Chasing a Slow Coolant Leak – 1996 80 Series, 190K, HG Done

[manofthewoods]

Could the oil cooler be a possible culprit here? Since oil/coolant mix was confirmed, I’m trying to rule out anywhere the two intersect.

Has anyone had a bad oil cooler or cooler seal cause a slow internal coolant leak with no obvious external signs? What’s the best way to diagnose or rule out the oil cooler — pressure test, bypass, dye, etc?

Appreciate any firsthand experience.

I replaced my complete oil cooler after 2 Blackstone reports showed coolant in the oil. The quantity was small. This was after a complete rebuild. Initially I thought it might have been a hairline Crack in the head. My machinist insisted it wasn’t.
Replacing the oil cooler is amazingly easy.
Solved my issue 100%

 

[jpoole]

@MissahPapaJ

Regarding the freeze plug location(s) here's a text string you can copy and paste into Google to run a search for related posts on the forum:

freeze plug leak 1fzfe site:forum.ih8mud.com​

​

Or you can open this link to the search results directly:

Google Search

I see related posts, with pictures, high in the results. I've not experience this issue myself and don't know if it's specific to a single plug or if there are more than one that may display these symptoms. A little thread reading via the above search, or similar, should get you up to speed I'd guess.

 

[LimitingFactor]

I pull the plugs and checked the tops of the pistons in all but cylinder 4 since it was under the intake, ill do a final check when I have more time to pull it apart more. none of the plugs or tops of the pistons looked to be 'steam cleaned' indicating coolant getting into the combustion chamber. The bore socpe I was using doesnt have the best resolution or FOV but it gives a good enough idea of it looks like inside.

#1 plug:

#2 plug:

#3 plug:

#5 plug

#6 plug:

 

[LimitingFactor]

And the tops of the piston surfaces from the scope. Generally they dont look amazing it might be the color which is making the carbon look like rust but its got 200k on the clock so not sure what I should expect them to look like?
#1 piston:

#2 piston:

#3 piston:

#5 piston:

#6 piston:

 

[LimitingFactor]

I replaced my complete oil cooler after 2 Blackstone reports showed coolant in the oil. The quantity was small. This was after a complete rebuild. Initially I thought it might have been a hairline Crack in the head. My machinist insisted it wasn’t.
Replacing the oil cooler is amazingly easy.
Solved my issue 100%

Did you find that the oil cooler had failed or did you just replace the gasket/seals?

 

[MissahPapaJ]

snack lah blur

 

[LimitingFactor]

I read through a few oil cooler threads like this one: Restoration: 1FZ-FE Oil Cooler - https://forum.ih8mud.com/threads/restoration-1fz-fe-oil-cooler.1028222/
I can say after checking under the front heatsheild the housing looks perfect, no residue, no pink staining. has anyone ever seen the interal oil cooler fail. I have ready people will swap them out for PM but only from a grime/sludge perspective.
I also sanity checked the freeze plugs for cracks and those seem fine as well.
At a loss now not sure when else to test/check. I didnt get a chance to check cyl #4 yet.

 

[manofthewoods]

Did you find that the oil cooler had failed or did you just replace the gasket/seals?

The first thing I did was replace the gaskets, even though they weren't leaking visibly. That didn't fix the issue.
That's when I deduced that the leak was internal and replaced the entire unit.

In my case the loss of coolant wasn't drastic, but consistent

 

[rhc404]

Have you tried the compression and leak down tests?

 

[LimitingFactor]

Have you tried the compression and leak down tests?

Not yet, trying to determine what will be the best value:
-Cooling system pressure check
-Leak down
-compression

 

[Marvelicious]

And the tops of the piston surfaces from the scope. Generally they dont look amazing it might be the color which is making the carbon look like rust but its got 200k on the clock so not sure what I should expect them to look like?
#1 piston:
View attachment 4080143

#2 piston:
View attachment 4080142

#3 piston:
View attachment 4080141

#5 piston:
View attachment 4080140

#6 piston:
View attachment 4080139

Not sure if you intended to skip #4 or they all looked roughly the same, but that just looks like carbon to me. You have to remember that your light source is basically right at your lens, so the reflection can do funny things to the color.

