psycho heater

What's up with the heater in my 1980 300TD? It takes forever to warm-up -- stays cold even way after the engine temp is up. Then, if you take your foot off the gas, COLD air. Only blows hot if the gas is down about 1/2 way or more. Any ideas?? I am freezing my a$$ off!

[jeffsr]

You may have a big lump of air in the cooling system, blocking flow thru the heater core, or the core could be obstructed, or.. your coolant level might be low.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..

[unkl300d]

Just a wild card guess, but could it be your
A/C servo unit gone psychO?
Don't know how to check it other than inspect
for leaks. 1980 was last year with this
muck device. Assuming your vehicle was included.
good luck.
aside: that's why bench seats were engineered.
(girl friend)

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1979 300D
133K miles
1989 300SE
1995 C280

[Jim H]

I'm with Jeff on the air in the core, due to low coolant level.

I just had that happen with the Suburban; no heat. The system was 3-4 quarts low! I topped up the radiator and the overflow bottle and drove it. It took about 2-3 start-up cycles for the air to be purged out of the system. I think a few corners helped.

The inside heater core is higher than the radiator, so it will drain first when draining the system. Of course, it will also starve for hot water first

If you find it's low coolant, top her up and be sure to then re-check often. The Suburban had a small leak in the radiator, causing the fluid level in the reservoir bottle to drop, then disappear, and then the level in the heater core dropped.

I didn't discover this for a while, because I was not checking under the hood, and who uses heat in the summer and early fall?

BCingU, JIm

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'96 E300D 61k mi (wife's daily ride)
'95 Audi 90 120k mi (my daily ride...)
'92 GMC Suburban 140k mi
'85 300SD 235k mi KIA/RIP

[johnofaunce]

Isn't the heater coolant flow through heater core pumped with an auxiliary pump on that merc. 300 td.?

[theairboy]

Also check you vacuum pump out put at low rpm's and your vac lines going to the doors on the heater box. Can't hurt.

[Jerry_Mullen]

Pretty common condition caused by the monovalve failing, what happens is its (internal) rubber seal tears causing coolant to be on both sides of the valve slowing or stopping its operation. The mono valve is controlled by the climate control system where as you increase the temperature setting, the voltage to the valve solenoid pulses faster and for longer intervals allowing more heated coolant into the heater core. Replace the monovalve core and your heat will be fine, cost is about $25-30. The aux water pump moves coolant through the heater core during idle, at over 1500 engine RPM, the main water pump provides enough flow so the aux isn't necessary. If you turn the key on to the run pos. with the engine off and the climate control set for heat, you should be able to hear the pump running, if not check its plug, take a voltage reading and replace it if there is voltage present but it doesn't run, last time I bought one it was around $80.

Good luck

[occb2]

I had the same problem on my 84 300TD. Its the monovalve, no doubt. I got the replacement kit on ebay and fixed it in 20 minutes. All the heat you need.
Rob

[kerry]

I'm pretty sure an 80 TD does not have a monovalve, but the ACC servo. It could be a bad servo, BUT I believe it does have an auxilliary heater pump, the failture of which would account for why you have heat at high rpm's with a lot of circulation from the main water pump, but no heat at low rpm's with the no additional circulation from the auxilliary pump. I'd check the pump first, and then the ACC servo. Pray it's the pump, the servo is $$$$$.


I just realized--why are we responding to a thread from 2000?