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Discussion Starter #1 (Edited)
Some information on the X cooling system including the easy way to add the necessary amount to improve cooling system and heater performance. I intentionally removed coolant from a properly filled system, 2 quarts one time and then 2.5 quarts. More information will be given later.

The only way to accurately observe the cooling level is when the engine is running, at any other time the visible level in the reservoir may give a misleading high reading. Also, it will be higher when hot than cool. The only time you can get an accurate level is when the engine is cool and running. The safest way to add coolant is when the system is cool. Keeping the coolant level up to the MAX level ensures maximum cooling system and heater performance.

With the system 2 quarts low this is the level in the reservoir with the engine hot and on. The indent to the left of the MIN text is a baffle in the reservoir. The outlet of the reservoir is at the bottom right of the chamber.



Hot and off, it is essentially the same after cooling off.



After cooling off and starting the engine. The baffled chamber is empty.



The following procedures to add coolant should take less than 5 minutes to do if you have the coolant mix ready. It may take two or 3 time to completely fill the system but you don’t have to wait for anything to happen. The system will purge and separate air naturally as coolant flows.

If after the engine heats up and your heater isn’t blowing hot air at an idle but is blowing hot air at higher engine speeds, the first thing to check is the coolant level. Check the level in the reservoir with the engine hot and running. If the level is low and the heater isn’t working properly, this is my recommended procedure. This will get the system close to full and should only need to be topped off after running for a while. Remember, the engine should be cool.

1. Have a 50:50 mix of antifreeze and water available, at least 3 quarts. A funnel will help but isn’t necessary if you are able to pour into the small opening of the reservoir and the radiator. Ensure that the pressure relief cap is on the reservoir and the plain cap is on the radiator.
2. Remove the radiator cap, fill the radiator to the bottom of the neck, and replace the cap.
3. Remove the reservoir cap, add coolant to the MAX line, and replace cap.
4. Drive normally for at least 5 miles after the engine has reached operating temperature before proceeding to complete topping off the system.
5. Even if the heater performance has improved, the coolant level is probably below the optimum amount. Finish filling with the next procedure.

Use this procedure for checking the level and just topping off the cooling system. The engine should be cool.

1. Have a 50:50 mix of antifreeze and water available, at least 1 quart. A funnel will help but isn’t necessary if you are able to pour into the small opening of the reservoir.
2. Start the engine and leave running.
3. Check the level in the reservoir.
4. If low, fill to the MAX line.
5. Replace the pressure relief cap and close the hood.
6. Drive normally.
7. After cooling off for at least a few hours or the next day, repeat starting at step 2. If the level is up the MAX line, you're done.

The cooling system on the X is a closed system that doesn’t normally breathe or release pressure during heating and cooling cycles. The reservoir is part of the pressurized volume and acts as both a container for the increased volume of liquid coolant as the temperature increases and a gas separator for liquid coolant. Some coolant normally flows from the top of the radiator through the reservoir to the return path to the inlet to the water pump when the engine is running, whether the thermostat is open or closed. There are baffles in the reservoir to encourage separation of any air mixed with liquid coolant if there is sufficient liquid coolant present.

The pressure relief cap on the reservoir doesn’t release pressure in the system during normal heating. This is different than more common systems that use an overflow bottle at atmospheric pressure. They are filled with liquid and the pressure relief cap opens regularly during heating and allows the coolant to flow into the overflow bottle. As the system cools and the pressure drops the coolant flows from the bottle back into the pressurized volume. These types of systems are normally filled with liquid and don’t have a gaseous volume above the liquid. The pressure build up is more a function of the liquid expanding rather than just the vapor pressure increasing as the temperature increases.

The pressure relief cap on the X has a 16 psi relief pressure. The cap should stay sealed with a 50:50 water to antifreeze mix easily past 250F. If the cap is releasing pressure at a lower pressure then both air and gaseous coolant, both water and antifreeze, are being released from the system. A small volume loss of coolant won’t affect the cooling performance of the system. But if the loss continues it will result in higher temperatures which will accelerate the coolant loss. A leak in the system will have the same effect.

My cooling system was filled to the MAX line on the reservoir when cold. I removed 2 quarts of coolant from the reservoir and drove the X several times from a cold start. The heater didn’t work properly and behaved the way many people have commented. At an idle the heater temperature dropped and then would increase when the engine speed was higher. In addition, I monitored the coolant temperature with SuperChips and under light load the temperature would quickly climb to 210F under light load but would cool off to 190F at idle. This was in 35F to 45F air temperatures. The factory temperature gauge increased to the 4 o’clock position at about 160F and stayed there up to 210F.

After sitting overnight I started the engine and added coolant to the MAX line. I drove it long enough for the thermostat to open and let it sit for a few hours. Then started it and added more coolant, there was a small amount left in the bottle when it was filled to the MAX line. Since I knew it was the correct amount I just added the remainder. It was less than ½ inch above the MAX line. It was then driven for a while. The coolant temperature ran between 190 and 197.

