Coolant Loss. Dip stick clean, and no leaks

[RG67MA]

I've got a 1990 with the 4.0L and the AW4 with the original closed cooling system. Early last year I replaced radiator because it developed a pin hole and installed a new Mopar thermostat.

It's been running great until now. I need to top off the coolant bottle every day. No coolant drips on ground. The oil dip stick only has clean oil on it. I do smell coolant from the engine area when I open hood.

Any ideas on what could be going on?

[RG67MA]

One more thing...I pressure tested the system yesterday and the pressure drops slowly, but no obvious leaks under pressure.

 

It had a 16 radiator cap on new radiator last year, but it would weep, so I replaced it shortly after (May 2024) with a 20 and the weeping stopped. It is not weeping from cap now.

[AnotherOldJeepGuy]

This seems like the right post to tack on to.  I am noticing the same thing.  I am starting some data collection, set a baseline a the midpoint between MIN-MAX on the overflow bottle.  This was set in the morning after setting over night, ambient temperature around 90.

 

Jeep is not overheating.  Drove around yesterday with AC on MAX, outside temp in the mid 90s.

 

Worth noting, about 3 weeks ago when I was working on my lengthy issue (MayDay!! - Hard Accel Killed Idle), one time and one time only when I started I notice a spurt of liquid from somewhere.  I never was able to recreate this and I cannot find any leak.  I've started many times since then.  I had recently tried spraying the engine with water testing for vacuum leaks and the only guess I have it it was because of that.  I have little confidence in this guess but with nothing else to go on, it's all I got.

 

I have a first question also.  I noticed that coolant is flowing at what appears to be a full rate into the overflow tank immediately at start from cold and AFAIK it never stops.  I don't really know what causes this to flow or not flow, and the only thing I know of that regulates coolant flow is the thermostat.  I put a new one in maybe 6 months ago when I put in a new water pump.  What controls when coolant flows into the overflow tank and should it be flowing at cold start?

 

[fiatslug87]

That’s not an overflow tank, it’s the pressure bottle. It should be filled to the top of the inner post when cold.

[AnotherOldJeepGuy]

Wasn't aware of the post, took a peak.  The level I have right now is pretty close, it's maybe 1/16" to 1/8" inch down so easy to top off and I'll start a new baseline.

 

Yeah my terminology is very often off a bit.  Pressure bottle sound good to me.  I've also see "Expansion Tank" and Dorman just calls it a "Coolant Reservoir".  But whatever name tag it's wearing, still wondering about the flow question.

 

[llhat]

IIRC,  the pressure bottle on the firewall has connections on the water pump and thermostat housing...  I'd expect some type of flow depending on how the connection on the thermostat housing is constructed ( "behind" the thermosat in the block or on the radiator side )...  

I also need to top my 89 off periodically, but "I" have some certainty that "I" have a weeping heater core, though my floor is dry as a bone.

 

 

[88towmanche]

Could be going through intake and out exhaust - there's a dye kit on Amazon you can check that pretty easy - if it was substantial, never mind it would be obvious, but I had it happen on a v6 Astro van - engine survived another 20k miles I had it and was upfront with the buyer when I sold it.  Went through maybe a pint every 1000 miles or so...I slowed it down periodically with magic sauce from the auto parts store - engine block in a can sort of thing.

 

[AnotherOldJeepGuy]

3 hours ago, llhat said:

IIRC,  the pressure bottle on the firewall has connections on the water pump and thermostat housing...  I'd expect some type of flow depending on how the connection on the thermostat housing is constructed ( "behind" the thermosat in the block or on the radiator side )...  

I also need to top my 89 off periodically, but "I" have some certainty that "I" have a weeping heater core, though my floor is dry as a bone.

 

 

I guess I'll see if I can draw it out, see what that reveals if anything.  I sure seem to remember that in the past I didn't see any flow into the bottle until the engine had warmed up, but I have slept since that observation was (or was not) memorized so who knows.

 

As for the heater core, my heater hasn't really worked for 10-20 years!  But living in Texas that generally isn't much of a problem since we have very few days where a heater is a necessity.

[AnotherOldJeepGuy]

1 hour ago, 88towmanche said:

Could be going through intake and out exhaust - there's a dye kit on Amazon you can check that pretty easy - if it was substantial, never mind it would be obvious, but I had it happen on a v6 Astro van - engine survived another 20k miles I had it and was upfront with the buyer when I sold it.  Went through maybe a pint every 1000 miles or so...I slowed it down periodically with magic sauce from the auto parts store - engine block in a can sort of thing.

