1990 mercrusier 4.3 overheating issue

[al0311]

hi guys, so here is my problem I recently purchased this boat a few weeks ago and have been working on it none stop just about and now another issue has come up.
The other day I finally had the boat up and running good on muffs when I notied the temp gague not working on the console. So I test the gauge and see it ping. so I Then assume the thermosat is bad. once i go into the housing and I pull the gasket off and look down there is no thermostat at all lol? why would someone remove the thermostat?
everything I have looked up on the boat looks like it requires a thermostat.
I replace / put a new thermostat in and get the boat running for a while and notice the temps climping well past the mid line gague almost to the redline.
I shut it down and let it cool for the day and try it again the next day but this time i take a thermo gun with me and check the temp at the sending up. After running for like 15 minutes and running the rpms up to 3500 I get a temp of 170 at the sending unit and its almost at the red line on the gauge.
I check the temps on the risers and there 90's and cool to the touch I can leave my hand there for awhile without it feeling overwhelmingly hot.
the next day I pull the risers and the manifolds any way to have a look and the risesrs were pretty rusted but not occluding any of the water jackets so Iam assuming there okay? likewise withe the manifolds. I cleaned them up and put them back on. Ran the boat and it still was overheating.
What would be the next step? should I still replace the risers and manifold?
like I said they were rusted pretty good around the outsied but down the inside of the risers there were no cracks or leaks or rust.
not sure when the impeller was replaced last btw. I figure since its pumping water all the way through the engine it should be working fine?
im kinda lost.
thanks for any input.

 

[Baylinerchuck]

Maybe a dumb question, but what temp thermostat did you put in it?

 

[al0311]

Maybe a dumb question, but what temp thermostat did you put in it?

it is a 140

 

[alldodge]

running good on muffs

After running for like 15 minutes and running the rpms up to 3500 I get a temp of 170 at the sending unit

Ever remember the Clint Eastwood movie, The Good, The Bad, and The Ugly?

The Good: Your correct in the motor should have a thermostat and installing a 140 degree is good

The Bad: 3500 rpms on muffs for 15 minutes, 1500 rpm max

The Ugly: Need to replace your impeller and maybe even the housing

 

[Rick Stephens]

Yup, first thing you do with a new to you Mercruiser is pull the outdrive, inspect the u-joints (and grease em), check the gimbal bearing is tight and totally smooth running, drain the drive oil, split the upper from the lower and inspect and replace the impeller and any other water pump components as indicated. Do not trust anyone's naked word where the impeller is concerned.

 

[al0311]

Hey guys so quick update got out today and dropped the lower unit inspected the impeller no rips or tears that I can see. Should I replace it all anyway ?
Where should I go next ?
If I can maybe tomorrow or the next day I'll pull the risers again and post some pics.
Is there any tests I can do to ensure that it's getting adequate water flow through the motor ?
On a side note I had to replace the thermostat housing due to seized bolts and replaced it with a different unit the old design

http://www.mercruiserparts.com/bam/subassembly/31478/951/150

Luckily I was able to find the housing that was on it and is on its way in the mail. Im guessing these two designs are compatible or they wouldn't be sold like that ? Could that be the problem ?

 

[alldodge]

the impeller no rips or tears that I can see

Wow, that's one in a million
Put a clear piece of hose between the intake hose coming from the drive to the thermostat housing

 

[Rick Stephens]

What wear and tear there is on an impeller is hardening of the rubber and a set to the spokes. They get permanently bent and they become less functional.

I would recommend you never pull the just the lower off. It is a bear to lift it back on and too easy to damage an oil seal. Annual maintenance includes inspection and greasing of u-joints, inspection of gimbal and checking there is no oil or water in the bellows.

 

[Baylinerchuck]

I learned from experience that if you get the wrong housing on an alpha I gen I, lower unit lube will leak past the o-ring seal and puke out all over the floor. Both housings are similar, except the way it mates to the lower unit. Not paying attention to that cost me a pint of lube, and the right housing.