Air pocket in cooling system

[Hobie]

I've been chasing a heating issue and finally solved it.  Didn't make sense so I'll tell you my saga.

After shutting off the engine, I noticed a small momentary gurgle in the radiator.  Passed it off as a normal sound and just hadn't noticed it before.  But installed a new cap anyway.  No more gurgle, but still heating.  Water pump was replaced 1 year ago and had problems with the fan clutch before, so in goes a new fan clutch.  Flowed more air but still slowly engine got hot.   

Hoses were strong and did not collapse in my tests and the radiator was clean.  Backflushed system anyway.  No change.  New thermostat.  Still no change.

Viewed into the radiator with the cap off to check flow and noticed it was foaming and very little flow.  Ding!  Bad water pump I figured and in went a new pump. ( However the old pump still had all its vanes and had a firm fit on the shaft. Hmm, maybe not the pump.)  filled it back up and still the heating.

For years I had always drilled a small 1/8" hole in the flange of the thermostat to allow air to pass when the stat was cold and closed.  Especially when filling the system.  With this trick I never had a problem for years in many different cars.

Then I remembered someone here said to be sure their was no air pocket in the system and to fill from the top radiator hose to prevent one from forming.  So I pulled the bypass hose on the water pump and used my garden hose to slowly refill the system.  ( Fyi, for all my tests I have been using tap water ) 

Noticed a few bubbles emerge from the bypass outlet. After it ran clear then pressed my thumb over it to fill the radiator.   Buckled everything back up and .......... problem solved. No heating!

Indeed I had an air pocket.  First, formed by a bad radiator cap.  And subsequently by the usual method of filling the system through the top of the radiator. 

Live and learn!

[TomC]

I know on my engine, there are two small hoses coming from the top most part of the engine to the overflow tank-this is to constantly aerate the radiator to keep the air bubbles out.  If you don't have these small hoses, then either look at another installation or confer with your mechanic on how to install them.  Good Luck, TomC

[DavidInWilmNC]

I know on my engine, there are two small hoses coming from the top most part of the engine to the overflow tank-this is to constantly aerate the radiator to keep the air bubbles out.  If you don't have these small hoses, then either look at another installation or confer with your mechanic on how to install them.  Good Luck, TomC

My MCI has one of those hoses on each radiator.  They also connect to the overflow tank way up high.  I didn't realize they were there and initially couldn't figure out why the radiator wouldn't come out.  On a similar note, I see what look like drain cocks on several places on the engine.  Some of them, like the one on the bypass pipe between the thermostats, are obviously not drains.  Will the rest of the engine purge itself from air pockets simply by filling the overflow tank?  On MC-8's, the tanks leads directly to the lowest point in the cooling system.  I would assume this, plus the ones on the heater cores, will let out all the air... is this a safe assumption?  Thanks.

David