The Mystery Of The Thermostat And The Fuel Consumption

[Hawkwind]

I replaced the thermostat on my w208 CLK320 the other day, simple job and all was well, heater nice and toasty.

I was feeling a little smug and pleased with my handiwork, basking in the warm air wafting from my vents.

Today I had to drive to Heathrow, mid afternoon, plenty of time, so stuck to 70 for most of the way and as I got closer to the airport I drove even slower, tucked in behind a lorry.

Glancing at my speedo, I noticed my average MPG was 29.5 for the journey and here in lies the mystery. Before I changed the thermostat I would regularly see 32/33 MPG and on one very gentle run I even saw 38!

Surely, if the engine is running warmer it is more efficient and should therefore use less petrol?

I'm seriously considering refitting the old thermostat, which made the engine run cold

Anyone got any theories about this?

 

[geraldrobins]

You can't really compare mpg based on one journey as there are many factors to consider. Wind direction and external temperature for instance. Also it looks like you're relying on your OBC which may not be accurate. You are only talking of a 10% difference anyway.
I used to travel the same journey every day and every afternoon coming home, and mainly motorway but I noticed that journey mpg could vary by 10% for no real apparent reason.

 

[dozypillock]

I replaced the thermostat on my w208 CLK320 the other day, simple job and all was well, heater nice and toasty.

I was feeling a little smug and pleased with my handiwork, basking in the warm air wafting from my vents.

Today I had to drive to Heathrow, mid afternoon, plenty of time, so stuck to 70 for most of the way and as I got closer to the airport I drove even slower, tucked in behind a lorry.

Glancing at my speedo, I noticed my average MPG was 29.5 for the journey and here in lies the mystery. Before I changed the thermostat I would regularly see 32/33 MPG and on one very gentle run I even saw 38!

Surely, if the engine is running warmer it is more efficient and should therefore use less petrol?

I'm seriously considering refitting the old thermostat, which made the engine run cold

Anyone got any theories about this?

I have a C209 320CLK and have never even had 30mpg so maybe something is wrong with mine? That, or I need to drive like an "old man".

Neil

 

[brucemillar]

Did you fit the same temperature thermostat as your previous one?

As above 1 journey may not be enough to give a realistic figure.

 

[Ted]

I have a C209 320CLK and have never even had 30mpg so maybe something is wrong with mine? That, or I need to drive like an "old man". Neil

No need to drive like Miss Daisy to get decent mpg on a 320 clk



View attachment 54881

 

[markmifsud]

No need to drive like Miss Daisy to get decent mpg on a 320 clk



View attachment 54881

I saw 33.4mpg once, that was heathrow to essex, M25 roadworks 50mph all the way

 

[HB]

The quicker your engine warms up the more efficient. You don't want a cold engine.

 

[MancMike]

Surely it's because you were taking so much warm air from the engine?

For the first couple of miles of my morning commute, I hit the off button. Trying to get warm air into the cabin from the offset makes it take longer to get heat into the engine.

I wait until it's up to temperature from my short motorway blast before whacking it up to 11.

 

[grober]

What was the opening temperature marked on the old and new thermostats?

 

[Auto-mobile]

My 2000 e320 returned 36mpg once, was the atmospheric temperature the same? Same amount of fuel? Any idle time? It all makes a difference, I doubt it's connected to be honest.

 

[Hawkwind]

The thermostat is an OEM one so if memory serves it is designed to open at 87 degrees, or there about.

The engine now runs a good 25 degrees warmer than before and the journey is one I have made innumerable times, returning 32/33 MPG when driven faster/harder. On a run like the one described in the original post I would normally have expected to see more like 36 MPG.

Yes I am getting this information from the on board computer, but I assume that would be fairly accurate

I will obviously continue to monitor things.

 

[brucemillar]

I really am **** about engine temps. Get an IR thermometer and aim it at your stat housing to check engine temps. Your new stat could be goosed. Try it in a pan of water on a cooker. See if it opens.

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