TDI mpg vs engine temp

[lloydy]

This will only really relate to ETDI's
Just had a fantastic 3 week euro tour in the van, which gave me a chance to monitor mpg in different climates and engine temps. My engine temp in normal running like motorways around town ect is around the 75 degree mark, which I thought was nice and cool. And at those temps it would return 30mpg no matter how I drove it, but when smelling the exhaust it was quite fumey, like it was running rich. Driving through Italy where the ambient temps were in the high 40's and every road seems to be on a slight incline, the engine temp would settle in the 85 degree region (all temps taken from scangauge, which reads from the coolant temp sensor) On 3 consecutive fill ups I returned just over 40mpg each time
After a bit of reading on the tdi forums have found that the ecu over fuels the engine until it gets over 80 odd degrees.
Cool running is caused either by the stat going soft and opening to early or the coolant temp sensor not reading right making the ecu think the engine is cold. Both faults really hit your mpg figures.
I'm going to be fitting a new sensor and stat, as I reckon my stat will be the original one. I will be going for a 85 or 90 degree stat (leaning towards 90) even on the steepest and longest mountain passes the engine temp never went above 92, the first stage fan kicks in at that temp and brings it down to 86, so I'm not worried about it running too hot.
I'll report back what I find, but it seems to me that these engines need to be hot to give you good economy

[ewenmaclean]

Hi Lloyd,

those are impressive figures and useful info. You can change this a bit in vag-com adaptation a bit. For what it's worth, what I did was to put two relays in and when starting I'd tell the ECU it was colder than it was (resistor in series), and one with a resistor in parallel - what this does is to aid good starting, since I had a JX starter and wanted to give it every chance, and then once running on the motorway I'd change it so that the ECU thought it was a bit warmer and reduce fuelling. I wrongly fitted an 80 degree thermostat, and completely agree with getting a stock tdi one (87 I think).

Hope you had a good holiday and glad to hear the van was well!

Ewen

[lloydy]

the increase in mpg freaked me out a bit, thought the gauge was faulty at first (normally very accurate) wasn't until i refilled after doing 526km and only getting 33L in! Admittedly Italian fuel is a bit frothy and its hard to get a full tank in, but even if you allow for an extra 5-6L it still works out at around 40mpg. And the next two fills confirmed that. As soon as i got back over the swiss alps and the cooler temps it soon dropped back to 30mpg. I'd be happy with 35-38mpg which i think is achievable if i can increase the running temp by 10 degrees

[HarryMann]

Hi Lloyd, Please keep all the raw data , continuous against mileometer, so we can put this in the Wiki when you get back... that'd be good for comparisons.

Running temps should never be that low, and amazing if that's how the ECU responds (from 30ish to 40ish)... most engines drop mpg running cold, but purely open-loop.

A few notes at each brim to say what sort of driving and conditions would be useful too, to account for any discrepancies.

Hot driving conditions (ambient air temps) increase flat road cruising mpg a wee bit (10%), due lower air drag.

What speeds are you cruising at BTW ?

Glad you're finally having a good holiday

[lloydy]

Sorry clive, nothing noted down just observing...
I'll see if i can pull anything off the scanguage, as for the high difference in mpg it's all to do with the ecu overfueling to heat the engine up. Lot's of threads on the tdi forums about it.
cruising speed is 62mph using cruise control, rpm is 3200.

[HarryMann]

Ah shame, as we have been looking for some genuine brim to brim Vs mileometer figures to compare to Mark's for quite some time...

Yes, can see that, have corrected my post on that water temp point, it didn't quite read right. Hopefully the ecu knows above about 85 to ignore cold running corrections

[lloydy]

Yeah, sorry should have recorded it.. the only one I wrote down was 526km and 33l of fuel used, which I think worked out at 44mpg. The other two high ambient refills were 41mpg and 39mpg.
To put that 526km into perspective, normally I'm looking at refilling pretty soon when I'm at or approaching 600km and it normally takes 55L if I refill at that mileage.
On the scangauge it will give you an average litres per hundred kilometers. This varied betwen 10-12LHK. I'll have a look at the scangauge next time I'm running the engine and see how much info it has stored in the trip memory, as it records each journey with ave speeds and I think fuel used
I'll keep some better records when I fit the New stat, just to confirm what I have read

[HarryMann]

Oops, forgot to say..

