E36 M52B28 - Cooling Pressure when cold

[Massnz 2]

Hello

1996 E36 M52B28....I fully bled my cooling system by jacking the front end up about 500mm and it seemed to be successful.  Hot Air coming from the heater and no visual coolant loss or overheating.

Drove the car about 30kms to drop it off at the Car Transporter as we were moving Islands, everything seemed to be all good.  5 days later I go to collect my car from the drop off point, open the radiator cap to check the coolant level prior to driving and there was a very loud pop/boom like opening a Corked Wine Bottle but with a much deeper tone.  No coolant splashed out so i assume this was under vacuum?

I then drove it home (6kms) and parked it up.  Next morning (12 hours later) the hoses still felt a little rigid (about halfway between when the car is running and cold with the cap off).  Opened the cap and it made an audible sucking/gurgling noise with a little bit of coolant streaming out from the small upper radiator hose into the expansion tank.

Is this likely to be due to an Air Pocket being present in the system creating a vacuum or could I potentially looking at something a little more serious?  

 

[NZ00Z3 187]

I don't know the answer but will apply a bit if science to see if it helps.

The pressure in the cooling system will cycle between hot (high pressure) and cold (low pressure) conditions. If its a good condition cooling system with no leaks, weeps etc, then the pressure difference should be very close to the atmospheric pressure when you last opened it and this time when you opened it.

If you have a small leak/weep, then you will be loosing a little bit of water when the engine is hot.  The weep may not be two way and not draw air back into the cooling system when the engine is cold, hence causing a small vacuum.

Maybe time to do a full cooling system inspection to look for weeps?

[Massnz 2]

Still having some issues.  Fully bleed the system after i put in a new waterpump (cars been sitting around for a while), took it for a good blast yesterday afternoon with the heaters blowing hot and parked it up for the day.

 Checked the cooling hoses 26 hours later and they were hard (just a little less than operating rigidity), cracked the rad cap open and a couple hundred mls of coolant came gushing out.  Reservoir level is still sitting at the cold line.

 

I performed a few tk tests over the weekend which appeared to come up all good and a compression test a month ago came up 180 on all 6 cylinders so a bit confused.  

Am I just being overly paranoid? 

 

 

[Rubix 434]

Pressurised coolant system when the car is cold is not a good sign - possible cause is combustion gases in the system, which is a symptom of a failing Head gasket. Do a leak down test.

[Massnz 2]

Yeah that's what I was thinking but after performing several teekay tests for combustion gases and having them come up all good I'm not sure.  

Performed the tests from cold to hot and gave it several blasts of the throttle but the fluid stayed blue.

I'm not having any coolant push out past the rad cap while the car is running which makes me wonder if I'm just being paranoid.

[Eagle 1662]

Have you tested the cap? It's one of first things that should be done.

[Massnz 2]

Funny I spent half commute home reading up on rad cap symptoms.  Short answer is no so I'll give that a go.  

 

I assume the cap is designed to allow coolant to flow from the overflow pipe into the expansion tank once the system reaches operating pressure, so if it's stuck then pressure builds up in the rad and has no where to go particularly when the system cools down...  until I open the cap?

[Massnz 2]

Obligatory update (its a pain in the arse when you read threads while diagnosing only to get to the last page and have no update from the OP), got myself a new expansion cap and gave the system another good bleeding.  Warmed it up to temperature and took it for a good hard drive the same route I normally do and the hoses didn't feel as hard as usual so i'll let it cool down and see how it is. 

The new cap seemed to spin a lot more freely and had more play in it than my old cap which would jam up if you twisted the stalk around, so maybe the cap was sticking and not allowing the system to adequately vent into the overflow.  I always thought the failure mode on a rad cap was excessive venting due to a failure in the spring mechanism which is why I overlooked this item.