Might have a heater core job coming up on my '94 pickup because I've recently flushed it and it's intermittently smelled a little like coolant inside for the past decade of ownership.

I don't like the idea of the 2-piece heater cores that the pickups have, where there's a separate pipe joined to the core under the dash with an o-ring and a clip that holds the flanges together.

Strangely, Toyota seemed to do the preferred 1-piece design on about half of their models, generally being lower-end ones (Corolla, Tercel, Celica) whereas the more flagship models (Camry, Cressida, Supra, Land Cruiser) all got 2 or 3 piece heater cores.

Also strange is that the '90-'95 4Runner got a 1-piece heater core, unlike the '89-'95 pickup that of course uses the same dash.

Comparing pics of the heater core boxes on ebay shows they are different, with the 4Runner holding the core parallel to the firewall and the pickup holding it at an angle, but the mounting areas and duct connections appear like they might be compatible. Any idea if one could easily swap a 4Runner heater core box (ie, the whole assembly) into an '89-'95 pickup?

Pics for comparison:

 

Bump.

 

Welp, here we are about 4 months after making this thread and my '94 pickup's heater core just started leaking on my passenger floor board last night.

Really a best-case-scenario for me, cause my fiance caught it right as we were leaving the house for dinner, so it was no problem to turn around immediately to swap to my '87 4Runner. Plus, I've got some of those Husky hard plastic floormats, which contained the coolant like a tray so it didn't soak the carpet.

A month or 2 ago, I tried flushing my heater core with hose water, and shortly thereafter I tried adding some CLR cleaner in there to let it sit for 2 rounds of 30 minutes each. Saw some small, dark flakes come out, but nothing wild. Seemed to flow ok. Probably made the leak happen sooner.

Was hoping to make the heater a bit hotter since it's always been just kinda warm, especially compared to my '87 4Runner's heat, which is by far the hottest of any vehicle I've ever been in, and it's delightful. Both 22RE.

Never heard back from this thread regarding swapping to the 4Runner heater box, so still don't know if they'd fit.

Plan is to just replace the core with an FVP brand from RockAuto (already ordered; was the last one remaining at this time). It's just a regular pickup-specific core. Also ordered an OEM o-ring on eBay for $10 shipped. Just disconnected the firewall hoses to start the removal process.

If I run into some big issue with this pickup's heater box and/or the replacement core, I think I'm gonna take a swing at trying the 4Runner heater box by purchasing one on eBay since there tends to be 2 or 3 for sale at any given moment. But for now, just for simplicity, I'm gonna stick with the normal pickup heater box arrangement. Fingers crossed that the pipe to the heater core is in ok shape.

 

Sorry I don’t have the answer but my heater in my 91 is atomic!!!! Maybe it is clogged and a new one will do the trick, but I can tell you it should be atomic, have you made sure the linkages are going full on to heat? I have never had to mess with the heat valve unit(not sure what it’s called) on the firewall? Idk just throwing stuff out there let us know what happens!!

 

Yeah, all the HVAC control cables have been in good order, and the coolant heater valve on the firewall has been working a-ok.

I actually just replaced the original valve when I did that 2nd flush job over a month ago. -- Reason being, when I did the first flush like a week or 2 prior, I noticed the tip of one of the 2 plastic nipples on the valve had broken off. There was still enough nipple left for me to slip the hose on, but that let me know the plastic had unsurprisingly become brittle, so I ordered a non-OEM replacement valve online and kept driving with the original one for a few weeks till it arrived to then swap it on. The end of the other nipple broke off when I was removing the valve. Coolant flow through both the old and new valves looked to be excellent.

Unsurprisingly, replacing those valves is a piece of cake. If you've still possibly got the original early 90s plastic heater valve on your pickup, I'd definitely consider replacing it preemptively. The exterior of mine looked like it was still in great shape, but it's the non-visible surfaces that undergo the harshness of hot coolant.

Not sure if anybody's had one of these valves fail in a way where they let the coolant out, but I could totally seeing that being possible, like one of the nipples breaking completely off and letting coolant spew out everywhere. Also, I became worried when that first nipple end broke off that it might be clogging stuff up somewhere in the system.

 

I still have original heater cores in my 86 and 88 pickup. The only leaks were from the O-rings. I measured them and looked up the best material options on McMaster-Carr and ordered a bag of them.

Lack of heat can be either lack of coolant flow through them or lack of air across them. In my case it was always from the Evaporator being turned into a dirt brick over the years choking off any air through the system. Whenever I have the entire box out for rebuild/reseal I run phosphoric acid in the heater core. That was enough to get mine clean anyway. Old school radiator shops can rod out heater cores just like they do with a radiator but hard to find those shops these days. I'm sure many of you already know this, just throwing it out there.

I have seen some forum posts where guys soldered the tubes to the heater core. IDK.....I guess it works but it's not that hard to get a couple O-rings for another 20-30 years of use. Personal preference I guess. These are the O-rings I used. https://www.mcmaster.com/1295N241/#:...n,16%20mm%20ID

I'd like to hear if the other heater box will fit as well.

 

I've got my OEM original 1994 heater core out of my pickup. Haven't tested it to see if it's leaking or if it was possibly just leaking at the o-ring, but the top surface of the core was NAZZTY looking. Also, some of the fins are jacked up looking at the bottom of the core. I'm not trashing it since it might still be usable, and could possibly end up being my best option for all I know.

I disassembled the pickup heater core box yesterday, cleaned it up and reassembled it. Tried installing the new RockAuto FVP brand heater core. The core's welded on tube needed a little bit of hand bending to correct its shape so that it'd align with the saddle shaped protrusion of the heater core box.

