Bubbles in expansion tank

[Beezer]

So lately I've noticed my temp gauge getting a little higher than normal at stop lights and decided to check on some things. As soon as i start the engine im getting small bubbles in the expansion tank. I just recently did a HG job on it not more then 3k miles ago. I cant remember before the HG job and after if there was bubbles in the expansion tank while engine was running.

I did perform a block test on it twice at different times and both times the block tester fluid stayed blue. Before i replaced the HG the block tester fluid turned yellow after a few quick pumps of the bulb.

Im hoping air got in there somehow and its not the gaskets again. I do see some dried coolant that leaked a tiny bit out of the small coolant line that runs from the top of the expansion tank (not the overflow line) to the engine.

I've been meaning to bleed the coolant but haven't had time to get to it yet.

My main question is; is there supposed to be bubbles in the expansion tank while engine is operating or not?

 

[Honcho]

So lately I've noticed my temp gauge getting a little higher than normal at stop lights and decided to check on some things. As soon as i start the engine im getting small bubbles in the expansion tank. I just recently did a HG job on it not more then 3k miles ago. I cant remember before the HG job and after if there was bubbles in the expansion tank while engine was running.

I did perform a block test on it twice at different times and both times the block tester fluid stayed blue. Before i replaced the HG the block tester fluid turned yellow after a few quick pumps of the bulb.

Im hoping air got in there somehow and its not the gaskets again. I do see some dried coolant that leaked a tiny bit out of the small coolant line that runs from the top of the expansion tank (not the overflow line) to the engine.

I've been meaning to bleed the coolant but haven't had time to get to it yet.

My main question is; is there supposed to be bubbles in the expansion tank while engine is operating or not?

Yes, sometimes. The expansion tank also serves the purpose of separating any trapped air bubbles in the system and allowing them to escape into the upper section of the tank. In a properly functioning system, you'll see some air bubbles after a coolant change as the system gradually burps and purges the leftover air out of the lines. However, after a few drives, you should NOT see any more air. If you continue to see a steady amount of bubbles and your coolant temp is edging high, it's likely you have a small air leak at one of the hoses. When the car cools down, it can draw air into this leak and thus into the system. In addition, the leak will not allow the system to pressurize, so cooling performance will be reduced and the car will run slightly hot.

Before pointing the finger at the head gasket (which can also cause this behavior, though you would be getting the yellow fluid on the test), I would pressure test the coolant system to eliminate an air leak as a cause first. Sometimes it's something as silly as forgetting to tighten the bleed plug on the radiator fully.

 

[Old Guy]

I've been meaning to bleed the coolant but haven't had time to get to it yet.

I am a little confused. Does that mean that you never carried out the bleeding process after the gasket replacement or you are thinking about getting around to repeating the process?

My main question is; is there supposed to be bubbles in the expansion tank while engine is operating or not?

If you never did a complete bleed after the heads were off you could have a lot of air in the system which might take until the cows come home before it was extracted by circulation through the expansion tank. As Honcho notes, bubbles immediately after a coolant replacement would not be unusual. Otherwise, long term not normal.

Before attempting a bleed, I would do as Honcho suggests and pressure test the system. If you have the original expansion tank you might have a crack in the seam which is not unheard of. If you can find the correct adapter also pressure test the expansion tank cap and its gasket. If you have a leak in the expansion tank it can admit air without resulting in coolant loss; but, I would not expect that air to become entrained in the coolant. Whatever, do the pressure test to determine if you have a leak and if you do, fix it before doing the bleed.

 

[Beezer]

I am a little confused. Does that mean that you never carried out the bleeding process after the gasket replacement or you are thinking about getting around to repeating the process?



If you never did a complete bleed after the heads were off you could have a lot of air in the system which might take until the cows come home before it was extracted by circulation through the expansion tank. As Honcho notes, bubbles immediately after a coolant replacement would not be unusual. Otherwise, long term not normal.

