Coolant Loss. Dip stick clean, and no leaks

[88towmanche]

Yep - the Astro was chevy and used the orange DEX-COOL

 

The way I remember it working was you pour the bottle into the radiator and continue normal driving and just check the exhaust with a blacklight flashlight (don't remember if there was a set amount of miles that had to be driven or what) but eventually I could see residue on the tail pipe using that little flashlight showing that at least some coolant was coming through the exhaust.

 

The whole test cost like $20 with the flashlight and gave me some answers...I bought more of the dye fluid and then started trying different head gasket in a can to see if I could stop or slow down the leak...had some success, I'd pour a can of Kseal or something in there and then check with the dye for a while to see if there was still coolant coming through - sometimes it would be clear for thousands of miles, other times wouldn't get that many - told the guy I sold it to all about it and gave him the flashlight.

 

That old Astro never flinched and I abused it with heavy loads long after I notice the problem...didn't consume any oil or overheat and I went from like a pint of coolant loss every 1000 miles to maybe a pint every 5000

[AnotherOldJeepGuy]

38 minutes ago, 88towmanche said:

told the guy I sold it to all about it and gave him the flashlight.

[AnotherOldJeepGuy]

OK got my additive and flashlight, added the dye in today.  I made one mistake I'll note here for the benefit of anyone referencing, I neglected to snoop around with the flashlight before putting in the dye.  Why does that matter?  It matters because the black-light flashlight doesn't only work on the dye!  I can walk around my house and garage nowhere near my Jeep, and see things that will light up.  So, while I do see some specs here and there in my engine compartment, I really don't know they were not already there.  So for future reference, if you are doing this look around before you put the dye in and make note of any thing that already lights up!  If you see anything, if you can, clean it off so that it doesn't shine, then later if you see a shine you will be sure it came from the dye!  For me, this just means I need to clean up any specs I see, and drive around again.  I learn something new every day on this "keep my Jeep running" journey!

[AnotherOldJeepGuy]

Turns out I have quite a lot of specs that light up, and I expect a lot of them are not really something that can be cleaned away.  But what I was able to determine was that the only glow that matched the expected glow was a small area at the thermostat housing.  I was able to match this to the expected color by swabbing the inside of the dye bottle with a q-tip and comparing any glow I could find to the glow on the q-tip.  This area at the housing is not an active leak, I can't see it leak.  That housing was pretty ruff when I replaced the gasket a few months back when I put in a new water pump, and it did have to do it more than once to get (I though) installed with no leaks.

 

This leak appears to be so small that I don't have a lot of confidence it is my only coolant loss, so the additional chemical analysis suggested will most likely still be done, but I'll try to eliminate this known leak first and see what happens.  (note it's all wet because of some cleaning)

[AnotherOldJeepGuy]

Data Collection #1 complete.  I used 12oz. of fluid in 379 miles.

 

I described my data collection and seep-only leak (no visible dripping or residue) to my favorite ai companion (duck.ai), and asked if the amount of coolant loss I measured could reasonable be expected for a seeping leak at the thermostat housing over the miles seen.  The duck's math suggested it was.  I don't really know if that's accurate or not, but, I still plan to try and eliminate the known leak, retest, and go from there....

 

[89 MJ]

I wonder if it’s leaking more when the engine is up to temp and the cooling system is pressurized. 

[AnotherOldJeepGuy]

3 hours ago, 89 MJ said:

I wonder if it’s leaking more when the engine is up to temp and the cooling system is pressurized. 

I've watched while the temp came up from cold to around 190 and haven't been able to see it.   But since the dye is showing, and that's the best clue I have, I'll work on what I can find and see what happens.  When I have the thermostat housing off a few months back that housing wasn't in very good shape, and a new one isn't expensive so I guess I'll replace it and see what happens.

[AnotherOldJeepGuy]

So since I am going to put in a new tstat housing anyway, and as I look at Rock Auto for parts, I am wondering what the experts say about a few things.

 

Thermostat:

1.  180 or 195 Degree Thermostat?  The Rock Auto comments suggest 180 might be better for older/classic vehicles.

 

2.  "High Flow" Thermostat?  The Rock Auto comments suggests "a reduction in engine warm up times, allows trapped air in the cooling system to escape, and stabilizes the pressure of the cooling system overall".

 

3.  "Fail-Safe" Thermostat?  These have a lock-open feature in an overheat situation.  I see less useful here, if I get an overheat I'm not planning to keep driving so other than an emergency that would demand it, I don't know that I am interested in this.   I also note that I don't see both "Fail-Safe" and "High Flow", so if High Flow is a good thing, that's another reason I would not use the Fail-Safe one.

 

Gasket:

4.  On gasket, when I replaced my valve cover gasket it was Fel-Pro.  The instructions specifically noted to not use sealant so I didn't and it is sealing fine.  I tried to find the instructions that would come with the Fel-Pro thermostat housing gasket to see if they said the same thing but did not find that.  I did ask my ai buddy and it said to install it dry unless there were lots of pits or imperfections.  So, does the community agree with this, that is, if I get the Fel-Pro thermostat housing gasket, does it work without sealant?

[87MJTIM]

I installed the felpro T-housing gasket w/o rtv on my 93 4.0 in my CJ. No leaks. 

[fiatslug87]

195° t-stat, regular flow

[AnotherOldJeepGuy]

On 7/28/2026 at 10:14 AM, 87MJTIM said:

I installed the felpro T-housing gasket w/o rtv on my 93 4.0 in my CJ. No leaks.

I'll go with a Fel-Pro!

On 7/28/2026 at 10:19 AM, fiatslug87 said:

195° t-stat, regular flow

Got it!

 

Rock Auto sells a Steel T-housing by Motorad (CH4802) and an Aluminum from FourSeasons (84802).  Any benefit to one over the other?  Is the original steel or aluminum?  I didn't really take notice with I had it off last time.

 

EDIT: Duh, all you need is a magnet to answer that last one, it appears what is there now is aluminum because my magnet doesn't stick!  So I'll order the aluminum replacement.