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I have a 2001 Honda Prelude SH w/ 190k miles and a slow oil leak (~ 0.5-1 qt/week depending on how much I drive it) that I believe ultimately comes out the lower timing belt cover.

I also need a new timing belt. Now I was going to take it to a shop for the belt and the leak but I've decided to tackle the timing belt on my own. This means that I should also find the leak (if I don't, I might as well have the shop do the belt, too).

From the service diagrams and some reasoning it seems the leak is most likely from the cam or crank seals.

My question is: What techniques can I do to diagnose where the leak is coming from in an engine that has the timing belt removed and thus can't run? Is there anything I can do beyond just replacing the hypothetically bad seals, putting it all back together, then seeing if the leak stopped?

I don't want to have to take it all apart again if replacing the seals doesn't fix it, since this is already a pretty daunting task. My goal is to verify that the leak is fixed before reassembling it, and to not have this turn into a week long project.

What techniques are available to me? How can I definitively nail down the leak (or at least increase the chance that I've fixed it) while minimizing the possibility of having to repeat a bunch of disassembly work?

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    Definitively? See it leak (drip). That's the only sure way to tell where something is leaking. Once you have an intimate knowledge of an engine, you'll know where the "usual leaks" are at and can say with more confidence. Until then, visual inspection is where it's at. – Pᴀᴜʟsᴛᴇʀ2 Aug 7 '16 at 15:40
  • @Pᴀᴜʟsᴛᴇʀ2 Maybe I could take the timing belt cover off then run it while looking at it. Sounds like, if I can get as many coverings off as possible while leaving the engine functional that will be the best approach? – Jason C Aug 7 '16 at 15:41
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    I'd bet the bottom cover is behind the crank pulley, which makes this problematic. I'd bet, the crank seal is your issue, though, if it's dripping off the bottom of the engine there. – Pᴀᴜʟsᴛᴇʀ2 Aug 7 '16 at 15:42
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On that engine the cam(2), crank(1) and balance shaft(1) seals are the only place that can leak oil behind the timing cover.

At 190K I guarantee they are all leaking, Crank usually leaks the worst first.

Putting UV dye in the oil, the run it for a few days, then use a uv light to see if anything visible outside the cover is leaking.

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There's three things you need to consider:

  1. As has been pointed out, seeing the leak is the only sure way to diagnose the orgin of the leak. UV dye in the oil is one way, giving the engine compartment a good cleaning and then watch for where fresh oil is coming from is another.
  2. You can't test a seal. Fluids inside an engine become under some pressure and some agitation while the engine is running. Either the seal seals, or the seal doesn't seal.
  3. As has also been pointed out, this car has a lot of mileage and age, if all your seals aren't already leaking, they will be soon, especially after you change a leaking one. A leaking seal will let some of the pressure out of whatever it is sealing, meaning after the repair, that system will be back to it's normal pressure, meaning the old seals you haven't changed will now have more pressure to contend with all of a sudden. You can replace the one or ones that are definitely leaking, or bite the bullet and do all of them (Moab pointed you to 4 of them). New seals buy you piece of mind.
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Replace every seal that you can when you do the timing belt. It just isn't worth not doing them. Most seals are relatively inexpensive and you're doing almost all of the work needed to replace them when you do the timing belt.

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