No heat!

A/c works GREAT! Blower motor blows strong on all vent settings, but a barely gets warm. It's an '89 Renix 4.0L and the previous owner converted it to an open cooling system(he did not bypass the heater core).

Other than a heater core possibly being plugged up, what else should I check?

I would check the blend door under the dash (watch the cable while you move the hot-cold slider) and also check the heater valve under the hood.

ehall said:

I would check the blend door under the dash (watch the cable while you move the hot-cold slider) and also check the heater valve under the hood.

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Also check the vacuum to that blend door.

I have a vac. Gauge, what kind of pressure should I look for?

I don't know what vacuum he is talking about. I thought the RENIX used a bicycle cable for the blend door, the same as my 91.

For the heater control valve under the hood, just move the slider between hot and cold and see if the valve lever changes position. Should be able to test with visual. I believe the valve is normally open which means it is pulled closed by vacuum pressure when the slider is on the cold setting. Should be able to tell with your fingertip if you have vacuum pull on the vacuum leading to the valve, but it is normally really low pressure like just a couple of hg inches.

If its blowing through all the settings and just not getting warm I dont think its a vaccum issue. Just unhook the two hoses that go to the heater core, drain them, then refill with a 50/50 mix of white vinegar and water. Let it sit for a while, then flush it out through both hoses and watch the nastyness flow out.

When I did mine It was so bad I let it sit overnight. Blew nice and hot after I flushed it out and put it back together. Cheaper and easier than replacing the heater core, especially if its not leaking.

There are two damper doors, the blend door (your bicycle cable...., and a fresh air /recirculate inside air door. If vacuum is lost to the recirculate door vacuum valve, it is spring loaded to open and allow cold outside air in to the system, making it take much longer to warm up the inside. Also if he has the OEM 89 Vacuum valve on the coolant feed to the heater under the hood, it may not be enough vacuum to open either, if there is a small vacuum leak. My Vacuum valve had a bad diaphragm, had to replace the valve, passenger side near the door, under the dash. Also had to fix several vacuum tubing leaks here and there.

This applies to 85-89 at least.

thermostat stuck open is also a possibility

Well I would just like to report that I tried the vinegar thing. 50/50 mix, and let it sit for an hour. Then blew everything out with the air compressor.... WORKED GREAT!! Thanks for the help!

thermostat stuck open is also a possibility

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This would only result in the heat taking LONGER to warm up. Once the engine is at temp, the thermostat wouldn't matter. If you're in antarctic temps where the thermostat would be more active, then it's a possibility...