Hey all!

Both discos have been issue-free for several months for the first time in a long time! ...but that season might be coming to an end. The 2002 is having a weird cooling system issue I can't quite diagnose.

Recap: 188,000 miles currently. The cooling system, the entire thing except the heater core (which is still certainly gummed up with Dexcool remnants) is new and has been gurgle free for about a year. Then the ubiquitous waterfall noise began not too long ago. I saw that it was a bad water pump (leaking out weep hole). Installed a new Airtex. Problem solved.

Now: The expansion tank drops within a couple short drives from the cold mark to about an inch below it where it just sits and doesn't go below. This is not normal for the Disco. I'll fill it up to the cold mark and then get tons of waterfall noises for a drive or two. Check the level... an inch below. No more waterfall noise when it's at that level.

There isn't a hiss when opening the expansion tank cap. The system is bled. I've pressure tested it a few times and nothing ever leaks during the test. I've block tested with the chemical test to check for exhaust gases in the coolant. Results are always negative.

One suspicion I have, but not sure if it's related or not - there's a small bit of congealed coolant on the passenger side, rear of the block, sort of in the vicinity of the O2 sensor plug. But that's been there ever since I've owned the disco, and it's gone through seasons of no cooling system issues whatsoever.

I would consider myself at this point, as close to an expert on these Disco cooling systems as a non-professional can get. I've had them fully apart and know them pretty much inside and out. And I'm stumped. What am I missing here? It's so strange that ever since the water pump replacement, the disco seems to be happiest with the coolant level an inch below the cold mark.

 

I had some odd coolant level issues in my 03. Changed the expansion tank cap and that solved it. Didn't have the waterfall noises though.

Another time it was the bleeder cap on T of the coolant lines starting to fail. That would release pressure when hot and suck in air when cooling down. Thinking you would notice this with pressure testing.

 

If you are not building some pressure when the engine is hot I'd suspect that you have an air leak on a hose into/out of the heater near the fire wall. something that will suck in air but not really show any signs of weeping. You might just need some new clamps.

 

There is pressure when it’s hot. There’s no pressure release when I remove the cap when it’s cold, which is expected.

 

You answered your own question, no hiss when removing the cap but no leaks when pressure testing you have a bad cap. BTW, I run my caps loose one 360 degree turn so that the system does not build pressure, stressing your hoses and radiator.

 

This confuses me more though… there should be a hiss when opening the cap when the engine is warm. There should not be a hiss when opening the cap when the engine is cold.

Also, if for some reason this ends up being a bad cap after all, I’m going to lose my mind here… this is the fifth faulty cap I would’ve had to replace in three years. The previous 4 were lucky8 garbage, this one is OEM from the dealer.

 

Depends how tight your system is. It should hold the pressure that built up while the engine was running, at least for a few hours.

 

When I say I can remove the cap without hearing a hiss, when cold, I mean… it’s after the car has been sitting overnight. Like. Stone cold.

 

If your system is tight, you could still have pressure after sitting overnight.

 

I have added UV dye to my coolant and it makes it pretty obvious when it is leaking and where.

Based on your description it sounds like there is a really small leak somewhere that maybe doesn't leak until like 15PSI or something. With mostly coolant in the system, it builds up a bit more pressure and leaks out until there is enough air In the system to act as a buffer?

It is a bit odd that the pressure test is good.