yes get a new impeller or atleast check the one in the engine now.
look for it being a spun hub or make sure the key is in the correct place.
before anyone tells you they cannot spin a hub on the impellor look at this one.
as you can see it was hardly used, yet the vulanising came away.
resulting in poor amount of water.
sometimes this can see to be ok yet as soon as the engine goes into gear no water.
if its not that then you have a cylinderhead gasket blowing or the exhaust chamber has a hole in it.

have you done a compression test ? I would check impeller again and definitely do compression check.
if these come up ok then the exhaust side cover needs to come of and a very good inspection will be needed to find a pin hole or a crack.
These engines are very well known for exhaust housing to wear away.

hope some of this helps you

see pic of spun hub impeller, notice it looked perfect when fitted !!!!
.

phill

Thank you for responding! I appreciate your help very much and wanted you to know.

Compression is good (and equal) on all 3 cylinders. The motor runs perfectly without any misses, noises, or abnormal smoking.

I had read that there are some poppet valves and thermostat in there but that the bolts are prone to breakage upon dis***embly. I will look first at the impeller but the condition seems to be somehow mechanical tied to the linkage, as though there are coolant p***ages that get switched in and out depending on what you are doing with the motor.

If I am sitting in neutral, water is coming out the tell tale like normal. If I take the shifter and go into range and back to neutral as fast as possible, the steam comes out for like 5 to 10 seconds before water resumes. The water (out of the tell tale) isn't hot, but it isn't cold either.

Again, thank you :-)

that sounds like head gasket or exhaust gas is leaking into the water p***age.
these older engines are prone to the exhaust problem.
some people get the holes welded up some just look around for good used parts.
it was a terrible design that really was doomed to cause problems straight from new.
when you look inside the exhaust cover youl see what can go bad.
sometimes it can simply be the gasket on the exhaust cover that goes bad due to calcium /salt build up pushing the gasket out of place.

there is definitely nothing mechanical that can cause the water p***age to be blocked off.
its either head gasket or exhaust gas entering the system.
the steam coming out for ten seci=onds is due to the engine being under load causing more pressure and in about 8 seconds the water pressure in the engine has built up enough to force the water past the steam etc.
this needs to be checked out before major damage is done.

unfortunately the bolts on the exhaust are also a pain and often break when trying to undo them.
give each one a wallop with a hammer before putting a spanner on them, it can help in breaking the rust and corrosion around the bolts.

also try to get a torch and look into each cylinder ( I know its going to be hard) through the plug hole, if one cylinder looks like new its a head gasket 100% .

hope this helps

phill

Just to follow up, I took it apart this weekend and found the thermostat and poppet valves full of silt. I cleaned everything real good and tested the thermostat in a pot of water and it works good so I put it all back together and it works perfectly.

Until I got a close look at it I was sweating it a little but it was real real easy to get open.