Bleeding air out of my '75

[ghhatley]

There's air in my cooling system, I can not seem to get it out. Suggestions?

[TJ Hopland]

I have never had any issues getting the air out of the cooling systems on these cars.   I just let em warm up with the cap off.  Once the stat is open top off the radiator and install cap.   Fill the recovery tank about half way and within one of two heating / cooling cycles the level in the rad stays full and the level in the bottle drops to the full cold level and all is well. 

In general that was the basic procedure for most cars and trucks till the 90's when they really got into the curved hoods and the radiators started not being the high point in the systems.  Once they did that then there started being specific bleeding procedures.   There were exceptions in the earlier days that were tricky,  Corvette is one that comes to mind.  In those the radiator was pretty low and at an angle.   Seems like there were a couple of the earlier transverse FWD cars that had the cap on the engine that were a little different too.

What issues have you been having?   Recently replaced some parts?   Whats the recent history of the car?

[Scot Minesinger]

Refilled a lot of cooling systems from cars of the 60's and 70's and never had any issues getting the air out.

Why do you think air is in the cooling system, because bubbles come out of radiator neck when you first start it up with cap off, and later it overheats?

Tell what are the negative symptoms and circumstances that lead you to write about air in the cooling system.

[ghhatley]

Here are the symptoms I've had
The upper radiator hose doesn't seem to pressurize after driving for 20-30 mins. Also I've noticed the air this car is blowing is cold even with the the heat set all the way over to hot. Thoughts?

[savemy67]

Hello ghhatley,

Is the upper radiator hose warm or hot after 20 - 30 minutes of driving?  If not, I would suspect a stuck thermostat.  If the hose is warm/hot, try TJ Hopland's procedure to see if the situation improves.  No heat from the Climate Control could be another problem.

Christopher Winter

[Scot Minesinger]

Sounds like as Chris says a stuck open thermostat.  During weather much below 60'F, the car will not get much past 150'F and the heat will not be warm.  Your car is not overheating, it is run away too much cooling.  In the summer I bet the hose would pressurize and heat would be warm.  This just happened to me on a 72 Eldorado.  Thought I was doing a good thing buying a fail open thermostat (most fail closed), but it failed after a week.  After it failed suffered same symptoms as you describe. 

[TJ Hopland]

Maybe a missing T stat from someone trying to make it run cooler?     How much of this cars history do you know?  Is it new to you?

Start it cold with the cap off and look in the radiator.    A normal system the coolant should be pretty much up to the cap in the radiator and not really change when the engine starts cold.   It may drop slightly but it shouldn't drop much and you really should not see anything that looks like flow.    If you feel the upper hose it should be at ambient temp.   

IF the level in the radiator drops when the engine starts I would guess you will also see signs of flow.   If the level drops enough you can usually see coolant coming through the tubes.   This is what its supposed to look like when the engine warms up and the stat opens.   If its happening right way the stat is stuck open or missing. 

Normal would be level stays high and no flow.   Upper hose won't get warm at all till the stat opens which should only take a few minutes.   You will usually feel the heat in the engine after a couple minutes and if you keep checking the upper hose it will start to get warm and you usually see little bits of steam coming from the radiator cap area.   Once that starts to happen within about a minute the hose should be hot enough that its uncomfortable to hold and you should feel the heat in the air coming from the fan.   Level should drop slightly and you should see flow in the cap area.    At that point put the cap back on and give it a few minutes to see if pressure starts building.   I usually take it for a quick test drive then check for pressure.  During all this testing you may get little dribbles and splashes out the cap area, this is normal and have a pan under it if you are somewhere you don't want a little coolant on the ground. 

If the upper hose very slowly warms up its another sign of no or bad stat.    When you get a replacement make sure its got the lower disc on it.   The basic size of the stat is very common and fits lots of engines.  The proper stat for these engines looks like all the rest but has an extra disc on the bottom about the size of a quarter.     Lots of low quality stats out there these days so don't go for the cheapest one your vendor has.    I have read that John Deere use the same stat in some of their engines and still have quality parts.   I got mine from MTS/500cid.com.   I try to buy from places like that because they sort through the crap and only sell good stuff plus they do a lot to keep these engines on the road.

Changing the stat is not too bad on these.    If the AC compressor is still there one of the bolts is at a funny angle but its not bad.   Bolts go into the iron block so they usually don't get stuck. 

[35-709]

A picture of what your 'stat should look like if it is there and the correct Carquest number for 180*.  The factory supplied thermostat when new was 195*.  The 180 and 195 'stats are the only ones available for your engine in the proper configuration shown below ---

[35-709]

In my limited experience, air trapped in the cooling system (air bound) will cause overheating, not running too cool, which is why some folks drill an 1/8" hole in one or two places around the flange (not covered by the gasket) to allow air to bleed out.  This does not impede proper system operation.

[TJ Hopland]

On my above description of what is normal for a cooling system I forgot that this era Cad has the heater core return going to the radiator not too far from where the cap is so you will / should see the coolant moving around when you look in there even when the engine is cold.    The last couple cars I have worked on didn't have that so I sort of forgot about that little detail.

I also wonder if this is why most of us have not had bleeding issues with these engines?   Thinking about it the heater outlet is in the back of one of the heads which is a high spot in the system, possibly even higher than the T stat.  The air tends to end up heading for the high spots which in this case would be the heater outlet which always gets circulation so any time the engine is running air is going to tend to get pushed through the heater core and straight to the radiator near where we usually have the cap off so it gets burped out fairly quickly?  I never really thought that hard about it before but I bet that is what happens.   I have drilled holes in stats before but never on a Cad or anything that had the heater hose return to the rad.