Well... I’m probably jinxing it by saying this, but I think it’s fixed....

The OEM heater core has been discontinued, so yesterday I ended up putting another aftermarket in it. The one in it was a spectra 93028 - should’ve been the right one - I put a 4 seasons 98028 in it.

While it was apart I also thoroughly inspected the blend and mode doors (all fine) as well as the evaporator. The 2 things that stuck out to me were that the heater core was supposed to have a foam seal all the way around it, but instead there were just a few strips of PVC foam (like what you use for sealing toppers or camper shells against the bed rails on a pickup) that had been placed by the previous shop and it wasn’t even enough to completely seal as the heater core could move a little with the case assembled. So I used similar foam to build up the seal on the top and bottom and make sure air couldn’t move without passing through the fins.

I also found the evaporator was between 25-50 percent plugged with dirt and debris - the previous shop clearly didn’t bother even trying to shop vac it. So I cleaned it as thoroughly as I could as well.

After reassembly, I used our fluid extractor as a vacuum on the heater outlet hose (a method I’d also tried on the other heater core) to try bleeding it without relying on the system flow to do it properly. Before it was even halfway up to temp, the heat was working so much better than it has since it showed up - I measured 135*F at the vents with the heat on high and high blower speed.

Of course it was almost 70* yesterday so it’s not like it had to work that hard. But my test for checking it more thoroughly was to put it on defrost so the AC turned on - there was a noticeable drop in temp with the AC on, but it was still much better than it has been.

The forecast shows a few nights coming up with temps near freezing again - I’ll test it one of those mornings to really see how it does.

 

Was there much of a design difference between the two heater cores? Fin density? Tubing size? Anything at all? Or was it just ****-poor sealing?

I could see how the restricted evap could reduce FLOW, but, it should reduce temps.......

 

No obvious differences I could see between the designs. I think it was a combination of the evaporator being partially restricted and the heater core not being sealed against the case. I’m tempted to dissect the old core to see if there’s an internal issue, just because it seems unlikely those were the only things causing the problem.

However I can see how not making sure the first replacement was sealed correctly would be misleading in diagnosis.

As to the evaporator being restricted, air flow doesn’t directly affect the output temp of the heater core, but a reduction in air flow through the core can affect overall heater performance.

For the sake of discussion let’s say the blower moves 10 cubic ft per minute - so with everything working correctly and no restrictions, that’s how much air is moving through the heater core. But with the evaporator being plugged even 25% that reduces flow to 7.5 CFM - then considering the heater core not being sealed, another 1-2 CFM isn’t getting heated either (or at least isn’t getting fully heated). By the time the air passes through the core, only 5-6 CFM is getting heated adequately.

Obviously those numbers aren’t entirely accurate but it’s easy to see how these 2 factors combined could affect heater output, especially considering the below zero temps we’ve had here lately.

 

Ok, that makes sense.

It's warming up here now...... all the snow is gone, and it's supposed to rain tomorrow. My truck could use a good rinse.

 

And the warmer temps are the reason I’m not yet fully convinced it’s fixed. I live in Iowa and spring is always a roller coaster of weather so we’re bound to see at least one more round of freezing temps - that’ll be the real test.

Maybe then I can stop

 

Yeah, I never expect the weather to be consistent. I am in Michigan.... and the beginning of March was REALLY nice.... (temps in the 50's....) so, I fully expect to see snow in the first part of April. Just the way it goes.

 

So I’m officially calling it fixed.

It was 34* when I got to work this morning and the first thing I did was start it up and let it idle to warm up. I checked it 5 minutes later - the temp gauge was reading about 150 and it was blowing hot air on high. I checked it again after another 10 minutes - it was up to temp and the air was still toasty, as was the whole cabin.

I tried setting it on defrost again to see if it mattered - again there was a little bit of a drop in temp (by feel only) but it was still hot.

Considering there were a few times that I had it running in the shop (when it was bitterly cold out) and it couldn’t produce hot air from the already heated air in the shop, I’d definitely call it fixed.

As frustrating as this has been, it’s one of those things that will always come to mind in the future when diagnosing heater performance. The thing that gets me is just how much of a difference sealing the core properly made - I know I’ve replaced heater cores without sealing them against the case before (or at least not completely sealing them) and there was no noticeable problem with the heater. But, in the future, I think I’ll make it a point to make sure it’s done.

Thanks again for helping me work through this.

 

Awesome. You wouldn't think that those seals were quite that important..... Lesson Learned.