When do I loosen the vent plug to let air out of the coolant system?

I would be inclined to suspect the radiator cap. If it is not holding the stated pressure the water temperature will be higher and the expansion (overflow) tank will fill to a higher level than normal when hot.

That's not the point. The thermostat is intended to keep the engine at the designed/engineered operating temperature.

Richard

You will be much further ahead to have the thing repaired properly.... Opening the cooling system when the engine is hot - particularly when you have little understanding of the system - is an invitation to disaster. The boiling point of the coolant is raised 7 degrees F for every pound of increased pressure.... If the cap is rated at 10 PSI and your engine is close to boiling - your coolant temp at sea level will be somewhere near 280 degrees F.... Hot enough to blast the meat right off your bones, so to speak...

There are many things that could give the symptoms you describe.... allowing the condition to perpetuate will create unecessary damage if left unattended (if the system hasn't been compromised already).

Since the problem appears to occur at low speeds - do you hear the cooling fan kick in at all (IIRC, this is an electric fan on your car). Check the coolant level in the radiator when the motor is cold.... is the rad full? Is the thermostat more than 3 or 4 years old? Are the radiator fins free of obstruction?

So many questions - so little time...

Goes back and forth? It ought to get up to operational temperature and pretty much stay put.

No. You don't want to open the system when it's hot.

The thing is, the coolant system is designed so the path to the radiator is near the top. If you've got air, that's what ought to get shoved out first. I'd be very surprised if the problem were the mix.

I'm sorry, but the most common problem I've seen with cooling systems (especially with odd behavior in the overflow bottle) has been blown head gaskets.

Yes, but don't. It's not the problem. The thermostat keeps coolant from going through the radiator until its hot enough, then it allows it. If you've got a 195 degree thermostat and you're running at 220, the thermostat is already wide open. Taking it out will make your engine heat up slower, but it will still reach the same temperatures. Find the real problem, and fix it. Actually, it's not impossible that you've got a bad thermostat -- put it in a pan of hot water on the stove and see what temperature it opens at; if it's bad, put in a new one.

But I'd start with a compression check (both hot and cold) and comparing the spark plugs to each other.

You may have already gotten it too hot..but:..

On a cold engine, remove the vent bolt, add fluid to the radiator till it comes out the vent. Plug the vent tightly, replace cap on radiator. Bring engine back up to running temperature. Let it cool down and do the vent, fill thing again. As little as 1/2 pint of air in the block with cause large temp swing..

If you drive long distances from "home" I might would go ahead and have a mechanic do a compression check.. to check for warped head, blown head gaskets, which this engine is known for..

Hose down all spilled antifreeze that you spill to keep it away from the cats and dogs..

Chuck (in SC)

Well, that's a lot of questions and things to try, not just from Chuck, but everyone involved.

Let me start by telling everyone what I have done so far:

I noticed that the washer or gasket under the drain plug was cracked and coming apart, and there was some fluid leaking from there, so I replaced it with a nylon washer (hope this is the right substance). I also filled up the radiator with the vent plug removed, as this allowed me to get more fluid into it. Then I capped the radiator, and added fluid to the vent plug opening (the vent plug sits higher than the radiator cap), while squeezing the radiator hose in an attempt to push out any trapped air.

In answer to some questions posted here:

The electric cooling fan appears to work. I have seen it kick in when the car is idling. The radiator fins are free of obstruction. The thermostat is probably 5 years old, however, the the temp gauge goes back and forth. What I mean by this is that it typically goes from C (cold) to around the M in Normal (a little more than half way), then drops back to the N. Then it usually hovers between the R and the M.

It seems to be doing better now, however, I haven't actually had the opportunity to drive it much in non-highway use. I'm still keeping an eye on it. I don't want to replace the radiator cap or the thermostat or any other parts, unless I know it is going to help fix the problem.

Well, yesterday was a very hot day (97 degrees), and I had a few stops to make after getting off the highway. The temp gauge got very close to the red while I was stuck in traffic. It seems that it only has a problem when I am stopped, although the fan is definitely kicking in. I made it home and popped the hood with car still on, and the fan was running, and the overflow tank was at the "hot" line. It is not leaking at the vent plug anymore since I replaced the washer.

So do I have to just guess and start replacing parts or is there something else I can do for diagnostics? I see that some people have suggested a "compression check", but doesn't that just check for leaks? It doesn't appear to be leaking since the level of coolant is not going down.

I was one of the ones who suggested a compression check before; a compression check is looking for leaks in the head gasket, not external cooling system leaks. I've seen slight head gasket leaks cause similar symptoms to yours, because hot combustion gasses have been getting vented into the cooling system.

Another response you got that I didn't respond to at the time actually fits your symptoms better than my suggestion (why did I make my suggestion? I didn't think of his. Why didn't I comment then? I hate "me too" posts) -- anyway, he thought it sounded like a bad radiator cap failing to keep pressure in the coolant system. Having the radiator cap tested would be a really good first step; at the price of radiator caps, that's something you can afford to just replace and see what happens before you get serious about diagnostics.

Is this a stock electric fan, or something that was added aftermarket? Many, many years ago I tried replacing my stock clutch fan (318 Plymouth) with a pair of 'so called' power saving electric fans. Bottom line; they were a complete waste of time and money. It is also possible that the electric motor on your current set up could be going bad and slowing down, so even though it is running it might not be pulling enough CFM to do the job. Since the problem seems to be happening in stop & go traffic that would point more towards an air flow problem.