How to Clear a Clogged Auto Radiator Without Taking It Off

Written by richard rowe
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How to Clear a Clogged Auto Radiator Without Taking It Off
A clogged radiator can easily put you on the fast track to nowhere. (Noel Hendrickson/Digital Vision/Getty Images)

Cooling systems lead a pretty hard life, contending with any number of harsh chemicals and minerals under extremely high temperature and pressure conditions. Such conditions will inevitably cause dissolved minerals to adhere to and collect on anything they touch. The radiator, with its many small passages and high temperature concentrations, is a prime target for such system-clogging deposits.

Skill level:
Moderately Easy

Things you need

  • Flathead screwdriver
  • Drain pan
  • PVC pipe plug
  • Calcium-lime-rust remover
  • Towel

Show MoreHide


  1. 1

    Allow the engine to cool. Place a drain pan under the radiator, turn the petcock valve on the bottom of the radiator and drain the coolant. If your radiator doesn't have a petcock valve, proceed to Step 2.

  2. 2

    Loosen the clamp on the lower radiator hose where it attaches to the engine, and gently pull the hose free. Keep the drain pan underneath and be aware that coolant will flow from the hose and the engine.

  3. 3

    Remove the lower radiator hose from the radiator to drain all of the coolant, and then reinstall the hose onto the radiator. Tighten the clamp.

  4. 4

    Install a PVC pipe plug into the other end of your lower hose and install a hose clamp. You might have to shop around a little to find the right size pipe plug; if you can't find one, you can use a C-clamp and two small blocks of wood to pinch the hose end shut. Close the petcock, if applicable.

  5. 5

    Fill the radiator up with an undiluted calcium, lime and rust remover. Commonly sold in hardware and department stores and used to clean bathrooms and plumbing, CLR will efficiently dissolve most of the calcium and rust build-up in the radiator. Allow the CLR-filled radiator to sit for three to four hours. In addition to the brand-name Original CLR, you can use any CLR equivalent, like Lime-A-Way, Gloves Off CLR remover or Johnson's CLR remover.

  6. 6

    Remove the pipe plug or clamp from the radiator hose and drain the CLR into your drain pan; it'll most likely come out looking like chocolate milk.

  7. 7

    Push a garden hose about one foot into the radiator hose and stuff a towel into the gap between the garden hose and the radiator hose to seal it. Hold the hose in place while an assistant turns the hose on full blast. In short time, the radiator will backflush and fill up with water.

  8. 8

    Place your hand over the radiator filler cap and use it to direct the overflowing water toward the front of the car and away from your cooling fans and engine. Continue backflushing for five minutes or until the water comes out clean and debris-free.

  9. 9

    Reattach the lower radiator hose to the engine and refill the radiator with coolant. Start the car and allow it to run while you add more coolant. Wait for the car to warm up and the thermostat to open; you'll know it opens when water starts flowing in from the upper radiator hose. Continue adding coolant, and squeeze the upper radiator hose lines a few times to work air bubbles toward the radiator. This purging procedure is vital for removing air trapped in your cooling system.

  10. 10

    Put the radiator cap on, drive the car for a few miles and recheck your coolant level to ensure that it's still topped off.

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