I have a problem with my 98 1.6 sohc honda civic . There seems to be grey sludge that looks just like cement in the water and radiator but the car is not over heating and seems to be running perfect . Would any 1 know the cause and how I could prevent it? Thanks


There are a few things that foul coolant. Grey stuff in coolant is usually a stop-leak additive, especially if any of it is solid or sludgy. The heaviest buildup is usually around radiator cap.

I suspect your car leaked (or still leaks) coolant somewhere, and one of its owners or "mechanics" decided to use a stop-leak additive to fix the leak. This is the lazy-mans fix for a small leak somewhere in the cooling system.

A head gasket failure is possible, but the coolant is usually fouled brown in this case. I also wouldn't describe that brown stuff as "sludgy" either. (Though the stop leak may have been added to deal with a failed head gasket.)

You may want to open up another question, "How do I properly deal with a coolant system full of stop leak?" It's a problem that deserves its own treatment.


Although more than one possibility exists, grey sludge could be oil or combustion gases in the coolant. This would be due to a leak in the head gasket. To learn more, you can try observing the coolant expansion tank with the cap off while the engine is running. Be sure to start from a cold engine so you do not overheat the engine, and don't run the engine for very long. If you see bubbles, this could mean combustion gases, and possibly oil, are entering from the head gasket.

I'm not sure this can be prevented any way besides regular maintenance. If it is indeed the head gasket, that is a very involved repair.

  • Head gasket failure typically foul the coolant with brown sludge, not grey. See here.
    – kdbanman
    Jan 27 '15 at 15:48
  • If there is oil in the coolant, chances are you will also find coolant in the oil. Perhaps easier just to lift the dipstick and look for "mayonnaise"?
    Jun 26 '15 at 19:41

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