Mercedes b180 petrol temp fluctuating

Mercedes b180 petrol temp fluctuating

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nav456

Original Poster:

113 posts

208 months

Tuesday 13th February
quotequote all
My dad just got a mercedes benz b class

Has a 1.6 blueefficency petrol engine.

We took it for a long 2 hr drive

I noticed the temp gauge drops up and down from 85-70 depends on if we cruising along or in traffic.

When at a stand still and parked up engine on it sits at 90.

Never goes over 90 so it is not over heating

Never evee had or driving a mercedes until now.

All cars i have had temp always sits at 90. Wont fluctuate.

Is there something wrong here?

Robertb

2,095 posts

245 months

Tuesday 13th February
quotequote all
Worth getting the thermostat operation and temperature sender checked.

I'd not expect the water temperature to drop back down below 90c in normal operation.

nav456

Original Poster:

113 posts

208 months

Tuesday 13th February
quotequote all
Bloody pain. Some forums online says its normal due to electronic thermostat. But this makes zero sense.

As you said how can water temp go down once operating at 90.

The car was freshly serviced i wonder if they did not bleed it.

Ill try get it in a garage to check it out.

GreenV8S

30,482 posts

291 months

Tuesday 13th February
quotequote all
Sometimes analog gauges can misread due to issues in the supply voltage, earthing or sensor wiring.

If you want to ensure the engine is being cooled, turn the heater on hot. If it continues blowing hot this proves there is at least some coolant circulating round the engine - if it cools off after a minute ot so this indicates there's no coolant flow in the heater matrix and possibly none in the engine.

E-bmw

9,976 posts

159 months

Tuesday 13th February
quotequote all
I have spent some time recently (due to an issue) looking into this on a completely different car, but maybe my experience may be of use to you.

The built in gauge on the car always (apart from when I had my issue) went straight up to mid range & stayed there until the engine overheated & went to "high coolant temperature reduce speed" which was the problem.

During the diagnosis I plugged in an OBD gauge to show actual coolant temperature & saw something completely different.

At 62 degrees the coolant showed "mid-range" on the dash gauge and firmly remained there until 114 degrees when it rapidly climbed & went "high temperature".

The thermostat on the car opened at 83 degrees as per manufacturers spec & was changed in case it was a problem.

While the problem didn't reappear when driving the temperature (after warming up) went from 79 to 87 all day every day. This I assumed to be the range of closed circuit cooling & stat open, which proved to be correct.

What I am trying to say is that most temperature gauges read a (very de-sensitised) mid-range reading so as not to distract the driver to what is a normal operating temperature range.

It is VERY likely that this is what you are seeing, but for some reason merc have decided that you should see a correct reading rather than a simple reading.

nav456

Original Poster:

113 posts

208 months

Tuesday 13th February
quotequote all
So i did just that. Dug out the old obd reader got the coolant temp graph up and drove around.

What was showing on the graph matched the needle..off by few degrees but nothing major.

Which was odd. Personally i always thought that the operating temp was 90 and stayed there.

But as you just confirmed that is not the case. On my vw it fluctuates slightly too on the graph but the needle in the car will not drop or go up from 90 once the temp is at operating temp.

Im curious if the mercedes system is a more true operating temp reading. Which personally i have never seen. Aparently this is normal in mazda's. Which also does what this mercedes is doing.

I will do few more tests over next few days and see if i can find a bleed valve for the coolant system in the morning.

As said the needle and readings never went above 90 at all. So its not overheating at all.

Tony1963

5,331 posts

169 months

Thursday 15th February
quotequote all
Does the indicated temperature drop when, for example, coasting downhill at 60mph?

Does the indicated temperature sit at ‘Normal’ when driving at 70mph?

Does it sit at ‘Normal’ after idling for a while?

If all the above are true, then it’s a stuck open thermostat.

The temperature drops when there’s little heat being generated by the engine, but lots of cooling from high speed air through the radiator.

nav456

Original Poster:

113 posts

208 months

Thursday 15th February
quotequote all
No it seems to not really stay still. On the 2 hr motorway journey when was coasting it would still fluctuate and while cruising at 70mph would go to 85..drop to 80 etc after few mins. Would not really stay in one place for more then 5-10 mins.

I will try bleeding it today and see how that goes. The car is due a service so if the bleeding dont help will just get the thermostat changed while its in for a service.

Tony1963

5,331 posts

169 months

Thursday 15th February
quotequote all
Maybe a temperature sender, in that case. There may be more than one.

tux850

1,863 posts

96 months

Thursday 15th February
quotequote all
My gut feeling is what you have here is a 'feature' rather than a 'fault'.

As another poster has said, we have become accustomed to heavily-damped gauges which in some cases (e.g. my wife's Fiesta) might as well be a traffic light display limited to indicating 'warming up', 'normal' and 'meltdown' for all it's worth.

The temperatures you are seeing on the gauge (and reading via OBD) wouldn't surprise me for an engine of relatively small displacement, aluminium construction (head and perhaps block too?), everything-variable control system, low coolant capacities etc. Furthermore, mechanical thermostats, assuming that's what it's got, are extremely crude in operation and have very high hysteresis so are going to be very laggy and thus won't do all that good a job of maintaining a stable temperature with all that's going on elsewhere, and nor do they need to.

Edited by tux850 on Friday 16th February 12:02

nav456

Original Poster:

113 posts

208 months

Thursday 15th February
quotequote all
Yes for now car is driving fine. Heaters working fine. Not overheating etc

When it goes in for a service soon will get it checked out. First time we have had a mercedes benz. Ans reading online these have some electrical thermostate or something. Not what you get in any other normal cars.

Prob because it is a blueefficency engine has these stupid readings and gauge. The MPG is great. So all signs that would indicate something is wrong are not there which is what is very odd.

Will keep you updated incase other mercedes owners come accross this

E-bmw

9,976 posts

159 months

Thursday 15th February
quotequote all
As I said earlier, I think all it is, is you are seeing a true reading whereas most show a heavily damped reading.

The temperatures you are seeing are perfectly normal it is just that most temperature gauges are very heavily damped around normal running range.

It is for exactly the reason you are asking the question that they are normally displayed that way, it stops people worrying about it.

Mine shows the needle exactly in the middle from 62 to 114 actual reading.

nav456

Original Poster:

113 posts

208 months

Friday 16th February
quotequote all
Yh i noticed that too..so i put in obd reader in my vw 2020 touran..and once the pbd reading was going above 75 the needle in the car was at 90. Even at few points the obd read above 90 and below etc..not by much tho as the mercedes just by 5 degrees here and there but the needle stayed 90. So yh defo think this may be a feature

Chris32345

2,116 posts

69 months

Friday 16th February
quotequote all
Robertb said:
Worth getting the thermostat operation and temperature sender checked.

I'd not expect the water temperature to drop back down below 90c in normal operation.
Quite common if water just to temp and it's cold outside and oils not fully warm

Especially if you have the heater on high fan speed

nav456

Original Poster:

113 posts

208 months

Saturday 17th February
quotequote all
So tried to "bleed" the system..left the cap off for about 10 mins before car to full temp. No bubbles etc. So there does not seem to be any air bubbles. Coolant level also has not dropped at all. Very odd.

I think the needle is showing real tempreture. So its a feature.

Will tell them to check it out when it goes for a service in 2 weeks.

I have done all my checks i could physically do myself