In the 535i it will do this if the ambient temperature is below ~6☌. Simply over cooled.Įven in the warm up phase, when thermostats are firmly closed, I can see a drop back in ECT and oil temperature, simply descending down to sea level on a light throttle, over a mile or so. Seen it in too many diesels, to know it is not a thermostat issue. Personally I don’t change the thermostats until they start dropping back as low as the low 80s though, that’s when you tend to notice a slight hit in mpg. I regularly used to check the coolant temps on these E90 diesel cars as the stats failed so often/easily. From experience of changing lazy thermostats in 2 E90 diesels where it was common for both engine and EGR stats to stick open a little after only a few years use, I can honestly say both my cars never dropped below 88c once fully warmed up properly with new genuine thermostats fitted, and yes we do sometimes get sub zero temps during winter here in the midlands With the new stats fitted they used to sit around smack on 90c with occasionally going a little higher under really heavy load or occasionally dropping back to 88/89c under very light load in cold weather. Pete, in my opinion the thermostats a bit lazy if it drops down as far as 85c once the engine is ‘fully heat soaked’ even in sub zero temps. It is also overcooled at light loads in winter temperatures. Drive through the village at 30mph and it still wouldn't regain the optimum ECT, until out of the village again.Įven the N55 in the 535i will do the same, drop back a few degrees below the running temperature. I'd drop down into the village, off throttle, and I'd often see the ECT drop back to ~85☌. My M57 330d (with new thermostats), I'd still see it dropping back, after the thermostat had clearly opened (~88☌) and been running say 8 miles ~90☌.
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