Coolant and head gasket question

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Volvo 1998 S70 non-turbo
Three months ago, the Volvo dealer replaced the original radiator with a new one.
Two months ago, I had the original cylinder head repaired with new valves, hydraulic lifters, valve stems seals and a new head gasket[water pump seized, broke the timing belt which caused the bent valves]. The independent Volvo specialists garage said the head was flat so no machine work. I have been keeping an eye on the pressurized overflow tank with a new 22psi overflow cap.

Last week, another garage flushed the cooling system, pressured checked it, no chemicals where needed and filled with Peak Global Gold Lifetime which was last Friday(7/25). On Saturday, I added about 12oz of Peak to the level mark. Every nite after a 2hr cooled down, I noticed the overflow tank level never really moved up or down. So tonite, I opened the pressurized cap, looking for any signs of oil or discolorization. So far so good and put the cap back on. The antifreeze level was the same level as previous nites. Next, I opened the oil cap on the engine.

When I did this the coolant level rose in the overflow tank. I removed about 8oz to get the level back to the original Saturday level mark. Can someone explain why the level rose again? Is this a head gasket leak issue? Do I need to be concerned?

Thanks
 
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1, the independent Volvo garage where the cylinder head was repaired said it wasn't that dirty. I saw aluminum bits & particles in the overflow tank and had my local work garage do a complete flush.

2. I only have one car and I work 40 miles from home. I have a local home garage, a local work garage, a Volvo dealer and a Volvo independent specialist. The local garages will not do complicated Volvo service since the learning curve & tools yield neglible profit margin.
 
I would say nothing more than a coincidence. Probably an air bubble in the cooling system displacing coolant at the same time as you removed the oil cap.
I would not have removed the what appeared to be excess coolant, chances are it will be low by that amount after a drive cycle.
 
With four hours of cool down, the current antifreeze level is at the appropriate level that allows for expansion and contraction after a drive cycle. I have the excess fluid in a cup and can added back to the overflow tank if neded.
 
Did you run the heater and bleed the system? Some air bubbles can take a while for find their way out.
I don't think you have any problem, no oil in the coolant or coolant in the oil and its not gushing from the overflow like a geyser which can all be symptoms of a blown HG.

The oil cap holds no real vacuum or pressure in the crankcase either from blow by or PCV and non at all with the engine off. If it did you would have more problems than this.
 
Yes, the garage who did the flush and Peak refill ran the heater. This week, I also ran the heater on two separate occasions four times total for about 10 minutes on the highway going to work. I am not sure if the garage bleed the system.

I am planning to do a UOA at the next OCI (4K). After the cylinder head repair, an oil change was done when it reached the first 1K mile mark.

Update: 8/01 7:35AM EST Just got to work (40mi one hour highway drive)Opened the hood and visually checked the overflow tank level. No level change from what I observed last nite. Did not open overflow tank cap. The upper radiator hose is fully pressurized. And the input hose to the overflow tank is hot which appears to be normal. In the Volvo design the input hose to the overflow tank is connected to the engine thermostat housing.

Trav, thanks for insight and comments.
 
Looks like Trav has provided sufficient additional info here. It sounds like everything is back to normal. Good luck!
 
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