Coolant not returning to radiator (new cap, new overflow hose, new radiator, etc)

Simply put, my issue is my coolant is not flowing back into my radiator from the overflow at the end of the day. I recently replaced the radiator and coolant temp sensor.

So far to fix it I have: replaced the radiator cap and replaced the overflow hose between the reservoir and radiator. I have gathered in my looking around online that this is occurring because for some reason there’s a lack of vacuum in the coolant system. I’m not sure where the air could be coming from but, a couple important things to mention:

-I dont see any of the usual evidence that the head gasket is leaking.

  • The engine does not overheat anymore now that I’ve replaced the coolant sensor

It does, however, read a lower temp than it used to before the radiator was replaced. and this will fluctuate up and down between warm and not-so-warm. lol Despite this, hot air does come out of the vents when i turn the temp up.

So anyone got any idea what the solution to this mystery is?

You may have installed a faulty radiator cap.

Just for yuks and giggles, I once took my pressure tester to the parts store when purchasing a new radiator cap.

The fifth one the guy behind the counter laid held the proper pressure.


Make sure all air is expelled from the cooling system: elevate the front of the car; turn the heat control to Hot (fan not needed); take off the radiator cap. Start engine and let it idle until all warmed up. Add coolant mix if level goes down in the rad. (Be ready to capture and safely get rid of any that spills over.)

Turn off engine. Cap the radiator. Get level OK in overflow tank. Check rad and overflow tank again in the morning when engine is cold, top up if needed.

i’ve already done this several times. it just doesn’t make sense to me that I replaced the radiator 2 months ago and that there could STILL be air in the system, when the coolant is constantly flowing.

the other cap I had on it worked just fine before i replaced the radiator, then after that it didn’t seem to have any problem yet this problem started. So I’m now on the second radiator cap that doesn’t seem to have anything wrong with it yet something’s occurring.

What year and engine is your Honda Accord??

2001 v6 3.0 liter

I’ll try the air release thing but somehow I have a feel that it won’t change anything.

Do you have the correct rad cap for the rad?? Possible wrong rad??

Some took a deeper rad cap depth then others… 0.85" deep inlet, or 0.9" deep inlet…

w/ Denso radiator

w/ Valeo radiator

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Did you follow the Honda coolant fill procedure?

  1. Set heater temp control dial to max heat.
  2. Loosen the bleed screw on the thermostat housing.
  3. Fill coolant to bottom of filler neck.
  4. Tighten the bleed screw as soon as coolant starts to run out in a steady stream.
  5. With the radiator cap off, start engine and run until warmed up (radiator fan comes on at least twice). Add more coolant if necessary to bring the coolant back up to the bottom of the filler neck.
  6. Replace radiator cap.

You do have to do the correct bleed procedure. I would recommend that you get a new radiator cap from the Honda dealer.

I also suggest that you remove the overflow tank and wash it out, thoroughly. Pay close attention to the molded in tube that goes down the side and enters at the bottom. A small piece of gunk can clog this hole and block the coolants return to the radiator.

Instead of a molded tube in the side of the tank, you may have a tube that attaches to the underside of the overflow tanks cap and goes to the bottom of the tank. Probably not but if it is this design, the tube should be replaced so there are no leaks where it attaches to the cap and it too must be clear of debris.

yes those look exactly like the caps I’ve been using. I don’t think i used the wrong radiator. Everything on it fit correctly and when i searched on amazon for it I made sure to match my car info with it. (like you can do on the O’Reilly website)

This model does not have a bleed screw. I already checked.

how is it possible that the coolant and flow through both overflow hoses to fill the reservoir, but not go back through to the radiator? the flow should be able to go the other way if it comes in, yes? in other words I’m saying that that probably is not the problem because if it were, fluid wouldn’t be able to come IN to the reservoir and fill it.

Debris in the bottom of the reservoir can act like a one way valve. Of it is the hose from the cap design and the hose does not seal around the cap, it wll bleed off the vacuum from the radiator before it draws any coolant back.

i shall replace the in-tank hose like i replaced the other one. is it possible the cap on the reservoir being “loose” also can cause the issue? because my cap IS loose where the outside hose piece connects to it.

The cap does not need to seal to the reservoir, in fact it needs to be able to pass a little air. Think of the hose from the radiator, through the cap and into the bottom of the overflow reservoir as a drinking straw. If there is a leak anywhere above the water line, you can’t create the vacuum needed to draw the water up. The coolant recovery line must be sealed and continuous from the radiator to the bottom (almost bottom) of the reservoir.

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What brand radiator did you install?



You need to burp the system by parking the car on an incline Nose up hill and running the system and adding coolant. If possible replace the stock cap with a leaver cap that allows for venting.