Car
1994 Honda Del Sol Si; 127k miles; 5 speed

Symptoms
With the engine running, I observed a slow stream of air bubbles coming through the tube in the coolant reservoir.
Increasing the RPM does not increase the flow of bubbles, while the radiator cap is securely on.
I first noticed it while flushing the coolant. I had refilled with water and left the cap off. In this case, the air bubble purge
Rate was dependent on the engine RPM. It smelled like exhaust gas. The CEL is not on.

Pertinent related info
My first thought was a head gasket leak, but...
1) I checked the cylinder compression; all cylinders were acceptable and within 5% of each other.
2) There is NO water in the oil; the oil is not like chocolate milk.
3) There is NO white smoke coming out of the tail pipe and it doesn't smell sweet.
4) There is no loss of power, especially under hard acceleration and high RPMs.

When I first purchased the car, I found the coolant reservoir filled with a black gunk. I have had to clean it out a couple
Of times since. The gunk could be oil, though it doesn't behave like oil. It doesn't float on top of the water. It sticks to
The sides of the plastic in the reservoir. I also checked the other radiator hoses and found all of them completely clean
On the inside and around the fittings. I also do not observe any gunk in the radiator itself or in the thermostat
Housing. It seems as though the gunk is being formed in the coolant reservoir.

Any suggestions would be most appreciated.

Thanks.

Thought:

The previous owner added an anti-leak product, possibly to seal the head gasket. Over time, some of it ends up in your overflow tank. Ensure it has not affected your radiator cap. There are two valves inside the cap that can fail. One allows hot coolant to flow into the overflow tank, the other allows coolant to flow from the overflow tank to the radiator, as the engine cools. If the coolant can't flow back into the engine, there won't be enough coolant and the engine can overheat. It's best to use a high quality, preferably OEM, cap. You need to get to the bottom of this situation--it may be serious. I'd go over the cooling system thoroughly.

I already replaced with an OEM cap. The engine does not overheat. I have put over 5k miles since the purchase. The coolant level is always full in the radiator. I have flushed the system 3 times now. What would cause air bubbles in the reservoir?

I had been leaning towards replacing the head gasket, but read somewhere on these pages something similar where the radiator was believed to be at fault. The air bubbles have me troubled. Thanks for your advice.

Air in the system. To refill Hondas, it's important to open the bleeder bolt, add coolant until it comes out of the bleeder bolt, tighten the bleeder bolt, turn the heater control to high, turn the fan off, run the engine until the thermostatically controlled radiator fan comes on, add additional coolant until the radiator is full, and rinse out and refill the coolant reservoir.

I am aware of the bleeder bolt and how to flush the system. I think I'm going to get a test to determine whether the gas is exhaust or not.

Good idea.

Hi, did you ever find out what caused that stream of bubbles into your reservoir? I have the same problem with my 94 civic dx

Thanks

Marc