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2002 Tundra .. The temp. gauge in the cluster rises to normal (maybe quicker than it used to) and is steady. On the other hand the heat is not coming out of the registers after 10 min. The valve on the firewall is moving between Hot and Cold but the heater hose is only warm. No change in temperature across the valve when open.
 

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How's the rad and overflow bottle levels, are they low?
 

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Mark the overflow bottle and monitor how much you are losing. It could be leaking from anywhere and if its a slow leak it might take a long time to find. Could be the weep hole at the water pump, a hose somewhere you dont see, maybe even the head gasket. Thats the one you dont want. Get a bright flashlight and just look all over from top to bottom. Could be hitting something hot like the manifold, engine, or tranny and burns off which means you wont necessarily see a puddle on the ground.
Here's an example of one. Where the guy saw fluid and where the leak ended up being was totally different.

 

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Discussion Starter #6
Mark the overflow bottle and monitor how much you are losing. It could be leaking from anywhere and if its a slow leak it might take a long time to find. Could be the weep hole at the water pump, a hose somewhere you dont see, maybe even the head gasket. Thats the one you dont want. Get a bright flashlight and just look all over from top to bottom. Could be hitting something hot like the manifold, engine, or tranny and burns off which means you wont necessarily see a puddle on the ground.
Here's an example of one. Where the guy saw fluid and where the leak ended up being was totally different.

Thanks cheapskate. I'll keep a more frequent check of the overflow bottle. It's a closed system so nothing should evaporate. Must be a leak somewhere.
 

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the bad news is if you dont find a leak and it still goes away then its likely a very slow but sure head gasket leak. I just had exact issue with my civic and no evidence , trails, white smoke, anything except heater not heating after 10 mins. Changed the head gasket and all good. Hope yours is just a hose.
 

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i was thinking check the oil dipstick. ifnit is high and grey, likely blown headgasket. IIRC, if you have a diesel and a blown head gasket, there will be diesel in the radiator. were it me, I would change the thermostat, most likely it is malfunctioning if all else is ok plus a thermostat is cheap, usually under 5 bucks last time I bought one.
 

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Discussion Starter #9
Checked the oil first thing, it's all oil, no water. Half quart low. Topped that off. Now that cooling system is full again, heat works normally. No noticable change in level in resevoir. So far so good but with no oder, no signs, I'm starting to wonder about a pinhole leak in the head gasket.
 

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I had my dealer tell me my radiator has a slow leak in the plastic housing......... I noticed that my bottle level would go low but take a LONG time to do that......They wanted $1200 to replace the radiator and serpentine belt. I declined and changed the belt myself for under $30 with Rockauto.com supplying the belt.

Now the level on the bottle still goes low but it takes at least a year and my dealer will top me off for free anytime I just drive into their customer service welcome bay. So............ until I have them replace my timing belt next year I will not worry about my leaking radiator. It's a easy job which I researched on YouTube and again using Rockauto under $200...........so I will tackle that myself...
 
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