Hi guys new here,

I’ve been battling with a strange issue with my car. I’ve had some major coolant work done on it to fix an overheating problem that I had. My radiator had failed coolant was blocked. Causing no proper coolant circulation. Thermostat and water pump was also changed. All genuine parts changed by Mercedes specialist. Head gasket tests have also been completed and have been cleared.

It doesn’t happen all the time but when driving on the motorway for around 30 mins my temp gauge started fluctuating like crazy. Going from 80-100 to 120 and then back down to 80 and even touching 60. Did it for around 10 mins and then stopped. I stopped driving when it hit 120 and turned the car off. No warning lights, no coolant leaks nothing. Coolant was also circulating properly too.

Do I have a bad sensor or is my car genuinely overheating ? This started after an ECU update was done.

mileage 57k full service history.

cheers

 

Hmmm, something like this?
https://mbworld.org/forums/c450-c43-...ave-swung.html

 

Unfortunately no, nothing like this.

 

Um... any chance there are any stored codes?

 

No codes as of yet.

 

Unless you actually physically measure not sure how to tell if the car is actually overheating or sensors data is inaccurate.

 

it might well have to be a trip to the local Mercedes dealer to get it plugged in and test sensors etc

 

Sorry that I wasn't able to help.

 

If it got down to 60C then the stat was open. I'd imagine it's possible the ECU could open the stat, but I seriously doubt it. A broken stat, which is common, will hang part way open which allows too much cooling at light loads, and not enough at heavier loads. So this is what I suspect. I know you said you replaced it, but imo all fingers point at the stat. So I guess the trick here is to pay attention to when it gets cool and when it gets hot. My car, for example, will run coolest cruising on the freeway (AC off), and hottest with the AC on and driving slow (40mph or less) up a grade, which I do every day. When stock it would also get rather hot at idle, easily above 105C and sometimes above 110C which I was very uncomfortable with.

A good stat should open ~102-103C (no ECU input). Temp can still go above that, as mentioned earlier, because the ECU isn't giving it enough, or any, fan. I modded my stat and fan so now my temp is usually ~85-90C and is very difficult to get it past 93C. I don't think my "fix" will help your issue, but if you want to lower your normal temp it is possible, which I can explain if you'd like.

If the temp sensor was bad I think it would behave differently, and trip a code and possibly trigger the fan to go 100%. I think you should physically check it when it says it's hot (or 60C). If not by feel, then an IR thermometer. I bought one ages ago for like $20, very handy for lots of things. Also, you can't trust the gauge on the dash. Not sure about the digital ones, since I've never had one, but the analog are, imo, a rough estimate worse than the gas gauge.

As for the update; I always tell people with computers: The best way to F-up a PC is to update it. Since you said it happened after the update I'd be wary. I kinda doubt it's the problem because odds are it was emissions or some petty change, but the timing is very suspect... If it were me I'd try to narrow down a mechanical problem, then if that fails I'd consider undoing the update. I suppose your "Mercedes specialist" did it? I seriously doubt he saved your old copy, but I'd imagine copies are available from the same source as the new one was, for such occasions.

Air in the system can cause drastic temp changes as you described, but as a general rule the car will burp the air quickly and that will go away, assuming you've topped off the water. My current car/engine doesn't have this issue and your eng is the same design so I'd assume you wont have air pockets either, assuming the air vent works. No venting and I'd image it would be a disaster. The vent should be a small hose on the overflow tank, leading to the front center of the engine. At least that's where mine is... It simply vents air directly to the tank.

The fan not working is something to keep in mind, but it won't let it cool to 60C, only the stat can allow that. A fan can give clues, like screaming at 100% when it's not actually hot tells you something is wrong. A different example is my previous car was one day getting hot-ish when stopped at lights, but it cooled down fast when moving so I assumed the fan died. I discovered a Rat had wedged a seed pod between the fan and shroud which locked it up! No codes, no CEL, no damage to the fan. So technically I probably could've drive it forever like that and the only real clues would be the temp swings and occasionally getting too hot. Not just a fan fyi, but a Rat fyi because they F up all kinds of stuff and who knows what some random chewed wire will do. Rats are getting so bad where I live that my neighbors are leaving their hoods open 24/7 when parked, even when garaged. One guy has that plus one of those strobe lights under the hood that also emits a chirp, that claims to repel rodents. I've had more than my share of damage as well. I don't open the hood, but instead I have certain areas they like to target covered in wire mesh, and I keep Rat repellent under the hood.