Coolant bubbling over reservoir

[92 5 oh]

Hey everyone, my fiancée has a 01 civic, a few months ago we were driving home on the highway and I noticed the temp gauge was climbing to the red as we were pulling in to the driveway. There was coolant on the top of the radiator and it appeared that it was leaking out of the cap. So I replaced it since it was the original. Drove it for a few days and noticed it was still borderline overheating. Next I did a pressure test on the cooling system and cap. It held ~18psi for a few minutes and slowly bled off. I couldn't locate any coolant leaks anywhere. So I drove the car around a bit and realized it would lose coolant when driving or running it hard (as opposed to sitting at idle). This is when I figure out the coolant was being pushed in to the overflow and leaking out of. The upper radiator hose felt like it has quite a bit of pressure, I'm not sure how much but it was definitely more than when doing the pressure test. So I got a leak down kit and ran it on the motor cold twice, then let the motor idle for about 30 minutes and did another leak down after it was up to operating temp. All 3 tests showed no more than 15% leak down. I'm also not able to find any signs of oil in the coolant, coolant in the oil, the plugs don't seem contaminated and the exhaust doesn't smell different or smoke, the coolant isn't rusty or appear to have any garbage in it.
I went ahead and replaced the thermostat and flushed the cooling system. While doing so I noticed the low speed fan wasn't coming on at all. The fan switch (located in the thermostat housing) tested bad, so I replaced it and the fan started operating as expected. I thought the problem was fixed, but wasn't completely convinced since the coolant problem was happening at driving speeds. And as far as I know radiator fans don't work at speeds above ~25 mph. So I drove the car around town, errands and such for the last two weeks with no problems. Took it on the highway when possible too and drove it harder than my fiancée would. I was just thinking to myself that it's fixed.
I had to drive about an hour away for work yesterday so I took the civic. Did fine on the way there but as I'm pulling in to my destination I noticed the temp gauge was starting to creep up. Checked under the hood and sure enough, the over flow was full and starting to leak out of the top. So when I left for the day I added some water, then drove home. It did fine but as I pulling up to my house the gauge was creeping again and the over flow was full/starting to leak out.
So as best as I can tell, the problem happens when driving for extended periods (45+ minutes) and/or from extended higher RPM's. It doesn't have a tach but I'd guess running ~70 mph it's around 2800 RPM's.
The only thing I can think of doing at this point is to top off the cooling system again and run it in the driveway at 2500 rpm's for 45 minutes or until the coolant fills the over flow and then do a leak down. I'm not sure if these are symptoms of a failing head gasket, warped head or clogged cooling system?
I'd like to try and get this fixed over the weekend. Any other thoughts or suggestions?

Thanks and sorry for the novel.

Steven

 

[RonD]

You have a cylinder to cooling system leak, common term is blown head gasket.

Free test for that, the Glove test.
Cold engine
Disconnect coil or spark plug wires, you want a no start
remove rad cap
remove overflow hose, and plug that port, you can use your finger if you have someone to crank the engine.
get a latex glove and rubber band, put glove over rad cap opening and seal it with rubber band, you can use a balloon or even a condom instead of the glove.

With cooling system sealed crank the engine, watch the glove
If if bounces then you have a cylinder leaking pressure into the cooling system.

If it just lays there then you don't

If it does bounce then pull out one spark plug at a time and crank engine
When glove stops bouncing last spark plug removed was from the cylinder that is leaking pressure, this can be helpful after head is off.

Cylinder pressure is 150+PSI when cranking and 900+PSI when cylinder fires, rad cap is rated for 14-16psi
Reason that it is hit and miss for over heating is that the leak is small now, it will get worse, if the "air"(and pressure) being pushed into the cooling system from the cylinder can make it to the top of the radiator then the pressure will push it out, bubbling overflow tank, but most of the coolant remains in the engine.
But if "air" gets trapped in the head then coolant gets pushed out the top of the radiator into overflow tank, it gets filled to top and overflows, and engine overheats because coolant level is now low.

 

[enjr44]

The only thing that will push coolant out of the radiator (more than the normal amount from warming a cold engine) is air. And the only way that I know of to get air into the cooling system is either a cracked head or bad head gasket.

 

[jeremysdad]

And it is possible to have either with 0 coolant getting in the oil or vice versa.

 

[AllanD]

It is also possible to have a cylinder-to-cylinder leak that is otherwise undetectable via cooling system diagnostic methods...

Remember that a head gasket contains and separates several media.
1) Compression/cylinder pressure you know
2)Coolant you know
3)Crank-case ventilation tends to get forgotten, but coolant leaks can leak into this and coolant then contaminates the lube oil.
4)On many engines pressurized oil feeds to the valvetrain, this can leak into the oil return
(which is hard to detect until parts wear out) but leaking pressurized oil into a cylinder is hard to miss (Enormous clouds of oily smelling Blue-white smoke)
and finally pressurized oil leaks into the coolant jacket which make a mess, typically seen as "Grey snot" inside the radiator cap.

Some engines can leak compression to the crankcase ventilation or oil returns "chain galleries" or even overboard.

late 80's 2.3's actually had a recall for leaking oil pressure overboard, typically as the drivers side rear corner of the cylinder head...

I've had a 4.0 that leaked coolant overboard with no other symptoms.
('93-94's are prone to this if they are consistently run hard & hot
(As mine was in an AWD Aerostar))

 

[92 5 oh]

Thanks everyone for the suggestions. I had a gut feeling it was a head gasket. I did as Ron suggested and the glove pumped up. The local parts store didn't have the gasket in stock so I'll do it this weekend. Thanks for the explanations and details.

 

[AllanD]

Be sure to buy new head-bolts for the job unless you want to be extracting broken bolts and doing the job again in a couple weeks.

The Head-bolts in a 4.0 are a single use item.

Also, I hope you have a T55 because the head-bolts are T-55 Torx

 

[enjr44]

Be sure to buy new head-bolts for the job unless you want to be extracting broken bolts and doing the job again in a couple weeks.

The Head-bolts in a 4.0 are a single use item.

Also, I hope you have a T55 because the head-bolts are T-55 Torx

I think he said this is a Honda?

 

[jeremysdad]

Indeed it is, but most engines use tty head bolts these days. Still would check into it and get new if that is confirmed for his application.