[Tony B]

Sorry - a fairly long post...there are 3 questions buried at the end.

I have had my E91 330D for 13 years (gasp), from 7k miles and it is now on 64k miles, full service history (still under extended warranty).

In the last few months (probably a year) it has got significantly more fuel thirsty, having gone from 37/38mpg to 31/32 now, in the same usage.

On checking data via the BMW Drive Analyser dongle and app, it appeared to be taking a long time for the coolant to warm-up (25 minutes from cold start), and catalyst temperatures rarely got above 350C even after 20 minutes at motorway speeds.

My dealer replaced 6 glow-plugs and the GP control unit because error codes indicated faults here. I am not sure how these would actually impact warm-up time or running temperatures, so pressed for them to replace thermostats (main and EGR).

Reluctantly the dealer replaced the main thermostat, but not the EGR stat.

After these changes, on the first run home from the dealer all seemed good - in fact OBC fuel consumption was showing 41mpg for that trip - it has not been that good for ages.

However, since then it has rapidly reverted to its typical (recent) consumption level of 31/32mpg.

Checking other data, the catalyst was rarely getting up to expected temps, and air/fuel ratio most of the time is constant at Lambda 1.4 - now having done much research recently I understand that diesel A/F ratios should be far more variable than petrol A/F ratios, so I would expect this to vary over a fairly wide range during general use. It should change significantly with throttle/accelerator movement, but it was not.

Given the continuing heavy fuel consumption and slow warm-ups I contacted the dealer, and I arranged to take the car back in today, for further diagnostics.

Driving to the dealer this morning, something unusual (to me) happened.

Cold start - Lambda = 0.6 (which has been typical recently) and then, within a mile or so, it flipped to 1.4 (again typical of recent behaviour) and the coolant temperature was coming up only slowly.

However...as the auto box was letting revs drop below 1k under load, I downshifted using the - paddle (I don't normally bother, except in these circumstances) and immediately Lambda numbers started to be far more throttle sensitive - all of a sudden increasing/decreasing with small throttle movements, as I would have expected all along.

At the same time, catalyst temperature increased rapidly, getting up to the high 500s inside the next half mile.

Question 1 - was this the catalyst/DPF regenerating?

Within another mile or so, coolant temperature was up to the mid 80s.

All good.

For the remainder of the journey to the dealer, everything was fine, coolant in running in the high 80s. Maybe I should have cancelled, as I should have known better - if there are no error codes they will not find anything wrong.

Computer says "No"...and £150 wasted for a non-diagnosis.

I was told that the coolant temperature increased from cold in-line with normal expectations. I asked how they tested this, and it appears that they just let the engine idle - so no cooling air going through the radiator....no other other test or diagnosis attempted.

Clearly, an idling engine with no airflow through the radiator is going to warm-up at a different rate from a car that is moving at 30 to 40 mph.

Question 2 - is anyone aware of a standard idling warm-up test used by BMW - is there anything in Insta (?) covering this?

I will pick the car up tomorrow, and I will be testing it a lot more in sport or manual transmission modes to see if this triggers the ECU into Cat/DPF regen mode and proper running temperatures.

Question 3 Has anyone come across anything similar to this before?

[Silver Streek]

When you change the thermostat always change both thermostat, if you are getting slow warm up times it's generally due to bad stats. I know you have recently changed the main stat, but you do occasionally buy a bad one that fails straight out of the box or soon after.

My advice is don't use the dealers they can be lazy and expensive working on older cars. Find a good independent, or better yet change the stats yourself, it's pretty straightforward. I always do mine myself on my M57 diesel.

It should be running up to temp and staying around 87 to 90 degs. Mine runs a pretty constant 87 to 88 degs. Any lower and your stats aren't working properly.

Low temps really affect your fuel consumption on diesels, eventually affecting your DPF too. The DPF needs an coolant to of 75C minimum to regenerate.

[Tony B]

Originally Posted by Silver Streek

When you change the thermostat always change both thermostat, if you are getting slow warm up times it's generally due to bad stats. I know you have recently changed the main stat, but you do occasionally buy a bad one that fails straight out of the box or soon after.

My advice is don't use the dealers they can be lazy and expensive working on older cars. Find a good independent, or better yet change the stats yourself, it's pretty straightforward. I always do mine myself on my M57 diesel.

It should be running up to temp and staying around 87 to 90 degs. Mine runs a pretty constant 87 to 88 degs. Any lower and your stats aren't working properly.

Low temps really affect your fuel consumption on diesels, eventually affecting your DPF too. The DPF needs an coolant to of 75C minimum to regenerate.

I still have the BMW warranty on the car so doing it myself is not an option - I would invalidate it. I might well try to find a local independent to do the EGR stat.

[Silver Streek]

Do the EGR stay yourself, it's a very easy job. Buy a genuine BMW one and theres no way BMW could know you swapped it out yourself. It's a Les than 20min job on the M57 diesel.

If you do buy an EGR thermostat make sure you get the original seal with it, it sits between the EGR stat and the mounting point.