dmatre

After about 5 years of the C230Sedan getting 33-35mpg on the highway, my mileage had dropped to 30-31mpg over the same roads.

I had just attributed it to the cold weather, combined with the 10% ethanol that they recently started adding to the gasoline, and went on about my business.

Two days ago, I got a check engine light, but no sputtering, stuttering, noises, etc. Took it to the shop yesterday, and they reported that I had a bad thermostat.

Seems it was stuck open, and didn't allow the car to reach the proper temperature (running too cool). After spending far too much to replace the thermostat, I collected the car this morning and headed out on my usual route.

Over my 103 mile route today, I averaged 34.0 mpg - including some stop & go getting out of the city.

I'd noticed that a couple of folks here in the 203 forum recently complaining about poor fuel mileage. It might not be a bad idea to check the thermostat.

Looking back, it did take longer for the car to warm up in the mornings, and it was running just below 80C, where it ran closer to 85C when new - but I never thought anything about it. I guess it finally got bad enough on Monday to throw a CEL - which led to this discovery.

Hope this helps somebody else out - as I've been helped numerous times by other folks here.

Benzben

I had also noticed some poor gas mileage over the past several months (mine is mostly city driving). I got the CEL (P0128) about a bad thermostat and fortunately was able to have mine replaced under the CPO. I've noticed that my mileage has returned to the 21-23 mpg (city) that I had been accustomed to.

Do thermostats normally go bad around the 5 year mark? Or are we looking at a run of potentially defective thermostats? My car is an '05 C230 (M271) but I'm not sure if the thermostats are different for other W203 engines. It would seem like they are a fairly universal sort of part.

Glyn M Ruck

YES YES YES !!!!!!!!!!!! Good post - This engine needs 87 degree thermostat to run efficiently -

daiso'

wow just in time im getting mine repaired for $150 from a guy in another forum but he's legit cuz the stealership wanted like $250 CAD

T.O_c240

This is pretty interesting - I find i get horrible mileage in the winter months compared to the summer, so its possible my thermostat might be out of whack. Is there anyway to check your thermostat asides from looking at the temperature output which should be at 87*C like Gyln said.
Thanks guys

Glyn M Ruck

You are probably also getting winter gasoline which has a revised distillation curve with more light ends which is lower density & will likely give worse mileage.

daiso'

AKA PETRO CANADA!!!!

ChaRgeC230

I too have been having bad gas millage just checked my cel code. it said it was the thermostat. Is this easy to fix?

Glyn M Ruck

Yes - very easy. Splinter has posted the exploded diagrams a number of times. Just buy a new one & pop it in.

LILBENZ230

What is "bad" gas mileage for an M271? It seems like a relative term. I get 25-26mpg in combined driving, 29-30 on the highway. 22ish in the city. I have no CEL and my car's temp gauge looks to point about to 87 when it's at normal temp so I guess I don't have any signs of this problem.

What mileage are you 2005s at?

Glyn M Ruck

You can say that

Glyn M Ruck

"Bad" gas mileage is the car not doing the mpg that the owner is accustomed to in his or her set of circumstances

T.O_c240

LOL but i always fill up at ESSO

Anyways thanks for the diagram...i may give this fix a try when the weather warms up here