Cooling system mystery

[hansa]

Been stumped for a little bit and want to know what people who are a bit more knowledgeable reckon.

My MY11 SV6 (late 2010/early series II/LLT) got done in for a logbook service early in July (was the 170,000km service if I remember correctly, had it done at roughly 168k on the odo, car currently is at around 170,680km). Standard stuff, oil and coolant change, new radiator cap, etc.
About three weeks ago, it started overheating a bit. First thing I noticed was the fans would come on at full blast, but the temp gauge stayed at normal levels so didn't think much of it. A little bit afterwards I noticed that it would occasionally spike up, at most 3/4 up the gauge, before coming back down. Never hit the red end, but was still enough to concern me. Kept an eye on the temperature when running and found out it was going upwards of 110C, occasionally in the 120s. This mainly happened at low speeds (i.e., side streets, parking garages) but would occasionally happen on main roads too. Been trying to minimise driving it since then but since I need to go to work and uni, I've occasionally needed to use it, just being wary of the temperatures.

At this point, I'd been keeping an eye on symptoms and trying to figure out what it was, and tried out some DIY fixes. Tried a rudimentary bleed of the system (ran it for ~20 mins until it got to 90C with the rad cap off), didn't really do that much. No signs of leaks, coolant levels looked alright, only thing that stuck out was the top radiator hose being very hot with the bottom being warm at most.
I thought it was either the water pump or the thermostat then, since I know one mechanic told me that the former may need to be replaced sometime in the future back in January and the latter is a common fault point. Looked around for a mechanic to get a thermostat replacement done, the ones responsible for the July service wanted $1k (LOL) to do it, the other one I knew quoted $400 but they were booked out until the 27th of September. Ultimately went for the second one, and decided to figure out alternate ways of getting around; although I still used my car occasionally if I absolutely needed to.
While I was waiting, I found out that the new radiator cap the mechanic supplied me with was non-OEM, so I ordered a genuine GM one to see if it would help. I did notice that the OEM cap (left) had a noticeably thicker rubber seal(?), but I'm not sure if that has anything to do with it.

Here's where things get weird; first uni trip I did with the new cap on Friday to test it out, trip there it started running hot again, okay it's probably not the radiator cap.
Trip back home however, it started running perfectly fine. Okay, maybe it is the radiator cap. Tested it out again yesterday, still running fine. On a whim I decided to check the coolant level after it had been sitting parked at the place I went to for a while - the reservoir was basically empty. Huh?. Double checked the engine bay, no signs of leaks, still coolant in the main system, oil looks fine. Filled it back up and went on my way. Checked it again when I got home later in the evening, seemed to be holding its level alright, still running at perfectly good operating temperatures. Went to church this morning, came back and checked coolant level - it had dropped again! But once again, everything looks in perfect running order, and no signs of leaks or coolant in oil anywhere, no unusual noises, nothing. Once again filled the reservoir, and here I am.

Anyone have an idea what's going on? Running cooler after supposedly losing coolant is probably the most confusing paradox ever. The only thing my mate and I could come up with that sounds remotely logical is the service guys didn't bleed the thing properly, the air pockets were causing the temperature spikes, the new OEM radiator cap somehow allowed the thing to bleed itself properly and in the process of filling the empty space drained the coolant in the reservoir. Is this even possible? If anyone could provide some insight on what might be going on, it would be very much appreciated, I'm at a complete loss right now.

 

[Immortality]

Most likely it just wasn't bleed properly and there was still air in the system. Next time run the engine, get it up to full temp, get the heater on full hot and then check coolant levels.

 

[gossie]

Using RED coolant?

 

[greenacc]

Yes, your logic sounds good and agree the after market caps are shite and the system would have needed bleeding by the time you put a decent cap on it.
Keep watching the level in the res, and in the engine until this gets sorted. If it keeps on drinking fluid from the reservoir you still have a problem, leak or something.
Check the carpet on both front seat areas to see if the heater is leaking coolant into the cabin, and check the heater hoses and the back of the engine for coolant leaks. Look under the engine as well for pink coolant stains doing down.

 

[hansa]

Most likely it just wasn't bleed properly and there was still air in the system. Next time run the engine, get it up to full temp, get the heater on full hot and then check coolant levels.

Yeah was considering doing a proper bleed with a full funnel setup, wouldn't hurt to do it anyway

Using RED coolant?

Yeah it's the red stuff. When I first got the car around a year ago I think it had green coolant in it, although that logbook service must have flushed it and replaced it with red, since that's what it was running when the overheating started. Been topping it up with Penrite 50/50 mix.

Yes, your logic sounds good and agree the after market caps are shite and the system would have needed bleeding by the time you put a decent cap on it.
Keep watching the level in the res, and in the engine until this gets sorted. If it keeps on drinking fluid from the reservoir you still have a problem, leak or something.
Check the carpet on both front seat areas to see if the heater is leaking coolant into the cabin, and check the heater hoses and the back of the engine for coolant leaks. Look under the engine as well for pink coolant stains doing down.

Definitely going to be keeping an eye on the coolant levels after every trip. Thanks for the advice on where to look for leaks though, never knew coolant could actually leak into the cabin.

 

[hademall]

Yeah was considering doing a proper bleed with a full funnel setup, wouldn't hurt to do it anyway


Yeah it's the red stuff. When I first got the car around a year ago I think it had green coolant in it, although that logbook service must have flushed it and replaced it with red, since that's what it was running when the overheating started. Been topping it up with Penrite 50/50 mix.


Definitely going to be keeping an eye on the coolant levels after every trip. Thanks for the advice on where to look for leaks though, never knew coolant could actually leak into the cabin.

I agree with the above suggestions and your theory. Also remember when leaving the cap off and running the engine, it is not going to get it up to temperature and may be the reason for your bottom hose only getting warm. And the other thing I thought might lead to the disappearance of coolant might have been due to a faulty radiator cap with coolant being released as steam which is not clearly obvious to the eye.

I had a new falcon wagon back in 86 (XF) which lost coolant, turned out to be a minute hole in one of the radiator fins. It took over twelve months to discover it by a radiator guy, and of course the twelve month warranty was over. The dealer told me on a couple of occasions that I needed to check the hoses my self by tightening them with a screwdriver. Needless to say I told him what I was more likely to do with the screwdriver

Ah, Jefferson Fraud back in the day, such quality service

 

[Fu Manchu]

Bleed it with a funnel/bleed bucket. $30 for one and solves a lot of problems.

New cap is a good move.

 

[Fu Manchu]

What’s happening is the air has not been bled out properly. As air is purged from the system under pressure, that volume of coolant is drawn in to replace it and so the reservoir level drops. There can be litres of air trapped. Then when empty, the air is reintroduced to the system and around you go

 

[Fu Manchu]

The correct way to bleed the stystem.
It can’t be done with just the cap off.
https://forums.justcommodores.com.au/threads/vz-ve-vf-alloytec-cooling-system-repairs.279976/

 

[Fu Manchu]

The car will get up to temp doing low pressure air bleeds and even after the first thermo opening, there is a Shinto load of air that comes through. Then after the second, a bit less and the third is fine bubbles.