[Thomeeno]

Hi all,

my e90 has serious overcooling. On idle blows lukewarm, while driving blows completelly cold.. I literally swapped everything which may cause this problem - i swapped two thermostats (one SKV, one Wahler), electrical water pump (Pierburg) and i have a new radiator (old one plastic hose cracked). Diagnostics has no fail codes, but live dada is measuring COOLANT TEMPERATURE gets to 96 - 97 C and sits there properly, but RADIATOR OUTLET TEMPERATURE on idle periodically goes from around 40 C to around 75 C and again back to 40. It draws kind of "sinusoid" on live data and never gets over 80. Once you drive outlet temperature goes to something around 12 (outside was slightly below 0) and of course blowing crazy cold... I would like to try another Tstat but its crazy this will be third!!! Someone has any clues?

[TunafishE93]

What radiator did you use? There are performance radiators that won't let you get up to temp. The stock radiator is already quite efficient, I have trouble getting to full temp in winter but run higher than you at 220f.

[e90yyc]

Clogged heater core?

[JonOhh]

Low radiator outlet temps are completely normal, and are necessary for the cooling system to function properly.

Engine temperature is regulated by controlling the amount of coolant flow through the radiator (read: not the engine, flow through the engine mainly alters coolant temp deltas between the engine inlet and outlet), once everything is said and done, this is of course, the job of the thermostat.

When the engine coolant temperature is below target, coolant flow to the radiator is reduced, and thus, less energy is rejected to the environment, and more stays within the coolant. As coolant flow to the radiator is reduced, and with the same amount of airflow across the radiator, radiator outlet temps will indeed plummet to very low temperatures, but because this flow rate is so low, this does not mean the temperature of the coolant entering the block is equivalent. The hot coolant from the radiator bypass, and the cold coolant from the radiator outlet mix together before going into the engine, resulting in some greater than radiator out temp, and less than radiator bypass temp at the engine inlet. If your engine coolant temperature reads 97c most of the time as you say, it is being cooled as it is designed to.


So, I would look away from the operation of the engine cooling side of the system, and instead look towards the heating system specifically.

First, run the cooling system bleed procedure (even if you've already done it once). Monitor coolant level before and after to see if there was indeed air in the system. Trapped air in a pipe system can significantly restrict flow, so this is a good place to start.

After this, recently, some e90 owners have had issues with clogged heater cores which can usually be unclogged, so if running the bleed procedure yields nothing, check back in and we'll point you to some of those threads.


And as a final note, I know this is simple, so excuse me if you know all of this, but make sure the temperature selector between the center vents is on middle or left, and make sure the climate control is set to 84f when trying to diagnose the heating system. For whatever reason, these cars blow lukewarm air through the vents up until 84f, when it decides to go full sauna. I guess it's a part of the auto temp regulation strategy.

[Thomeeno]

I get BEHR radiator.But still problem was even with old OEM radiator. We managed on idle properly heat the engine and get flow to the radiator. Vents are blowing air according this "outlet temp" - once temp is on 75 C its blowing lukewarm, but if it goes down to 40 blows rather cold which means there is flow through the radiator. All this is going on idle. What actually means this sinusoid outlet temp is drawing? Both heater core pipes are warm on touch...

[TunafishE93]

This is all at idle? I know i can sit forever and not be up to temp at idle. I will pull up the heat temp check off of ISTA+ in the AM to see if you are actually out of spec.

[Thomeeno]

Originally Posted by TunafishE93

This is all at idle? I know i can sit forever and not be up to temp at idle. I will pull up the heat temp check off of ISTA+ in the AM to see if you are actually out of spec.

Outlet temp is drawing sinusoid on idle, once you drive outlet temp get messed to 10 according your driving style.... Through weekend we try to change new, BEHR thermostat (just for sure) as all this looks like thermostat related failure and they are prone to fail. Also we will try to measure if there is current on thermostat heating cable.

during last bleeding procedure (gas pedal on 10 sec with ignition off) there was such pressure from water pump there was a big fountain from bleeding screw for few mins... Literally coolant everywhere....

[TunafishE93]

You need to get some repair manual since your bleeding procedure is incorrect. The bleeder screw shouldn't be open when pump is running.

You fill coolant to fill line in expansions tank.

You open expansions bleeder screw and fill until fluid comes out of bleeder port, than tighten the screw.

You put expansion cap back on.

You then run the 10 second gas pedal/pump circulation procedure.

You than open expansions cap and fill to full mark. Done.

[6Inarowmakesitgo!]

I am having this same issue. I am pretty sure there is something wrong with my DME. Been chasing this rabbit for awhile

[TheMidnightNarwhal]

I would just look at the coolant temp. Which engine do you have? On my car I have no outlet sensor temp yet it still gives ma some data, it's like a difference of 7 always between the 2.

[Thomeeno]

Problem solved - there was clogged radiator with a bit of flow which makes BOTH radiator hoses warm (or even hot!) on touch. Especially over long idle tries hoses managed to heat themselves to hot on touch which was very confusing!!
Im surprised as i read about this literally just from 328i owners, as if this is something special for these models. May this be special related to N51 SULEV with different radiator?
Thanks to everyone for replies and i hope this could help anybody