I am having issues with the coolant backing up into the overflow tank and seeping out of the top. The upper radiator hose gets very tight when at full operating temperature and air always gets released when I open the radiator cap. Overheating only occurs when the car is low enough on coolant that the thermostat won't open anymore to trigger the cooling fan at idle. I have also noticed that the pressure buildup seems to be worse on a 30 min highway drive than a 30 min drive through traffic.

Upon doing some research, the signs point to a compromised head gasket allowing exhaust gases to escape into the cooling system. The car has no coolant in the oil and does not burn any off through the tailpipe. I bought some K-Seal to try and repair the gasket but quickly realized this wouldn't work as it would have to be a leak into the gasket and not our of the gasket for the K-Seal to settle and do it's job. So, I figured if I did a compression test to find out where the leak was, I could pull the ignition and fuel from that cylinder long enough at idle for the gasket seal to settle and do it's magic. However, when I did the compression test, the motor appears to not only not have a leak but is actually in very strong condition for having 320k miles. For what it's worth, it also feels that way when I drive it with no noticeable loss of power or inconsistencies in the power band. Here are the numbers for the compression test:

D 165 160 160 170

W 185 175 175 185

The car has had the thermostat, radiator, water pump and heater core replaced within the last couple years so all parts of the cooling system are relatively new. I recently flushed the entire system and burped the car as well. At this point I'm all out of ideas and would appreciate input or criticism of my methods so far.

Thanks in advance!

 

A leakdown test would confirm a head gasket problem.

Also a block test kit would test for exhaust gasses entering the cooling system.

DO NOT use any stop leak snake oil solution, it will only lead to clogging up your cooling jackets and you'll have to rebuild the engine entirely.


Your D/W differences show your rings are worn, but you're still within serviceable compression limits iirc, so it's not necessary to pull and rebuild just yet.

 

+1 NEVER EVER use a "stop leak" product. These engines often blow gaskets at least once during their life, but if you don't let it burn up from overheating or clog it up with stop leak, slapping in a new gasket is quite simple and will get you many more miles of use.

Head gasket denial is really big on car forums. The only symptom of a small leak is what you describe-- coolant being displaced out of the engine and radiator and replaced by air, eventually resulting in overheating. Stuff like loss of compression, water in the oil, and white smoke means you have a big leak.

A simple, no cost, and relatively conclusive test is to get the system as full as you can and then let it cool off completely. Loosen the radiator cap to release any pressure, then replace it. Squeeze the top radiator hose, it should be soft. Now start the engine (cold) and rev it and keep feeling the hose. If you feel pressure build up quickly, there is a leak. Normally, pressure should not build up until the engine has warmed up considerably.

 

Thank you both for the feedback!

I mean, can you blame me for not wanting it to be a headgasket failure? Honestly, the only reason I still had my doubts is because I thought there should be clear signs of compression loss with even a leaky head gasket. I do have some follow up questions then.

How long can I reasonably drive the car like this before replacing the gasket becomes necessary or I put the engine at risk (driving it gingerly of course)? Will the high pressure conditions put other components of the cooling system at risk for failure? I would love for this car to last me another 3-6 months to allow me keep saving to buy a new car.

Are there any mitigation techniques I can use so that the coolant does not keep overflowing? Currently, I use a turkey baster to transfer from the overflow tank back into the radiator before my drives every day. I keep the coolant at a low enough level so that it fills up but doesn't overflow the tank and still have enough running in the system to not overheat. It is effective but tedious.

I will start doing my research now on replacing the gasket. I can't dump the money to have a mechanic do it so it will have to be on my own even though it will be the most complicated DIY I have attempted.

 

don't be intimidated by the job. i have done tons of d series head jobs, but my first one was in a parking lot with limited tools and experience. keep it simple the first time, mark the cam gear and timing belt with a paint pen/white out, slip the belt off the gear, and reinstall with your marks lined up. you don't even need to remove your intake manifold, but you will need to disconnect the bracket that secures it to the block (2x bolts, kind of a pain to reach). the only "specialty" tool you need is a torque wrench for reinstalling the head. if you're on a budget, harbor freight sells one for around $10 that will get the job done.