Bad Thermostat = Lousy Fuel Mileage
Thread Starter
MBWorld Fanatic!
Joined: Feb 2003
Posts: 1,414
Likes: 339
From: Charlotte, NC
Had: 1987 300TD, Had: 2004 C230 Sport Sedan, Have: 2014 E350 Sport, Have: 2019 S450
Bad Thermostat = Lousy Fuel Mileage
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.
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.
Member
Joined: Mar 2008
Posts: 219
Likes: 1
From: Kansas City, MO
'05 C230K SS
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.
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.
Super Moderator
Joined: Jun 2008
Posts: 19,942
Likes: 192
From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
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.
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.
Member
Joined: Jan 2007
Posts: 226
Likes: 2
From: Toronto, Ontario
'15 C400, 03' C240, '79 240D
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
Thanks guys
Super Moderator
Joined: Jun 2008
Posts: 19,942
Likes: 192
From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
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.
Member
Joined: Apr 2008
Posts: 194
Likes: 1
2005 c230 sport, k7 gsxr
Trending Topics
MB World Stories
The Best of Mercedes & AMG
Manual Mercedes? 6 Times Sindelfingen Let Drivers Have All The Fun
Verdad Gallardo
Mercedes SLR McLaren 722 S Is Extremely Rare Example Modified by McLaren
Verdad Gallardo
8 Classic Boxy Mercedes Designs That Have Aged Like Fine Wine
Verdad Gallardo
Flawlessly Restored Mercedes 190E Evo II Heads to Auction
Verdad Gallardo
Electric Mercedes C-Class Unveiled: 11 Things You Need to Know
Verdad Gallardo
Mercedes EQS Gets A Major Update: Everything You Need to Know
Verdad Gallardo
5 Underrated Mercedes-Benz Models That Don't Get the Love They Deserve
Verdad Gallardo
Mercedes 300D Has Pushed Well Past 1 Million Miles and It Ain't Stopping
Verdad Gallardo
10 Most Reliable Mercedes-Benz Models You Can Buy Used
Verdad Gallardo
Super Moderator
Joined: Jun 2008
Posts: 19,942
Likes: 192
From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
Yes - very easy. Splinter has posted the exploded diagrams a number of times. Just buy a new one & pop it in.
MBWorld Fanatic!
Joined: Jul 2008
Posts: 8,384
Likes: 800
2019 G70 6MT & 2022 Ford Maverick XL
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?
What mileage are you 2005s at?
Super Moderator
Joined: Jun 2008
Posts: 19,942
Likes: 192
From: Llandudno, Cape Town, South Africa
late 2009 CLK 350 Coupe Elegance, '65 Jaguar S Type wires
"Bad" gas mileage is the car not doing the mpg that the owner is accustomed to in his or her set of circumstances
Member
Joined: Jan 2007
Posts: 226
Likes: 2
From: Toronto, Ontario
'15 C400, 03' C240, '79 240D
