slight water boil-off from loose coolant cap?

dgeist · **Posted:** Tue Feb 04, 2014 9:48 am

After getting my timing belt service a couple months back, I did some other wear item replacements in the engine bay and apparently left the radiator expansion cap un-tightened a bit (loose enough that I could lift it right off this morning). I was topping off the coolant since it looked a little low and I was getting the nag light.

Can an incomplete seal do either of the following (I suspect yes, but would like confirmation):
- prevent the temp from getting all the way to proper run-level due to lower pressure in the system
- allow some of the solvent (water) to boil off, causing the low-fluid condition

It's been a while since I calculated boiling points at various atmospheric pressures, so I don't know if normal engine temp is enough to boil the water if the cap is left cracked. If that's not the case, then I need to start hunting for leaks.

Dan

papaindigo · **Posted:** Tue Feb 04, 2014 10:58 am

Remember the way the KJ CRD coolant system is designed that cap is essentially the same as a regular radiator cap (e.g. the cap on top of the physical radiator core). So imagine, if you will, an old style radiator with no overflow bottle. On those systems the cap is/was designed to hold coolant up to a certain pressure to 1) ensure a higher boiling point, 2) proper system pressure to ensure coolant circulation, and 3) regulate system pressure by allowing the cap to popoff and coolant to vent to the ground as you drove. That is why on those systems you had to check and top off the radiator at EVERY fuel fill up.

On our vehicles, if you left that cap loose as coolant expanded it would bubble out thru the loose cap and flow onto the ground as you drove, just like in the old system, and probably circulate poorly due to the lack of system pressure.

So the answers to your questions are all Yes but no to hunting for a leak.

Essentially in lay terms you left a loose lid on a boiling pot.

dgeist · **Posted:** Tue Feb 04, 2014 5:54 pm

Yep. That was my suspicion as well. I just wanted to make sure since I didn't see any residue around the cap. I suppose the 50/50 solution separates out as the state change of the water takes place.

Dan

flman · **Posted:** Tue Feb 04, 2014 7:20 pm

Put on a new cap, drive, let it cool overnight, pull the cap in the morning, if the system has pressure cold you have a HG leak.

DocB · **Posted:** Wed Feb 05, 2014 1:06 am

I just posted a question about this in another thread, but just so I understand correctly:

On a cold engine, there should be NO pressure under the coolant overflow tank cap???

None?

racertracer · **Posted:** Wed Feb 05, 2014 5:00 am

DocB wrote:

I just posted a question about this in another thread, but just so I understand correctly:

On a cold engine, there should be NO pressure under the coolant overflow tank cap???

None?

Yes, None.

dgeist · **Posted:** Wed Feb 05, 2014 11:19 am

I appreciate the notes on head gasket stuff (and agree). There are plenty of threads available for that already, so lets not add another one

I performed those tests before my TB change (to know if I needed to open up the top end). I have no evidence of oil or coolant contamination or other HG-related signs, just suspiciously low on coolant after the TB change resolving the normal slow loss due to an old water pump. The slight boil-off due to the cap not being tight explains it. I just need to watch it for a few weeks and make sure (and possibly get a new cap since having it steaming for a month or so may have damaged the seal).

Thanks!

Dan

racertracer · **Posted:** Wed Feb 05, 2014 1:53 pm

Dan,

I wish you luck with that.

slight water boil-off from loose coolant cap?

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