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I tried to read all the cooling threads and learned a lot. So i did spend hrs searching. I want to throw my issue out and see if you guys have any ideas.
Ok so i bought an 04 ex 4dr 5spd with 130k. Pretty clean and runs good. Well after owning it for 2 days and driving it 300 miles I was driving on the highway to Boston and went around a few corners and noticed the temp gauge feral up to 3/4 then once going straight it would go to just below half. It cont. To happen around almost all the corners. Got to my destination and found the coolant sprayed from the expansion tank and it was full.
Ok so i drove it home and it was fine for probably 100 miles then a random corner at highways speeds and boom..went to 3/4 then back down. This went on for prob 300 miles and the coolant was fill the expansion tank and then go down in level randomly.

I went to Honda , bought a thermostat, rad cap and coolant. Did a flush n changed the thermostat. Drove 50 miles and noiced the coolant was slowly filling the expansion tank and not going back in the engine.
I did the thermostat n flush yesterday.
I did a leak test after flushing the coolant and driving about 20 miles. It was negative.

Even with the tank full it shows the temp is good and hasn't gone up even though the rad is empty almost due to the expansion tank being 3/4 full.

So..I know everyone says head gasket but why could I drove hundreds of miles and it be ok and then only a few times jump to 3/4 and jump back down. ?

Since new thermo and flush and cap..its ok on the gauge but filled the expansion tank and not going back.

So frustrating and don't want to just tear into the hg without it needing it

Any suggestions

 
Rep Power: 518

I did a leak test .... It was negative.

You didn't do the test that would prove it needs the head gasket job.

why could I drove hundreds of miles and it be ok and then only a few times jump to 3/4 and jump back down. ?

Airflow at road speed kept it cool enough to not register high on the gauge. When you slowed down, the airflow through the radiator was gone, and the cooling fan can't move nearly as much air as 80MPH on the highway can.

but filled the expansion tank and not going back.

Typical head gasket (assuming the radiator, cap, and tubing is proper).

So frustrating and don't want to just tear into the hg without it needing it
Any suggestions

Pick a test that would prove it.

HTH

 
Rep Power: 313

No point of me posting since ^ this guy is on it

 
Rep Power: 186

Not unique. Not even kind of unique.

Uview Combustion Leak Tester is one tester for just this situation. A couple vids are on youtube on how to use it, but it is designed for problems just like yours, which is most likely a blown HG based on your symptoms.

ezone is all over it.

Good luck.

 
Rep Power: 0

When I said leak test I did the combustion test and it was negative.

 
Rep Power: 518

Gee, that wasn't the "leak test" I had pictured in my head.

You borrow that tester from ChinaZone or something?

Quite often, that test doesn't work on these when the leak is slow. Yours is apparently actively leaking when you are running 80MPH on the highway (under a load).
But almost no leak activity at idle, when that tester is used.
No leak during the test, no bad results.

See where I'm going with this?


That isn't the test I use to PROVE the head gasket is leaking.

 
Rep Power: 0

So what test do you do to Prove the hg is bad?

It doesn't raise the gauge after the car is driving. Its very random. Mostly notice it when driving around a corner at a speed above 20mph. That's why I said its uniqu, I didn't read that anywhere.

Thanks.

 
Rep Power: 518

That IS unique, to me. Could be that the CTS is getting uncovered by sloshing coolant. IDK.
Since the gauge on the dash is operated by data from the PCM, I'd want to see a datalist so I know what the PCM is thinking.
Could be a separate, unrelated issue too.
Fix the main problem and see if that goes away.


The test I use is similar to a cylinder leakdown test, but without the gauge set.
Roll a cylinder to TDC (exact) with the valves closed.
I stick the funnel-fill radiator funnel on the radiator neck and add just enough coolant to get the level up to the skinniest part of the stem, then I connect 170PSI+ shop air pressure directly into the cylinder, and watch for any change in the radiator level.
Any change or movement in the coolant level means there is a breach between the combustion chamber and the water jacket.
Repeat for each cylinder until all bad ones are found.

This method has not failed me yet.
I don't even use the combustion detector test because it can't identify individual cylinder issues. I need that data because I inspect once it is apart.

HTH

 
Rep Power: 313

Too bad I don't have access to shop air. Would 110 PSI do the trick?

 
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Another way of doing it is to remove the cap when the engine is cold. Make sure the rad neck is full and start the engine. The combination of the cold engine and the pressure in the cylinders when the engine starts will spray coolant out of the rad when the engine starts. If coolant comes out, it is being pushed out by gases coming from the blown cylinder head.

 
Rep Power: 518

Yours has issues too?

You know for certain that the OP only has 110 PSI available?




I know the regulated 30PSI or so that a cylinder leakage tester uses won't reveal a leak unless it is a huge leak.

110 PSI might be enough, but it would depend on the size of the leak. Try it and see. We already know it is a head gasket problem in this case, so if 110 doesn't show it then you need to use more pressure.

170 is closer to real pressures that happen inside the cylinder. I think 175 is as high as the compressor goes at work.



Something else I forgot to mention is I start this test on a fully warmed up engine. That way the checks can be redone again after the engine cools off, without extra work.
Some leaks might not show up on a hot engine, and vice-versa.


HTH

 
Rep Power: 313

I don't have issues with mine I replaced it a couple years ago and it resolved the issue. I was just more or less wondering so I know for future reference

 
Rep Power: 0

Ok off to find a shop to help with the pressure test TDC idea.

Thanks guys.