MPG losses

[chrisg_s2k]

As the price of fuel is currently ridiculous I map my usage with an app - gives me tank by tank economy calculations.

The last few fill ups have all been ~5mpg less than expected. I do essentially the same journey every day (Bristol-Gloucester round trip) on the motorways and I have been leaving the cruise control on as I usually do.

Normally I get about 41mpg but the last few fill ups have calculated at 37ish and I can't figure out why. E90 330d m-sport so I guess I should be delighted with what I'm getting (average over 13'670miles is 41.1mpg) but it is the sudden drop I don't understand.

Tyres are at the correct pressure
Brakes aren't locking/rubbing etc
Wheels are balanced
Engine sounds like it is running smoothly
Oil is correct level
Clutch not slipping (as far as I can tell)

I'm due a 75'000 service in April but am not expecting anything exciting to come up, although I know the air filter is due to be changed.

Does anyone have any ideas?
Many thanks,
Chris

ps the only thing I have changed is the addition of a PH smiley and l certainly hope that isn't the cause!

[gizlaroc]

Thermostat??

How long is it taking to get up to temp? Hard to tell on the E90 as it has no temp guage, but the redline should disappear within 2 miles or 4 minutes, if it is taking longer than I would change that.

[gizlaroc]

Should just add, my car has also dropped from around 34/35 average to 30/31 over the last 2-3 weeks, I am in an E320cdi Merc.

I have been wondering what it is on mine, my temperature seems to get up quite quickly and seems to be sat around 85ºc, should be 87ºc, so pretty close, so don't know what else to think?

I am wondering if the fuel companies are using a lower grade fuel or something, as the weather is warmer if anything, so can't think what else it is.

[Deva Link]

Maybe these cars are getting bunged up as they're being "babied" everywhere and they need a good thrashing now and again?

[gizlaroc]

No, I do boot it around a fair bit.

I am going to bypass the EGR valve over the weekend and see how much of a gain that gives, I reckon I will see 3mpg more and more importantly it will be far better for the car.

[Deva Link]

Engines are more efficient the colder the air temp, but in normal use that should be easily cancelled out by faster engine warm up etc.

[chrisg_s2k]

How would I check if the engine is getting up to temp? I hadn't noticed the redline changing? Is it like on the S2000 where you can't go into VTEC until the engine is warm? I tend not to boot it in the first few miles/minutes anyway as I live in the middle of a city (and I have mechanical sympathy!) I guess the thermostat will be checked at the service?

Secondly, what is an EGR bypass?

I do 25000miles a year so given the current fuel prices losing 5mpg is not to be ignored!

[chrisg_s2k]

Might have found the solution!

Via the hidden OBC menu (found here) I checked the temp over 30mins of mixed city and fast motorway driving. Max temp was 86 at red lights just after a fast bit. Mostly seemed to sit at 78. I understand the desired is 88-90 so I guess I've got a problem.

Will report back after I've fixed it.

[BMWBen]

chrisg_s2k said:

Might have found the solution!

Via the hidden OBC menu (found here) I checked the temp over 30mins of mixed city and fast motorway driving. Max temp was 86 at red lights just after a fast bit. Mostly seemed to sit at 78. I understand the desired is 88-90 so I guess I've got a problem.

Will report back after I've fixed it.

Is the thermostat not monitored by the ECU? I would've thought it would throw a fault code and the engine light would come on if it was broken...

[gizlaroc]

BMWBen said:

Is the thermostat not monitored by the ECU? I would've thought it would throw a fault code and the engine light would come on if it was broken...

No, it's not.

This is the main cause of poor mpg on these cars, it just thinks it is cold so over fuels to compensate.

[chrisg_s2k]

This above is also how I understand it and is the reason why MPG has been declining.

I checked my temps again this morning during my commute and after 40miles on the M5 I still only had 78degrees.

Both coolant thermostat and EGR thermostat together cost £70 from BMW and are a DIY replacement (I have been assured).

I urge you to check yours out fellow BMW diesel owners.

[BMWBen]

gizlaroc said:

No, it's not.

This is the main cause of poor mpg on these cars, it just thinks it is cold so over fuels to compensate.

What temp should it be reading after the engine has fully warmed up?

[gizlaroc]

I think the thermostat is a 87ºc one on that car from memory.
But it should get to temperature in 2 miles or around 4 minutes while the outside temp is around 6ºc.
If it is taking longer than that there is probably an issue.

[chrisg_s2k]

That's about what I have been told: 88-90degrees.

[briers]

I think the target is 86 as a normal operating temp.


My 335d sits at around 83/84 and takes about 10 miles to reach this. Occasionally it reaches high 80's and highest has been 92 that i have seen.

[chrisg_s2k]

Just spoken to a very helpful chap (Mike) at BMW Bristol. E90 thermostat is set for 88 degrees and normal operating temp is 88-90.

Anything less than this is a problem.
Chris

[GR4]

Question for any E60 owners who have had a thermostat issue, How did you check it? Is it good enough to see how quickly the red line moves, or do you need to access the OBC 'hidden menu'?

[phumy]

briers said:

I think the target is 86 as a normal operating temp.


My 335d sits at around 83/84 and takes about 10 miles to reach this. Occasionally it reaches high 80's and highest has been 92 that i have seen.

Can you tell me how i find the water temp on my 335d? Cos i dont have any gauge to tell me that on mine.

[chrisg_s2k]

See my post above, I assume yours will be the same as mine.

Briefly though:
Turn engine on
Hold down trip computer button for >10 secs
It will display 01:-- then will show some writing including serial number
Note serial number, it will be in format XY12345
Add all the numbers together to get your passcode, (mine is 29 annoyingly)
Press and hold button until it goes back to 01:--
Single press until you get to 19:--
Wait a second and then single press until you reach your passcode
It will say sonething positive about unlocking and then go back to 01:--
Press until you get to 07:--
It should then display something like KRET-MOM and then the current temp in degrees Celsius

Good luck, apologies if this doesn't make sense-on my phone.

[phumy]

chrisg_s2k said:

See my post above, I assume yours will be the same as mine.

Briefly though:
Turn engine on
Hold down trip computer button for >10 secs
It will display 01:-- then will show some writing including serial number
Note serial number, it will be in format XY12345
Add all the numbers together to get your passcode, (mine is 29 annoyingly)
Press and hold button until it goes back to 01:--
Single press until you get to 19:--
Wait a second and then single press until you reach your passcode
It will say sonething positive about unlocking and then go back to 01:--
Press until you get to 07:--
It should then display something like KRET-MOM and then the current temp in degrees Celsius

Good luck, apologies if this doesn't make sense-on my phone.

Are you sure this is to get the water temperature? whats with all the "pass codes", its not as though were trying to crack the code for the ECU its only a temperature FFS, that is a ridiculous amount of "switchery" and "jiggery pokery" just to see how warm the water is.