Chasing a Slow Coolant Leak – 1996 80 Series, 190K, HG Done (5 Viewers)

[LimitingFactor]

Coolant Level Slowly Dropping – Chasing Down the Cause (‘96 80 Series, 190K)

Hey folks — hoping to tap into the collective knowledge here.
I’ve been noticing a gradual coolant loss on my 1996 80 Series (1FZ-FE, 190K miles). Nothing major or catastrophic, but it’s enough that I’m topping off the overflow every couple of weeks. No overheating, no puddles — just a slow, consistent drop. It did have 1 'hot' event which spawned the trouble shooting, it ended up taking about 0.5 gal coolant to top off the radiator. temps have been normal since.

• Coolant level in overflow tank drops slowly
• No visible leaks under the truck
• Temps stay normal
• No white smoke, no milky oil
• No coolant smell inside the cabin
• Small amount of milky (oil?) residue under the rad. cap - I have read this can be normal?

Background:​

• PO said the head gasket was done ~3 years ago
• Stock cooling system as far as I know
• Radiator looks original

What I’ve Checked:​

• All hoses and clamps look tight
• Radiator cap is OEM — thinking about swapping just in case
• Water pump weep hole is dry
• Heater core seems fine — no wet carpet or smell
• Haven’t done a pressure test yet, but it’s on the list
• Not seeing bubbles in overflow or rad. while the truck is idling

Theories So Far:​

• Micro-leak under pressure somewhere not obvious
• Radiator cap not holding pressure anymore
• Could be early signs of a head gasket issue (but no clear symptoms yet)
• Maybe something sneaky like the heater control valve or rear heat lines?

Planning to:

• Do a cooling system pressure test
• Swap in a new cap just to eliminate that variable

​

Would love to hear where you found your leak if you’ve chased a similar issue. Especially interested in things that are easy to miss. I’ll update this thread with anything I find in case it helps the next guy.

Thanks in advance!

 

[jpoole]

I'd consider pulling the spark plugs and scoping the cylinders as a step to perform in your information gathering process. If the coolant is disappearing into a cylinder at a high enough rate that cylinder/plug will be cleaner than the others and could help you determine if that's where the coolant is going.

That foam on the rad cap is concerning to me as it should not normally be there based on my experience. It's possible that oil is leaking into the coolant. It's also possible that when the HG work was done that they didn't clean the cooling system out sufficiently and that you are seeing that foam as a result of residual oil/contamination that was left in the system. It would be lower down on my list but I would plan to flush the coolant system or at least change the fluid out once you figure out what's going on and have completed the other necessary repairs.

Failure in the oil cooler can allow oil and coolant to mix. I'm not sure which way the fluids would flow but someone here probably knows that. If it's just oil into the coolant though, causing the foam, that wouldn't account for the loss of coolant you are seeing so this seems less likely than a HG failure where the coolant is getting into the cylinders.

Some folks have had freeze plug or small crack leaks on these where a slow leak while the engine is at temp was causing the coolant to evaporate and not puddle on the ground. Something like that could be going on here and account for you not seeing coolant on the ground. I think I recall that there's a freeze plug or similar at the back end of the head that can lead to this scenario.

 

[rebelcruiser]

Mine was a head gasket

 

[Kernal]

Agree, unless someone has added something other than coolant to the cooling system (?sealer), if that foam is a new/recent development it's concerning

You could collect a sample of the current engine oil and send it to Blackstone Laboratories for an Engine Oil Analysis (EOA),
if coolant is getting into the oil it will show up on that test.

Also you could use one of the kits that sniff the vapor in the cooling system to see if it detects combustion gases.

Or as mentioned you could hook up a cooling system pressure tester while the spark plugs are out then look/listen for a leak.

Or, all/some of the above.

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