i plow snow with my 91 ctd and the heat defrost has always worked great, but just recently it's been blowing luke warm from the defrost and hot from the floor?? which doesn't make sense to me, also if i rev the engine up a little it heats up some out of the defrost as soon as i let off the gas it goes back to luke warm. i just did a flush and fill on the coolant system, and checked the
'little flapper door and it;s doing what it's supposed to. anybody have any ideas?? any help would be appreciated.

 

Sounds like you might have plugged your heater core up beyond it's heating capacity when you did your flush and fill. Over time the heater core will slowly get plugged up, decreasing it's efficiency. With a new improved and slightly larger heater core from NAPA you'll have more heat than you did previously.

I have 3 of these 1st gen diesel pickups. Two of which have the NAPA heater core. The other is stock as far as I know, never installed one. It acts exactly like yours is doing now. Give it a little rpm, the water pump is sending more hot coolant to overcome the cooling capacity of the blower fan. At idle the fan is cooling the heater core faster than it can receive hot coolant. That is just my theory.

With the two NAPA heater core trucks, I receive much more heat at a much lower water temp, say 120* vs. 150* with a stock heater core. At operating temp it will really supply you with the heat.

Of course there are many 1st gen trucks running around with OEM untouched heater cores that have super heat. Two things, the heat can be objective, individual to individual. Two, I think it depends on how much the heater core is restricted.

That doesn't help much in your situation if you don't feel like doing a heater core job and spending the $56 on a new NAPA core. Me, I always prefer ditching the POS vacuum actuated hot/cold valve that opens or closes to allow or not allow coolant flow to the heater core. Once you have that figured out, you might try flushing your system again real good.

 

a couple of years ago i pulled off the hoses to the heater core blew all the water out then plugged the heater core outlet and filled it with some kind of industrial acid for about 20 seconds, then flushed the acid out with water and all sorts of crap came out of the heater core! is the heater core accessible from the glove bos or is a pain the the butt to change? thanks, pete

 

Personally I don't think it's a pain. I've done a few. I think there's a write-up in a sticky somewhere on this site. You might try flushing it out the same way again, might just work.

 

i'mstill scratching my head about blowing hotter out the fllor than the defrost??

 

1.- Clean out the heater box. See the sticky or drill a big hole an plug it with a grommet when done.

2.- New thermostat. Your's may be staying open a bit.

 

Dad did the heater core on my truck when he bought it before selling it to me. It was doing exactly what you mention here. The new core fixed it. Better to replace the core than just flush it, if you are going to hassle with getting it out of there, may as well replace it. If its plugged its probably also corroded and will soon start leaking.

 

I've found ussually debris is on the air and in the coolant side of the heat exchanger. If your going to flush, I'd suggest citric acid instead of an "industrial acid".
Mine, I'm just going to replace it with a new one from NAPA.