406 1.8 16V coolant temp, uphill climb with A/C on

[LFY]

I solved the problem!

Let me explain:

The problem had nothing to do with A/C pressure switch, it was a matter of fans didn't go into high speed mode (as i had guessed correctly).

The fan control unit of my car is a Bitron unit and it is designed to turn on low speed mode @ 95c and high speed mode @ around 103c. That's why when i drove steep uphill with A/C on the temperature reached about 100c or a little higher.

I bought another fan control unit with the following formula:

Low speed mode @ 91c
High speed mode @ 95c

I installed this unit and went for a test drive at that famous long uphill road in the middle of summer when outside temperature was very hot (40c!).

My average speed was about 65 km/h. At first the temp reached about 91c and stayed there a little. Then it started to rise slowly until it finally reached 95c! and this was when the big thing happened!! I watched temp gauge and expected it to reach around 100c like in my previous tests but to my surprise, the temp gauge needle started to go down and it reached 91c in the middle of the road where it was still uphill! I immediately realized that high speed mode had kicked in and brought temp down! Success!

I tested it again and it never reached 100c again and fluctuated between 91 and 95c.

I planned to use this fan control unit only in summer and reconnect the original Bitron unit in autumn and winter.

So, what do you think? any ideas?

[Hell Razor5543]

I would be inclined to check what your 406 SHOULD have had (fan unit, I mean), and if the new one is the correct one I would leave it in permanently.

[white exec]

It's now obviously working as it should:
- AC always brings on fans at slow speed
- without AC, fans cut in first at low speed, then at high speed
...all be it at lower temperatures than originally designed, but quite acceptable.

I don't think the original temperatures you quoted (gauge rising to a bit over 100C when the car is climbing lengthily, at a high ambient temperature) were excessively high, nor should be a cause of concern.

Many modern engines and cooling systems are designed to work at these temperatures. Our 2.5 XM (a different engine, I know) normally shows 90-95C, but functions quite happily at up to 120C, and will routinely rise to 110C under the conditions you describe, with fans operating as designed - first speed cutting in at 98C, second speed at 101C. At 118C the high coolant temp dash warning operates, and at approx. 125-130C fuelling is automatically and progressively reduced to prevent any further increase in temperature.

Good quality coolant and a pressurised system allows all this to happen without the cooling water boiling. These sorts of temperatures would once have been the subject of heart attacks on much older cars. Once upon a time, thermostats were often removed, to keep system temperatures away from even 90C! Not much good for winter, nor for petrol engine efficiency.

[LFY]

Thanks for your posts!

I would be inclined to check what you 406 SHOULD have had (fan unit, I mean), and if the new one is the correct one I would leave it in permanently.

It had the original Bitron unit (made in Italy) and still has it, but I think this unit was not much suited to my area of living especially in summer. That's why I decided to use another unit to best suit the weather conditions. High speed mode (when A/C is on and going uphill) is a fundamental factor to keep engine from reaching above 100c.

It's now obviously working as it should:
- AC always brings on fans at slow speed
- without AC, fans cut in first at low speed, then at high speed
...all be it at lower temperatures than originally designed, but quite acceptable.

I think in these hot summer days in which ambient temperature even reaches above 40c, my coolant temperature range of 91 - 95c is a normal operating temperature. It is neither cold nor hot.

Coolant temperatures of above 100c may be within engine's tolerance but i think frequent reaching to that temperatures shortens the life of head gasket and cylinder head in a wet liner engine. That's why i chose to use that unit only in hot summer days to be able to use my A/C without worrying about engine temperature.

But if you think my coolant temperature is considered cold, i should switch back to Bitron or some other unit. In fact the original Bitron unit on the car has quite acceptable low speed design of 95c and my only problem with it was the high speed mode which didn't kick in until 103c!

I have read about carbon build up in combustion chamber due to engine running cold but i don't think that's the case with my car because it is running only 4c below original factory setup and considering the fact that in very hot summer days, oil temperature gets hot enough, running at 91c without A/C is considered acceptable. And with A/C on and going uphill, my coolant temperature is exactly the same with my previous unit and A/C off.

[white exec]

I think you will be fine with your new, slightly lower temperature control.

Interestingly, on the 2.5TD XM, if you are hill-climbing, the temp gauge actually runs about 10deg lower with the AC on.
This looks as if it should defy common sense - with AC on, engine is working harder, and there's additional heat coming from the AC rad immediately in front of the engine rads (it has two).

But that's not the case, because the AC automatically brings on the cooling fans at slow speed, thus providing extra temp reduction, despite the extra heat being generated.

Cooling fan on the 2.5 is actually controlled by the engine (injection) ECU, which progressively reduces the amount of AC available at very high engine temps, in order to further reduce engine bay heat. Clever, clever stuff.

On our old BX19RD (no AC) we upped the spec to a pair of cooling fans (originally one, but all the slots there for two), and added a dash switch to allow manual operation if needed, either a low or high speed. The car did spend a lot of its time up and down Alps and Pyrenees, often with 650kg of caravan in tow!

[LFY]

Interestingly, on the 2.5TD XM, if you are hill-climbing, the temp gauge actually runs about 10deg lower with the AC on.
This looks as if it should defy common sense - with AC on, engine is working harder, and there's additional heat coming from the AC rad immediately in front of the engine rads (it has two).

It is exactly the case with my car at other regions but i live in an area that has a very polluted and dry air!

Ten days ago i was in another city which was quite as hot as my hometown (around 37-40c) but it had cleaner weather and there was some humidity in the air because of being close to sea.

In this city despite very hot ambient temperature, i haven't had any problem with my A/C on and the original Bitron unit. In fact when i turned on A/C, coolant temperature was lower than without A/C! even in uphills and high engine loads!

That's the reason i switched to slightly cooler fan unit to overcome summer days in my region! Because it seems that the manufacturer didn't take my situations into account!

[white exec]

You could also check that the engine thermostat is correct for your climate/region.

[LFY]

You could also check that the engine thermostat is correct for your climate/region.

I replaced my t-stat with a brand new OEM (83c) last year. It works fine and my temperature stays at about 80-83c when driving in autobahn or high speed cruising.