Inexplicable Coolant Loss

Jan 2, 2004
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"
The original hypothesis behind Dex-Sludge and sudden coolant loss on GMs in the 1990s-early 2000s is that Dex-Cool attacked the IMGs - GM used a glass-filled nylon gasket with a printed silicone bead. However, glass-filled nylon fittings have been used with modern coolants for a while and even the Japanese are starting to use them with their coolants. Toyota used plastic impellers on their water pumps in the 1990s and Mercedes was first to switch to plastic-tanked radiators - which all have stood the test of time but plastic gets brittle over time. A boneheaded move - using RTV to seal coolant passages. Toyota loves that practice - but silicones don’t tolerate plasticizer exposure, all the organic acid salts in today’s coolants are plasticizers(benzoate less so than sebacate/2-EHA).

If it was me, I’d do a overnight pressurize of my cooling system and add UV dye to the coolant beforehand, drive the truck. That way, you can find a leak where a mirror can’t pick up on.
Oct 26, 2008
Northern, NY
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.

If the heater core was leaking you would know it immediately due to the smell in the cab. I suspect you have a slow leak at one of the seams in the radiator. It is a common problem and rarely leaks enough to leave a spot on the ground.

As suggested above, add UV dye to the system and you will be able to find the leak.
Oct 28, 2014
On the road Midwest


Nov 13, 2020
And looked for a leak out of the heater core drain tube, quote "not a drip".
Guess it's not there.
Did he see if the pressure went down over time?
A proper pressure test is needed.