Could oily sludge in cooling system affect engine temprature?

Question

I have a BMW 530i 2007, the engine gets hot up o 115 degree of celcius, it actually fluctuate between 95 to 115(which is a little bit higher than normal specially when the outside temperature is 5 c)

I checked the water pump, it was working steady with no strange noise, and there was no air in the system(bleeded). I don't know if the water pressure was normal, but its like a human male is peeing with high speed in water reservoir😂

I replaced my burnt headgasket last year, and after getting it fixed, I still see green sludge under the water reservoir cap.it seems that the mechanic that replaced headgasket forgot or was lazy to flush the cooling liquid.

Now that the engine gets hot, I am guessing the oily sludges and residuals are blocking the liquid current somewhere in coolant system. Could it be the case? How much oily sludge in coolant system could prevent circulation of water?

The ventilated air is cold sometimes, when it is expected to be hot, for example while the engine is 90 degree of celcius.

I also think that the thermostat is failing cause the radiator fan start to spin with high speed at the morning while the engine is cold and is just started, though the fan does not continue with high speed and it keeps switching between low and high speeds.

With above comprehension, what is(are) the problem(s) with my car?

Answer 1

Deposits(grease, rust or whatever) in your cooling system can restrict the waterflow, but that's not necessarily the (only) cause. These deposits also restrict the heat transfer from the iron block to the coolant. They have an insulating effect just like your clothes have.

Your radiator being clogged and restricting coolantflow is a more plausible cause. It's also possible that the outside of your radiator is covered with deposits. Again it's the heat transfer that mainly suffers because of this, and not the flow . (You can believe me, i've got an english oldtimer. If there's one type of car that has an undersized coolsystem and cooling problems, it's definitely this one.)

Your fan coming on right after a cold start doesn't have anything to do with the temperature. The coolant and the block don't heat up so fast that the fan is needed right after starting. It can be that the fan spins up shortly to check if it's working. If it keeps spinning at high speed, it's probably in open-loop mode because one or more sensors are not working. The fan then spins at full speed to prevent any damage due to possible overheating that it can't detect. Overcooling is still better than overheating.

Your temperatures being so high could have multiple reasons. Deposits could be one reason. A faulty sensor sending the wrong temperature could be a reason, but that's far from plausible. The best thing i can come up with is a stuck, broken, or clogged thermostat. A thermostat is way better at blocking the coolantflow than clogged coolantsystem. They're generally easy and not expensive to replace. I'd look at that first. You can find tutorials on the internet on how to test them. All the best.