Overheating

[Buckoh9]

Hoping someone can help me figure this out. My 325e has been overheating recently. Seems that It gets bled and the temp stays fine at either 1/4 or just under half temp. After about 250 miles of driving with no issue, the temp starts to creep up. Today it crept up to almost red so I pulled over and just like the first time I saw the coolant was bubbling through the reservoir cap. I let the car cool down and then opened the cap, let it finish bubbling, and then opened the bleed screw a little and let the hissing noise which I assume was air, get out. Then I was able to drive it home and instead of going up to red it just stayed steady at 3/4. I'm going to run through the diagnostic threads I found here but wanted to ask if this sounds like anything that anyone is familiar with. I noticed that when driving up hill the temp will rise a little and when going downhill it will lower. I was thinking it's the radiator? What else would cause air to keep entering the system?

Also I had my o2 sensor replaced to solve a problem I was having with the car revving up and down whenever in neutral. Tonight while driving it started to do the revving again - I'm assuming the higher temp may have burned out the o2 sensor or maybe some of the coolant that was pouring out ended up messing it up?

Any ideas? It drives great for the 200-250 miles between overheating episodes. Not sure what could be happening in that time to cause it. Not driving the car hard either.

Thanks

[SlamedIAm]

I would start by making sure the car is bled properly. However it sounds like there is a leak allowing air to enter the system. Have you noticed any loss of coolant?

[Dirty325ix]

The one time I saw my temp gauge go past 3/4 I blew my head gasket(never seen it hit red)
I don't think I have ever drivin the car much over the half way mark on the temp gauge
Pull and resurface the head and slap a new gasket down

[jlevie]

That sounds like a head gasket failure that is pumping air into the cooling system.

[NoM54me]

There are several ways for air to enter your cooling system, like cracked hoses. For me, it was the nipple on the bottom of the thermostat housing that runs to the throttle body. It was cracked just enough to vent and cause the coolant to boil, which I assumed-incorrectly- meant that I had air somehow getting in the cooling system. I'd replace the hoses and cap first because they're cheaper and easier than head gasket. The oxygen sensor won't affect the idle speed of the engine. I don't know how wildly your RPM's fluctuate, but that's typically a bad idle air control valve, or IAC module.