Changed the thermostat and still running under 90°

My 456 is doing that too, and I am suspecting a broken oil thermostat (stuck open). It should be either that, or (as most people on the 456 forum told me) absolutely normal behavior.

 

Sounds like your gauges are marked in Celsius? What number is on the 12 o'clock position?

 

That's where they should be for a euro or Canadian gauge.

 

Your coolant temp should be no lower than 82°C (179.6°F) after full warm up. Any lower and the ECU will enrich the Air Fuel Ratio according to the lower coolant temperature. Which will sometimes manifest in a SDL, hotter exhaust temps. And if the car is run like this for extended periods, it will cause increased wear on the cats, among other bad things.

 

Yes Goth, I have read with great interest you previous posts. I have the same symptoms as the OP (changed water thermostat and the temperature falls to 80°C when running, but climbs nicely to the middle of the gauge when idling). I tested both thermostats and they opened as they should - identical).
So, why should the temperature fall when driving, even on a hot day? Could it be (as suggested) the oil thermostat (part 235648). Or what?
I would greatly value your opinion on this.

 

Coolant temperature is measured by two different sensors. The temperature indicated on the dashboard might not be the same value that the Motronic ECU is seeing. I would start by verifying the numbers.

 

Well assuming your Celsius gauge is the same as the Fahrenheit gauge, excepting of course the Celsius translated markings. Ideally the gauge would stay between the 8-11 mark after full warm up (+/- one click on the diagram below). Sadly I have see some OEM thermostats to be out of tolerance right out of the box. I have had good luck with the BEHR brand thermostats from 'GT car parts' in AZ. They are less expensive but for some reason, have been good every-time so far. Maybe because they are German instead of Italian?? who knows ...
How is your idle? Is it steady or burbly after warm-up.
What are the circumstances of the SDL? random or during similar driving/idle conditions?
If you have a hand held pyrometer, check your exhaust temps at the end of the manifold. At the area maybe 2-3 inches forward of the axle. See if there are any differences between left and right banks.

Going down to 80°C is not too far off, I would say that should be within the ok range. Are you having any other issues, any SDLs? Check the diagram below, between 8-11 is where the temps should be after full warm up. With 8 being on a steady mid to lower RPM cruise on a cool day.
Also, if you have a pyrometer check your exhaust temps as I described above.

Yes, this is true, there are two, sensor for ECU /sender for temp gauge. So taking that into account, there may be a discrepancy in what the gauge is seeing vs what the ECU is seeing. The good part is that both sensors are located on the same coolant passage, only a few inches apart, so the difference, if any would be between the electrical assemblies themselves. I agree there may be some delta between them. but up until this point every 355 that I have seen or did a thermostat change on, the temp gauge was spot on or within the useful range of indicating a problem (not off by more than one click on the diagram below). However as you indicated, I am sure there may be quite a few 355s out there, that may have a bigger discrepancy. But with 'online troubleshooting', we have to start somewhere ....
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What is this thermostat for? Any part number? I swapped the thermostat this past winter with a OEM new one and zero change in temperature behavior. I drop down to the 5-6 range when cruising in cool weather..... still

 

Yes, I had the same problem with new OEM thermostats as well. That is one of the reasons imo the 355 has had so many cat problems. Shops change them with a new OEM and think all is good ........
I believe the one I use from GT car parts also fits a 328. I don't have a part number for the BEHR version, but just contact them and ask for the 'BEHR 82°C thermostat for the 355' and they will send you the right one

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If it goes down to 6 any time after full warm up, it is too low. Ideally 8 while cruising on the highway and 11 in traffic. Between 7-11 is also acceptable because of possible variation in the sensors.

How long does it stay at 6?
Does this happen 'momentarily', maybe when turning the heater on, while on the highway?
Did it go to 6 from the very beginning, right after changing the t-stat?

 

Haha, well i suppose you could not drive at 240 so much and you will be fine

 

ok, now I get it, I thought you were saying you had put 150 'odometer' miles on the new t-stat, before it started going down to 6 ... .......... I have not tested my 355 to that speed with the new BEHR t-stat .... yet ..... ... so I do not know what sort of numbers I would see ..... hopefully it will stay at 8

But if the lowest you see is 7 during most other driving, after full warm up, then you are only one click off the ideal, which may be within sensor variation deltas.

Under what conditions do you see the SDL?

+1 ....

 

But surely this remark from Tim is the most interesting. Maybe the temperature gauge on the dash is for us to look at but the ECU gets its measurement elsewhere?

 

I have never had any SDL's either but have exactly the same symptoms as you - the faster I go, the lower the reading on the in car gauge.
I think that Tim's remark is very interesting.
Does the temperature switch at the top of the left hand radiator feed info to the gauge on the dashboard? If so, maybe we should be changing the switch and not the thermostat.
Surely it's not the temperature of the water at the top of the radiator which is used by the ECU to determine amount of fuel etc.
Very illuminating all this!

 

Dumb remark as the switch just turns the fan on and off. Temperature sender must be in the block I suppose..............

 

Both temp sensors are in the valley between the heads. I replaced both of them with new oem at the same time i replaced the thermostat. Nothing changed.

 

Probably a good place to start would be to plug in a scan tool (SD2 if you have one) and look at coolant temp with the engine warmed up. This would be the temperature that the ECU is seeing.

 

Nothing is perfectly accurate, however the gauge is useful and can indicate a problem if the readings are 'way off' normal.
I thought you posted you are getting SDLs? If you are NOT getting SDL and your temps are as you said, then you do NOT have a coolant temp problem.

+1 on the sensor locations! The fact you changed both your sensor and sender and nothing changed, would show the calibration between the old and new sensors are very similar in calibration.

If your temps still go down to 6 after full warm up on the highway, you should check your exhaust manifold temps right after you get there on the gauge. Are your manifolds shielded? Do you have a hand held pyrometer?

Yes that is a great way to get additional data ... though as you mentioned not everyone has one of those on hand.

Maybe it was a language translation barrier with rumen1, I thought his earlier posts said he was getting random SDLs, his later posts say the opposite. So I am guessing he never really had a problem and was just trying to confirm that fact.