Here's the write-up I did for a TJ Tech Thread on doing a heater core flush on the TJ. I posted it here as well so that I have it and can find it easily
As the winter months near, and temperatures drop throughout New England, I finally got off my butt and decided to move forward with the famed "heater core flush" as described by many people on the forum as an easy and potentially great solution for luke-warm air from the heating system. I can honestly say that I do remember a time, albeit a few years ago, when the heater would blow out burning hot air. Over the past few winters it has gotten much cooler, to the point of keeping gloves, earmuffs, and extra jackets in the Jeep. Before the cold really sets in, I executed this process.
One of the problems I had found was that I am a very visual learner, and nobody had posted a photo guide to this on the forum here. I watched a YouTube video with a guy that showed the process, and went from there. Here's my guide with photos.
Step 1: Locate the two hoses that travel from the radiator/engine to the firewall on the passenger side of the engine bay. These are the two that you will be passing water/air through to try and unclog and discharge any stray substance from. I have circled the appropriate hoses in the photos.
Step 2: Locate the ends of the hose (meaning: don't remove the hose from the firewall, use the opposite end) Remove the hose clamps from the end of the hose, and move them further back on the hose. I use a pair of large pliers, clamped the tabs, and wiggled the clamp further down the line to loosen the grip and allow me to slide the hoses off of their positions. Remember, they may be old/dried out, so carefully remove them to avoid problems. The lower hose of my Jeep took a little extra love to get off. Also, have a catch-bucket nearby to collect any of the fluid that may come out upon hose removal.
Step 3: Fasten your hose of water/air compressor into the Jeep hoses. It doesn't really matter which you start with, although I began with the upper hose. Situate the catch-pan/bottle at the end of the opposite hose, as that is where the water & debris will flow out of. I used a milk gallon at the flushing end, a funnel at the end of the hose, and ran the highest pressure water flow I could with my hose.
*For steps 3 and 4 - flush water through the system until it flows out the other end (into the catch bucket) in a clear form)*
Step 4: Reverse the process. Much alike fish scales, pieces of debris will not be freed by water moving in the direction it normally does. By reversing the process, water will be more likely to catch debris in the opposite direction and remove it from the hose. Don't forget to move the milk jug device to the other hose.
Step 5 (optional): The next step is one that some people say to do, and some people opt not to go through with. In an effort to make sure that all debris is removed, people use a vehicle appropriate cleaning product in the hoses. As noted below, please make sure that the substance you wash the lines out with is safe for aluminum (the material of the heater core) and all of the hoses themselves. Start by pouring the substance down the hose (matters not which) and letting it sit for 10 or so minutes. Then go ahead and flush it in either - or both - directions.
The most important part of this step is to make sure that all cleaning substances have been removed from the hoses, as it is not good for the radiator to receive any left-overs once the car is started again. Run water through after this step until you are absolutely sure that there is no substance left in the hoses.
Step 6: Re-attach hoses. Affix them to the appropriate tips on the engine, and replace the hose clamps to their original positions. Remember to spread the clamps as wide as possible so it can slide freely without scraping apart the hose.
Step 7: Replace any coolant fluid that may have been lost during this process. I picked up a gallon of Prestone 50/50 coolant at AutoZone for $13.99 before starting this process. If you don't have a visible loss of fluid, turn the vehicle on and depress the gas to about 2,000-2,500 RPMs for 30 seconds or so. At that point, re-check the fluid level and add appropriately.
That's it!!! Hope this helps some people get an idea as to how easy this kind of thing is to do, and give them an idea as to how to go about it! Feel free to post any thoughts, suggestions, or questions below. I'd be happy to add/change any parts of this if people have suggestions that have worked for them.
As always, PLEASE DISPOSE OF COOLANT AND ALL VEHICLE FLUIDS APPROPRIATELY. MOST AUTOZONE/ADVANCE AUTO PARTS STORES WILL DISPOSE OF YOUR FLUID IF YOU BRING IT TO THEM IN AN APPROPRIATE CONTAINER.