W210 E320 1997 rough idle/terrible gas mileage
Thread Starter
Newbie
Joined: Dec 2025
Posts: 1
Likes: 0
W210 E320 1997
W210 E320 1997 rough idle/terrible gas mileage
Hey! Hope y'all doing good. Since I’ve tried pretty much everything, this forum is my last hope. Any help will be much appreciated.
So, I’ve got myself a w210 like 5 months ago or so. Overall it runs like champ, little to no rust, feels smooth and just in general is a great car. But there’s one little issue I’ve been dealing with since I got it. Terrible gas mileage (10-13mpg) and a haunting idle (fluctuates around 50rpms up and down). I would assume these are related somehow, but I’m not entirely sure. A few words as for idle fluctuation:
Right after a cold start it goes up to ~1200rpms and then drops to 800 after a few seconds. The idle at 800rpms stays still until the secondary air pump stops blowing. It starts to fluctuate a little afterwards, which becomes more and more prominent as soon as the engine temp reached ~50 degrees C. That’s when the rpm needle starts moving up and down. Once the engine’s warmed up to a working temp (~87 C) it stabilises, however if you look close enough at the gauges, you can still see a bit of a fluctuation. Theres nothing wrong with acceleration nor does it shake after you got it up to some speed.
There’s an audible air hiss somewhere in the engine bay. After multiple smoke tests, no leaks were found. Purge valve works as it should.
The following parts were replaced:
MAF
O2 sensor (upstream)
All coils and wires
New spark plugs
Valve cover gasket (was leaking)
Fuel filter
It doesn’t throw any code, the dash is clean. Thanks for reading this. Any help will be much appreciated.
P.S. one thing I forgot to mention earlier: the engine is m104.995 3.2L l6
So, I’ve got myself a w210 like 5 months ago or so. Overall it runs like champ, little to no rust, feels smooth and just in general is a great car. But there’s one little issue I’ve been dealing with since I got it. Terrible gas mileage (10-13mpg) and a haunting idle (fluctuates around 50rpms up and down). I would assume these are related somehow, but I’m not entirely sure. A few words as for idle fluctuation:
Right after a cold start it goes up to ~1200rpms and then drops to 800 after a few seconds. The idle at 800rpms stays still until the secondary air pump stops blowing. It starts to fluctuate a little afterwards, which becomes more and more prominent as soon as the engine temp reached ~50 degrees C. That’s when the rpm needle starts moving up and down. Once the engine’s warmed up to a working temp (~87 C) it stabilises, however if you look close enough at the gauges, you can still see a bit of a fluctuation. Theres nothing wrong with acceleration nor does it shake after you got it up to some speed.
There’s an audible air hiss somewhere in the engine bay. After multiple smoke tests, no leaks were found. Purge valve works as it should.
The following parts were replaced:
MAF
O2 sensor (upstream)
All coils and wires
New spark plugs
Valve cover gasket (was leaking)
Fuel filter
It doesn’t throw any code, the dash is clean. Thanks for reading this. Any help will be much appreciated.
P.S. one thing I forgot to mention earlier: the engine is m104.995 3.2L l6
Super Member
Joined: Aug 2008
Posts: 689
Likes: 30
Seems like you have covered most of the bases. Might be worthwhile getting it to a dealer and having them do a scan. Or at least an Indy who has MB's Xentry. and knows how to use it. Interesting that if it is using a lot of gas that you don't get any codes.
MBWorld Fanatic!
Joined: Jan 2014
Posts: 3,966
Likes: 1,315
From: Republic of Texas
'99 and '05 E55 AMG
The instrument cluster gauge shows 87C but is the Motor Electronics module receiving the same data? Symptoms seem to describe ME "seeing" cold coolant temperature and dumping extra fuel into the engine to enrich the mixture for cold running.
Just went through this with my '99 E55 with a bad (open) thermostat.
Take a look at your schematics and see if there are separate feeds to the gauge and the Motor Electronics. I would hypothesize the coolant sensor feed to the ME is defective.
Just went through this with my '99 E55 with a bad (open) thermostat.
Take a look at your schematics and see if there are separate feeds to the gauge and the Motor Electronics. I would hypothesize the coolant sensor feed to the ME is defective.