However, when attempting to fit the OEM pipe to the FVP core (using a new OEM o-ring) and reuse the C-clip that fastens them together, the 2 flanges don't seem to get close enough for the clip to grab them. The FVP heater core's flange is thicker than the OEM core's. Either that added flange thickness, or possibly a shallower depth to the FVP core's "cup", or some combination of the 2 is preventing the flanges from getting close enough to each other.

I added some grease to the o-ring, then pushed the pipe into the FVP core while twisting to see if I could get it to seat deeper in there, but no luck.

I then tested the pipe back to the OEM core (now with the new OEM o-ring) and the flanges just mate up perfectly and the clip slips right back into place.

Not a fan of this o-ring c-clip tube design. Ended up ordering a clean '90-'95 4Runner heater box on eBay last night (which still has a heater core in it that's probably OEM original from the '94 4Runner that it was removed from), along with a new RockAuto Four Seasons heater core.

Time to see if this is a viable option. Before ordering, I confirmed the eBay seller accepts returns just in case. There's just gotta be something that interferes or doesn't fit, right? There has to be a reason for Toyota to have gone to the trouble to produce a different heater core box design, what with all the separate tooling/molds needed for the different plastic box shape, and the different spec'd heater core. Maybe something to do with the different A/C evaporator and evaporator box that the 4Runners use.

Also ordered a new A/C receiver/dryer and new o-rings since I might remove my evaporator to clean it. I broke the top left bolt hole ear off the A/C evaporator box (where it overlaps the heater core box) while I was removing the heater core box due to having read a post here on YotaTech saying that you don't have to remove the evaporator box to get the heater core box out. Well, just a heads up to anybody reading this, you might want to remove it anyway, or else you risk damaging the evaporator box like I did. Used replacements are EXPENSIVE since everybody wants to add A/C to their pickups. I'm just gonna leave mine broken since surely everything's packed in there so damn tight anyway that I doubt it'd matter, plus I'm sick of this job and ready for it to be over despite not being fully at the halfway mark yet.

 

Unfortunately, the 4Runner heater core box will not work in the pickup, at least not without also swapping to:
- 4Runner evaporator box
- 4Runner blower box
- (possibly) 4Runner refrigerant hard line from the receiver/drier to the evaporator box liquid line
- (possibly) 4Runner refrigerant suction hose from evaporator to compressor manifold
- (possibly) some wiring change since the 4Runner pressure switch is a 3 stage to run the auxiliary condenser fan, whereas the pickup’s is just a 2 stage, and the sensors use different threads so you can’t just swap your pickup’s sensor onto the 4Runner liquid line (ideally one would go all the way and get that fan fitted too), but this might not be an issue since the wiring sub harness on the evaporator box and AC amplifier might have this covered.

Everything lines up for the 4Runner heater core box to fit into the pickup from what I could tell, but the box’s rectangular inlet where it receives air from the evaporator box is much wider compared to the pickup’s.

One could possibly hack up the plastic of the evaporator box and fiberglass a section to fill it in, but I don’t like the sound of that route.

Plus, after carefully filing down the new RockAuto heater core flange (both in thickness and in diameter), I was able to get the heater core pipe to fit with the C-clip in place and the new OEM o-ring.

So I’m going to just use that, re-foam the doors inside my pickup’s heater box, and return this 4Runner heater core box to the seller on eBay and return the new 4Runner heater core to RockAuto.

Here’s some comparison pics (4Runner heater core box on the left, pickup box on the right) along with some test fit pics in the pickup showing the mismatch to the pickup evaporator box. Hopefully this will be helpful to any future owners that ever consider this route.

 

Sorry to see all the trouble you're going through trying to get the correct parts. I have a couple heater/A/C units and ducts in my stash but they are from an 86 and 88 pickup.

 

I appreciate the sentiment man! I think my original pickup heater core box and A/C evaporator box are still in good condition. The evaporator box has that broken piece at the top left (visible in the pics test fitting the 4Runner heater core box in the cab). It's right below the slot for the upper bolt that joins it to the heater core box. I might try to glue it and make a sheet metal doubler reinforcement to reattach the broken plastic piece, but it might be ok left as-is.

I just wire-wheel'd all the old foam adhesive off the heater box doors and I've got some nice Blend Door USA adhesive foam that I'm going to stick on.

Here's a video of leak testing the original heater core with the original OEM o-ring:

 

Also will be relieving to have a fresh heater core in there so that I know this job won't be needed for presumably a couple more decades (even if this non-OEM core doesn't last as long as the OEM one did). I'm excited to see if the new heater core + re-foaming the HVAC doors results in hotter heat out the vents.

Similarly curious to see if my replacement evaporator gives me some improved cooling.
Unfortunately it's also non-OEM/Denso as they're discontinued just like for the heater cores...
But I do have a brand new Denso expansion valve that I bought 7 years ago before they were discontinued.

I hate to use it since I don't think my original expansion valve is bad, but seeing that the new one is marked specifically for R134a (which I retrofitted my truck to years ago) makes me think perhaps it's tweaked/calibrated to work better with the higher pressures of R134a than the old original R12 valve.

Don't know if my old OEM original evaporator was leaking but I've basically never gone a year without having to add more refrigerant at least 1 or 2 times, even back when it was still R12.

The evaporator was dirty (like they all are lol). I had blown compressed air in there 3 years ago to get a lot of debris out by removing the blower motor.

Just the other day, with the evaporator on the work bench, I tried cleaning it but there's some gunk between the fins that neither the Simple Green HVAC coil cleaner nor the brake parts cleaner could get off, even when gently worked in with a plastic bristle brush. Not sure if it's mold but it's definitely not helping. I'll keep it as a backup along with the original expansion valve.