On Tue, 14 Oct 2003 11:04:46 -0700, "Peter Duniho"
wrote:


I'm not sure what exactly is going on here, but I don't think it's
"hydraulic lock". As a previous post hinted at, "hydraulic lock" is
something that is specific to only certain kinds of engines (like radials)
and not those found in C172's. It happens when oil drains into the
cylinder, putting an incompressible substance into a space that wants to
compress when the engine turns.

If you are having trouble with the starter not being able to turn the engine
on a C172, you've either got a dead battery, or something worse is going on.

When you first wrote "hydraulic lock", I actually assumed you meant "vapor
lock", which is what happens when the fuel in the fuel lines is heated to
the point of vaporization, preventing it from being pumped to the engine.
This is the usual problem when starting hot engines, especially fuel
injected ones (carbureted engines usually have enough fuel in the float bowl
to let things get sorted out during engine start, so even a little vapor in
the fuel lines won't cause a problem with starting).

But now it sounds like you did actually mean something other than "vapor
lock". I'm not sure what it was, but I'm pretty certain that, given that
you're talking about a C172, it wasn't "hydraulic lock". Even if hydraulic
lock were possible on a C172, it wouldn't be related to a hot engine start.

There are actually two fuel pumps, one works with the engine turning,
the other is the electric auxiliary pump. So fuel will be pumped to
the injectors the moment the engine starts turning.

I'm not familiar with the new fuel-injected C172's, but it's entirely
possible that with the electric fuel pump running, fuel will be pumped to
the injectors and even into the engine the moment the fuel pump is turned
on, regardless of whether the engine is turning or not. That's how the
fuel-injection system for the Lycoming 540 engine in my plane works.

Pete

Pete, I'm assuming it's hydraulic lock because the starter cranked the
prop around half a turn and then jerked abruptly to a stop. I
re-engaged the starter and it tried to turn the prop but wouldn't. I
waited a second and tried again and it turned the prop another half to
3/4's of a turn and jerked to a stop again.

I did not get out an look to see if gas was pouring out of the exhaust
stacks. But once I advanced to full throttle, the starter, after a
few more attempts, began to turn the engine over at a normal rate.
After a few seconds of this, one of the plugs fired and I thought we'd
be ok, but it still didn't start.

I waited another several seconds and re-engaged the starter and this
time, as it spun the prop, it actually briefly fired up on all
cylinders. I moved quickly to slap the mixture in, as the engine had
quit again but the prop was still windmilling, and just before the
prop stopped, the engine caught for good.

I was an auto mechanic in a previous life and have seen hydraulic lock
before. I know no other way of describing it.

Corky Scott