What are the things that can cause an engine to overheat?

Here's a partial list. What am I missing? I'm stumped on my Toyota Tacoma.


  • Make sure fan is rotating
  • Check/replace the fan belt
  • Check any fan fluid drive joint


  • Make sure radiator is clean and clear of debris

  • Replace radiator (as it might be internally clogged or corroded)

  • Replace radiator cap (and get one with the correct pressure settings for the vehicle)

  • Tube to recovery tank with a slight vacuum leak thus leaving radiator not full as engine cools down


  • Visually look for coolant leaks

  • Pressure and/or vacuum test cooling system

  • Flush system with a garden hose until only clear water emerges

  • Check the coolant type for the vehicle

  • Replace the coolant

  • Test the coolant to water ratio (at 50% in my case for glycol)


  • Replace thermostat. Consider that you might have been given the wrong thermostat. Also they are available in different opening temperatures. For my vehicle I saw 180 degrees as standard, and 170 degrees as optional.

  • Test thermostat it in a pot of water on the stove with a thermometer and caliper to meet factory service manual specs, i.e. what temp it's suppose to start to open at, and how much it's suppose to open. NOTE: I live at 6000 feet elevation, so water here boils at about 200 degrees, which means the thermostat never reaches fully open in the pan on the stove, but it's close.

  • Make sure the thermostat is in the right way (i.e. flow direction is correct, and wiggle hole is up)

  • Make sure the thermostat is the right thermostat. Try to verify that both diameter, length and temperature set-point are correct. Somehow I got the wrong part. It was too long. Even though it opened correctly in a pot of hot water to manufacture's specs, it wouldn't open in the truck as the end that closed off the bypass would impede the end that let the main amount of water flow. But thinking that since it was both new and had been tested twice, this then caused me major headaches looking in vein for overheating causes coming from other things.


  • Wiggle the shaft to see if it's bearing is bad

  • Replace it

  • (How can one tell if it might have cavitation creating bubbles?)

POSSIBLE HEAD LEAK (exhaust leaking to coolant creating bubbles)

  • Check block to head joint, (in my case head was resurfaced by a quality machine shop)

  • Check compression (to be to spec in all cylinders)

  • Smell exhaust for a sweet smell

  • Chemically test coolant for exhaust gas in it (with an indicator-solution test kit which turns from blue to pink if exhaust is leaking into coolant)

  • Replace head gasket

  • Replace head bolts (in my case of the stretching type)

  • Carefully torque and spring load the head bolts
    (this includes checking a whole long list of things like carefully visually inspect block and head flatness and smoothness, check bolt and hole length, and that washer with correct side up, and oil is applied to each bolt and washer as installed, and that hole is blown out first, and factory packing sealant is applied as directed, and bolts are smoothly tightened in the factory specified order, and with a dial angle gauge)

  • 2
    I've seen errors in the instrument clusters gauges before causing it to appear overheated when its actually not. Don't know if that counts for your list tho.
    – narkeleptk
    Nov 11, 2019 at 4:40
  • @narkeleptk, Yea I think that's a good one. Thanks. Nov 11, 2019 at 6:00
  • My Frontier was overheating when I got it. Turned out previous owner had put the Thermostat in backwards. Not saying this is your problem, just that sometimes it's something you least expect. Nov 11, 2019 at 10:42
  • Haven't seen it mentioned here yet, but make sure you are using DISTILLED WATER in your system, not regular tap water. Tap water is ok for flushing the system, but it contains chemicals and minerals which will decrease the boiling point of the water, and may cause the chemicals and minerals to build up in your system. So after you flush with tap water, you need to flush the tap water out using distilled water. This racing website has a very detailed scientific guide to antifreeze and water in your cooling system.
    – sam
    Nov 11, 2019 at 20:10
  • Quick and easy suggestion: buy a new temp sensor ($15), and a digital temp gauge ($20). After you install the new temp sensor, wire the digital gauge to the temp sensor. Keep the digital gauge in the cabin. Your gauge on the dashboard will no longer work, but you will now be able to see the exact temp of the coolant. Best case scenario is that your original temp sensor or dashboard gauge was broken.
    – sam
    Nov 11, 2019 at 21:24

4 Answers 4


Because you live at a high altitude, you may want to consider getting a thermostat which opens at a lower temperature.

