Rough idle after replacing thermostat housing
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2012 Mercedes C250 1.8L turbo
Rough idle after replacing thermostat housing
On my wife's 2012 Mercedes C250 1.8l turbo I replaced the thermostat housing because she had no heat in the cabin. That fixed the issue but now I'm having problems with the idle of the vehicle. Here's the details.
Does anyone have any other thoughts on this?
Richard
- Starts fine but rough idle (bouncing around 800 to 900 RPM)
- P0108 code which I never had before - cleared the codes and it came back.
- Drives great
- Coolant level good and bled the system (with the vehicle front raised) and no air in the system
- Double checked all connections are plugged in correctly
- I replaced the front large intake pipe because rubber had hardened and couldn't get it on the turbo inlet
- Used the old MAF sensor from the old pipe - no difference
- All vacuum hoses tight
- Tried using brake cleaner to find a leak (nothing)
- While car is running removed the plug on the MAF sensor (the one on the large intake tube) and no change in idle
- I tried testing the wires on the MAF sensor (used a good ground) and saw no volts on any of the pins
- Expect to see at least 5V on one of the PINS with the key in the ON position and while the vehicle is running
- I remove the ground wire (the brown wire on the harness), cleaned and reinstalled
- Checked the fuses and all are good
Does anyone have any other thoughts on this?
Richard
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Joined: Apr 2019
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2012 Mercedes C250 1.8L turbo
Thanks. I actually drove the car for like 20 mins with the intake hose at the turbo inlet end not correctly attached. I think that the engine has 'adjusted' to this and now it doesn't run well at idle. So my thinking is that I have to reset the ECU (disconnect battery for 30 mins) and now that everything is connected correctly the ECU will reprogram with the throttle body correctly and the idle hopefully will be good.
Does that make sense?
Richard
Does that make sense?
Richard
Last edited by Richard Koop; Dec 26, 2025 at 11:15 PM.
Thread Starter
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2012 Mercedes C250 1.8L turbo
Getting the ECU to correctly relearn the vacuum/throttle body position worked! 
For benefit of others here's the process I did for the ECU to relearn...
I disconnected the negative from the battery for 30 mins, ensured all vacuum lines/connectors were tight, reconnected the battery, turned the vehicle to the ON position, waited about a minute, started the vehicle, waited for a minute (idle was rough), turned on the AC, waited for a minute, turned off the AC, and went for a regular (not hard) drive for about 20 mins. All was good and when I got back idle is great and no issues or stalling when going slow and turning the wheel (which is when it stalled before).
Richard
For benefit of others here's the process I did for the ECU to relearn...
I disconnected the negative from the battery for 30 mins, ensured all vacuum lines/connectors were tight, reconnected the battery, turned the vehicle to the ON position, waited about a minute, started the vehicle, waited for a minute (idle was rough), turned on the AC, waited for a minute, turned off the AC, and went for a regular (not hard) drive for about 20 mins. All was good and when I got back idle is great and no issues or stalling when going slow and turning the wheel (which is when it stalled before).
Richard
Thread Starter
Member
Joined: Apr 2019
Posts: 111
Likes: 19
2012 Mercedes C250 1.8L turbo
I spoke too soon...
I let the car cool down for a few hours and when I started it again I had a rough idle. No stalling or anything. Maybe I just need to drive it more to adjust since it's fine when it's warmed up. Other than that at this point I'm thinking...
I let the car cool down for a few hours and when I started it again I had a rough idle. No stalling or anything. Maybe I just need to drive it more to adjust since it's fine when it's warmed up. Other than that at this point I'm thinking...
- Small vacuum leak when the car is cold - probably do a smoke test
- MAP sensor gone bad - seems coincidental but I still get the P0108 code
- Bad wiring - maybe I broke a wire somewhere but all was fine before I didn't yank on any wires.