Before attempting a bleed, I would do as Honcho suggests and pressure test the system. If you have the original expansion tank you might have a crack in the seam which is not unheard of. If you can find the correct adapter also pressure test the expansion tank cap and its gasket. If you have a leak in the expansion tank it can admit air without resulting in coolant loss; but, I would not expect that air to become entrained in the coolant. Whatever, do the pressure test to determine if you have a leak and if you do, fix it before doing the bleed.

Yes i bled the coolant right after the HG job and it was fine for about 2k miles. What i meant was ive been meaning to repeat the process.

Yes i am still using the original expansion tank and it does have some cracks at one of the mounting tabs but i dont see any coolant seeping through.

 

[Beezer]

Yes, sometimes. The expansion tank also serves the purpose of separating any trapped air bubbles in the system and allowing them to escape into the upper section of the tank. In a properly functioning system, you'll see some air bubbles after a coolant change as the system gradually burps and purges the leftover air out of the lines. However, after a few drives, you should NOT see any more air. If you continue to see a steady amount of bubbles and your coolant temp is edging high, it's likely you have a small air leak at one of the hoses. When the car cools down, it can draw air into this leak and thus into the system. In addition, the leak will not allow the system to pressurize, so cooling performance will be reduced and the car will run slightly hot.

Before pointing the finger at the head gasket (which can also cause this behavior, though you would be getting the yellow fluid on the test), I would pressure test the coolant system to eliminate an air leak as a cause first. Sometimes it's something as silly as forgetting to tighten the bleed plug on the radiator fully.

This was what i suspected first but i was thinking i would see more coolant loss or some wet coolant loss where the leak would be at. While doing the HG i replaced all the hoses up top in the engine bay and the 2 in the front. I didn't even look at the ones that run underneath the car.

 

[Old Guy]

Yes i am still using the original expansion tank and it does have some cracks at one of the mounting tabs but i dont see any coolant seeping through.

View attachment 193826

The cracks in the tabs I would not worry about for now. The pressure test will tell whether you have a leakage issue.

 

[Beezer]

Ok so i did a pressure test and set the pressure to 18psi (i know the cap states 16psi) and the first few mins it dropped down to 17psi. Its been around 3.5 hours and its dropped down to 16psi. I dont hear any noises nor see any leaks.

Is it normal for the pressure to drop slowly or is it supposed to hold firm?

 

[Old Guy]

The drop can be due to the hoses stretching a bit. If there was a significant ambient temperature drop during the test that could also contribute to the change in reading. Your test would probably qualify as a pass based upon most service manuals. Most pressure testers will deliver pressures up to at least 30 - 35 psi. I might be inclined to repeat with an elevated test pressure of 25-30 psi and see what happens. The higher test pressure may show a leak.

In your first post you mentioned spotting dried coolant; but, you made no mention of having to top up coolant. If you have not had to add coolant perhaps the dried coolant and the potential leak is all a red herring. If you have been topping up with coolant then the leak theory allowing air to be sucked into the system may be valid. If you have been adding coolant and the cooling system passes the pressure test, I would
- test the pressure cap to make sure that it is not the source of the leakage
- add UV dye to the coolant to check for leaks

If all of this tests out good then bleed the cooling system as per Kaz's method and then watch to see what happens in terms of bubbles in the cooling system.

Cooling System Bleeding Procedure 01 - Blogs - NSXCB Forums

Cooling System Bleeding Procedure 02 Today at Atelier Kaz - ex-Honda R&D, F1, Indy/CART engineer

www.nsxcb.co.uk

Make sure that your elevated coolant temperature is not something completely unrelated. Since you report that the higher temperatures are occurring at stop lights I assume that means when the car is moving temperatures are normal? If so, how is your radiator? Are the fins bent over or plugged with dead bugs? When you come to a stop light can you hear the radiator fan switch on? Do you have anything that might be impeding air flow through the radiator?

 

[Honcho]

Great advice by @Old Guy. We've been seeing a lot more radiator failures as these cars get into their third decade. A small rad leak is hard to spot and can cause the behavior you're seeing.