air and fuel) are in good order?

Just a few thoughts anyway to help you rule out items if nothing else.
 
Thanks Paul,

yes the car is regularly serviced by Awesome Gti, last done in September, and I'm quite sure there's no brakes binding, but all good points, so thanks.
 
Thanks Paul,

yes the car is regularly serviced by Awesome Gti, last done in September, and I'm quite sure there's no brakes binding, but all good points, so thanks.
We had bad mpg, then noticed the coolant temperature was not getting up to 90degC. Time and count how many miles it takes to get up to 90C. It should do it within 6-8 miles or 10 minutes -ish in this weather. It also should maintain 90C on the motorway with the heater on full blast too. When our car couldn't do the above and was returning 40-45mpg on the motorway we had the thermostat and water temp sensor changed and that put it back up to 55-65mpg. The ECU throws more fuel at the engine to try to heat itself up if the thermostat is stuck or the sensor is reporting the wrong temp.

Also check you have an undertray fitted and that your tracking is as best it can be.


Edit:
Do the described test with the heater turned off.

Sent from my SM-G930F using Tapatalk
 
Hello all,

My TDi90 Sport normally returns around 50-54mpg in daily use (commute is a mix of motorway and stop-start heavy congestion). On the last tankful, this dropped to around 45mpg, which I put down to a load of really, really bad congestion (average speed for the tankful was down 10mph too) due to some major roadworks near where I work.

But the current tankful is showing around 44mpg, and if anything, traffic has been better than average lately. I do notice that in very cold weather (cold for Manchester, anyway) the average often drops to around 48-49 anyway, so the current figures are unusual. It may be just a blip and a combination of unusual traffic, but I'm loathe to just ignore this sort of anomaly.

I do recall something similar happening one winter about 8 years ago. No fault found by Audi, then about a month later, the turbo blew. May be just a coincidence, obviously, but when Awesome replaced the turbo, it transpired that the hose from the intercooler was leaking and I suspect this had caused an overspeed. I don't hear any obvious extraneous noises this time, and am alert to the possibility this time around. I'll monitor things for another tankful over the holidays, and see where we end up, but some thoughts from the assembled luminaries wouldn't go amiss.

Thanks.

Steve
Mines the same.
With the latest weather,4.30am starts with the electric heater and lights on plus driving at different revs due to snowy icy roads I put it down to that.


Sent from my SM-G935F using Tapatalk
 
We had bad mpg, then noticed the coolant temperature was not getting up to 90degC. Time and count how many miles it takes to get up to 90C. It should do it within 6-8 miles or 10 minutes -ish in this weather. It also should maintain 90C on the motorway with the heater on full blast too. When our car couldn't do the above and was returning 40-45mpg on the motorway we had the thermostat and water temp sensor changed and that put it back up to 55-65mpg. The ECU throws more fuel at the engine to try to heat itself up if the thermostat is stuck or the sensor is reporting the wrong temp.

Also check you have an undertray fitted and that your tracking is as best it can be.


Edit:
Do the described test with the heater turned off.

Sent from my SM-G930F using Tapatalk

Thanks, you're right, coolant is taking longer than usual to get up to temp, and is dropping lower than 90 degrees when not working so hard (something it never normally does). I'm not sure if the thermostat or the temp sender are at fault (had both replaced a few years back for similar reasons. I'm suspecting the temp sender though, as one oddity of late is that, now that the weather is cold, the DIS comes up with the 'stop, engine temp problem' light within seconds of cold startup, and this goes away after maybe a minute or so, never to return (until next time). I'm assuming some sort of spurious temperature message is getting to the ECU, so the sender could probably do with a replace.

I believe it's an easy DIY job, and a cheap part. Can anybody point me in the direction of a thread on it? Engine is ATL (2004 TDI90).

Thanks

Steve
 
Steve,

Happy New Year.

This is a good thread on the temperature sender:
http://www.a2oc.net/forum/showthread.php?27989-Temperature-Sensor

This thread is mainly about thermostat replacement but mentions some hints for the temp sender too at the end:
http://www.a2oc.net/forum/showthread.php?31595-Thermostat-Replacement-1-4-TDI-75BHP-2002/page3

To my mind you have indications that both the temperature sender and the thermostat are faulty.
Changing both at the same time means you only refill the coolant and check leaks etc. once so would be worthwhile given the small parts cost involved.

Note that the temperature sender actually has 2 temperature sensors (thermistors) inside it, which is why it has 4 pins. One tells the dashboard gauge the temperature, the other one tells the engine controller the temperature. It is common for one sensor only to fail, so the engine controller can act up whilst the gauge shows normal activity. Intermittent electrical contact inside a failed sender is also common, due to cracking and expansion of parts with temperature.

Get Audi or OE part (Beru?) for the sender. For the thermostat I'd go with any well-known brand.

All the best,

Matt
 
Last edited:
Thanks Matt, yes, planning to replace both at the same time, as they're not expensive. Will be ordering them up from a local Audi parts department in the next day or so.

Steve
 
We replaced both the sender and the stat (at the same time as Paul (depronman) upgraded the map (thanks Paul), and all the indications are that mpg has returned to normal (a 'brisk' run back from Paul's, plus a similarly quick run yesterday are showing an overall mpg of about 52, where the previous couple of weeks of feather-footed driving was giving me no better than 48). It' s going to be interesting to see what a week of my regular commute does for those figures.
 
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