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Diagnosing failure to start on a 2001 Honda Prelude SH, still in brainstorming phase. Leaving out list of other thoughts because this question is specifically about this possibility, rather than generally troubleshooting the start failure.

A few weeks ago, on attempt to start, the engine started, remained running normally for about 10 seconds, slowly dropped RPMs, then stalled. All further attempts to start have failed, leading to cranking but no start at all (note: battery is a non-issue, charge has been maintained through start attempts via charger/jump attempts), car has been garaged in frustration since.

At the same time, I've had a long-standing (and rather severe, as of late) coolant leak. When examining more closely I pulled the air filter and noticed about half of it was covered in dry coolant (the coolant leak is near the intake filter). Filter and surrounding seals are in otherwise good condition. It is difficult to tell if it made it through the filter due to the highly visible bright green staining the white filter. A fresh filter did not solve the issue which rules out a clogged filter obstructing the airflow.

I have two questions:

  1. Can coolant actually make it through an air filter?
  2. If it does, could it somehow lead to a situation where the car continues to fail to start even after the coolant → intake source is corrected? If so, how, and what would have been damaged?

I am trying to determine if I should leave this on my list of possibilities, or scratch it off.

  • Not about your coolant leak, but with that description I would want to verify that the fuel pump is energized/working. – BowlOfRed Apr 2 '17 at 5:09
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If sufficient amounts of coolant made it into the intake, it's possible for the engine to be hydrolocked. While air can be compressed by the cylinders in the engine, water cannot be compressed. So something else has to give, which can result in extreme engine damage.

As for whether coolant (or water) can make it through an air filter, yes it can. Air filters are not water tight.

That being said, the symptoms you describe (the engine slowly dropping RPM and then stalling) are not consistent with hydro lock.

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Can coolant actually make it through an air filter?

The answer to this is "yes". The bigger question is, though, how would it get there and where did it come from? If you have the stock intake on your Prelude, the filter element is downhill from any place which could provide coolant to it. Most newer fuel injected vehicles have a coolant line running through the throttle body which provides some means of keeping the throttle body from freezing during cold weather. This seems the most likely place where coolant might have came from, which then ran downhill toward the air filter. I'm making a few assumptions here, but it seems the logical place where it might have come from. It is nearly impossible for the coolant to have flowed the opposite direction.

If it does, could it somehow lead to a situation ...

The basic answer to this is no. The reason I say this is because it takes a lot of coolant for it to actually stall an engine. If you get that much coolant in you system, it would be enough to hydrolock the engine. Even with a decent sized head gasket leak, the engine will continue to run, though it would have volumes of white smoke coming out of the exhaust.

For what it's worth, I think you are barking up the wrong tree. I'll see what your next theory is and see where we need to go from here.

  • "How would it get there and where did it come from?" -- Yeah... I left out details there because I was only concerned with its effects. There's too many variables for me to definitively say where it came from but: There's actually a few leaks; one is near the reservoir tank and it tends to run down behind the battery, where it is upstream from the intake. Also any coolant that drips out of anywhere around the radiator on that car tends to sit in the splash shield, and flows around over near the filter intake. Plus any coolant I may have spilled. It also could have been a one time thing. – Jason C Apr 1 '17 at 23:59
  • But yeah scratching this off my list. I've got a few other obvious things to try first. Also I can't shake the feeling that it's somehow related to the random odd cylinder misfires, which I actually never solved, and that behavior never changed. – Jason C Apr 2 '17 at 0:12
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Something that seems worth checking: If that motor has a spark plug, remove it long enough to check whether any liquid is bridging the gap of that spark plug.

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