 

I took a wag at an Amazon search, found a whole bunch of different things.  If you feel like looking up a link for what it is you are recommending, that would be awesome.

[88towmanche]

Think it was this - you have to buy the blacklight flashlight also

[AnotherOldJeepGuy]

I'll take a look.

 

What about these kits, any good?

[AnotherOldJeepGuy]

Data point 1.  I'll keep collecting for a bit, but this data point is not real encouraging.. 

 

[fiatslug87]

You can send your oil to Blackstone Labs for analysis. They will give you the amounts of different metals, fuel, coolant, etc. in the oil along with a brief analysis of the results.

[llhat]

i'd guess the dye is a good process, as 'some' external coolant leaks will disperse and not puddle or accumulate in cavities

[AnotherOldJeepGuy]

5 hours ago, fiatslug87 said:

You can send your oil to Blackstone Labs for analysis

Interesting.

[AnotherOldJeepGuy]

On 7/14/2026 at 6:38 PM, 88towmanche said:

I slowed it down periodically with magic sauce from the auto parts store

Is this something like what you are referring to?

https://www.amazon.com/K-Seal-ST3501-Permanent-Gasket-Repair/dp/B00QYZIVA6/ref=pd_bxgy_thbs_d_sccl_2/143-8891960-5551068?pd_rd_w=hUGRC&content-id=amzn1.sym.9bef5913-5870-4504-8883-3ba89d7f8e39&pf_rd_p=9bef5913-5870-4504-8883-3ba89d7f8e39&pf_rd_r=NSR08A2EZ2XP0FB1457E&pd_rd_wg=g9m2v&pd_rd_r=1b1904b5-2886-4ccf-add6-ad927b46cdd2&pd_rd_i=B00QYZIVA6&psc=1

 

I generally think if it's too good to be true it probably isn't, but on the other hand, mostly good reviews, so hmm..

[fiatslug87]

Before I pour anything into my engine I would make sure I knew why I needed it. Have you done a compression test (dry and wet) and a leak down test?

[watchamakalit]

I had a small leak at my water pump to lower hose connection for years due to a faulty hose clamp.  Didn't find it till I pressure tested the system. It never dripped on the shop floor.  It was only evident by mysteriously disappearing coolant.  I even did a probably unnecessary head gasket trying to fix it. 

[AZJeff]

6 hours ago, fiatslug87 said:

You can send your oil to Blackstone Labs for analysis. They will give you the amounts of different metals, fuel, coolant, etc. in the oil along with a brief analysis of the results.

I HIGHLY encourage you to do this whenever you have a mystery coolant loss.  It’s about as definitive as it gets, and will give you an answer (via an emailed report) in a few days.

[Ωhm]

8 hours ago, AnotherOldJeepGuy said:

Data point 1.  I'll keep collecting for a bit, but this data point is not real encouraging..

 

Coolant temperature affects volume.

[88towmanche]

Yeah I think Kseal was one of the ones I used...but keep in mind I didn't care on that old Astro was just looking to buy time (and did) - same as you saw a lot of positive reviews and figured had nothing to lose...far as I know it did no damage and the van still cruised along until I sold...got at least 20k out of it after the problem popped up

[AnotherOldJeepGuy]

13 hours ago, fiatslug87 said:

Before I pour anything into my engine I would make sure I knew why I needed it. Have you done a compression test (dry and wet) and a leak down test?

I thought I knew what a compression test was, a tool (that I don't have) that is inserted in to each spark plug hole to measure pressure?  But first I ever heard of there being a wet and a dry version.  So perhaps I know even less than I thought I did. 

 

Never heard of a "leak down" test. 

[AnotherOldJeepGuy]

12 hours ago, watchamakalit said:

I had a small leak at my water pump to lower hose connection for years due to a faulty hose clamp.  Didn't find it till I pressure tested the system. It never dripped on the shop floor.  It was only evident by mysteriously disappearing coolant.  I even did a probably unnecessary head gasket trying to fix it. 

I am planning to get some kind of dye to attempt to confirm there is or is not any visible leak.

[AnotherOldJeepGuy]

11 hours ago, Ωhm said:

 

Coolant temperature affects volume.

Right.  I am collecting my comparisons first thing in the morning after engine has set overnight.

[AnotherOldJeepGuy]

On 7/14/2026 at 8:43 PM, 88towmanche said:

Think it was this - you have to buy the blacklight flashlight also

Well apparently this can only be used on "orange" coolants:

 

But it appears this one can be used with the "green" coolant I have.  I'll start with this diagnostic and see if I can find a visible leak.