Cooling - totally stumped - help me

[havand]

S50. M42 rad.

Here is the situation. The car will just not stay bled. It is ok for a few weeks then gets worse again. As the weeks go, the radiator hoses start staying pressurized after cooldown. Last time I was on the highway and turned the AC on and the temp started to climb. Once it passes a certain point, it likes to 'hang' above the normal mark. When I release the cap, i hear whoosh and the water level in the overflow tank usually rises. I can squeeze the upper heater hose and feel it has air in it.

Replaced rad hoses, waterpump, thermostat. Checked heatercore for leak. Don't see any leaks. When I had pump out and thermostat I put the hose in the engine output and flushed it in reverse to dislodge anything that may be blocking the channels. I did the same with the radiator and it seemed to flow out the other side without issue.

I just checked the coolant with a combustion gas checker. Nothing.


Anyone have any thoughts? I wasn't hoping it was a headgasket leak (trust me), but at least it would explain something.


Edit: The only thing I can think of is there is a sealing leak on the suction side of the pump and it pulls in air, but doesn't leak. OR the heatercore has a small leak and it sucks in air as the system cools...but then why doesn't it leak when it is pressurized?

[LivingLegend06]

Does your coolant level seem to drop over the course of a few weeks or is it staying constant?

Have you had the system pressure checked?

I would expect that any air leaking slowly into the system would bleed itself out as you drive.

Is a simple re-bleed all it takes to fix it for several days?

Even the suction side of the water pump will be above atmospheric pressure when the car is up to temp so I don't think it's possible to "suck" any air into the system that way.

Headgasket seems to be the most likely solution but it's still not a perfect fit.

[mentaleeill]

try a new radiator cap.

[havand]

Does your coolant level seem to drop over the course of a few weeks or is it staying constant?

Have you had the system pressure checked?

I would expect that any air leaking slowly into the system would bleed itself out as you drive.

Is a simple re-bleed all it takes to fix it for several days?

Even the suction side of the water pump will be above atmospheric pressure when the car is up to temp so I don't think it's possible to "suck" any air into the system that way.

Headgasket seems to be the most likely solution but it's still not a perfect fit.

The coolant level doesn't seem to drop...just move around? It seems like maybe the air moves from the reservoir to the block somehow. When you open the top, the coolant level rises in the reservoir. It isn't consistent.

I have not had the system pressure checked.

I agree with you about the air getting in....I'm just reaching at straws because it doesn't make sense to me.

Usually a rebleed (takes me a while, have to burp the hoses to really bleed it worth anything) does fix it for a few weeks.


I already put another cap on... What are you thinking? That the cap was bad allowing the coolant to boil and push it out? Wouldn't that put the system under vacuum when it cooled?

[mentaleeill]

i have seen plenty of caps go bad and give symptoms of a headgasket failure.

[JRKOUPE]

I had pressurized hoses after track sessions.......rock hard and some coolant puke. I actually just got head back from shop and it revealed a light warp..but nothing obvious on the hg......Im putting her back together this week....

now in retrospect i see my expansion tank cap is bit iffy...........

I wonder if this hg $$$$ job was only a bad cap?....car was 100% fine in regular street duty....

[golde30]

Hmm, this thread is interesting, never thought about the expansion chamber cap, I have some strange cooling system issues once in awhile, and everything is new on the car, except the coolant expansion tank cap...

[havand]

Well, the weird part for me about the cap is for the system to be pressurized, there has to be more of something in it than before. A bad cap, I would think, would let stuff out of the system when hot. Wouldn't that cause a vacuum situation when cold?

Never the less, ill try replacing the cap. I think in have the old one from my e36 328 around.

[edsel1134]

Check your thermostat housing.. If it's plastic oem replace with the 11$ alum one.. Then from there check the heater core coolant intake bung on the back of the head.. If it's warped or bent it will do this.. My 1995 m3 had same symptoms.. Also could be easy as rad cap.. Or warped waterpump pulley.. Good luck

[havand]

Aluminum one. When I replaced the pump and thermostat I cleaned up the housing/engine and put a thin layer of sensor safe high temp RTV on it. Have not noticed any leaks.

I noticed something weird today. i have an 80c thermostat installed, so the needle should run between the 1/4 and 1/2 mark. Today it was creeping up nearly to the 1/2 mark in some warmer weather. I switched on my spal cooling fan and it came down...at highway speed. THAT doesn't seem right. The air @ 70mph should be providing much more cooling than the fan could ever hope to. I am wondering if my condenser is clogged and preventing good airflow to the radiator? I got my AC working last year or the year before but was never really happy with the performance. I have taken out the radiator since then and did not see much debris between the condenser and radiator, but if the condenser fins are clogged...

Any thoughts?

[oldmann]

The condensor on my 318 was plugged behind the fan, at least a good hand full of what looked like a birds nest, got to cleaning it and the bottom 1/3 was blocked with road crud.