No heat while radiator cap is on

Hi, i have a no heat problem. Its a 2004 chrysler pacifica. It doesnt have access to the radiator but it does have an expansion tank with a radiator cap in it. The thing is that while the cap is on i get no heat kn the cabin when the heater is on, however as soon as i remove the radiator cap from the expansion tank, i immediately start getting heat throigh the vents. I flushed the sustem, flushed the heater core and burped the air. Seems like i burped the system for like 2 hours.

Replace the thermostat

Already replaced it, i should have mentioned that. It seems like i have done everything that could be done, and i get heat only when the cap is off the expansion tank.

Ok, this leads me to think that the radiator cap is defective and is not keeping system pressure.If that doesn’t work,consider towing your car to a nearby general mechanic.

Lol im obviously not good at posting questions because i should have also mentioned that i put a new cap on also. Like i said i have gone throught every possible thing that could seemingly be done. It has something to do with pressure but im just picking brains to see if i get any ideas. Thanks

Seems that When i remove pressure from the system, the coolant goes through the heater core. When i put pressure back on by putting th cap back on the flow to the heater core cuts off. I know the radiato cap works to release pressure and let coolant flow but in this case its in the expansion tank so that pressur ehas no where to go

Could a thermostat installed backwards or upside down do this?

What we’re missing is the fact that when the pressure is released by taking the cap off, water flow is restored. The only time I’ve had that is when I had a head gasket leak. It would form an air bubble and not allow circulation. With the radiator cap off, the pressure was released and allow circulation. Sorry but a thermostat likely is not going to cause this and you need to get it to a shop to check for combustion gas in the coolant.


Yes, I have seen this with a head gasket problem as well. With the engine COLD, have the cap off and start the engine. Do you see bubbles coming out of the coolant? Does it smell like engine exhaust? If so you have a bad head gasket. DO NOT DO THIS WHEN IT IS HOT as I will burn you.

You will normally see some movement in the reservoir. Actual bubbles are what you need to be concerned about.

They also have combusion gas test kits on loan at Autozone and such to test for this problem. I once got one on loan and bought the chemical after my GF began to comment about some issues her car was having and I wanted to make sure the HG was OK. I ended up not even using it because by that time it was so obvious that the head gasket was faulty.

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This is the tool you need. It is called a block tester.

There is also a chemical indicator you will need to go along with this.

Thanks so much, you are all saving me some brain damage and time. I will definitely check the coolant for exhaust and combustion. Can uou guys recomend a good sealer if thats the case?

The normal fix is to remove the head, have it checked for cracks, and install a new gasket. If you are going to junk the car anyway in a few months, maybe try a sealer, but it can also seal other passages that you don’t want sealed. I have no experience with them though.

The sealers are a last ditch effort to keep an engine running and they usually don’t last long when the sealer does work at all. To improve the poor chances of a sealer working locating the cylinder(s) where the exhaust is leaking and removing the spark plugs while operating the engine for a few minutes helps.

I think that there’s still air trapped in the cooling system.

But if it is a head gasket leak, the best stop-leak is sodium silicate.

You acquire this at your local pharmacy. It’s about $25.00 for a 12 oz bottle.

I keep a bottle of this in the shop just for this purpose.


Use a sealer if you plan to just drive the car until it dies and then junk it but not if you want to keep it. Fix it right if you want to keep it for years and years.

Also be sure this is actually the problem before you do something so drastic. Yes, you can seal things that you don’t want sealed like radiator passages. This crap is permanently in the system once you pour it in so be sure this is really the problem and how you want to solve it. Stop leak is also nasty. It just forms a sludge in the system and again, this is hard to remove.

Definitely make sure that there is circulation in the radiator and that no air is in the system. do you see any coolant under the car? This is often the case with a head gasket as the system becomes over pressurized and it either has to release some of the pressure or something will break. Often a blown radiator hose or even a radiator is not unusual because of all the extra pressure from the exhaust gasses.

Also realize that auto manufacturers add a stop leak product to cooling systems on the factory floor on new vehicles.

Here’s what GM adds to cooling systems on their new vehicles. And it’s OE!


I think I’ve still got some of those. The book called for using them when changing coolant in the Olds. Pretty mild though I think.

That is interesting to need a form of sealer, even if mild, from the factory. It seems like they are masking shoddy machining or something. One of my buddies works in a high precision machine shop and tells me stories about making fittings that are so perfect that no gaskets or sealants are required. Apparently the auto-industry isn’t one of those places. I always coat gaskets in a film of RTV as well just to be safe.

No, all the manufacturers use a mild sealant in their coolant from what I’ve been told. I do not believe these are as aggressive though as the other sealers. They aren’t going to solve a head gasket issue but just help with the dozens of coolant connections in a new car.