After cooling off 2.5 quarts were removed from the reservoir, as much as could be removed. The engine cooling and heater performance was noticeably worse than when 2 quarts were removed. Hot air from the heater would cool off very rapidly at an idle, and even when driving slowly or coasting. The coolant temperature got up to 210F during light load driving with the ambient temperature below 40F, but the electric fan did turn on. The level in the reservoir looked similar to the levels observed at 2 quarts low.

When the engine was cool it was started and about 1.5 quarts was added to fill the reservoir to the MAX line. It is still about 1 quart low based on what was removed. The cooling and heater performance were noticeably improved. The heater would cool off slightly at an idle but I’m not sure I would have noticed if I wasn’t looking for a change. Coolant temperatures were more in line with the full system.

The level in the reservoir is random and misleading at any time other than when cool and running. On one cool start the baffled chamber completely emptied at the start but then filled to the MIN line. The level is at or just above the MIN line running.

1 qt low, Engine cool and off.



1 qt low, Engine cool and just started, baffled chamber is empty



1 qt low, Engine cool and running, coolant level is higher



1 qt low, Engine hot and off




Next step was to add coolant to bring the level up to the MAX line, almost 1 quart. There is still some coolant left in the bottle. There is clearly enough coolant now to be considered full. I’ll do the cool fill tomorrow just to see if the level has settled with air separated out of the system

Some thoughts and Observations

The cooling system is essentially a closed volume with two phases, liquid and gas. The liquid is a water and antifreeze mixture and the gas is an air and coolant vapor mix. If the system is in good repair and properly charged with coolant liquid will be circulated by the pump and gas will collect in the reservoir chamber above the liquid. The reservoir is baffled to separate gas trapped in the liquid and prevent it from flowing back to the pump.

If there isn’t adequate liquid, the gas will continue to circulate with the liquid and compromise both the cooling function to remove heat from the engine and heater core’s function to provide hot air. The reduced performance of the heater is probably the first indication that there is a problem.

From observing the system with reduced coolant it is obvious to me that air is circulating through the system, not collecting in some pocket and preventing flow. If it was collecting and preventing flow, an increase from 600 to 1200 rpm wouldn’t clear the pocket nor would slowing from 1200 to 600 rpm allow enough time for air to collect. The temperature change of air from the heater occurs to quickly for air to be collecting in the core. If the is air circulating, it is directed to the heater core from the manifold at the back of the heads before it can reach the radiator. From there it circulates back to the pump. Without adequate liquid the air will continue to circulate

It takes less than 1 quart to fill the reservoir form the MIN line to the MAX line. After removing 2 or more quarts from the reservoir, the level in the reservoir hot is still around the MIN line. That means there is at least 1 quart of air circulating with liquid. Without installing clear hoses to view the coolant flow it is not possible to positively say where the air is distributed.

After adding coolant to the reservoir it does take some time for the air to separate and collect in the reservoir. Just driving the X is the easiest way to let the separation occur. The engine speed will be higher and will allow the separation to take place more efficiently than just letting it idle. Plus, you aren’t wasting time waiting for the separation to occur while staring at the engine idling. After driving for a day, just add more coolant and drive.

If you are having to add coolant regularly there is a problem that should be corrected. The way the X cooling system is designed there should be minimal loss over time, the system is essentially sealed during normal use. If the system is leak-free and the pressure relief cap is holding the design pressure, the system shouldn’t be releasing air and vapor. The system will pressurize during heating and will de-pressurize on cooling without venting to the atmosphere.

If you’re adding 1 quart a year I’d check the system carefully for leaks. I’ve had leaks occur at the gasket between the radiator core and the header tank. They only leaked when the engine was cold and would seal after heating up. The small amount of coolant lost would evaporate before dripping off of the radiator.

A pressure relief cap that is either leaking at the seal or isn’t holding the design pressure will release coolant over time in the form of vapor. There may not be evidence of coolant being lost so this type of failure is harder to detect. New quality pressure relief caps aren’t that expensive and are readily available.

Jeff
 

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Yet another helpful post, Jeff ... thanks. I'll probably need this later, but haven't looked into it at all yet.

Hope I'm not the only one that appreciates your thorough and detailed how-tos and informative posts ... though I do feel pretty secure in saying that I'm not.
 

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Thanks so much Jeff. I appreciate the time and quality you put into this post. I'm going to check mine here at lunch time since the X is still new to me. Plus its great to get a understanding of it so I can diagnose and fix this myself if need be. Definitely bookmarked and printed for my X folder :)
 

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Discussion Starter #6
Thanks everyone. I've added the results of running with reduced coolant and some observations based on the testing.

Jeff
 

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thanks for this. i was having the rushing water sound from the dash, and decided to just fill the overflow tank to the top and let it sort the rest. i guess it worked since i no longer have the toilet flush sound from the dash.
 

