It has now happened twice to me that after adding coolant to the radiator, when I turn the car on the RPM's will bounce up and down and the engine is reving (between 1k and 1.5k RPM's). Normal idle is 750 RPM's. After a while it will eventually idle fine, but usually only after driving it for a bit.

Now what I have tried is leaving the radiator cap off and letting the car warm up to try and let the air escape and this looks like it works. The coolant level rises all the way to the top and will eventually 'burp' and go back down. I've let it run for about 20 minutes. But there still seems to be air trapped inside as startups still has rough idling.

I have seen some references to a bleeder valve, but my car doesn't seem to have this (2002 Honda Accord). Is there any better way to get the air out?

  • 1
    I don't think the surging and adding coolant are related. Why do you thinking they are?
    – NitroxDM
    Nov 5, 2012 at 2:19
  • @NitroxDM Sorry for the late reply. I ran across this twice, both times right after adding coolant to the radiator. I think I poured the coolant too fast.
    – Despertar
    Jun 6, 2013 at 5:23
  • Are you burping this with the heater running? Hester needs to be open to get all air out.
    – Dee
    Feb 24, 2016 at 3:02

6 Answers 6


Here's a thread from honda-tech.com. Sounds like you should have one from the factory, but it's possible an aftermarket thermostat housing was installed minus the valve.

If you really have no bleeder, you could install a thermostat housing that has one, or pull yours and install one yourself.

Otherwise, you're on the right track, though you might try running the heater and squeezing the hoses to move air bubbles along. Be careful poking around the engine bay with the engine running, don't lose any fingers. Gentle prods from a rubber mallet might be a safer strategy. You also need to wait for the engine to warm up and for the thermostat to open, otherwise you could have some air trapped in the block with no way out.


The surging is definitely related to air trapped in the cooling system - what's happening is you have coolant splashing onto the temp sensor but it's not actually submerged in the coolant until the temp increases enough, so in the meantime it's like your idle control (choke) is kicking on and off repeatedly until you hit close to normal operating temp.

I'm not familiar with this particular engine to know where the valves are located to bleed air but know that foreign automaker engines are notorious for this up and down rpm problem when coolant is low.

If all else fails, you could try jacking up the front of the car to elevate the radiator and then run the engine for a good 20 minutes or so (make sure you hit full temp) with the radiator cap off to let all air work out of the system. If you see coolant level drop, add more coolant with the engine running until it's topped off and staying full.


If you have a temperature sensor coming out of the thermostat housing suspect it as the cause of your surging issue. This doesn't even set a CEL light. You can change it same time as the thermostat replacement. Read the Honda groups on the Internet.

The low level of coolant won't cause engine surging, nor will air in the system. It is good to try to get it out though.

  • Are you sure it couldn't cause surging? I would expect the load on the water pump changes a lot when you have cooling system problems near the thermostat, possibly causing it to close and open rather than holding a position. You might also be flopping back and forth between moving water and moving air.. Jul 27, 2013 at 17:01

The rev surging might be an idle-control valve trying to do what it does. Have a good mechanic take a look.

  • This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post.
    – cinelli
    Jul 27, 2013 at 20:15
  • 1
    I am neither critiquing nor requesting clarification. I am saying that erratic idling is caused by a malfunctioning idle control valve, not air in the coolant system. Which is the most probable solution to his problem. Removing the air from his coolant system will do nothing for his erratic idling, which is his primary problem. Jul 28, 2013 at 11:20
  • 1
    Maybe you should elaborate the above within your answer. Otherwise, essentially one-sentence reply is not quality.
    – theUg
    Aug 16, 2013 at 2:59
  • Of course it is. I indicated what the problem is most likely to be AND the best solution. Aug 16, 2013 at 7:34
  • @JuannStrauss, your answer got flagged by the auto-mod system as potentially low-quality because of its short length, hence the attention you're getting. Personally I think you've added something to the conversation, but I also feel that 'take it to a mechanic' falls a bit short.
    – mac
    Sep 3, 2013 at 19:59

I had a bad idle as well. Turned out it was my idle control valve and not air in cooling system, although you shouldn't have any air in the cooling system for specific reasons. Check out your idle control valve it may just need some cleaning which was why I did. I was getting close to the point where my car almost stalled. I drive a 2000 Honda Accord SE (6th gen) btw and no more idle problems :)


Just as a friendly tip check out these so called vacuum fillers:

Here's a hit on the product at Amazon UVIEW 550500 AirLift II Economy Cooling System Refiller

And here's a video of a fella demonstrating one.

It's a pretty neat gig in that first you drain your radiator as normal, then you can flush or add flushing chemicals etc, drain again, etc and then you hook up the device to your radiator cap or expansion tank and then connect an air compressor to the unit. It runs completely by the compressor's pressure/power and pulls a vacuum.

Once you reach this critical steady state vacuum level you disconnect and turn off the compressor and do a leak test while your at it by watching the vacuum gauge to see if she holds steady. If the vacuum holds then you have your premixed coolant handy in a clean bucket or other container and you stick one of the hoses into it. Some folks prefill the hose so that the small amount of air occupying the hose is avoided. Then you open a valve and the hose is now exposed to the vacuum set in the coolant system which basically causes the hose to suck in all the coolant until equilibrium or your run out of coolant so have enough handy. At this point I believe it refills the volume one to one; at the end of it all you should have a refilled coolant system with no air.

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