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Discussion Starter #1
Long time member, first post in a while. I had this happen this evening, and I was trying to determine the root cause or if another member had experienced the same anomaly. This evening, I had to run a short errand, and drove the van from a dead cold start (40°F) for about 15 minutes to my destination, and while this was occurring, I was trying to troubleshoot. Set the heater temp to 80°, and once vehicle temp was warm enough for the thermostat to open, the climate control started blowing hot air. Mode was on recirculation, fan speed at medium low, vent selected. As I was driving along, the air was coming out warm, and then I approached my first traffic light. The air started blowing cooler the longer that I sat at the traffic light. When the light turned green and I started moving, the air temperature started warming up. I approached another traffic light, and the same scenario repeated itself, which really puzzled me.

Since the climate control was set on recirculation, I figured it was cold air being drawn in from the cooler part of the cabin, so I set the mixer to fresh air. I'm a retired mechanic, so I don't know why I thought that the fact that the air coming over the heater core from either source mattered, but I was trying to troubleshoot the situation and make it replicable so I could eliminate some variables.

I set the temp to Hi, and still encountered the same situation where the temp of the air would cool down to ambient while I was not moving, but then would heat up when I started moving. Engine RPM seems to affect the temperature. I checked the coolant level a week ago when I was under the hood checking fluid levels. My thoughts are air bubble in the coolant system. I'm planning on changing T/B and associated components soon, so if there's a fault with the W/P, it'll be addressed then. I'm sure it would be a good idea to change the rad cap too, since I've never changed it because I haven't had any reason to.

I'm puzzled, because I've owned the van for 7 years, and performed all of the PM and regular maintenance on it, and this is a new quirk that popped up tonight.

Anyone have a similar experience? If so, what was the cause?
 

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Check the coolant level now. Much can change in a week.
Also check for coolant flow. Pump may be getting weak. also check for restrictions in the heater hoses.
M
 

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Sounds like you are simply not getting enough coolant flow through the heater core. Perhaps air in the system or a partially clogged heater core. Wrong coolant? Flush the heater core and see if that resolves the issue.
 

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You need to check the coolant level in the radiator directly with the cap off - not the overflow tank. This has all the symptoms of low coolant level, but you need to top off the radiator directly, possibly multiple times. I have had this happen on both Odys and my accord, both times low actual coolant level in the radiator / system. On my '07 Ody, I can leave my garage with the van cold soaked at ~19 F temp of 0F outside and will be getting warm air out of the vents within 60-90 seconds of driving. The thermostat does not need to open to send warm coolant to the heater core. The thermostat opening sends the coolant flow through the radiator (to cool it down). If your level is good, then check the coolant flow valve on the firewall and make sure it is opening the whole way and is not obstructed, and then heater core blockage as noted. But I would start simple.
 

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Discussion Starter #6
Thanks for the replies. I checked the coolant level in the radiator yesterday before I started the engine, and it was indeed low. I added about 2 quarts of Honda 50/50 coolant(I had a gallon left over from my last timing belt replacement I did). This makes me wonder where the coolant went, because with every vehicle I've worked on in the past, I use my Spill-Free funnel to bleed air out of the system. Those things are awesome, but now I've got to figure out where the coolant went, and why I had air in the system considering I had coolant still remaining in the expansion tank. If there was an air pocket in the system from the last belt change I performed(over 5 years ago), it should have been replaced by coolant in the expansion tank.

I'm due for another T/B replacement soon, so perhaps my water pump is seeping; I couldn't see anything when I looked at the cover. Regardless, the heat now works properly with a full radiator 😁
 

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Nice, sometimes the overflow tank will not suck coolant back into the rad depending on fittings.....
 
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