The brief
The E250 came in because the temperature gauge had started sitting higher than normal, worst when the car was crawling in traffic. There was no puddle under it and the expansion tank was holding its level, so nothing was obviously leaking, which is exactly the kind of thing that makes an owner nervous enough to bring it in before it boils over.
The coolant pump is what pushes coolant around the engine and through the radiator. On this engine it's an electronically controlled unit. When it can't move coolant properly, the engine runs hot even though everything looks full and dry. So that's where the diagnosis headed.
The diagnosis
We pressure-tested the cooling system and it held, confirming there was no external leak. The system was bled to make sure no air was trapped.
On the STAR diagnostic tester the live data told the story: coolant temperature climbing past where it should sit, and a stored fault code, P2B837A, logged against the coolant pump, flagging that its speed was running too high and that a seal had failed. In plain terms the pump was failing internally and no longer circulating coolant the way it should.
So the fix was the pump itself, not a leak repair and not a thermostat.
The work
The old electric coolant pump came off, along with the plastic connecting pipe that runs with it, and a new Mercedes-spec pump and pipe went on with fresh seals.
We refilled the system with the correct coolant, ran a proper vacuum bleed so there were no air pockets, then pressure-tested it again to make sure it was sealed and full.
Last step was clearing the stored code on STAR and taking the car out, including some slow stop-start running, to watch the gauge behave.
The outcome
The temperature gauge sits dead centre again, and it stays there in traffic, which is where the problem used to show. No fault code has come back, and the system is holding pressure and level with no leak.
The owner drove out with the cooling system sorted properly at the root, the pump replaced rather than the symptom chased, and the peace of mind of not watching the temperature needle every time the car slows down.