History:
new water pump
newer fan clutch
newer thermostat
new coolant
newer radiator
new belts
new aux fan
newer headgasket

Symptoms:
On recent cold days (30-40F) coolant temp needle will occasionally creep up past the middle once the car is warmed up - 1 needle width+ above the middle - never above that. Normally, the car stays at 1 needle width below the middle. The nieedle tends to jump around a bit. One second I will look at it and it will be past the middle, the next second it will be at the middle or below.

I tried bleeding the system 2x today at various times throughout my trip to work. No change.

I pulled the return line going from the radiator to the expansion tank and blew it out with air. Then I blew air through the nipple on the expansion tank to make sure it was clear. All good.

Problem persists.

Tonight it happened again (it's cold out still) on the way home from work. I tried bleeding it again. No change. I put my hand on the top hose and it was normal temp. I put my hand on the bottom hose and it was COOL/COLD. WTF. I checked the aux fan operation while the car was past the middle thinking it would be on, but it wasn't. I have to double check the temp that it kicks on. Maybe it wasn't hot enough. Also have to jump it to make sure it working correctly.

Having already gone through a headgasket replacement, I constantly watch the temp gauge as I'm driving, so this is bugging me.

I have plenty of heat in the cabin. Heat is on the whole time.

Coolant level has been constant level the whole time.


Hypothesis:

(1) Been reading about the aux water pump and heater valves and wondered if these have anything to do with the car running hot. I dunno. Thoughts?

(2) Maybe there is still air in the system? Seems like whenever I bleed this car NO air comes out.

(3) Temp sender/sensor is flakey? It is original afterall and has gone thru 1 total overheat/headgasket job.

(4) Maybe I should drill a hole in the thermostat to help the system bleed itself of air?

(5) New thermostat?

(6) Air is being sucked into the system overnight in cold weather. This might not happen in warmer weather.

(7) New coolant cap??

(8) Crap clogging up the return line to the expansion tank when it's hot?


Let me know what you think. I know it's not a total overheat deal, but I swear it's like driving on the edge when you have a cooling system peculiarity. Kills the driving experience.

Are you moving when the temp goes up or stopped and idling?

Are you moving when the temp goes up or stopped and idling?

Oops. Forgot that. It happens when I'm moving and when I'm idle.

same here, but i have an 318i radiator could this be my problem??

same here, but i have an 318i radiator could this be my problem??

Could be. The 318i rad is smaller, is it not?

Anyone else here change out their temp sensor/sender?...ever?

Problem fixed. It was a leaky heater core. On cold days I'm guessing the 18 year old heater core was allowing air into the system. On warm days I wasn't seeing the problem. On warm days the temp needle would be a notch below the middle.

After the heater core replacement, the needle sits directly in the middle. It's never done that since I've taken ownership.

For all those with the needle on your coolant temp gauge a notch below the middle - beware. It might be because of a leaky heater core.

same here, but i have an 318i radiator could this be my problem??

i hope you didnt put the radiator in the car... :nono

Can Someone Confirm The Heater Core Replacement Actually Fixed This Problem?

It Would Seem To Be A Bad Heater Core Would Have Been Leaky. And Since It's A Pressurized System, How Did It "suck" Air Into It?

It Would Seem To Me That You Jhust Had Air In The System That Inadvertantly (sp) Was Fixed When You Changed The Heater Core.......

Can Someone Confirm The Heater Core Replacement Actually Fixed This Problem?

It Would Seem To Be A Bad Heater Core Would Have Been Leaky. And Since It's A Pressurized System, How Did It "suck" Air Into It?

It Would Seem To Me That You Jhust Had Air In The System That Inadvertantly (sp) Was Fixed When You Changed The Heater Core.......

Yea, it was leaky - enough to let air in and coolant out. Not enough to notice anything leaking.

Again, I only saw the problem on recent cold days. My guess is that on cold days, the seals shrunk allowing air into the system as the coolant cooled off. Hot coolant in a closed system, but when it cools it creates a vacuum sucking air in.

I got no air bleeding off the thermostat housing which to me means there was no air up front. I could see air bubbles coming out of heater core outlet going to the coolant reservoir, but only on cold days. I thought it was just reminants of opening the system, not I kept getting bubbles.

I even pressure tested it both hot and room temp (heated garage), but never left the car outside overnight AND then pressure tested. My guess is I would have seen a drop in pressure.

Drove it to work today...120 miles...needle @ middle of gauge.