Inexplicable Coolant Loss

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May 10, 2005
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Last winter, in the dead of winter in February, the coolant level is the surge tank started going down. I had to put in a half litre of coolant every 500 kms. This only lasted for less than a month and the truck stopped using coolant after that. I carefully checked all the hoses for leaks, checked the tailpipe for wetness and found nothing. No drips from the drain pipe for the heater core compartment. I ordered a heater core from Rockauto, figuring I would replace the core come spring after the weather moderated. In April I pressurized the cooling system to 15psi with compressed air and watched the drain tube from the heater core compartment and did not see a single drop. I have not replaced the heater core yet.

I never smelled any coolant in the cab. I use Dexcool, replaced every five years. No white smoke from the exhaust. I just went and checked the coolant level, it is full and I have not topped it up in four months.

Wondering if I should just replace the heater core. It is a fairly involved job, requiring dash removal.
 
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A Barrier Island
If it ain't broke why fix it? But on the other hand, better do it now than when it's 20 below.
+1 Good advice.

Long ago when I lived in the rust belt it seemed to me that once the winter clamped down on you the chances of a mechanical failure increased astronomically.

Winter.jpg
 
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Dec 19, 2013
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Fort Worth, Texas
GMT800 trucks are not known for Heater Core failures, The plastic heater hose fittings are known to leak/seep then one day blow off altogether. And the seepage can be temperature dependent.

Before anyone says Intake Gaskets.....'06 4.3L's have updated gaskets.
 

George7941

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"Possible air pocket(s) worked its/their way out? No way I would do that much work unless I was ABSOLUTELY SURE the heater core was bad…"
Don't think so. It has been over a year since I last worked on the cooling system

"The plastic heater hose fittings are known to leak/seep then one day blow off altogether. And the seepage can be temperature dependent."
One of the two heater hose fittings started to leak about a year and a half ago. I posted about it and followed Chris142's advice and got rid of the Quick Connect fittings and clamped the heater hoses directly on to the heater core barbs.
 
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Mar 19, 2022
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Any kind of coolant leak is found with a long term (a day or 2) pressure test. Remove spark plugs if the pressure drops and you don't see leaks externally. Look into the cylinders with a bore scope if necessary for coolant.

Check the oil for any froth, which would be coolant in it.

If it holds pressure for 2 days then, don't worry about it!

A heater core is very easy to troubleshoot, the cab will smell like coolant with the heat on.
 
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I had the same problem with one car and brought it to a shop to pressure test overnight, it turns out the leak is in the coolant bypass hose between the engine and the firewall. Do you have a pressure test tool and a way to test it "overnight"?
 

Astro14

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Leaking o-ring or hose. In the extreme cold, parts like o rings shrink and you had a tiny leak that went away with warm weather.

The fact that you didn’t smell it suggests the leak is under the hood.

I reckon you’ll find it again when winter comes.
 
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Jul 13, 2003
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Tracy, CA
There was no indication of coolant loss. No drop in coolant level, nothing on the ground, maybe because of the engine under cover but I never noticed any leakage when removing the cover for engine oil changes.

Found this when I did the timing belt on the Lexus (at 96k miles). Seepage from the water pump weep hole.
20180822_203627_resized.jpg
 

George7941

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I have not had a UOA done in about a year, will do it at the next oil change. Oil on dipstick looks fine, with no hint of contamination.
 
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On the road Midwest
Just FWIW, this has actually happened to me. I was losing a little coolant and kept having to top up; no sign of wetness on driveway (clean concrete), no wet spot I could find on engine. Pressure check held for hours. Found the leak (seep) with a BRIGHT flashlight and a swivel-head 2 1/4" dia. telescoping inspection mirror, while the motor was warm and running. Bottom side of plastic elbow leaking very small amount which would evaporate off the top of the warm timing cover while engine was running. Seemed like it only leaked with the engine up to temp; I LOOKED at this spot with engine off and cold. I hate any plastic on any engine; especially on a pressure part. Maybe OK for a wiring hangar.
 
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D60

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{snip}I hate any plastic on any engine; especially on a pressure part. Maybe OK for a wiring hangar.
You should apply with Ford and help them 'cause they love it. There's the notorious 4.0 V6 plastic thermostat housing, and the 2.0 in the '08-11 Focii use a weird (and relatively huge) coolant manifold that inevitably fails.

In defense of the engineers, though, these plastic cooling system parts do last a surprisingly long time and much longer than any manufacturer would consider the "lifetime of the vehicle"
 
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