Coolant Level Anomaly

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jonheld

jonheld

10 Year 15 Year
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This is going to be long winded, so relax for a while.

Over the last several months I have noticed a coolant level anomaly that I can't put my finger on.
97 LX450 280K+ miles
Has NEVER lost coolant over the last 14 years.
Oil is clean, coolant is clean.
Rear heater deleted.

What's happening is the coolant level rises in the expansion tank as usual, but when the engine cools, coolant is not getting drawn back in to the radiator.
My first thought, besides having to rebuild the motor, was to pressure test the system. It holds 13 psi all day.
My next thought was the radiator cap. Purchased a new OEM cap and cleaned the mating surfaces of the filler neck which made no difference.
Next I thought there must be some sort of clog in the overflow piping. Pulled it all apart, cleaned it up and I noticed that the overflow cap has a gasket that was deformed and twisted. So of course I said, "Ahh ha! This must be the issue."
However, thinking about it, the overflow tubing is a direct connection to the radiator filler neck and the tank cap is open to the atmosphere. Even if the cap was off the overflow bottle, the radiator cap would still allow coolant to be drawn back in as long at the tube was sitting in coolant.

Today I poured the excess coolant that was in the overflow bottle back into the radiator, and it was the EXACT amount to fill the radiator back to the top of the filler neck as the level in the overflow was at the FULL mark.

Not exactly sure where to go from here, so I ordered a new OEM complete overflow bottle assembly from our friends in Dubai.

So as opposed to rebuilding the motor, I'm willing to entertain suggestions.
 
Overflow hose from the cap still attached well?

1772321063653.webp
 
I'm assuming you pressure tested while cold, perhaps you have a temperature dependent opening in your system?
 
60 toy ota said:
I'm assuming you pressure tested while cold, perhaps you have a temperature dependent opening in your system?
Click to expand...
If that was the case then the overflow level wouldn't rise with engine temp. Unless something is acting as a one way valve...
 
One scenario as mentioned, the overflow reservoir piping is blocked acting like a one-way valve (has the reservoir cap/hose sub-assembly 16405-66020
been replaced?)

Another darker scenario, the cooling system is overpressurised by ??

Don't want to send you down a rabbit hole so FWIW:

My 95 model had a (unknown) crack in the #6 cylinder wall which overpressurised the cooling system forcing coolant out of the radiator into the overflow tank/reservoir. There was no mixing of oil/coolant, engine ran fine, no other symptoms/signs of what was happening.

Only symptom initially was the reservoir filling up and not going back to the Full mark after the engine cooled down. When that situation got worse it eventually blew enough of the coolant out of the block/water jacket to cause the engine to overheat.
 
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This may seem blatantly simple, but just mentioning it; the Overflow Bottle cap is plumbed "directional" to the Overflow of coolant from the radiator, and if installed backward, it will allow the Overflow to dump into the bottle, but as the inlet (*incorrectly plumbed) is at the top, there's no way for the cooling radiator to suck the coolant back in because the inlet isn't submerged. This incorrect routing also results in any pressure from the radiator resulting in coolant flowing out of the submerged tube and overboard - IF you have a perfect seal at the Overflow Bottle cap (*most aren't, and don't need to be air-tight).

Screenshot_20260228-173135.webp

Hard to see in the drawing above, but the Overflow line from the Radiator should go into the plastic pipe that's CENTERED on the cap; the overboard vent is slightly off-center. The rubber hose which extends down into the bottle is from the centered inlet.

Apologies if you already know this - and not trying to insult you, but it's easy to get the plumbing wrong if you've ever pulled the bottle out for some reason.

Also, make sure the rubber hose inside the tank doesn't have any cracks or pinhole leaks, but I think you already pressure tested that. It should be an air-tight fit to the plastic bongs, too.
 
Could prove telling to put negative pressure on the overflow tube to verify coolant gets drawn from the reservoir, then time lapse a pressure gauge on the radiator overflow nipple as it cools
 
LongDuck said:
Apologies if you already know this - and not trying to insult you, but it's easy to get the plumbing wrong if you've ever pulled the bottle out for some reason.
Click to expand...
No apology necessary. I do stupid stuff all the time. That's just not one of them...this time anyway.
 
@jonheld
You have some kind of air leak in the system that breaks any vacuum from building.
You've checked the overflow as stated above but it could be anywhere in the system. Pressure testing the system may be of no help as an air leak will be harder to detect than a fluid leak. Time to add some UV dye.
 
60 toy ota said:
Could prove telling to put negative pressure on the overflow tube to verify coolant gets drawn from the reservoir, then time lapse a pressure gauge on the radiator overflow nipple as it cools
Click to expand...
Yep, this is what I'm thinking too. I can picture a small nearly invisible nick in the hose holding positive pressure enough that the coolant goes into the reservoir, but when exposed to a vacuum, it sucks air.
 
I have been trying to track down a similar issue. I found a very small crack on the overflow line from radiator to overflow reservoir. Haven't had time to do a proper test drive and check over yet, butnon a short run it looks promising. Might be worth looking at since it wasnt obvious until I moved the hose around a bit.
 
UintaGriff said:
I have been trying to track down a similar issue. I found a very small crack on the overflow line from radiator to overflow reservoir. Haven't had time to do a proper test drive and check over yet, butnon a short run it looks promising. Might be worth looking at since it wasnt obvious until I moved the hose around a bit.
Click to expand...
I spent some time looking over all those associated parts and decided to just order a complete new OEM overflow assembly (16470-66020) . I figure after 29 years those parts don't owe me anything more. Should be here by the end of this week.

In the interim, I'm going to run out and get a combustion gas tester to either put my mind at ease or start preparing for a significant cost.
 
jonheld said:
I spent some time looking over all those associated parts and decided to just order a complete new OEM overflow assembly (16470-66020) . I figure after 29 years those parts don't owe me anything more. Should be here by the end of this week.

In the interim, I'm going to run out and get a combustion gas tester to either put my mind at ease or start preparing for a significant cost.
Click to expand...
the combustion gas tester are relatively inexpensive and good to have in your toolkit. They're not 100% full proof in determining head gasket issues, but a good step towards it. My only issue is that you should be doing it while the thermostat is open and engine warm, which is very hard to do with the cap off.
 
petegangies said:
the combustion gas tester are relatively inexpensive and good to have in your toolkit. They're not 100% full proof in determining head gasket issues, but a good step towards it. My only issue is that you should be doing it while the thermostat is open and engine warm, which is very hard to do with the cap off.
Click to expand...
Yea, I'm going to have to remove a decent amount of coolant from the radiator before starting this test. Probably best to be able to see the tops of the coils when cold, as the coolant will expand quite a bit once at operating temps.
 
jonheld said:
Yea, I'm going to have to remove a decent amount of coolant from the radiator before starting this test. Probably best to be able to see the tops of the coils when cold, as the coolant will expand quite a bit once at operating temps.
Click to expand...
yep, I used a large syringe to suck some out, otherwise you end up drawing up coolant into the tester which invalidates the test.
 
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