Heater Core Flush Procedure

Dodge RAM heater core issues.

We see common issues with a temperature differential between the driver and passenger sides of the RAM truck, or lack of heat on both sides.  The root cause of the problem is that the heater core is a parallel flow core, meaning the the water comes into the top and down multiple channels to the bottom.  Most cores are serpentine flow and you’ll get equal pressure and flow across the entire core.  You can check the flow of the heater core by letting the truck get to operating temperature with the heater off.  When you cut the heater on, if you get a burst of warm air and then it dissipates, this is a strong indication of the heater core not providing heat as fast as the fan is extracting it.

The core is the narrowest constriction in the flow path and will act almost like a filter for any gunk in the radiator system.  Dodge recommends routine radiator flushes, but no one pays attention and does this.  If you are as negligent as us, this could be a cause of the problem.  We also see issues when any work is done on the radiator system.  Just replacing components or wiggling heater hoses can product particulates.  You should always do a radiator flush anytime work is done on the coolant system.

The pressure is not equal across all the parallel channels and the back half will tend to “catch” the gunk and you’ll get a temperature gradient across the core which shows up as different temperature for the passenger and driver.  It will get progressively worse and you can get to a point where you don’t have good heat on either side.  Even with minimal flow the two heater hoses will get hot and you can’t really gauge the heat in the core by feeling the two hoses on a hot engine.  What you have to do is cut the heat/fan on full blast and feel the two hoses from a cold start.  The hoses should warm up at the same rate and get uncomfortably hot in a few minutes.  This is a common problem and just doing a good flush on the core is probably not a bad idea if you have any concerns with temperature.

Here’s what we recommend to get the core flushed correctly.  Since the flow is parallel, if you just get a few channels open, you’ll get good flow through the core.  You can’t put enough pressure on the core to clear all channels once a few are open.  To get to all the channels, you have to attack the problem chemically.  
    1.  Isolate the core by either removing the heater hoses or just buy two brass garden hose repair kits at Home Depot and cut the hoses.
    2.  Flush the core backwards and forwards with a garden hose.
    3.  Fill the heater core with Prestone radiator flush and let it sit a couple of hours or overnight.  We recommend the Prestone liquid flush because it works.  We met the Prestone VP of engineering at SEMA a couple of years ago and he agreed with our analysis and solution.  He said he would take our information back to their lab and look into a kit for just this issue, but I haven’t seen anything come of it.
    4.  Flush the chemicals out.
    5.  The trick is to swap the heater hoses such that the core will flow “backwards” when you reconnect.  
    6.  If there is still gunk floating around in the radiator, the problem will occur again.  We’d suggest just connecting the two heater hoses together and do a good flush on the radiator independent of the heater core.
    7.  Going forward it’s a good idea to do a flush every 12-18 months and swapping the hoses back each time.
    8.  If this helps, but not enough.  It’s like the old High School Joke: “It’s a long way to the cafeteria, you have to flush twice…”

If this doesn’t help, the next concern is the blend doors and our replacement is a complete fix for any door issues.  This is about a thousand dollars worth of free advice and we’d appreciate you keeping us in mind when you do see door issues.

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