V70: Engine Plummeting fuel consumption

[yostumpy]

Bring up mpg avg on dash, press reset on the lower part of the left stalk, and Bingo, Bob’s your aunties fancy man. No need for Vida. BUT do it when you fill up, and clear it every time you fill up, you get to see if somethings wrong very quickly.

[simboc2004]

My average fuel consumption has dropped to saying 29mpg following a 700 mile round trip towing my caravan (and with roof box on) to France and it has remained at that figure since reverting back to "normal" car. It has also said 290 miles remaining for the last tank of fuel - from full down to 1/4 of a tank! So, about to try reset suggested by Yostumpy - and will try to remember to do that for every refill...
Interestingly, I discovered in that trip above - with temperatures well over 30C for the whole time - a new error message on the dash display: "Shift down due to Heavy Load" that appeared fairly frequently (as well as "Slow down due to high engine temperature" on one occasion). Had never seen either message before, though both made sense - the car seemed quite happy in a higher gear, but I guess the engine management system didn't agree!

[SnineT]

Originally Posted by simboc2004

- a new error message on the dash display: "Shift down due to Heavy Load" that appeared fairly frequently (as well as "Slow down due to high engine temperature" on one occasion). Had never seen either message before, though both made sense - the car seemed quite happy in a higher gear, but I guess the engine management system didn't agree!

If you're automatic that's the gearbox overheating. Might be time to have it's oil changed or ensure you aren't towing something too heavy for the car.

[simboc2004]

It's a manual and we tow an Eriba - 1000kg max loaded and a pop top roof, so much smaller profile when towing. I use cruise control a lot - which may well be the reason... plus those high temperatures.

But then... my fuel consumption is still staying below 30mpg average (on the instrument panel), so I'm thinking there may be an error in the engine management somewhere? Time to find where I put my VIDA Dice...

[100K+]

Have towed a Compass 510/5 caravan for 20 years with 4 different volvo V70. Always travel at legal max mostly A roads and have never had overheating issues. Strangely the diesels lost the most Mpg when towing over the petrol.(all 5/6 speed manuals) current 215HP D5 will return 33/5 mpg towing. (Computor) 50mpg solo.

Cheers Bob

[simboc2004]

I think my computer has gone doo-lally... I reset the average on the last fill up, it then told me I had 290 miles remaining for the entire time I was away (with the caravan) over the weekend (about 150 miles done). On return I have been into work and back a couple of times (60 mile round trip) and the tank gauge now shows just under 3/4 and the average reads 430 miles to empty... Bizarre

[Cloth Ears]

Originally Posted by simboc2004

I think my computer has gone doo-lally... I reset the average on the last fill up, it then told me I had 290 miles remaining for the entire time I was away (with the caravan) over the weekend (about 150 miles done). On return I have been into work and back a couple of times (60 mile round trip) and the tank gauge now shows just under 3/4 and the average reads 430 miles to empty... Bizarre

The 'miles to empty' extrapolates range from the mpg you achieved over the last 30 or so miles, I think. So if you reset it, and then drove around with a caravan on the back, your 'actual' mpg would be quite low and result in a low 'miles remaining' figure. As you put some miles on, that might get adjusted upwards a little bit if your actual trip average mpg was higher than what you started out at, but as you're using fuel too, the range to empty might not change much.

Then, when you remove the caravan, your mpg will rise, and after a few miles of that, your range on the tankful will start to climb, based on your 'average' mpg which has now climbed back towards normal (ie, once you've done >30 miles without the van on the back).

[simboc2004]

Yes, that makes sense, however it doesn't seem to change at all for several hundred miles, then leaps up (or down), so i'm not sure how often it actually makes the calculation.

I have just filled up and reset. Mileage travelled since last filling was 240 and fuel put in to first pump click was 5.7 gallons (26litres), which means an average of 41.9mpg - not the 29 mpg it told me I was averaging... and 110 miles of that was towing the caravan (from my caravan tracker).

[FreshAir]

Originally Posted by 100K+

Strangely the diesels lost the most Mpg when towing over the petrol.

Cheers Bob

This is not unexpected.

Diesel engines give great economy when running at very high Air to Fuel ratios (AFR). When you are towing the engine is under greater load, so lower AFR and therefore much less efficient.

Petrol engines under load run at the same AFR due to the catalytic converter requirement. Under more load the throttle opening is greater which reduces the pumping losses from the throttle restriction in the inlet. So more fuel needed for more power/torque, but reduced losses from inlet throttling.

[uknowiama]

I thought this was standard practise? With whatever car you have. Fill up tank. Reset trips. Reset average mpg. Always do this to keep an eye on things. How many miles gone before used 1/4 tank then 1/2 tank. How to keep an eye on health of engine - when factored in with type of driving being done - long runs, short slow town trips?

Hope you get to the bottom of it and sorted without too much more hassle.