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Discussion Starter #10
whats all the spaghetti wire all over the place????
Secret links to the 5th dimension to improve fuel economy. I get at least 40mpg driving in the mountains.

Seriously, they're thermocouple wires for monitoring ATF temperatures at various positions.

Jeff
 

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Those look like thermocouples - used to get temperature readings. I could be wrong. I'm never wrong though. Well, there was that time I thought I was wrong, but it turned out I was right. So yeah, I was wrong about that.
 

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Discussion Starter #14
I modified the first section of the first post. After completing the third and last addition of the removed coolant, a revision to my original post seemed in order. It isn't a major change and it will just make it easier if the coolant level is low enough to affect heater performance.

Jeff
 

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I had an issue with no heat, unless i was driving on the highway or up a steep hill. Then it would get hot. As soon as i would let off the gas, it would immediately go cold again. I checked the forums and found that low coolant could be the cause, and found that my reservoir coolant level was at the minimum mark. I didn't worry too much about it as it wasnt cold at the time (Octorber) and i had some other issues with the Xterra that i was focused on.

So i just followed the directions in this post today, and topped up the radiator itself and the reservoir to the max mark. It took about 1.5 litres of 50/50 coolant-water mix. Now can feel heat from the vents (although i also just developed an issue with the blower motor not working).

Anyways, my question is, if i was down that much coolant, would i have been doing any damage to the engine at all? Would there have been enough coolant to circulate around the engine to prevent overheating? My temperature gauge was always normal, around the 4 o'clock position. But i read something about the coolant temperature sensor being located high in the system and maybe it wasn't getting immersed in coolant if there wasn't enough in the system, so that perhaps some parts of the engine were actually getting hotter but it wasn't registering with the gauge.

Any thoughts would be appreciated, thanks!
 

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I had an issue with no heat, unless i was driving on the highway or up a steep hill. Then it would get hot. As soon as i would let off the gas, it would immediately go cold again. I checked the forums and found that low coolant could be the cause, and found that my reservoir coolant level was at the minimum mark. I didn't worry too much about it as it wasnt cold at the time (Octorber) and i had some other issues with the Xterra that i was focused on.

So i just followed the directions in this post today, and topped up the radiator itself and the reservoir to the max mark. It took about 1.5 litres of 50/50 coolant-water mix. Now can feel heat from the vents (although i also just developed an issue with the blower motor not working).

Anyways, my question is, if i was down that much coolant, would i have been doing any damage to the engine at all? Would there have been enough coolant to circulate around the engine to prevent overheating? My temperature gauge was always normal, around the 4 o'clock position. But i read something about the coolant temperature sensor being located high in the system and maybe it wasn't getting immersed in coolant if there wasn't enough in the system, so that perhaps some parts of the engine were actually getting hotter but it wasn't registering with the gauge.

Any thoughts would be appreciated, thanks!
Gauge never moved, doubt it ever got that hot.
The air bubbles would be in the top of the cooling system. That is the top of the heater and the top of the radiator. If you tried towing a trailer, up a mountain, in Arizona, in the middle of summer, as fast as it could go, then you may have had issues. For as little as you were low it wasn't that big of a deal to the engine.
 

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Hey Jeff,

I read your post a couple times and have topped off the coolant levels to the proper capacity IAW your post however I still only have warm air blowing out when hitting the gas (cool air when idling). I have also replaced the radiator (and topped that off) as well as tried to have the system flushed a couple times to try and rid the air bubble. I have since been told that it is probably the heating core?

Any suggestions on how to determine if it is actually the heating core without taking apart the whole dash and getting to the heating core itself. i was sure it was just coolant problems but believe i am at the max level in all the resevoirs when egine is running and cool?

also, is it possible to have too much coolant in the resevoir which would result in similar failures in heater performance?

thanks guys.
 

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Discussion Starter #18
Hey Jeff,

I read your post a couple times and have topped off the coolant levels to the proper capacity IAW your post however I still only have warm air blowing out when hitting the gas (cool air when idling). I have also replaced the radiator (and topped that off) as well as tried to have the system flushed a couple times to try and rid the air bubble. I have since been told that it is probably the heating core?

Any suggestions on how to determine if it is actually the heating core without taking apart the whole dash and getting to the heating core itself. i was sure it was just coolant problems but believe i am at the max level in all the resevoirs when egine is running and cool?

also, is it possible to have too much coolant in the resevoir which would result in similar failures in heater performance?

thanks guys.
Is the coolant level at the MAX level in the reservoir when the engine is running? Is the temperature gauge in the middle of the gauge? If the temperature gauge is normal, the heater core may be faulty. If the gauge is reading low the thermostat may be faulty and need replacing.

Jeff
 

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thanks jeff. yes the coolant level is at the MAX when engine is running and the temp gauge maintains in the middle normal range.

i think it may be the heater core but there is no way of checking except to take the whole dash apart and the replace the heater core so i am trying to cancel out everything else beforehand.
 
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