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Discussion Starter · #1 · (Edited)
Alright guys. I'll try to make this quick. Short story. First of its one of those rare 95 SE preludes. H23a1. We had one of those small radiator hoses in the back start slow leaking while under pressure. The car started to overheat. We caught it relatively quickly, then found the issue and replaced the hose. Coolant was off color. We refilled and moved on. Coolant seemed to disappear, and the person who did the work did not bleed the system. It overheated for a short time again. When i say short time i mean less than 30 seconds.
Drain coolant, It's milky. I'm thinking head gasket because of the overheat. I put those head gasket tester kits on it. Two different ones, neither showed positive for exhaust gases. The coolant in the radiator stays clean until it goes through the engine. I pressurized the coolant system to 20psi and used a bore scope to see if there was any coolant bleeding off into the cylinders. None. Held pressure for 30 minutes.

I'm thinking maybe the oil cooler is contaminating it somehow? We just changed the oil and transmission fluid after all this and no signs of water in it.

I'm at a loss without throwing parts at it. Right now The thermostat is suspect so I'm replacing it since its cheap insurance.

Any other ideas on what is causing the milky coolant? Could it be residual?

edit: thermostat was definitely stuck or sticking randomly. Could not open by hand when I replaced it. So now it's trying to find out coolant contamination. It looks like milk is being put in the coolant.
 

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Alright guys. I'll try to make this quick. Short story. First of its one of those rare 95 SE preludes. H23a1. We had one of those small radiator hoses in the back start slow leaking while under pressure. The car started to overheat. We caught it relatively quickly, then found the issue and replaced the hose. Coolant was off color. We refilled and moved on. Coolant seemed to disappear, and the person who did the work did not bleed the system. It overheated for a short time again. When i say short time i mean less than 30 seconds.
Drain coolant, It's milky. I'm thinking head gasket because of the overheat. I put those head gasket tester kits on it. Two different ones, neither showed positive for exhaust gases. The coolant in the radiator stays clean until it goes through the engine. I pressurized the coolant system to 20psi and used a bore scope to see if there was any coolant bleeding off into the cylinders. None. Held pressure for 30 minutes.

I'm thinking maybe the oil cooler is contaminating it somehow? We just changed the oil and transmission fluid after all this and no signs of water in it.

I'm at a loss without throwing parts at it. Right now The thermostat is suspect so I'm replacing it since its cheap insurance.

Any other ideas on what is causing the milky coolant? Could it be residual?

edit: thermostat was definitely stuck or sticking randomly. Could not open by hand when I replaced it. So now it's trying to find out coolant contamination. It looks like milk is being put in the coolant.
You've changed oil and trans fluid, why not the coolant. Should be flushed, not just changed every 2 years. Use only Honda coolant and distilled water. Milky coolant could be result of not allowing engine to warm up properly. Do you do a lot of short distance and/or city driving? Also make sure the water pump is doing its job. And yes, bleed the cooling system any time you open the system. Any air in the system can cause overheating. A big no-no with these aluminum heads.
 

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Discussion Starter · #3 · (Edited)
You've changed oil and trans fluid, why not the coolant. Should be flushed, not just changed every 2 years. Use only Honda coolant and distilled water. Milky coolant could be result of not allowing engine to warm up properly. Do you do a lot of short distance and/or city driving? Also make sure the water pump is doing its job. And yes, bleed the cooling system any time you open the system. Any air in the system can cause overheating. A big no-no with these aluminum heads.
Maybe I didn't word it properly. But we did a full drain and flush after the overheat. We also did a drain and refill after the initial issue. All the coolant in the system is new. It only had about 2000 miles on it when all of this started actually cause the radiator is new. So we have replace the coolant 3 times in the last 2300 miles or so. Which has all happened in the last 3 years. The second time after the initial incident, I did it myself. I did bleed the system, I did drain from the radiator and block, I did do a distilled water flush during all that. Where did you get 2 years from? I never previously mentioned my change intervals. Almost all the driving is Highway and almost no trips under 20 miles. That being said, it only gets driven maybe once a week or 2. Right now it's been sitting while I try to figure all this crap out. Never heard of short trips causing milky coolant, oil yes but not coolant. Doesn't matter how much water vapor gets in the coolant system. It's already mostly water. The water pump is working and only has about 5k miles on it.

Any other ideas? Preferably something that doesn't assume I"m an idiot? I know those kind of people are on this forum, but before I got my engineering degree I was an ASE certified mechanic. I pretty familiar with how this stuff goes. My biggest mistake was letting someone else do the coolant and the initial fix. Either way I appreciate the response and now that we've covered all the bases I'm hoping you have a good idea for me to look into.
 

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If it's an auto (you don't say), transmission oil could be mixing with the coolant in the radiator. Otherwise, the cooler like you suggest (though unlikely as they are pretty robust) or the head gasket, your tests for the HG would not definitively catch oil making its way into the coolant under a heat cycle.

I cant think of anywhere else oil and coolant could mix really.
 

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If it's an auto (you don't say), transmission oil could be mixing with the coolant in the radiator. Otherwise, the cooler like you suggest (though unlikely as they are pretty robust) or the head gasket, your tests for the HG would not definitively catch oil making its way into the coolant under a heat cycle.

I cant think of anywhere else oil and coolant could mix really.
It is an auto. Sorry I didn't mention that. I did a drain and fill of both transmission and oil and they look good. The transmission color was off, but since most of the fluid in it is so old (This vehicle i think just hit 90k), I wasn't too worried. It looked old, not like water was in it.

The radiator I put in myself about 3 years ago, and has at most 1000 miles on it. I couldn't imagine the transmission cooler being the source since they are built into the radiator and it's new. That being said I haven't actually ruled it out.

The exhaust gas tests I did while the engine was warm and circulating. However I can't remember if I did them right before or right after the thermostat. I'm thinking right before, maybe I should redo them and see if the thermostat was sticking. It's been cold so these little engines take a long time to overheat from sitting right now so It wasn't overheating during the tests or anything to signify it was faulty. I just decided to replace it as cheap insurance. thanks for the help and response as it got me thinking about what we've already done and how it was done. I might have made a mistake doing the tests before the thermostat so I'm going to do it again.

What you said and what I was thinking are the same. So I will redo the head gasket test, if that comes up negative I will take off the cooler and pressure test it, if that comes up negative I will pressurize the transmission cooler and see if that is the issue.

Thanks, I'll update when I know more in case this kind of crazy happens to someone else or none of the above fix it.
 
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