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So, I have a thermostat and water pump on their way....just to finish my cooling system project (and when I do those, the insulating fabric is going back onto the firewall)....
But.....odd timing a thermostat would throw this...
your chassis is getting ready to enjoy it's fresh tstat... yeah!!
On a different note, I think the whole coolant temp regulation has something wacky for us to discover.
> THE TEMP YO-YO...
-- We know Tstat is managed by mapped temps according to load, speed, temp and target strategy to evaporate oil contaminants.
There's something else making actual coolant temp poorly controlled... unstable.
-- OP too cold stored transient is an example of that.
-- then there's the opposite: the fan running in winter madness.
-- New T-stat DOES not fix that!
-- I wonder if this yo-yo has to do with Temp Sensor referenced by a painted GND hidden behind firewall???
I don't think basic sensor itself is bad and is reliably sealed by design unlike oily CPS.
This matches well a transient fault in OP
++++ INVITATION !!!
This is an invitation to go clean GND Post while dealing with firewall jacket.
Last edited by CaliBenzDriver; Jan 2, 2025 at 05:35 PM.
That is a STORED code, it could happen while you were doing the last leaky repair, when and if Ignition ON-Engne OFF performed by accident or when air bubble
still a lot and the ECT did not get/touch proper liquid coolant...yet.
When it comes and go one or two more times after proper coolant filling with your new water pump + thermostat assy and road test, you can start to dig deeper.
Sometimes battery disconnect at the wrong time and battery re-connect, it can also produce stored DTCs from modules which are not "ready",
and the DTC sometime come after some key cycles later.
If indeed that DTC have often occured, its usual cause is the mechanical thermostat stuck a bit open ( common ) and that will be cured with the new
thermostat coming soon
Does your scanner have the freeze frame data to show how many times the DTC has occured and the mileage the most recent DTC occured at ?
If your INPUT side radiator hose is easy to touch and feel, you can verify a semi-open stuck thermostat by hand, for its hot coolant to radiator input.
Typically a MAP controlled thermostat like M276/M278 will be a 95C mechanical thermostat starting to open and fully open at 103C, excluding heater (R48) assist of the thermostat.
I would think M157 is using the same cooling system temperature strategy.
From a COLD start, to about 90C ish actual* coolant temp ( *using OBD2 / scanner ) , the radiator input hose at radiator side should be NOT hot to touch.
M157 engine bay is so packed in front of the engine, I can't even see the said hose from photo of an open hood W212 with M157 , but I know that hose exited the thermostat
at below the cylinder heads and not as high as in M276 cylinder head height thermostat housing.
long before ECU finally flags the cold temperature fault, the lazy hard Tstat had opening assisted along by Tstat heater to stay within target temps (*).
This means Tstat is best considered as a scheduled maintenance item like ignition, lambda, tensioners... ie. don't wait for codes.
Then what about the oil Tstat lifespan since there's no oil sensors??
+++ which "mapped target temps" are suspected of suffering from a sensing circuit issue... ie. floating GND circuit leads to skewed temps besides ECU mapped numbers.
Last edited by CaliBenzDriver; Jan 4, 2025 at 03:32 PM.
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