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Old 01-13-2021, 10:06 AM   #1
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“Burping” engine after heater removal

Hi all!

This is my first post and excited to be apart of the community.
I need help!
I’m converting a bus with a CAT 3126 engine and have removed the heater, using barbed union to reroute the tubes into themselves and drained excess coolant. I know how to top off coolant, however my uncle told me when he stopped by that the air would need to be “burped out” before driving the bus again. My boyfriend has looked up videos but is having issues and hasn’t been able to figure out how to remove air. Has anyone been through this - is it necessary and how do we go about it?
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Old 01-13-2021, 10:34 AM   #2
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Year: 1984
Coachwork: Bluebird
Chassis: International 1753
Engine: 6.9 International
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It will help if you let us know the details about your bus, like whether it is a conventional, FE, or RE.

To "burp" it, all you have to do is hold the engine at a high idle until the thermostat opens. Once it opens, any trapped air in the heater loop or engine will be expelled to the radiator, where you would then top it off with coolant.

Some buses with expansion tanks don't need to do this. And some with booster pumps in the heater circuit need this pump on to get all the air out.

One can argue about it's necessity, but it's not hard to do, so you might as well do it. Afterwards, keep an eye on coolant level and top it off when necessary, it can take several drives before all the air will be purged and the coolant level stabilized.
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Old 01-13-2021, 11:55 AM   #3
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Join Date: Sep 2017
Location: Idaho
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Year: 1998
Coachwork: Bluebird
Chassis: TC2000
Engine: Cummins 5.9/Allison MD3060
FWIW it took forever for my Cummins 5.9 to finally burp all the air out. I thought I was done, then after a drive on Saturday I was a bit low on coolant. I hope I'm finally done, but I didn't notice any big issues with a couple bubbles in the system.
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Old 01-13-2021, 02:13 PM   #4
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Year: 1986
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I remember having to bleed out air from the cooling system in the "old Crown" after replacing a split hose. It is not difficult but being a smaller person is better. The engine's cooling system simply would not fill up until that was done. The "old Crown" has a Detroit 671 with turbo.
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Old 01-13-2021, 09:09 PM   #5
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It is vital that the engine cooling system is full of coolant -- water & anti-freeze -- with no significant pockets of air.

As mentioned already, you can run the engine and top off the water until you are certain.

Most thermostats have a small bypass hole, but I like to drill it out a bit. And two of them, top and bottom, if the thermostat is mounted vertically, as is the case on Millicent's Cummins 5.9.

Some buses have bleeder valves at the high points of heater hoses, where air becomes trapped when the hose is filled. Such bleeder valves can certainly be added.
If you still have heater hoses running over the left front wheel-well in a flat-nose bus, that's a high spot right there.

The in-dash heater may be a big one, depending on the bus.

Be sure to open all heater valves while "burping" the system.

Some years ago, I had replaced Millicent's water pump, and we were in a hurry to leave on a trip. Yup.... After about a mile, she overheated rather seriously.
That was when I added holes to the thermostat, to speed "burping".
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