All you need to do Lloyd, and it's dead simple and no hassle....

Just scribble the mileometer figure down on the fuel receipt (the VAT litres receipt) at every fill, and put a big 'B' after it, if it's an attempt at a Brim, if not no 'B', but write mileage down. Does not matter if the brim was accurate , just try for same if you can (e.g. first click whatever)

Do not use the tripmeter, just the continuous mileo figure (can use the trip for whatever else, instantaneous, but no good for genuine long term and accurate mpg).

Nothing else matters, then we have the raw figures over an increasing distance as the journey unfolds..

Even if you forget to write the mileo figure at fillup, brim or not, it doesn't matter, as long as you keep the fuel litres receipt, and the dates can be used to order them

To start: the next time you brim tank, will be the start datum, doesn't matter when or where, that receipt doesn't even matter either, but keep all subsequent ones, simple.

Subsequently, (or whenever you are feeling bored on a motorway ), you can check the mileometer calibration over 10, 20 or more k. 10k is usually good enough to get a fair idea what the correction factor should be on straightish motorway. You might also cross-calibrate with GPS, again on a straightish non-hilly section

Scangauge useful as a guide.

[jed the spread]

Thats really interesting with the MPG figures. I found the same and have been thinking about it for ages as it felt really weird I filled up in Italy near Milan and went through quite a few mountain passes in Switzerland , one of them was really high and has glaciers on the top and took ages to get up. It wasnt until we left The Black Forest in Germany that I realized we hadnt re fueled in nearly a week and thought I was cracking up and forgot one of the fuel stops or something. In my own mind I put it down to no wind resistance going up then using no fuel coming down with loads of wind resistance or some foolishness but I am keen on following on the changes your doing after you have tested it all out.

jed

[lloydy]

i'm picking up a genuine VW 87deg stat and sender tomorrow from driftbridge, i'll try to fit them before i go back to work

[..lee..]

Cruising at 60mph at 2450rpm I returned 35mpg with etdi and bigger nozzles. That's the best I've had so far. Need to work out the uk mpg as I was doing 70 / 75 this side. Probably closer to 28mpg

Would love to know how your getting 40mpg

Lee

[jed the spread]

Cruising at 60mph at 2450rpm I returned 35mpg with etdi and bigger nozzles. That's the best I've had so far. Need to work out the uk mpg as I was doing 70 / 75 this side. Probably closer to 28mpg

Would love to know how your getting 40mpg

Lee

Maybe its the bigger nozzles Lee, I know when I have been talking with Russel he says there is a economy compromise with the bigger ones and as he knows what I wanted from the van he kept mine standard.

jed

[jed the spread]

i'm picking up a genuine VW 87deg stat and sender tomorrow from driftbridge, i'll try to fit them before i go back to work

jed

[lloydy]

like i said, normally i get 30mpg, it was just the 3 refills when the engine was running 10 degrees hotter that i was around the 40mpg mark, if i change the stat and i get 35mpg i will be very happy, i will be more amazed than you if i get 40mpg. (but very very happy )

[randolph57]

Interesting..............
I'M getting around 32 mpg with modified crown wheel/pinion and 0.74 fifth driving at moderate 70-75 (3000rpm ish ) but cant resist using the power to stop being overtaken by k...bs who always want to cut in front just because i'm a 'slow' camper-I have the 80 deg thermostat in at the mo and can understand the overfuelling bit as vagcom always tells me engine is yet to reach temp on the TDI timing graph!Normal temp is bang on 80 rising to a max of 84 on some of the longer SW hills so am def overcooling.
Am off to France after Busfest so standard thermostat is going back in tommorrow so will be mega happy if economy is increased even a smidge!!.