I’m pretty sure he mentioned bypassing the heater valve. That’s why I suggested the pipe under the intake is blocked

I know his problem is legit, but it makes no sense.


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Even if the IAC valve hose clogged, if I am looking at the flow diagram properly, it looks like that port and the heater port are shared on the casting. Maybe I am wrong.

I just wouldn't think they have anything restricting that heater core feed except the heater core control valve. As folks would want heat as soon as possible, this is probably why the port is where it is.



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Quick update: like gamefreak mentioned, I was able to get the system up to pressure after driving around a little bit (still cleaning coolant out of all the crevices ).

And as I mentioned in the original post, the heater valve has been bypassed.

I studied that cooling system diagram before starting this thread and while it is good enough to give me a general idea of the circuit, the arrows are so poorly drawn that it leaves me scratching my head when I try to put together exactly whats going on.

I plan on putting some work into it this weekend.


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I agree it's definitely a head scratcher for sure.

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I am not sure if this would help but the part# for 96-98 is 87240-35050 and the part number for 99+ is 872403-5070.

So I think I’m one step closer to solving. I got the vacuum coolant refill kit snd it works great, confirmed the system has no leaks and is bled well. But did not solve the issue.

An additional symptom that I’ve been able to consistently recreate is that the heater upon startup/idle will actually works pretty good, but as soon as I start driving and (presumably) the thermostat opens up, the heat goes away.

I also pulled all the coolant lines between the engine and heater core to check for any blockages. No smoking gun, but while all the hoses were off i did stick a 14gauge solid wire as far down into the coolant bypass (under the intake) as i could and when i pulled it out it was filthy with a bunch of crap on it. When i bought this truck a year and a half ago the coolant looked absolutely atrocious and had tons of gunk everywhere, so this is not surprising.

To me, this is seeming like a clogged coolant bypass. My valve cover gaskets are leaking oil, so I’m seeing this as a great opportunity to replace those and inspect the bypass while I’m in there. I will order parts for that job this week.


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Is it possible that your thermostat is just bad? As a defective thermostat will do exactly what you are seeing. The works when first start up then after warm up no heat. Is sounding to me like the thermostat is sticking open. A stuck open one.

Do you know of the thermostat is oem or aftermarket?

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I could be wrong, but I don’t think it’s the thermostat. A few reasons:
- brand new OEM thermostat as of fall 2021
- that wouldn’t explain the lack of flow to the heater core
- my coolant temp always stays at ~188F, if it was stuck open I’d imagine it would fluctuate based on demand and ambient temperature


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Pull out the thermostat and force water through the coolant bypass and let it run until you see clean water and no chunks. It’s the only thing that made sense. Hopefully it’s the issue.

Potentially that crap will migrate into your new heater core so back flushing that is a good idea also.


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Sounds like somebody put a bottle of stop leak in there. That stuff ends up doing more damage than it helps.

+1 stop leak was used in the past. This would explain the clogging.
Agreed the thermostat would have the coolant temp fluctuate especially if sticking open. :-)

That stuff will plug all kinds of things up. It really should be completely pulled off the market. :/

Yes, I agree stop leak was likely used by a previous owner. Reason being, there is an external repair job on my radiator (that surprisingly doesn’t leak) that was probably done in tandem with the stop leak. Sometime in the future I’ll replace the radiator with a new Denso, but it’s not a priority for me right now.


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