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So my customer was complaining about having to add engine coolant every couple of weeks. First few times he came in, I found 2-3 leaky clamps. Little tightening, and  and he would come back and it somehow loosened up. It was brand new hoses put on by another shop, so things were still settling in, plus we were going into the winter season and I expected the rubber to shrink.

 

But it got progressively worse, until the point that the coolant bottle on top of the engine was always low, and the overflow bottle would empty out. He was adding a cup of coolant every 2 weeks. Well, I'm in the middle of a 100 hour, and he gets around to telling me that...

 

Cylinder 4 had a leaking exhaust valve, and pressure since the last inspection dropped 7-8 PSI (reading 78/86, down from 85/86 in 100 hours). My usual tricks didn't help, so I sent that cylinder in to LEAF for a checkup. In the mean time, I can't find any traces of leaking coolant! GRRR WHERE IS IT???

 

Anyways, got the head back, and was told it was the valve seat. It had some pitting that was developing, but since we caught it early, all it took was light grinding and lapping and it seals nicely again. While assembling, I come across this:

 

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Do you see it?

 

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How about now?

 

post-1078-0-52467300-1456472014_thumb.jpg

 

That's the hose from the bottle on the engine, to the overflow bottle on the firewall. Found the leak!

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I would bet that that hose didn't get replaced at the hose change. Likely the mechanic didn't see the need for replacing it, because it is not under pressure. For the record I always replace that hose when doing a hose change.

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No floor puddles. No cockpit smell. We're talking about 10+ hours a week flight training, with lots of start cycles, so it was a really sloooow leak. This hose actually wants to hold itself together. This probably would have not been found until the next hose change in a normal use aircraft. It was sheer luck that I found it to be honest... It was really hard to see until I bent it!

Remember: the coolant overflow bottle is not for topping up the system. It exists because all liquid cooled cooling systems have spots that will gas up from boiling (specifically, around the inside of a tight turn radius). This hose would have only leaked when the engine was hot, and in small amounts, as the cap in the coolant bottle (in the spider on the engine) has a spring loaded valve to only let coolant by when pressure gets too high or too low. When it cools, it would have drawn a little from the overflow bottle and some would have leaked out, but pretty insignificant.

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