Does anyone know if the heater core still runs after you bypass it? I bypassed it a few weeks ago but i still smell coolant when i turn on the AC. After a few seconds the coolant smell goes away and then the air smells like rust/metal. What I’m asking is does air still pass thru the heater core when the AC is turned on even though i bypassed it? Also can anyone recommend a product/substance to flush the heater core and get rid of the rust? I dont plan on replacing it anytime soon too expensive. Thank you!
If you bypass the heater core, coolant no longer runs thru it.
Tester
Air would pass through the bypassed heater core in heat mode, and probably in AC mode too. The part that makes the air cold in AC mode is called the evaporator, which is separate from the heater core. But the two may be arranged in series, so air passes through both of them in either AC mode or heat mode . Just guessing mind you, and probably varies car to car. Maybe try the experiment in heat mode. If you smell the same odor, likely that’s the cause.
You might smell leaked fluid and rust that’s in the nooks and crannies of the plastic box that contains the core, fan, and ducts.
2 Likes
Coolant doesn’t evaporate like water does, and it has a residual smell. It’s very likely there is still some coolant in the core or the box around the core, and it will smell. Try squirting Febreze or Lysol into the air intake while you have the fans running, so you suck the deodorant through the core fins. It might still stink, but it will stink different.
The leaky heater core could drip for a year or two, you might blow the coolant out of the heater core with compressed air or flush it out with water.
There are foam seals inside the HVAC assembly that can absorb fluids, these may take a few weeks to dry out.
change the cabin air filter too. what? you dont have one?
Just plug up the open ends of the heater core and you will gradually get rid of any smells. Leaving the ends open will cause smells form a very long time since the coolant does not readily evaporate.
I had a mouse condo in my 1979 Toyota truck’s heater after parking it in the country one winter. Removing and disassembling and cleaning with water and detergent and Lysol and airing it out in the sun for weeks took care of the odor.
Removing the heater is very involved in many cars, not so big a deal in an old pickup truck.
But it IS a big deal on many modern pickup trucks, because they’re typically in the cab now, not in the engine bay, as in days past
And many/most manufacturers do NOT design vehicles with ease of service in mind. Most trucks with the heater core in the cab do not have a “cut here” design . . . if you’ve done it, you’ll know what I’m talking about. In most cases, you have to move quite a few things out of the way to gain access
The last relatively easy model were the full frame rear drive full size GM models, Chevy, Pontiac, Olds and Buicks.
On the other hand, we had 1976 Ford Granada V8 352 with air and there was 8 hours of labor involved in removing and replacing the A/C and heater cores.
In a Mitsubishi Montero with air, the whole vehicle seems to be built around the heater and A/c core. Allow at least 10 hours labor in this case.
I remember a particular Benz roadster that was pretty much built around the heater case
The heater case couldn’t even be removed in one piece . . . !
The instrument panel carrier was not removable, so you had to undo about a hundred clips securing the top- and bottom halves of the heater case, then you removed first the bottom, then the top half.
I can understand why some guys might be tempted to cut carriers, then weld them back
But that’s outside of my comfort zone . . . I could have stated all sorts of unpleasant opinions about guys who do those kinds of things, but I didn’t 
Fortunately for me, the only heater core I’ve ever had to remove was in my '72 Vega. That one was easy, accessed via the front of the firewall, and in those days engine compartments were not yet usually crammed with stuff.
Ease of core removal was the good part. The reason I had to do so was the bad part. The heater hose came around the back of the engine in such a manner that it put lateral loads on the core’s inlet pipe, causing it to fracture at the seam. I was still in the Air Force at the time, and the guys in the airframe shop made an aluminum pipe for me (to my design) that routed the heater coolant around the back of the engine without putting lateral strain on the core’s pipe. Problem solved. Those guys made a few different things for me that solved design weaknesses of the car. If anyone reading this was in Airframes in the 319th Bomb Wing in the early '70s, your help with my many Vega fixes will always be remembered.