Coolant pressurised when cold

[dr_gn]

All, my wife has a 2001 Civic 1.6 SE auto. It’s fine, gets used locally and for work (10 mile motorway trip each way). It’s done c.120k miles. I changed the transmission and engine oil and filters, and checked the coolant last week in preparation for a 350 mile holiday trip. No problems, but the morning after we arrived, I checked the radiator and it seemed to have residual pressure (cold), accompanied with a gurgling noise - presumably fluid going into the un-pressurised expansion tank (the tank cap hissed) as I removed the rad cap. I don’t remember it doing it in the past, although I hardly ever have reason to check it.

So is it a head gasket problem?

No oil in water.

No water in oil.

No emulsified oil in the filler cap.

Seems to run fine.

Only anomaly is the temp gauge is at about 1/3 of its travel, not near 1/2 mark. It’s been like this as long as I remember.

ETA I opened the cap (cold) and ran the engine. A few bubbles now and then but mainly nothing. Guess that could be trapped air, or combustion gasses coming through the system?

Edited by dr_gn on Tuesday 28th May 16:28

[Super Sonic]

Temp gauge at 1/3 is normal on civics. I've had seven.

[dr_gn]

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

[Super Sonic]

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

[dr_gn]

Super Sonic said:

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

I’d have thought a small amount of combustion gas wouldn’t raise the temperature much?

I’m thinking the high pressure gas gets into the cooling system, any excess vents into the expansion tank. Then when the engines off, the cooling system remains at pressure even when cold because it can’t vent back into the cylinder at low pressure. I assume the hot liquid cools and contracts, but the pressurised gas in the system doesn’t contract as much, hence the residual pressure.

I can’t think of any other reason that a topped-up cold system would remain pressurised after getting hot and cooling again?

[E-bmw]

Super Sonic said:

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

Are you trying to say that the coolant passing through the hot engine wouldn't heat the coolant up itself?

If so you may need to revisit your physics from school.

Solid to water heat transfer would be much more efficient than gas to liquid heat transfer.

[E-bmw]

dr_gn said:

Super Sonic said:

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

I’d have thought a small amount of combustion gas wouldn’t raise the temperature much?

I’m thinking the high pressure gas gets into the cooling system, any excess vents into the expansion tank. Then when the engines off, the cooling system remains at pressure even when cold because it can’t vent back into the cylinder at low pressure. I assume the hot liquid cools and contracts, but the pressurised gas in the system doesn’t contract as much, hence the residual pressure.

I can’t think of any other reason that a topped-up cold system would remain pressurised after getting hot and cooling again?

Basically, you are correct sir.

As the coolant heats & cools it expands & contracts, this affects the pressure in the system.

These basically equal each other out, so if you were to top up at 20 deg C, run the engine up to temperature & then cool it back to 20 deg C there should be no residual pressure.

[dr_gn]

E-bmw said:

dr_gn said:

Super Sonic said:

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

I’d have thought a small amount of combustion gas wouldn’t raise the temperature much?

I’m thinking the high pressure gas gets into the cooling system, any excess vents into the expansion tank. Then when the engines off, the cooling system remains at pressure even when cold because it can’t vent back into the cylinder at low pressure. I assume the hot liquid cools and contracts, but the pressurised gas in the system doesn’t contract as much, hence the residual pressure.

I can’t think of any other reason that a topped-up cold system would remain pressurised after getting hot and cooling again?

Basically, you are correct sir.

As the coolant heats & cools it expands & contracts, this affects the pressure in the system.

These basically equal each other out, so if you were to top up at 20 deg C, run the engine up to temperature & then cool it back to 20 deg C there should be no residual pressure.

So…I’m definitely looking at head gasket failure?

[Super Sonic]

E-bmw said:

Are you trying to say that the coolant passing through the hot engine wouldn't heat the coolant up itself?

If so you may need to revisit your physics from school.

Solid to water heat transfer would be much more efficient than gas to liquid heat transfer.

No I'm not saying that at all. Not all of the heat from the combustion process is transferred to the engine block.

[dr_gn]

E-bmw said:

Super Sonic said:

dr_gn said:

Super Sonic said:

Temp gauge at 1/3 is normal on civics. I've had seven.

Thanks - well that’s something!

If you had combustion gasses escaping through the coolant, the coolant would get hot. Keep an eye on the temperature gauge and coolant level and you should be fine.

Are you trying to say that the coolant passing through the hot engine wouldn't heat the coolant up itself?

If so you may need to revisit your physics from school.

Solid to water heat transfer would be much more efficient than gas to liquid heat transfer.

I think they meant the leaking hot combustion gasses would heat the coolant to a higher temperature than normal.

[Super Sonic]

dr_gn said:

I think they meant the leaking hot combustion gasses would heat the coolant to a higher temperature than normal.

Correct.

[Super Sonic]

dr_gn said:

So…I’m definitely looking at head gasket failure?

What you describe is not a symptom of head gasket failure

[dr_gn]

Super Sonic said:

dr_gn said:

So…I’m definitely looking at head gasket failure?

What you describe is not a symptom of head gasket failure

OK, so where is the residual pressure coming from when cold, after the engine has run?

[Super Sonic]

How much pressure are you talking about? Enough to shoot the radiator cap into the air and shoot coolant out, or enough to make a bit of a hiss when you remove the cap?

[Super Sonic]

There is a test your local garage can do to see if combustion gas is getting into coolant, or can be done DIY if you're reasonably competent.

[dr_gn]

Super Sonic said:

How much pressure are you talking about? Enough to shoot the radiator cap into the air and shoot coolant out, or enough to make a bit of a hiss when you remove the cap?

About 3 seconds of hissing and gurgling. I think rapidly taking the cap off would result in having to keep a good hold on it. Basically it’s abnormally high pressure for a stone cold engine. Something’s up I can feel it in my bones.

[Super Sonic]

It may be worth getting it tasted as I described above. There are YouTube videos if you want to have a go.
I had a civic with a d17 and it leaked combustion gas into the coolant and it would overheat, but if the overheating is pressurising the system, logic says the pressure would drop with cooling.
Definitely keep an eye on the temperature gauge and coolant level til you can get it checked though.

[dr_gn]

So today we drove about 20 miles. Left the car for about 6 hours and just got back to it - it’s cold, but there’s a significant amount of pressure under the cap - bubbling noise as it’s released. There’s no overflow from the filler, not sure where it’s going. My guess is it’s gas, but I’m not sure why the weather level isn’t falling. When the cap is removed, coolant brims the filler.

I’m not sure how the expansion tank works in these - the overflow pipe seems to be in the filler neck, but below the rubber spring plunger. The tank itself doesn’t seem to be sealed, so not sure how the pressure can build up, unless it’s a special tank with a pressure and non pressure side.

Any ideas?

[GreenV8S]

Do a block test aka sniff test to see whether there is an internal leak into the cooling system.

[scoobydo123]

I have on other cars rigged up a bike pump and a tyre valve and pressurised the system to the rating on the cap. Left for a couple of days had a needle width of pressure drop but had a few drops.of coolant on top of the piston. Cause was MLS gasket without required RA on block. So you can purchase a pressure tester make one with either teeing in to expansion system or even make one with an old radiator cap and tyre pressure gauge. Or as others said try sniff test