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Discussion Starter #1
My new (99) rav4 is producing a good deal less heat than my old one. The previous owner ran some leak stop through it and I suspect didn't turn the heater off first, so maybe it's a bit clogged. But I was wondering if there's a procedure to bleed the cooling system in case that's the problem?

Car isn't losing coolant.
 

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I'd suggest disconnecting the heater hoses and reverse flushing it. I did that on a friend's older F-150 after he'd driven it for one winter w/o any heat. A bunch of debris came out and the heater worked very well afterward.

(Moved thread to more appropriate forum.)
 

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Agree with Dr. Dyno - the same remedy was done on a friend's car. Why did the previous owner put stop leak into the cooling system?
 

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Discussion Starter #4
It seems there was a small coolant leak from the engine, it's mention as an advisory on the last MoT. It certainly doesn't leak now so in that sense it's working fine.

Also found out about the heater flap so I'll check that too. Thanks for your help :)
 

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Most likely the heater core needs a back flush, and there are many YouTube videos which demonstrate the process. If your antifreeze is brown and muddy looking, rather than green and clear, that is usually a tip off that the heater core needs some cleaning. Of course it can still have a problem even with clean antifreeze after all these years.

Another likely cause is a defective engine thermostat which is preventing the engine from reaching the proper operating temperature. If you RAV has a temperature gauge you may notice that it reads lower than normal.
 

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As you suggest the situation may be caused by the heater flap. Obviously it's good that the cooling system doesn't leak now, but rather than the heater flap the addition of the stop leak to the cooling system may be the cause of the poor heater performance since it may have clogged the heater core. Hopefully reverse flushing the heater core will solve the problem. Personally I do not like to use stop leak since it can cause clogging, and have seen it do that on other peoples' vehicles.
 

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Discussion Starter #8
I agree, I'd never use stop leak on any of my own cars. At the least folk need to turn off the heater first.
 

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A good test for reduced heater core flow is to let the engine idle after the gauge is up to normal and set the temp to max but with the blower off. Even with low coolant flow the core will heat up fully. Then turn the blower on high. If the air comes out hot but cools off quickly it'll be due to low flow not being able to keep the core hot. A damper door issue will cause the air to be less hot but not cool off with the blower on high.
 

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If the air comes out hot but cools off quickly it'll be due to low flow not being able to keep the core hot. A damper door issue will cause the air to be less hot but not cool off with the blower on high.
Along the same lines is checking the two heater hoses under the hood for temperature. This can be done just by touching them, no special equipment is necessary.

Turn the temperature selector to high, and turn the blower on high. The input hose should be quite warm, but if the flow through the heater core is reduced, the blower will remove most of the heat from the coolant resulting in the return hose being significantly cooler to the touch.
 

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Along the same lines is checking the two heater hoses under the hood for temperature. This can be done just by touching them, no special equipment is necessary.

Turn the temperature selector to high, and turn the blower on high. The input hose should be quite warm, but if the flow through the heater core is reduced, the blower will remove most of the heat from the coolant resulting in the return hose being significantly cooler to the touch.

And if one has an infrared thermometer even the possibility for an inaccurate result is removed if one has little ability to feel the difference in the temperatures of the hoses. My hands are tough enough to not be able to feel non-extreme temperatures very well. Even an outdoor thermometer can be used if the bulb or sensor can be placed in contact with the hoses.
 

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Discussion Starter #12
Heater flap seemed to be working fine so today I flushed the heater matrix, very easy.

Both hoses seemed to be pretty warm so I was unsure which was the inlet, I flushed both to be safe.

An old hose coupling designed to fit my garden hose to a round tap was a perfect fit for the pipes so connection was easy. Ran the hose for a few minutes into each pipe and the heater is now toasty warm.
 
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