 

[petegangies]

Not yet, trying to determine what will be the best value:
-Cooling system pressure check
-Leak down
-compression

I see you've already done the TK head test (co2 sniff test). Even though yours is negative, it does not completely rule out issues with the headgasket if you have coolant in the oil. It detects exhaust gases in the combustion chamber, but also should be completed when the thermostat is open and when engine is hot (be careful of pressure in the cooling system). Usually if the sniff test is negative, the leak down test is likely to be as well, but still worth doing. Cooling system pressure test overnight would also be useful. Lisle 20250 Compression Tester off Amazon is cheap and easy to do. The more data the clearer the picture will become.

 

[MissahPapaJ]

Not yet, trying to determine what will be the best value:
-Cooling system pressure check
-Leak down
-compression

If you own old shizzzz you should have both compression and leak down testers and the knowledge to use them. And a coolant system pressure tester. But in all reality if a shop was going to preform a compression test and leak down I’d bank on over 5-600$ for both tests cause it’s really easy to do both at that point but it’s not worth doing one at a time cause the process takes the same amount of time once you get the plugs out. At my shop I’d say about 5-600$ maybe 700$ but if everything checks out or not it’s gonna need to be put back together so there’s that. You’ll at least need a throttle body gasket and whet ever else might come up.

 

[petegangies]

Not yet, trying to determine what will be the best value:
-Cooling system pressure check
-Leak down
-compression

any updates?

 

[LimitingFactor]

Update:
Did a cooling system pressure test — lost about 4 psi over ~20 minutes, so there’s definitely a slow leak somewhere.

Only thing I’ve found so far is a bit of pink coolant residue at the heater T hose, but it’s dry and pretty minor. Doesn’t seem like enough to explain the coolant loss I’m seeing, especially since I’ve been regularly topping off the coolant.

Current situation:

• Slow pressure loss confirmed
• Minor seep at heater T (maybe part of it, but probably not all)
• No visible external leaks
• Temps normal
• Prior oil analysis showed coolant in oil
• Coolant level continues to drop over time

Question for the group:
At this point, does it make sense to move forward with an oil cooler reseal and refreshing some cooling system components (water pump, thermostat, etc.) as a baseline? Or better to keep digging until I can positively ID the source?

Appreciate the input — this one’s been pretty subtle.

 

[petegangies]

Update:
Did a cooling system pressure test — lost about 4 psi over ~20 minutes, so there’s definitely a slow leak somewhere.

Only thing I’ve found so far is a bit of pink coolant residue at the heater T hose, but it’s dry and pretty minor. Doesn’t seem like enough to explain the coolant loss I’m seeing, especially since I’ve been regularly topping off the coolant.

Current situation:

• Slow pressure loss confirmed
• Minor seep at heater T (maybe part of it, but probably not all)
• No visible external leaks
• Temps normal
• Prior oil analysis showed coolant in oil
• Coolant level continues to drop over time

Question for the group:
At this point, does it make sense to move forward with an oil cooler reseal and refreshing some cooling system components (water pump, thermostat, etc.) as a baseline? Or better to keep digging until I can positively ID the source?

Appreciate the input — this one’s been pretty subtle.
View attachment 4115201

You'd be surprised how much you lose when its pressurized. Dont expect to see big puddles as most will evaporate. Fix leak and pressure test again. Removes guessing.

 

[LimitingFactor]

Update: Compression Test Results

Quick update after doing some more digging.

I ran a compression test across all cylinders to get a better sense of what’s going on. Results came back pretty consistent across the board — all cylinders measured between 190–197 psi.

Good sign overall. No obvious outliers or weak cylinders that would point to a clear failure. Combined with the previous tests (oil analysis showing coolant, inconclusive combustion gas test), it seems like if there is an internal leak, it’s likely small and not affecting compression significantly.

Given that, I’m going to move forward with investigating the oil cooler next and plan to re-seal it and baseline the other cooling system parts. It seems like a plausible source and worth ruling out.

Curious if anyone has seen similar compression numbers with a slow coolant intrusion issue on a 1FZ.

 

[Fj80oregon]

Did you fix the leaking hose? If not go right ahead but it would be silly to find a leak and not fix it, then Continue looking for another leak.

 

[LimitingFactor]

At this point the 80 is taking about a half gallon of coolant every 250 miles. I would be amazed that the residue on the hose was equivalent to that much leaking since the area around it is dry. I'm definitely going to fix that but I don't think it's the main source.

 

[LimitingFactor]

Got a bunch of OEM gaskets and seals to baseline the cooling system. Also got the hose kit to delete the rear heater as one of the T hoses is showing signs of failure.