CRD cooling system

BoarX · **Joined:** Sun Mar 20, 2016 12:44 pm **Posts:** 127

Looking at different sources I am seeing contradictory information on KJ CRD cooling system operation. Is it supposed to remain under positive pressure even on the cold engine??? Can someone please pop his coolant/degas tank cap and verify? In my situation it looks like $1000+ question!

Thanks!
A.

flash7210 · **Posted:** Fri Aug 25, 2017 10:27 am

No.
The cooling system is not supposed to retain any pressure when cold.
Under normal conditions, the water/coolant heats up and expands. This liquid expansion combined with vapor pressure in coolant tank pressurizes the cooling system. This pressure in the cooling system actually raises the boiling point of the water/coolant mixture.
The radiator cap is designed to hold this pressure up to 16psi.

When you are done driving for the day, the engine cools down to ambient temperature, the liquid contracts, and the vapors condense. Pressure in the cooling system returns to normal atmospheric pressure.

If the cooling system is still under pressure when cold, some other force is adding pressure to the cooling system (i.e. combustion gasses).

You would think that even with a head gasket leak, as the engine cools down that excess pressure would relieve itself through the leak point. This is not always the case.
Head gaskets leak because the head bolts AND gasket are not able to hold back the high cylinder pressures of combustion.
When the engine stops running, there is no more cylinder pressure, head and block cool and contract, and the leak almost seals itself back up.

The definitive test for a head gasket leak:
1. wait for the engine to cool down
2. pressurize the cooling system to 16psi with a cooling system pressure tester
3. remove all 4 glow plugs
4. unplug all 4 injectors
5. wait about 15 minutes
6. crank the engine
If water shoots out the glow plug holes, its time to pull the head.

WWDiesel · **Posted:** Fri Aug 25, 2017 1:49 pm

Let me add a little bit more to Flash's good description!
He is correct in the fact that the cooling system on any modern vehicle cooling system will not have any pressure after it has completely cooled to ambient temperature.
If it does, there are some other issues causing the pressure!

But there is more to it; as it heats the coolant up to the normal operating temperature, the liquid will expand in volume, if there is not enough space or volume capacity in the coolant tank as this heat expansion takes place to store the heated and expanded liquid, the radiator cap will allow any excess volume to bleed over into the smaller expansion overflow tank.

Now, after the engine is shut down, the coolant temperature will rise somewhat and the volume will actually slightly increase even more as there is no water pump action to circulate the hot liquid. Although some liquid movement will continue through the radiator as long as the thermostat remains open due to natural heat convection circulation.

As the liquid (coolant) starts to cool down as it cools all the way to ambient temperature, the volume of the liquid will actually start decreasing (shrink) in size and at some point actually creates a vacuum in the coolant expansion tank which will pull or suck any excess liquid back into the system from the expansion overflow tank if there is any.

All this back and forth movement is controlled on every temperature cycle of engine startup / shutdown /cooldown by the functions of the radiator cap! So you see a well functioning radiator cap becomes very important in the total overall proper operation of a healthy coolant system!

See this thread for diagrams of coolant flows and its ancillary components on the 2.8 VM diesel engine...
viewtopic.php?f=98&t=87021

:SOMBRERO:

BoarX · **Joined:** Sun Mar 20, 2016 12:44 pm **Posts:** 127

Thanks guys, you got my last doubt eliminated!
Good weekend everyone!
Alex

BoarX · **Joined:** Sun Mar 20, 2016 12:44 pm **Posts:** 127

Ok, while I am having fun disassembling the engine for the head gasket replacement, here pops yet another question about the CRD cooling system: what is the function of the electrical fan mounted in front of the AC condenser? Is it just for the AC operation or does it also complement the main clutch-coupled mechanical fan to cool the engine as well? Is it turned on/off by ECU or some independent sensor?

Thanks!
A.

GordnadoCRD · **Posted:** Mon Sep 04, 2017 12:46 pm

When you are driving at 30+ MPH, neither fan is making much of a difference unless you are towing something uphill.

When you are stopped in traffic, and in gear, the front fan becomes necessary because:

1) the engine fan isn't drawing a lot, as you are idling with your foot on the brake.
2) the air movement the engine fan does create, is as likely to be coming from the top, bottom, and sides of both fore and aft of the CRC core.
This leaves almost no movement at all through the AC condenser.

Because of this, you can see that the e-fan's primary purpose is to keep the AC system happy, and any benefit it ads to the other systems is gravy.
The fan should be on -low speed- at any time the AC compressor is running. High speed, I believe, is commanded by the ECM using the temp sensor.

flash7210 · **Posted:** Mon Sep 04, 2017 4:25 pm

BoarX wrote:

Ok, while I am having fun disassembling the engine for the head gasket replacement, here pops yet another question about the CRD cooling system: what is the function of the electrical fan mounted in front of the AC condenser? Is it just for the AC operation or does it also complement the main clutch-coupled mechanical fan to cool the engine as well? Is it turned on/off by ECU or some independent sensor?

Thanks!
A.

I removed my clutch fan over 100k miles ago and never put it back in. I only use the electric fan for cooling.
It's worked just fine for me and the engine has never overheated, even in a Florida summer.
I've also wired in a switch to turn it on manually if necessary.
But I almost never tow and don't have any mountains to drive up. So this may not be the best thing for all conditions.

CRD cooling system

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