Replaced my water pump, heater hoses, heater valve and coolant yesterday. All **seems** hunky-dory, no leaks found yet, etc. But I would like to know how long the cooling system should hold pressure after the car is fully warmed up by driving about 30 miles, then parked and shut off with the hood open. Having the hood open probably lets it cool a bit faster. It is about 65 to 70 deg. in my garage, and there is good circulation, but no direct breeze.

Judging by the highly unscientific method of squeezing the upper radiator hose, it holds pressure pretty well for at least 15 minutes after shutdown, maybe longer. But, by 2 hours later it has lost all pressure, and cooled enough to take the cap off of the expansion tank.

Is she cooling off and losing pressure too fast? Or, is this just about right? Should I become hysterical or just remain calm?
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

I've read your original post and the second one you submitted, and it all sounds well. Some points to make:
1) When you installed the thermostat three months ago, did you rotate it to make sure that the bleeder (a hole with a jiggle piece, or sometimes an actual ball-valve) in the rim is positioned at the high point? If not, it's probably why the system "burped" (coolant dropped) the first time when the thermostat finally opened -- it released an air pocket that was initially trapped when you first filled the system, something the bleeder would have gradually released even before the engine warmed up. In any case, once the system has burped, the position of the bleeder is moot (unless you develop a leak), so don't worry about rotating the thermostat now -- just remember for next time.
2) You should (or at least could) have first let the engine warm up with the overflow tank's cap off (it won't explode water all over you, honest, because the coolant won't boil), and then added mixture to the max level before putting on the cap -- that's what I do; and I would suggest doing this now (including putting the heat temperature at "Max" (but the blower fan is unnecessary), just to make sure that the level is proper.
3) In any case, it doesn't seem like you have any leak, headgasket or otherwise, because the system seems to hold pressure for a reasonable amount of time after shutdown.

So just fill (as in #2), and relax.
Regards,

Ken,

I'm in the same boat at N7. Just put new coolant parts on and made sure the thermostat jiggler was positioned at the top of its housing. i haven't started the car yet, but you're saying (in #2) that I should leave off the reservoir cap while it's heating up the first time and add coolant mix to top it up, as it gets drawn into the system?

re: "...but you're saying (in #2) that I should leave off the reservoir cap while it's heating up the first time and add coolant mix to top it up, as it gets drawn into the system?..."

Sure. Let it go through a complete cycle of heating up (for the first time, with the cabin heating set to full hot -- if there's a bubble caught, and it gets purged when the thermostat opens, there might be a little burp in the bottle -- you may want to keep the cap on very loosely to avoid a mess, but the burp will only occur the first time. Then top off to the proper mark, and close the cap tight. If all's well with the system, it should stay at the right cycle, going up and down within the marks from now on.

If I may reply for Ken: Yes, that is what he is saying.

Did you leave the t-stat housing loose when you filled, to allow air out? Then, when the coolant begins to run out of housing, stop filling, tighten t-stat housing down, continue filling to "MAX" line. Or you could just leave the upper radiator hose off the t-stat housing when filling, then put it back on when coolant flows out of there. Then continue filling to the "MAX" line.

After making sure that everything in the cooling system is properly tightened and there are no leaks, set the heater control inside the car to full hot, then start the car, cap off of the tank. When the t-stat opens you will hear noises that would be quite rude coming from a person, and the engine will suck down the coolant in the tank.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Oh I get it now. Theremostat housing loose to let air escape as you fill it up when cold because the thermostat will be closed. Then tighten it down and keep filling. Then start car.

Thanks.

I did have one more question: is there a torque specification for the hose clamps? I know that seems nit-picky, but I might as well do it at this point. Otherwise, just tighten them til it feels right?

I don't know of one for the hose clamps, but keep this in mind: where you are clamping to plastic, like on the radiator, go easy. You don't want to crush or crack the plastic tube that you are attaching the hose to.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Ken,

Thanks for the reassurance. If I may, to answer your questions.

1. Three months ago, I did not install the t-stat. It was installed by a local Volvo specialist, who I have since come to think of as a lousy mechanic and jerk. It was installed as part of having the head gasket replaced - hence my focus on and fear of a problem there. I did remove the t-stat when refilling the car, leaving the housing loose so that I could see it begin to spill over out of the housing when the head was filling. I was careful to put the jiggle pin at the top when reinstalling.

