Radiator expansion tank constantly boiling

[OlgaAK]

I had the radiator replaced a few weeks ago. There was a boiling sound with the old radiator also. I just learned that it is not normal and have been trying to figure out the source of the issue. There is no mechanic available in this area. I have driven the bus many thousands of miles and it has not broken down, nor has the engine temperature gauge ever gone above 220. The only time I felt concerned about overheating was when taking the bus up extremely steep and windy dirt roads. I've stopped doing that, at least until I get this issue figured out.

The engine is a Cummins 5.9. I idled the bus for about 60 seconds and turned it off. Soon after that, bubbles began appearing in the expansion tank. I opened the engine cover and felt all the tubes coming from the back of the radiator. I feel bubble-y movement in the thin tube marked in red.

Any ideas what this might mean?

Someone recommended trying to replace the thermostat, which I think is under the tube marked in blue. Can anyone verify that, and if it might be worthwhile? I can probably get that part tomorrow.

[Johnny Mullet]

If the bubbles are showing up and building high pressure before it even warms up then I suspect compression is getting into the cooling system from a head gasket or cracked head/block.

[Danjo]

I’m guessing thermostat unless the impeller on the water pump is not pumping.

[Danjo]

Originally Posted by Johnny Mullet

If the bubbles are showing up and building high pressure before it even warms up then I suspect compression is getting into the cooling system from a head gasket or cracked head/block.

Oooh yeah. I hope not. Does the underside of the oil cap look like a milkshake?

Of course you could blow into the coolant without hitting an oil gallery,, but if it looks like a cappuccino in there...

[OlgaAK]

Thanks. I don't have any other signs of head gasket like white smoke. How bad of a problem is this likely to be? Is there any downside to trying something like Blue Devil head gasket sealer?

Also, what is the purpose of the tube marked in red? What is the thing that is shown in the red circle?

[Danjo]

Originally Posted by OlgaAK

Thanks. I don't have any other signs of head gasket like white smoke. How bad of a problem is this likely to be? Is there any downside to trying something like Blue Devil head gasket sealer?

Also, what is the purpose of the tube marked in red? What is the thing that is shown in the red circle?

You don’t want to use the sealer if you don’t need it. A friend reports good results with blue devil. I am always skeptical of sealers like that, but his son has been driving in a pickup that blew a gasket and it seems to be working.

How long does it take for the bubbling to start from a cold engine?

[OlgaAK]

The bottom of the oil cap looks the same as it always has, as does the stick. The last oil change was pure shiny black oil.

[Danjo]

Originally Posted by OlgaAK

The bottom of the oil cap looks the same as it always has, as does the stick. The last oil change was pure shiny black oil.

And no white smoke out the tailpipe or water running out of it?

[OlgaAK]

Originally Posted by Danjo

I’m guessing thermostat unless the impeller on the water pump is not pumping.

A few people have told me to try first to replace the thermostat. It seems that to do this, I need to siphon out some coolant and then unbolt the metal thing that's shown in the blue circle, and the thermostat will be under there. And that this is a simple thing that is just basic tools and directly swapping a single item. Is that correct?

[Danjo]

Originally Posted by OlgaAK

A few people have told me to try first to replace the thermostat. It seems that to do this, I need to siphon out some coolant and then unbolt the metal thing that's shown in the blue circle, and the thermostat will be under there. And that this is a simple thing that is just basic tools and directly swapping a single item. Is that correct?

Yes. There should be a petcock on the bottom of the radiator. Sometimes they have a fitting to put a little piece of hose so you can drain it into a jug. If you take a gallon you should have enough room.

Have a new thermostat, gasket and form-a-gasket RTV sealant. Pay attention to which side of the thermostat faces the engine. You don’t want to install upside down.

When you’re done you need to purge the air from the coolant system by drinking it to temperature and gradually adding the coolant back in.

Don’t open the radiator hot! Have the cap off while you’re doing that.

[OlgaAK]

How long does it take for the bubbling to start from a cold engine?

I just timed it. I began running the engine from cold at idle. I stood in front with a flashlight shining into the opened expansion tank. After 2min40seconds, strong bubbles began.

It usually takes at least 15 min for this to end, sometimes more. It is still going now as I'm typing this.

[OlgaAK]

Originally Posted by Danjo

And no white smoke out the tailpipe or water running out of it?

None. The only smoke I see is when driving for a long time, the coolant will boil so heavily that it sputters out of the pinhole in the expansion tank and hits something hot and burns off. The exhaust seems fine and there's no water.

[OlgaAK]

Originally Posted by Danjo

Yes. There should be a petcock on the bottom of the radiator. Sometimes they have a fitting to put a little piece of hose so you can drain it into a jug. If you take a gallon you should have enough room.

Have a new thermostat, gasket and form-a-gasket RTV sealant. Pay attention to which side of the thermostat faces the engine. You don’t want to install upside down.

When you’re done you need to purge the air from the coolant system by drinking it to temperature and gradually adding the coolant back in.

Don’t open the radiator hot! Have the cap off while you’re doing that.

Thank you. How can I know what gasket to purchase? My understanding is that a shop like Napa will be able to provide the correct thermostat if I tell them the engine type. Is this accurate? Will they also know the appropriate gasket and form-a-gasket? Where in the process is the form-a-gasket used?

[bluebird90]

If the bubbles start within seconds of starting, I think you have a combustion leak when cold. Go to the parts store and get a block testing kit. That will tell if you have a blown head gasket or not.

[OlgaAK]

Originally Posted by bluebird90

If the bubbles start within seconds of starting, I think you have a combustion leak when cold. Go to the parts store and get a block testing kit. That will tell if you have a blown head gasket or not.

It is about three minutes. Do you mean literally seconds?

[Danjo]

Originally Posted by OlgaAK

Thank you. How can I know what gasket to purchase? My understanding is that a shop like Napa will be able to provide the correct thermostat if I tell them the engine type. Is this accurate? Will they also know the appropriate gasket and form-a-gasket? Where in the process is the form-a-gasket used?

Some thermostats come with a gasket. Check with the shop.

When you pull the housing off take note of the direction of the thermostat.

Use a razor blade to carefully scrape the old gasket and a clean rag to wipe off any residue.

Either the head or the housing will have a recess where the thermostat goes. It’s usually on the head. Then the gasket.

Some gaskets have adhesive on the gasket. This will face the thermostat. I always put a little RTV on the other side. If no adhesive, put RTV on both sides. It doesn’t take much. Spread it thin.

[Danjo]

Originally Posted by OlgaAK

It is about three minutes. Do you mean literally seconds?

Yeah, if it starts immediately on startup it’s a gasket or cracked head or block.

[Danjo]

Don’t run it anymore until you do the thermostat. $10 and a couple hours. Hopefully that’s it. If that’s not it, then you want to do a compression test.

[OlgaAK]

Originally Posted by Danjo

Yeah, if it starts immediately on startup it’s a gasket or cracked head or block.

What if it is three minutes? It looks like I can get a kit to test for combustion gases in the radiator at the same kind of shop that will have a thermostat. But I am anxious now and will appreciate any info.

[OlgaAK]

Originally Posted by Danjo

Don’t run it anymore until you do the thermostat. $10 and a couple hours. Hopefully that’s it. If that’s not it, then you want to do a compression test.

What is a compression test? I will need to run it to get to the store and back. About 60 minutes total driving.