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I have a 99 Nissian Altima! Fans work,Thermostat opens and closes,but my job is about 22 miles from my house and i take the highway; on the highway my cars temp stays on normal with no flunctuation.(The cars coolant does not leak at all and i do not have to keep refilling!) once i get off thw highway and stop at a redlight and proceed to accelerate the temp starts to increase and wont go back to normal at all. The bottom radiator hose after driving to work is cold to the touch and has no pressure but all other hoses are hot and carry pressure? I also noticed that my coolant resivoir is bubbling? Need help and dont want to spend money for a mechanic if i can do something like this myself? Can you please help?

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  • I would be suspicious of a clogged radiator. What does it look like inside (if you can see)? You could also have an impeller on your water pump which has separated from the shaft (on some engines this happens more frequently than others). If you stop mid-way on your journey and check the lower hose, what is the temperature like? It could be that at highway speeds the impeller is kept spinning enough, but at lower speeds there is not enough flow. A passive IR thermometer would help you get temps through your system as you diagnose.
    – mongo
    Commented May 3, 2017 at 15:17

3 Answers 3

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Here's my most probable diagnosis, based upon your symptoms as described: intermittent failure of the water pump. Sometimes impellers will separate from their shaft. They still bounce around, especially at higher RPMs and may provide some pumping action. At lower speeds, the impeller just glides over the shaft, with no resultant pumping action.

If you can take off the radiator cap off WHEN THE ENGINE IS COLD and check the flow after the engine warms up, that may give you an idea as to whether the coolant is getting circulated.

My next most probable diagnosis is a partially clogged radiator. Not as likely, because the lower hose would still have some warmth.

Since there is no loss of coolant, radiator caps, head gaskets, etc. are even less probable.

Add: A third possible, but not as likely cause is the collapse of the internals of a coolant hose. I say not as likely because at cruise, the draw would be the greatest, and therefore the propensity to collapse would be the greatest. That doesn't fit the stated symptoms.

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There aren't a lot of things that cause overheating.

  1. Insufficient coolant. But, you said it's full.

  2. Air pockets in the system. This happens when the coolant is replaced incorrectly.

  3. Bad head gasket. Check your oil, in the pan not just on the stick. If it looks like chocolate milk, you need to stop driving it and replace the gasket. $$$$

  4. Fans not running. Without air flow over the radiator, it's much less effective. Also, make sure there isn't any trash stuck in the front of the radiator. It happens.

  5. Coolant isn't moving. In order for the system to work, coolant has to move - via water pump and open thermostat - from the engine to the radiator and back.

So, assuming that you haven't replaced the coolant lately, and hoping for a good head gasket, and you're sure the fan is coming on, focus on coolant flow. That can be interrupted for a number of reasons, including:

  • Bad water pump. How many miles on the car?
  • bad thermostat
  • as mentioned in another answer, blocked radiator or hose.

It's a pretty simple system.

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There are a number of things that can cause this.

A coolant system pressure test will help you determine a lot. If the system can not hold pressure, it will boil. That can be caused by something as simple as a failed radiator cap, or complex as cooling system blockage or needing the water pump replaced.

Radiator caps are very inexpensive. I would start with that. If the problem persists, seek a pressure test.

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  • He says he is not loosing coolant. If that is true, how would holding pressure be an issue? Coolant can boil under a 15# pressure cap, just at a higher temperature. That is consistent with his symptoms.
    – mongo
    Commented May 3, 2017 at 15:26
  • Because the cap may not have completely failed. It may be holding some pressure, but not enough to keep it from boiling into the expansion tank.
    – CharlieRB
    Commented May 3, 2017 at 15:28
  • If there was going to be boiling, then it would occur during cruise, when the waste heat from the engine is higher. He doesn't have that. He has boiling at shutdown, which tells me there are hotspots in the engine. And his boiling problem is not severe enough to loose coolant, as he says he does not have to replenish it. Most engines will run just fine without pressure caps, the increased pressure helps deal with temperature surges, hotspots and the sealed cap helps seal the coolant system from leaks. He reports no leaks.
    – mongo
    Commented May 3, 2017 at 15:34
  • I've experienced what the OP has described. In my case a radiator cap (which cost me $6.00) fixed the problem. You surely know there is no way to definitively diagnose and resolve problems over the internet with information given. And you also probably realize there are some issues which can have multiple correct answers. The OP is only one who can say for certain what fixed the problem. Let's let the OP do that.
    – CharlieRB
    Commented May 3, 2017 at 18:39
  • I am fine with that, and I certainly agree that a radiator cap is easier to swap than a coolant pump. I would suggest that the OP give that simpler fix a try. Nevertheless, given what the OP tells us, I am inclined to see a zebra, rather than a horse (grin).
    – mongo
    Commented May 3, 2017 at 19:54

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