Re: [OMC-Boats] Instrument Question

On Mon, 2 Mar 2009, jd wrote:
> As far as the water sending unit, another interesting point you
> raise. I am wondering if mine is even in the spot it's supposed to
> be. That could be contributing to weirdness.

I can't recall where the temp sensor is on my 225, but I would generally
say that a water temp sensor should be in the engine's internal cooling
loop (the portion that has water cycling through it even when the
thermostat is closed). Typically this is near the thermostat, though
anywhere in the cylinder water jacket should be reasonable,

> The culmination
> of all this mess of not knowing if my engine is actually overheating
> or not came at the end of last season when i had to ditch the gauge
> setup and go with pointing a heat sensing gun at various points all
> over the engine. Not the easiest thing to do while underway!
> Anyway, assuming it was accurate, that seemed to work well and helped
> me see exactly which parts of the engine were running hot. The main
> part that seemed to be hot was the front left and right cylinders just
> above the spark plug hole. On both sides in those areas I was
> getting readings of around 200 - 215 after only running at 3/4 speed
> for a couple minutes. The weird part is above the other sparkplug
> holes and most other places, things seemed to stay about 170 or
> less. And this was all running the engine with clear tubing
> everywhere and being able to see the water move through. So
> knowing the hoses weren't clogged, I thought maybe the manifold was
> clogged towards the front, or maybe a port was blocked or
> something. Nope. But I got it cleaned out anyway at a machine
> shop - even though it didn't really need it. Also got the
> disassembled water pump cleaned. So now the engine is still semi
> torn down, but ready to reassemble with new gaskets. But I'm
> really hesitant to put things back together yet, because I don't
> think i've solved the problem (if there even is one). As crude as
> it seems, pointing a heat sensing gun at the top front areas of the
> engine and getting 210 temp readings doesn't seem good. What do
> you think? Temp gun too crude of a method to be able to conclude
> that?

Infrared thermometers are actually fairly accurate - I believe the Raytek
unit we used is specified for +-1.5% of the reading - assuming that you are
using it on a non-reflective surface. We generally put a small spot of
flat black spray-paint on whatever we're trying to measure - it reads way
off when used on shiny metal. Anyway, you should be able to measure
temperature within 5-10 degF that way with good repeatability. A technique
that might be easier if you're working alone is to use wax indicators -
it's basically a set of crayons that you use to mark the engine with - each
one melts at a specific temperature. I think we've used a brand called
"Tempil Sticks" before. You could also buy a cheap thermocouple reader.

I don't know what to think of temperatures of 210. That'd be very normal
for an automotive engine, but is seems rather high for an engine with a 140
or 160 degF thermostat. In areas where the water cooling jacket is between
the cylinder and the outside surface, you should never see a temperature
higher than the coolant temperature. Is it possible you are looking in a
place where there is no cooling jacket? Are you measuring temperature
while running or after shutdown - you typically will see an increase in
temperature after shutdown, as coolant flow ceases and temperatures
equilibrate. Could you determine if the cooling jacket was clogged up in
that area? You'd likely need to pull the heads to see, but perhaps you
already have things apart enough that that'd be easy. I don't think the
geometry is such that you could get bubbles trapped in that location
(assuming the engine is level) - getting air bled out of the
cooling system can be a challenge on some smaller high-performance engines.

Ethan

-- 
<a href="http://www.engr.wisc.edu/~brodskye/"> Ethan Brodsky </a>