How to Tell if a Radiator Is Clogged

by Jennifer PattersonUpdated June 30, 2023
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A clogged radiator can lead to engine damage if it is not fixed promptly. Radiators keep the engine cool by transmitting the coolant through the engine block. The coolant is then heated, before being passed on through the radiator, where it is cooled by evaporation. When a radiator is clogged, there may be signs to indicate it. It is imperative that you repair your radiator as soon as possible to prevent extensive engine damage. You can tell if you radiator is clogged with some simple tips.

1. Park your car and allow engine to cool.

Park your car, and turn off the engine. Allow the engine to cool for 3 hours. Open the hood of the car, and remove the radiator cap from the radiator.

2. After 3 hours, start your car and wait two minutes

Start your car, and wait at least 2 minutes for the thermostat to open. The coolant will start to circulate.

3. Use flashlight, look for uneven flow of coolant

Turn on your flashlight, and point it downwards into the radiator. Look for an uneven flow of coolant. If the radiator is blocked, the coolant will not flow evenly. Another quick way to check for a clogged radiator is to ask someone to rev the engine while you are squeezing the radiator hose. If the radiator hose stiffens and is difficult to squeeze as the speed of the engine increases, this means that the radiator is clogged.

Never open the radiator cap while the engine is hot to prevent serious burns.

Video: Clogged Radiator Symptoms

Helpful comments from the video:

  • Partially blocked can be harder to diagnose it may also stop heat from locking up a viscous fan causing a secondary symptom. Check radiator is hot all over the face (turn engine off). If any cool areas then your radiator is partially blocked. Hot water cannot flow into those channels. You will find that whilst the middle channels are badly blocked, the channels at the inlet pipe and at the far end would still be open and allowing a reduced flow which does not have time to cool properly under loaded conditions such as going up steep hills but may still run properly under normal driving conditions.
  • I just had mine done yesterday, the temp is cool down here in Mexico now, about 50 degrees , on the highway the car would not overheat, maybe because of the fan and the cool air, but as soon as I slowed down for traffic lights etc within the city the temp started to rise to about 3/4, I immediately took it to a shop, the guy actually opened up the radiator as shown here it was about 60 percen clogged, he cleaned every hole in the radiator, here labor is cheaper , it cost me $40 usd, and the radiator is in great shape now, an aftermarket one was around 150 usd for my trailblazer so yeah so far it was a good deal, Im good for at least another year .
  • Good info. Thx for sharing. Straight forward

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