Air in Coolant system

[Resolution Edit] - It was the Thermostat. Don't trust your thermostats.

Hello,

I've been running into an issue where my car (2007) will somehow get air into the coolant system very 5 or 6 times I drive. My car sits a few days a week since I don't drive it often, but there doesn't seem to be a correlation between when the air will get in and how often it is started up. It's a bit odd in that it doesn't overheat when driving hard, on long interstate drives, or around town. It will just decide that today is the day it wants to let some air into the system.

I'll notice on the AP that the coolant temp will climb a bit faster than normal, and begin to climb steadily past 190 up into the 220s+. When I see that, I let it get up to temp, park it off for about 3-5 minutes, gently crack the coolant cap to relieve a little pressure while it is still hot, and then seal it up and start the car up. At this point I can see a lot of air has gotten into the system as it bubbles into the reservoir and the level drops back down to normal. Temps are then fine and I have no further problems for the rest of the day. This is a guaranteed routine for me to fix the issue every time.

Car drives fine and runs perfectly, no discoloration of the coolant or oil contamination, and no sweet exhaust vapors or smoke, so I am doubtful it could be the head gasket. I have replaced the thermostat to no effect.

Possible causes: cracked hose, faulty coolant temp sensor / sensor seal allowing in air? I am inclined to believe it may be related to the sensor as the pressure relief + a start up bring the temperatures down sharply from the 200s down to 190 almost immediately, meaning air is sitting trapped in that section(?).

Curious on thoughts before I chase down a new hose kit or sensor.

 

Have you gone through the "official" process to bleed the air? Its (all numbers are ~ish) 2-3 minutes at 2.5k RPM with a blip up to 3.5k RPM, Rinse and repeat until satisfied. Warm the engine up fulled before doing this.

Why are you taking the cap off when the system is hot? Wont that pull a vacuum once the system begins to cool down?

One part you havent mentioned is the reservoir cap itself. Easy / cheap enough to replace.

 

That sounds like a good idea on the reservoir cap. I bled the system fully when I did the thermostat, the issue started up maybe a few weeks ago.

From what I can tell that vacuum created when I pop the cap a little while it is hot is the only way to get the air bubbles to travel to the reservoir and for temps to operate normally again. I may be misunderstanding it, but that tends to solve the issue for a few days, as it never happens in subsequent days, almost like it takes time to build up or pull air into the system.

[EDIT] The more I think about it the reservoir cap might actually be the issue, since popping it under pressure is what causes the bubbles to be drawn out from the system...

 

You didnt happen to replace the thermostat right before all of this started happening...did you?

Popping the cap is releasing pressure, not creating vacuum. That pressure is there to raise the boiling point of the fluid in the system, so when you remove the pressure you may be creating those bubbles yourself.

 

Check your coolant hose connections for leaks are you noticing your coolant levels drop? Can take some time to burp the system.

 

Coolant level has stayed consistent throughout, and the amount of air that I can see returning to the reservoir is almost always the same, which means air is getting in somewhere, but coolant isn't getting out.

The thermostat was replaced a probably a month or more before I noticed this happening, was rock-solid in terms of temperature beforehand. I guess it's definitely possible that the bubbles are a result of the boiling point getting compromised - is it possible that by popping the cap perhaps it is opening up / jostling open the thermostat, and the issue is that it is sticking closed until "manually" manipulated by the pressure change?

 

Its more likely that spots in your engine are above the boiling point of the fluid and when you reduce pressure you are boiling the fluid, which is making those bubbles you are seeing.

A month or more before noticing kinda points to that last thing touched being the issue, or near the issue. Have you eyeballed the thermostat area for any drips etc etc?

 

Yeah I've looked a few times and don't see anything obvious like a staining, dripping, etc. around the thermostat or the rad hoses coming off of it.

It makes sense that there are places where it is reaching boiling point; could it be possible that a leak would allow air in but not lose any coolant either?

 

So I ordered and installed a new OEM coolant reservoir cap and no dice, same issue occurring. A quick question for those who would know: in the reservoir should the left side be filling up with coolant in the same way that the right side does? I assume the left side is the overflow section, and the right side is the actual reservoir.

Could this indicate a compromised separating wall between the two sections in the reservoir?

 

Any visible stress cracks on the reservoir? Those gen 1 reservoirs are known to crack over time

 

I'll have to take a good look tomorrow and confirm. Nothing that jumps out, no coolant loss.

 

Give it a good look. I had 2 crack over 250k miles when I had my ms3. Neither leaked. But always let air in.

Both were cracked on the back side of the reservoir closest to the strut tower just above the max fill line.

 

Good catch, I forget about the coolant reservoir but now that you mention it my monies on that, my buddy had to replace his too although I believe he was actually leaking coolant.

 

Been a while since I have updated this thread - about a month ago the issue started to occur more frequently than it has been in the OP. It got to the point where I couldn't pop the cap and set it back to normal running temps, with issues every drive. I've replace the coolant reservoir with a new OEM, as well as a new OEM cap to no change. I also replaced the thermostat again, with no change, under the suspicion that the thermostat was getting stuck in some manner, due to acting like it was stuck open, and then more recently stuck closed causing temps to climb past acceptable levels on every drive. Also noting it is very difficult to get the car to bleed air properly after draining / refilling coolant.

Something that is potentially worth noting is that the bubbles when popping the cap always come from the two smaller hoses on the right side of the reservoir, running towards the right side of the engine and into what I guess is a distributor / coolant sensor housing.

I also today swapped in a new coolant temp sensor, and took the below pictures of the old sensor. One of my theories is that the air is somehow getting collected at the sensor location and not cycling out properly, as noted by the almost burnt look in the below pictures. Haven't yet started the car up with the new sensor, curious to see if that makes any difference, but not getting my hopes up there.






Definitely getting frustrated by the process since the car runs as if nothing is wrong at all - no coolant loss, no smoking exhaust, no obvious drips from anywhere, coolant drained was untainted with anything other than small bits of rubber, I assume from old hoses. If the sensor doesn't amount to much, I will take it to a local mechanic to have them pressure test the system for leaks / HG test. My stubbornness to fix it by myself has almost reached an end.

 

how are you hoses? I wonder if one is on it's way out and permeation is happening

 

With my juan and ms6, the only way to get it to burp properly has been to start the car cold with the radiator/reservoir cap off and all 4 wheels on the ground. Let it warm up to 187 degrees on its own. Then hold rpm at 3k for 30 seconds at a time until temp goes to 217 to kick the fans on and then let the temp come down

 

That's definitely a possibility - I tried to do a pressure test on it today, and for the life of me none of the fittings seemed to hold a seal and the universal cap it came with popped off at like 10psi. I really am sick of getting coolant everywhere, it may be worth my time to have a shop just take care of it and tell me what is going on. I miss my car.

I'm going to try this tomorrow when I go to start it with the new sensor, will reply back with results.

 

Keep some extra coolant on hand. Mine usually pushed out a pretty big air bubble and burps a good bit of coolant out

 

Picked up another gallon today, I managed to spill some more when I did my sensor, using up the last of my reserve.

 

No dice - followed Sho's method to get the car up to temp and it still exhibits overheating. What's weird is that when the fan kicks on, the temperature will stick at 219 / 217 for longer, but after awhile climbs up to 223 and beyond if I let it. It doesn't behave normally in that regard, i.e. dropping back down to acceptable temperatures. Blockage maybe?