Found coolant leak/temp fluctuation I've been chasing... looking for thoughts

[JustAWhisper]

Hello all,

I'm sure no ones remembers, but I've been posting in various threads on M3Forum trying to diagnose a weird coolant temp issue I've been having off and on for about two years. I've had two different shops look at it and I've been doing various suggested troubleshooting measures myself. Pressure tests never revealed a leak and the temperature fluctuations were always too random to isolate the cause. The shops didn't know what was going on, either. I had already replaced the water pump and thermostat, and I later ended up replacing the radiator when it started to leak. The issue persisted.

This summer I noticed the temp fluctuations were getting worse. I also noticed little bits of dried coolant showing up in odd places in the engine bay, as it appeared that coolant was occasionally dripping onto the belts or fan and were flung around the engine bay. But I could not find any dripping while idling in my driveway and pressure tests continued to find nothing (pressure held perfectly). The coolant level was not visibly dropping.

Finally, this weekend, I was poking around the engine bay looking at something else and I noticed significantly more coolant spots than before, and I was able to find a a trail pointing towards the thermostat housing area. I opened the coolant bottle and the level had dropped noticeably. After removing a bunch of stuff, it became obvious my thermostat housing is leaking. This is likely the source of all of my issues, as I now believe I've had a microleak in the thermostat housing seal for years, possibly since I replaced the thermostat itself almost 4 years ago. It's just taken this long for the leak to become significant.

I've removed the housing itself, and the seal does not have any obvious damage. I still do not know exactly why the leak happened. I'm thinking I should go ahead and replace the seal in case there's a microtear or something I can't see.. I figure I should also replace both coolant hoses since they're still original and quite swollen, though that's not related to this problem.

Is there anything else I should consider replacing or look at before I get the parts and put everything back together?

[heinzboehmer]

Consider replacing the thermostat housing itself as well. It's got a rubber seal on the inside that seals against the thermostat. A failing seal might lead to temperature fluctuations by allowing some coolant to bypass the thermostat.

[JustAWhisper]

Yeah, I talked to a friend who suggested the same thing.

A new thermostat housing, O-rings, radiator hoses, and fresh coolant is on its way. I'm glad this mystery will finally be behind me.

[S14]

On my euro S50 car I had an issue with the pipe at the back of the thermostat housing that is only held together with 0 rings on either end. I had a damaged one on re-assemble and it would suck in air but wouldn't leak so I constantly had air in the system. On the 3rd removal I finally found the little knick and it solved my problems.

Those O-rings should be replaced as they become pretty square in shape with so many heat cycles.

[JustAWhisper]

Thanks for the tip, S14. I was actually thinking about that so I ordered a spare gasket for the other end of that rear connecting pipe just in case. I'm not sure if it's worth the effort of getting that little pipe out, though. I gave it a tug and it seems to be in place pretty good. It stayed in when I replaced the thermostat 4 years ago, too, and I'm inclined to not mess with it if it's not leaking, which I don't see any evidence of.

Considering it's never been removed, do you think it would be important to do so now?

[S14]

yes, it's about as difficult to remove that pipe as pulling up a shift knob that has never de-mated from the shift lever.

[JustAWhisper]

Well, I'm now pulling my hair out. I got my parts and put everything back together last weekend. I did an initial test drive and everything checked out OK. Great!

Fast forward to today and I saw a characteristic wiggle in the water temp gauge. I opened the hood and sure enough, the fan shroud has the dried coolant mist on it again. Something is STILL leaking in the thermostat housing area. EVERYTHING has been replaced by this point. I don't even know what to do anymore.

[JustAWhisper]

Now I'm wondering if I'm overreacting. I just went poking around the engine bay with a mirror and I can't see dried coolant mist on the hoses or on the underside of the thermostat housing, which were replaced this go around and are new. I'm only seeing it on things like the fan shroud and in crevices where I had trouble reaching when I was cleaning things up before. I'm wondering if the coolant stains coming back is simply because they weren't cleaned well enough, as I was only using water and rags to clean up before. It's also possible some spilled coolant in some crevice from the repair job was splashing around. I'll continue to monitor.

[sapote]

Fast forward to today and I saw a characteristic wiggle in the water temp gauge.