If your thermostat is rated at, for example, 195 degrees F, it will begin to open at 195. It may not be fully open until 200 or 205.

As you say, water boils at 200 degrees F (94 C) at an elevation of 6000 feet (1829 meters). Thus, your coolant/water mixture may begin boiling before your thermostat is fully open. This is not taking into account the fact that antifreeze and pressure will increase the boiling point, but the final point remains the same: the air bubbles may be forming in your system. Air bubbles = very baaaad


One possible solution (to the symptoms) may be to upgrade the size of your radiator. Base-model trucks typically have a 1 inch radiator, but models with larger engines or "tow packages" will often have a 2 inch radiator. In my experience, radiators will use the same mounts, making swapping rads relatively easy. You may also have to get a shallower fan shroud which fits the new radiator. The fan shroud is a very important part of the cooling system.


You've checked almost everything there. Perhaps check the vehicles heater radiator for blockages which can restrict flow around the main system. Is the vehicle ACTUALLY overheating, or is there perhaps an electrical fault showing an overheat condition e.g. faulty temp sensors, gauge etc.

  • It's actually overheating, as also evidenced by radiator cap starting to leak steam, in addition to gauge too high, and once I had boiling that I could hear, so I am sure of the gauge. It comes up and regulates for awhile, but then a bit later overheats. BTW, I've ordered a 2nd new radiator cap, as possibly the 1st new one was not working correctly. Thanks. Nov 11, 2019 at 7:02

I. Solution: I had the wrong thermostat. Not sure how this happened. It was too long and even though it tested okay on the stove in a pot of water, it didn't have the space to open properly in the engine. So problem solved, finally! Thank you to all for looking at this.

II. To help diagnose an overheating engine, brake it down into these steps:

  1. First, the system must actually be overheating, i.e. you're not being fooled by an inaccurate gauge. Steam from the cap is a solid indication of actual overheating.

  2. There must be the correct fluid in the system. Water by itself conducts heat best, but will freeze. 100% glycol won't transport enough heat. So the mix is normally something like 50% of each.

  3. The fluid must be flowing, (i.e. the pump must be pushing the fluid, the thermostat must be the right one, and open wide enough, and the pathway must be otherwise clear in that there are no blockages).

  4. The engine must be conducting it's heat to this fluid. To accomplish this the fluid must not have, nor get, bubbles in it. As the fluid begins to heat up, it expands. This creates internal pressure. A spring in the cap holds this extra pressure in and regulates it to a maximum pressure. This pressure forces the fluid to boil at a higher temperature, thus allowing the engine to work a bit warmer than water would normally allow at atmospheric pressure. Thus the fluid does not develop boiling bubbles in it at normal working temperature and pressure. But if the pressure drops because something leaks this pressure out, like a leaking cap, or hoses, etc., then the fluid can develop bubbles in it as it starts to boil against the hot parts of the engine. Gas doesn't conduct heat nearly as well as fluid, so these bubbles reduce the fluid's ability to carry heat off of the engine.

    Also the fluid must not have any bubbles in it for any other reason. As the explosions are occurring in the cylinders, this exhaust gas must not be leaking or getting forced into the coolant because of a leaking head gasket or a cracked or improperly machined head/block surface, etc.

  5. The radiator must be cooling the fluid. In other words the fan must be blowing air across it, and the radiator must conduct the heat out of the hot water and into the blowing air.

So in summary, it has to have the right flowing fluid, under pressure as set by the cap, and it must not leak in or out, and the radiator must be cooling the hot fluid.

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