My car keeps running hot after about 15 mins. The coolant in the reservoir tank gets boiling hot.

I flushed the cooling system until clear water came out bottom radiator hose. I changed the water pump, thermostat, reservoir, & cap.

What could be the problem? How do I diagnose it?

UPDATE 12/9/2016

Ok i changed all of the gaskets including head gaskets. Its still overheating.

  • Take the rad cap off start it up and look for bubbling. If its bubbling its the head gasket/ cracked head. Does the heater run hot? if not its possibly an air lock stopping circulation of the coolant.
    – Mark
    Commented Aug 24, 2016 at 10:25
  • it's not just bubbles, it's literally boiling. ...hot to the touch
    – Will Mc
    Commented Aug 24, 2016 at 10:49
  • do the cooling fans come on?
    – Ben
    Commented Aug 24, 2016 at 14:26
  • After you flushed it, did you fill it with plain water? or 50/50 mix of coolant and distill water?
    – rana
    Commented Aug 24, 2016 at 14:43
  • first I used 50/50 & water it boiled,then I flushed again & used regular water,so that I wouldn't waste more 50/50,it boiled....like I can boil and egg in the reservoir
    – Will Mc
    Commented Aug 24, 2016 at 19:22

4 Answers 4


It could be a number of thing, first things to check are that the radiator is actually getting hot. There is thermostat that, if faulty, may prevent coolant from entering the radiator. It could be that the radiator is blocked with silt / rust / debris. It could be that the impeller has become detached from the input shaft on the water pump or has lost a fin or two so the coolant isn't circulating. If could be that the system is air-locked so there is a bubble somewhere preventing coolant from entering the radiator properly, than can occur when the coolant level is topped up without setting the EVAC to HOT or overfilling the system.

  • Ok i changed all of the gaskets including head gaskets. Its still overheating.smh
    – Will Mc
    Commented Dec 9, 2016 at 15:20

I would check:

  1. radiator in/out: check if it gets hot, and water flows out "clean"
  2. water pump: check if it is not sucking air by looking for leaks when engine is not working
  3. thermostat: take it out, put its assembly into the engine and run it without it for a check
  4. timing!: sometimes retarded timing increases engine heat a lot
  5. air in the system: when filling with coolant, squeeze the hoses like a hand pump, to let air go out, I sometimes add coolant with fresh started engine running while I add the liquid

When you uncapped the engine to put new head gasket, did you check for dirt in the coolant passages in both block and head?


Release the air valve from the water outlet you have air trapped.

  • I did bleed it there is only steam,reservoir boiling. ...how else do I get trapped air out? if it is trapped air, it's almost like there is no suction from the water pump but I used 2 new ones.i thought that the first pump was defective
    – Will Mc
    Commented Aug 26, 2016 at 5:12
  • 1
    This answer needs to be fleshed out a little. If there is an "air valve", where is it?
    – Zaid
    Commented Aug 26, 2016 at 11:44
  • rite on top of thermostat housing
    – Will Mc
    Commented Aug 26, 2016 at 12:10
  • Trapped air in the engine can be very difficult to get out, especially if air is trapped at the thermostat. Air does not heat up the thermostat like coolant does so it does not open and the engine reaches boiling. Some thermostats have a tiny hole to allow the air to be pushed into the radiator, but not all do. If your thermostat does not have a hole drill one and try again before you start changing all sorts of gaskets. Commented Oct 25, 2016 at 18:04
  • Ok i changed all of the gaskets including head gaskets. Its still overheating.smh
    – Will Mc
    Commented Dec 9, 2016 at 15:19

I have had the same problem once, this was what caused it. In your engine, you have so called core plugs. (often wrongly known as and called frost or welsh plugs) Sometimes they put up a fight when you try to pry them out, and lazy mechanics sometimes decide to just push it in the block to save time. It then gets stuck somewhere in you watercircuit and restricts waterflow. If this is somewhere near the cylinder, you have a local hot spot there, and the coolant may start to boil. I don't say this is the case with your engine, but it may be. It sounds like you have considered every other possibility.

  • Ok i changed all of the gaskets including head gaskets. Its still overheating.smh
    – Will Mc
    Commented Dec 9, 2016 at 15:19
  • I'm sorry but what do gaskets have to do with overheating? Overheating may be cause by poor tuning, but it's not plausible if it has ran fine before. I would suspect the radiator or cooling passages in your engine. Deposits on them restric heat transfer and can cause the engine to be unable to get rid of it's excess heat.
    – Bart
    Commented Dec 12, 2016 at 11:19

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