Temperature changes is normal.

if the white dried coolant spray appeared again, I would do this: connect the pressure tester to the tank but don’t pressurize it; run engine until reaching operating temperature, then pump the tester to increase the pressure slowly until the leak show up. But don’t pump higher than 35 psi.

In your original issue, using the pressure tester at operating temperature would have pinpointed the leak.

Testing at cold condition, the crack might not expand to rear its head.

[JustAWhisper]

Temperature changes is normal.

if the white dried coolant spray appeared again, I would do this: connect the pressure tester to the tank but don’t pressurize it; run engine until reaching operating temperature, then pump the tester to increase the pressure slowly until the leak show up. But don’t pump higher than 35 psi.

In your original issue, using the pressure tester at operating temperature would have pinpointed the leak.

Testing at cold condition, the crack might not expand to rear its head.

The "wiggle" I was referring to was a small needle excursion outside the upper end of the the band, which I believe is 100C. It was only a small wiggle, so I was only at 100.5-101C, and only for a couple seconds. My understanding is that it generally shouldn't happen in normal driving conditions, and indicates a pressure problem in the system.

Anyways, after watching all weekend, there does appear to be some fresh coolant leaking out of somewhere. It may be the same or a different leak to before (perhaps there was more than one all along). I am quite frustrated, but there's nothing to do other than continue to investigate.

[8000RPM]

What brand water pump and radiator did you buy? Just for sanity sake, did you check the play on the new water pump. I’ve had OEM fail pretty quickly in the past whereas the BMW brand will go 100k miles safely.

A well maintained cooling system should not have temperature swings. Once I got mine squared away, it didn’t move even in 110F Las Vegas summer.

[sapote]

The "wiggle" I was referring to was a small needle excursion outside the upper end of the the band, which I believe is 100C. It was only a small wiggle, so I was only at 100.5-101C, and only for a couple seconds. My understanding is that it generally shouldn't happen in normal driving conditions, and indicates a pressure problem in the system.

Nothing could change the coolant big thermal mass that much within a couple seconds. Either the sensor or the gauge is jumpy, not the coolant temperature!

[JustAWhisper]

I only use genuine OE BMW parts only for the cooling system. I agree, there should not be any excursions past dead center on the gauge with normal cooling system operation.

sapote, a pressure fluctuation can dork with the sensor. I guess "a couple" second could also be "a few." I would say it was definitely no more than 10 seconds, probably 5-7. You may be correct that there is an issue with the sensor/gauge. I've been chasing weird needle behavior for a while now before it became apparent there was a leak.

Anyways. There's clearly still "something" wrong. The question is what. I haven't had a chance to do any more work on it yet, so no new updates.

[sapote]

If there is small leak then focus on fixing it.
Re to gauge needle jumping, sudden change in pressure still cannot cause the temperature to jump. We are not talking idea gas here with PV/T = nRT.

And what could cause sudden jump in pressure? Nothing unless tank ruptured or exhaust gas leaked into coolant, but you didn’t have that. So I say it jumped bc electrical or mechanical in the gauge.

[JustAWhisper]

sapote, I don't think you're understanding what I'm trying to communicate. Maybe wiggle was a bad way to describe it, because it seems you're thinking the needle is bouncing around wildly.

What I've seen is moments where the coolant temp is hanging out right on the edge of the buffer, which is at 100C. The temperature is fluctuating slightly (it always fluctuates slightly, that's why BMW buffers the gauge), but it's averaging at a higher temperature than it should be (~100C instead of ~93C), and some of the fluctuation is spilling outside the buffer. These are slight needle movements. The reason the temperature is elevated is because there is a small pressure loss in the system from the leak. It's not a pressure jump, the entire system is running slightly outside it's design condition when a leak is introduced into it. This is why failing water pumps often cause small overheating problems before complete failure.

Speaking of water pumps, I've come to the conclusion that it is the likely culprit. I'm having trouble accepting it, as the water pump is less than 3 years old and only has about 14000 miles on it, but coolant is clearly getting on the pulleys and the thermostat housing is clean from what I've been able to see.
There are about 8 track days and 30+ autocross events on the pump, so maybe that has something to do with it, but I was really expecting more life out of it. It's also possible the shop that replaced the water pump lied when they said they put an OE BMW pump in. I never looked close enough as I trusted them at the time, but in retrospect I should have because I ended up having a bunch of issues with that shop.