Thermostats keep going bad? Any ideas? 2008 4.8i

[Corellian Corvette]

Super strange phenomenon - trying to figure out if maybe I'm doing something wrong (or looking in the wrong place).

Maybe someone can explain how the thermostat in these cars work and if they work differently then I expect.

Got a new (to me) 2008 4.8i. When I got the car, it had no heat. After some diagnosis I determined the problem was BOTH in the heater valve and the thermostat. Heater valve seals had failed, and I noticed that the lower radiator hose was very cool to the touch with the engine warm.

Replaced both with OEM parts from BMW (they had both in stock funny enough!) and things worked fine. Heat started working all was well.

A few days later, I noticed a leak coming from the lower radiator hose. Seems like now that the thermostat was working, the old lower radiator hose started to leak. Since the system seemed old, this weekend I did a full cooling system - radiator (nissens) as well as upper and lower hoses as well as expansion tank.

Everything seemed well until today. Noticed heat was only so-so, put in my diagnostic tool and the radiator outlet temp was only 70deg when engine temp was 220. Lower rad hose was cold - AGAIN.

It seems strange (but possible) to have a brand-new OE thermostat go bad so fast. I *assume* these thermostats are heat-controlled and the sensor is just for reading temp? Or is there possible another issue causing my thermostat not to open?

Going to throw in a cheap parts-store unit tonight just to confirm the failure.

Thanks!

[pshovest]

I don't see a thermostat problem here. The thermostat controls engine temp, it does not control lower rad hose temp. A cold radiator outlet temp means the radiator is doing it's job well of removing heat. An outlet temp 0-20F above ambient temp is not unusual when engine is not overheating.

[Corellian Corvette]

Typically I would agree with you, but for sure something is *off* about this situation.

I fired up my e63 650i with the same 4.8 and watched the temps; both engine temp and rad hose temp were within 20-30 deg. I also checked my wife's e70 3.0 and similarly, when engine was about 180deg, the inlet temp was about 160.

I just threw a parts-store t-stat on tonight and have the same same problem again; engine would get up to about 180deg, and the inlet temp was about 80.

The lower radiator hose, and radiator itself, is basically room temp. However the upper radiator hose is hot as heck. Moreso, when the engine was running about 220 I could feel the water percolating in the upper hose, when the lower hose was basically cold.

So *something* is going on here. 2 thermostats with the same problem worry me the problem is elsewhere.

[killcrap]

if you have the engine warmed up enough, does the heat start working if you keep the RPM's around 2500 at idle? If so the auxiliary coolant pump is faulty. also you might want to check the stratification setting in the radio menu.

[Nanniepoo]

When was the WP last replaced?

A complete shot in the dark...but slow flow, or less volume, through the radiator would allow the coolant to cool off a lot more during the time it's in the radiator. Which will give you a greater delta T. I suppose an obstruction in the system could do the same. Or a barely cracked t-stat, but you've replaced that a couple times.

[Nanniepoo]

Or...the t-stat is electronically controlled, right? At least there's an electrical connector. Perhaps the issue isn't the t-stat itself but upstream.