Yesterday I noticed small flow of bubbles rising in the coolant reservoir in my wife's 2007 XC90 3.2L 6-cyl (52,000 miles on the odo) while idling hot. This naturally suggests a leaking head gasket. The vehicle has never overheated. The coolant has been changed once at the dealer.

For several years there has been a very slow loss of coolant, maybe a cup per year or maybe not that much, but I never before noticed bubbling. The rate of bubbling increased somewhat with increasing engine speed, but I only raised it to about 1500 rpm. I was alone and used a wedged piece of wood to press on the accelerator pedal. My next step is to get a helper to see what the rate of bubbling is at say 2500 or higher rpm.

I see that there is a test apparatus with an indicator solution to see if combustion gases are coming out the reservoir. I may get this today.

If the bubbling starts immediately on cold start-up, is this an almost certain indication of combustion gases leaking into the coolant?

 

- Engine compression test on all 6 cylinders.
- Send an engine oil sample for analysis for glycol etc.

 

Can't say I've seen any of those head gaskets fail yet, but I would certainly check for combustion gasses with one of those kits with the blue fluid.

A compression test likely won't show anything if it's that small of a leak. A cylinder leakage test would be better, but a cylinder leakage tester is probably more expensive, plus the front crank plug and the special tool to turn the crank.

 

The test was negative. I got a NAPA BK. 700-1006 Universal combustion leak tester and the color stayed blue.

I drove the vehicle maybe 8 miles to the parts store and back with the test kit ($34.99 + $2.89 sales tax) and let it sit for maybe 20 min before doing the test. The vehicle had been driven gently and coolant was then somewhat down in the reservoir. Before starting the engine I removed the reservoir cap (heavy rubber elbow length glove), but there was no expulsion of coolant.

I started it to idle and the level of coolant rose right to the max line in the reservoir as it was heating up. I was able to get repeated draws through the indicator solution and there was no change in the color of the solution. When the engine reached operating temp the squeeze bulb was not able to draw gas from the reservoir, meaning that no gas was being expelled into the reservoir at that point. There was no visible bubbling at that point.

This is a gratifying result, but I wonder what the original bubbling was. Maybe the engine was not at operating temperature when I observed the bubbling originally. It could be that there is a loose hose or sticking check valve which is causing air to be sucked into the cooling system when the engine cools down.

I may test it again tomorrow from a fully cold state, but right now I am pretty confident that no combustion gases are getting into the coolant, so the head gasket must be OK.


I assume that there is a check valve is the reservoir cap which allow air to be pulled into the reservoir as the engine cools and the level of coolant drops in the reservoir. If this valve was not allowing air to be pulled back in and if there was a slightly loose hose connection, then air could be pulled into the coolant at at place where it would end up in the block or head. When the engine was next started and heated to operating temp, then this air could be expelled as bubbles in the reservoir. Maybe this is the explanation for the bubbles I observed. I might switch coolant caps with my V70 and see if this does anything.
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I am a bit embarrassed to confess that what I thought was bubbles rising from below was actually a disturbance caused by the stream of coolant from the small rubber hose connected at the top of the tank which comes from the head near the intake manifold.


The steam of coolant from this hose hits a baffle and is deflected down into the surface of the coolant in the reservoir making a disturbance including bubbles. I imagined that they were rising from the large hose at the bottom. Yikes!

 

In this case, I'd rather be a little embarrassed than be right and have a blown head gasket!

 

Very true and my overwhelming reaction is relief.


I was misled by comparing the operation of the coolant reservoir in my 2004 V70 (2.4L 5-cyl) which does not have this small coolant hose from the head (in the 3.2L 6-cyl) and so the reservoir in the V70 does not show any disturbance when the engine is started.