2. My fear of overheating damage, in this case due to localized or nucleate boiling blinded me, and for the first time ever in nearly 40 years of working on cars I put the cap on when warming up the engine for the first time. Jeez, when my fear gets hold of me it can make me screw up royally.

3. Thanks for the reassurance.

BTW, while I have a Stant cooling system tester, I lack the $60+ Volvo adapter, so this morning I conducted a very crude test to see if cap was holding the right amount of pressure. It was based on whether the upper rad. hose was stiffening the right amount when under pressure: I filled up one of my daughter's BMX bike tires (about the same diameter as the upper hose) to 22 psi, or roughly 1.5kPa, and compared "squeeziness." Within the range of human error, it felt exactly the same as the upper rad. hose when pressurized, so everything seems okay on that issue.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

I would say.... it is very hard to say, haha.
Anyway, that it is holding pressure for up to 2 hours is good.

That it is holding pressure for 15 minutes is good.

I would measure "goodness" by seeing what temp it is when it is under pressure 15 minutes later. As long as the temps are dropping (and not from boiling coolant evaporating) you don't need it to really retain pressure.

-Will

PS I would be more concerned with is it releasing pressure when the pressure is too high? Is the cap fairly new?
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1990 740 Turbo, on its way to stock specs, maybe beyond

Will,

The cap is genuine Volvo, and less than two weeks old.

Here is why I am concerned, perhaps over-concerned: After replacing the pump, heater hoses, and valve, I refilled it with Volvo coolant/distilled water mixture. Opened the heater valve (turned on the heater to full hot), started it and ran it, in Park, at idle, until the t-stat (3 months old, genuine Volvo) opened. When the t-stat opened, it gulped down some coolant, lowering the level from the Max line on the tank to the Min line. I let it cool off for about an hour or two, opened the cap (very tiny, short hiss, couldn't tell if it was going in or out) and topped it off. Drove it for about 3 miles, coolant level was stable.

Then I packed up and drove to the Friends of the Library Book Sale in Gainesville, about 35 miles one way. In Gainesville, the level seemed a bit lower, but it was hot so it was hard to tell, probably the equivalent of 1/2" below Max.

When it cooled off at home, later, after about a 85 mile round trip and some errands, it was actually 1/2" below Max, probably the same level it was at in Gainesville. Thus my concerns: should it have dropped at all again after being topped up as explained above. There are no apparent leaks, and I use an inspection mirror, flashlight, and where necessary crawl under it.

So, my concern is because I can, not yet anyway, find a leak, even minor, and if it is loosing coolant, there is really only one place it could be going, right? Head gasket = Ugh!

Sent it to work with my wife this morning, and will see what happens this evening. If no more drop, or very little drop (much less than yesterday) then it was just burping up little, residual, air bubbles or whatever. If even close to yesterday's drop, then look extra super carefully for signs of leakage, but most likely "Oh, Crap!"

Edit to add: BTW, from a cold start up, the upper radiator hose takes at several minutes to get stiffer due to internal pressure. It does not pump up quickly at all, so that seems okay. Also, there is no oil in the coolant, and no coolant in the oil. Guess I have to wait until it comes home tonight and see what has happened.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

The car has been home and parked in the garage for 1 hour & 15 minutes, so far. With the hood just open enough so that it is not latched (for easy access since I'm checking it every 15 to 30 min.), but nearly closed. The upper radiator hose is still stiff, coolant level is still about where it should be, I think. When cold this morning, level was just at the "MAX" line, when it came home, and now (still quite warm) it is just at the top of the letters "MAX." That is about 3/4" above the line. I figure that it will fully cool overnight and the level should be at the "MAX" line as it was this morning. I hope.

Of course, since I am completely fear and worry driven, I'm now concerned that it is holding pressure too long . . .
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

The cooling system is cooling down, coolant level is where it should be, and the upper radiator hose is finally, after about 3+ hours noticeably softer indicating that the system is returning to atmospheric pressure.

Amazingly it worked perfectly well today even with one of the heater hoses lacking a clamp on one end. See my post "The luckiest Volvo alive!"
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

This morning, with the engine cold, the coolant level was exactly where it was yesterday morning with the engine cold. As noted above, it seemed to hold pressure pretty well after it got home yesterday evening, too.

So, having found a hose clamp that I left off (see "The luckiest Volvo alive" thread), and with everything seeming to be okay, I may be able to breathe easier for a while. Will still keep an eye on everything for a few days, though. With a cooling system, you can never be too careful.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)