No Heat - Not a flap issue

Last year I had lukewarm heat and ended up repairing the degarded foam and taping over the holes of the heating ducts behind the radio. There was a DIY somewhere on here, and the heat worked great after the repair. I haven't used the heat too much this year, but I know it has been working ok during the cool mornings that we've had the past couple of weeks. I didn't have the heat on high, blasting, but I could feel warm air as i would have expected. This morning, i turn the heat on HI with the blower on HI and the air is straight up cold. Not warm or lukewarm...I couldn't get any heat to come out. Assuming that this has nothing to do with the degrading foam issue that I fixed, where would I look next to diagnose this type of thing? Nothing else seems abnormal, no coolant smells, no leaks, no issues whatsoever.

I know you said it is not a blend door issue, but did you tape both? There are 2 blend doors.

Quotes source post: 16127489

Heater core could be defective. There is a slight chance the thermostat within the controller is malfunctioning. I've experience both on other cars. Heater core could have a blockage.

Check your coolant level and manually lift the wire bail to burp out any air in the system. Leave it up for several drive cycles hot to cold. Continue to check coolant level. You can eliminate the blend doors from the equation by pulling out your pollen filter, wrapping it in a plastic bag and reinstalling. That will eliminate outside air being blended into the cabin air. Run heat on high and see what happens. If it doesn't get hot, probably the heater core is blocked, or a hose/pipe leading to it. When was your coolant last changed out? Remember, coolant is cycling through the heater core all the time. It is the blend doors in the air box that regulate the temperature.

After my IMS Retrofit, my indy had a hell of a time getting air out of the coolant system. During that time the heat wasn't functioning as it should. He finally got it but the heat is mediocre at best. How would one check to see if the heater core is blocked / bad?

Quotes source post: 16127787

Unhook the two hoses, clamp hoses to prevent spillage, remove heater core, and push water through it. If it doesn't pass water, then it's time for replacement. Or, just replace it. The part is like $135. Be mindful, you will have to remove air bubbles in the coolant system afterwards especially if you install a new unit.

is replacing the heater core a big pain in the *** type of job? Or is it pretty straight forward?

Quotes source post: 16129622

Depends on how you define PITA job. To me, this would be on the level of replacing the alternator or water pump. Less frustrating than replacing the coolant expansion tank. Get my drift? I think the hardest part is getting to the actual heater core because you have to remove the plastic cowl cover and wiper assembly. Follow these steps here on Renntech. There may be a DIY guide on Pelican. I'm not sure. https://www.renntech.org/topic/35708...val-996-99-c2/ Just a side note of concern, before you tackle this job, I'd rule out the blower flaps. That's the most common problem with heat on these systems. Good luck!

Thanks for the link....I'm going to get behind the dash again. I taped up the flaps pretty good last winter, but i'll double check to make sure they're sealed. Before I sealed the flaps, i was getting lukewarm air, it just wasn't hot. When i fixed the flaps, it got hot. When this issue popped up two days ago, it just started blowing cool air, not lukewarm at all. And it happened out of the blue, and was nothing gradual.

One more question....if i rule out the flaps, would the heater core be 100% the cause of zero heat in cabin? No other possibilities, right? If I do replace the heater core, should I do a complete flush and refill? I replaced the expansion tank a year ago, and added fresh anti-freeze during that time. I def do not want to deal with burping air pockets....I have a vacuum fill, so that's not a problem.

Quotes source post: 16130180

IMHO, if the flaps on the blower section are ruled out, then it could very well point to heater core. The heater core is the source of heat generation for the cabin - outside air is routed through cabin air filter into the heater core where it is heated. The AC controller (thermostat) controls air mixture at desired temperatures. If the thermostat "AC controller" is malfunctioning, then you will get similar results. Make sense? I've experience both problems. Also, if the heater core is confirmed faulty which more than likely is because of a blockage, then I would recommend a full flush of the system. You don't want any contaminants dislodging and blocking other parts of the cooling system. Also, if you flush, make sure you use a proper airlift system.