Heater only blowing Luke warm air

[Thatold350]

Last winter when I got the truck 1988 k2500 5.7 350 I replaced the heater core and flushed and back flushed the system numerous times and the heat would stay lukewarm well winter came quick and still have the problem. I checked the heater core both inlet and outlet tubes to warm to touch. Truck does have ac but doesn’t work, seems like every actuator clicks while being used. When changing the heat from cold to heat I can hear the actuator behind the radio make a sound , blower works good. Does anyone know how to diagnose the problem

 

[Caman96]

Bad thermostat?

 

[Thatold350]

I’m buying one right after work

 

[Caman96]

I’m buying one right after work

Yeah, so just checking, when you flushed system, was it changed then?

 

[Thatold350]

No didn’t get it

 

[movietvet]

The most basic form of cooling system maintenance, especially when doing a flush, is a thermostat.

 

[Schurkey]

Sounds kinda like my truck. '88 K1500, regular-cab, 205 degree thermostat, 35 miles to work, and I had to wear a snowmobile suit to not freeze.

I had the heater core replaced. No change.

Turned out there were rotted foam seals between the steel of the cowl, and the plastic of the HVAC fan/motor box. The cold air pouring in past the crappy foam seal overpowered the heat coming out of the heater system. It never was a "heater" problem.

Had to tear the heater box out, replaced the jelly-looking deteriorated foam with plain ol' home-improvement store foam-strip-on-a-roll. Now I gotta turn the heat down or I'm roasted out of the cab. It's also how I found out that the '88 Service manual has so many errors and omissions that they had to publish a Service Manual Supplement with lots (not all) of the errors corrected.

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The bright side of this mess is that I could pick most of the pine needles, leaves, and other debris out of the evaporator fins. Plugged fins reduce air flow.

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