1990 250DA Mercruiser Thermostat

Hey folks, brand new here and I have a question. I recently bought a 1990 Sea Ray 250DA with the 5.7 single motor/outdrive combination. When I picked up the boat from the marina and loaded it on my trailer, the "over temp" alarm began to sound off while idling to the ramp nearby. The previous owner had stated that he had seen the same issue at full throttle, but if he backed it off a little, the alarm went quiet. Well, it was sounding off for me at idle.
So, I decided to replace the impeller and housing in the SE106 outdrive. I did and found that it really wasn't in bad condition and still had some good flexibility, but I replaced it anyway. I serviced the lower unit with new grease as well, it needed it.
Then I decided to go ahead and replace the thermostat, because that's what he suspected was the issue. So I removed the hoses, then pulled the housing and to my surprise, there was no thermostat present. I also found that the alarm switch, not the temp switch, was damaged. The plastic shroud was missing from the switch but the wire was still attached. So, I'm replacing the gaskets and installing a new 140 degree thermostat this weekend. After examining the way the system operates, is it possible that the absence of the thermostat could make it run hot? Curious if the spring loaded balls going to the exhaust coolant lines weren't sealing well enough, if the exhaust was helping heat up the water and switch on a back feed? I know, goofy question, but I'm looking to solve the overheat alarm or possible actual overheating issue, before I put it back in the water. What else should I check as far as blockage? The belt is tight and the pump looks good externally and this boat has always been in fresh water. Is it wise to go ahead and replace the engine water pump? The boat is showing 767 hours on the meter. Thanks in advance!

what is the water pressure?

...mine, orig owner replaced impeller but had broken piece blocking system just enough to overheat on plane.. was a pain to find.

I'm not sure what the water pressure is? I just moved it from the slip to the ramp and have VERY little time running the boat. I'm going to hook up my water jacket on the outdrive and run it in front of my shop on the trailer, to wtach the temp after I replace the thermostat. I will know more by the first of the week.

sounds like you are on the right track to me.

Here is what I would do in this order until this issue is resolved:
1. insure all water lines are routed properly
2. insure drive belt is tensioned properly around circulating pump pulley
3. inspect cooling system for pieces of old impeller(s)
4. replace impeller (or entire pump if needed) in out drive
5. insure spring loaded balls in T'stat housing are free
6. insure T'stat housing is not clogged with rust
7. insure T'stat is correct temp rating
8. replace T'stat
9. back flush engine cooling passages and exhaust components with water
10. check ignition timing
11. check for bad head gasket
12. replace engine circulating pump
13. check exhaust manifolds, risers, elbows for restrictions
14. check for broken exhaust flapper in exhaust restricting outflow of exhaust water

good luck and welcome to CSR....

cliff

Sounds good. I am at step 8 right now. I will test after I wrap up the stat install, then proceed with your recommendations. I'm pretty certain that the head gaskets are fine. It runs extremely smooth and all the other tell tale signs with a blown head gasket don't exist. I think I will be down to back flushing and checking for restrictions. Possibly could be a jammed broken flapper, but I will check both sides if all else fails. Tuesday or so, I will be able to test with the water jacket and observe the gauges, post thermostat install.

Replaced the thermostat finally. Ran it with the water jacket in the driveway for about 45 minutes. Ran 142 -145 degrees. Perfect. Put her on the water and ran it out for about 3 hours with no issues! Max temp seen at full throttle was around 148 degrees.
Moved on to my a/c issue and found that the "March" water pump was not working properly. The coils weren't cooling to keep the compressor from tripping the reset. Should have this squared away in a week or so. Have her sitting in the slip now.
https://s33.postimg.cc/7m3nubsrj/IMG_4748.jpg

glad it was something simple.....

cliff

I'm having the same issue with my 88 Amberjack (5.7). So far Iv'e replaced the impeller, pulled and tested the thermostat, checked spring balls for freedom of movement, removed and inspected the circulating pump. The circulating pump turns freely and had no leaks. The impeller in it is steel and it didn't appear that there was any form of seal in the pump itself. I've never had one apart though so I have nothing to compare it to. I pulled the hose from the circulating pump from both engines as they ran and it appeared that they were pumping at a similar rate although I did not take a flow measurement. This has the 6 hose Warm Water System. Should this be a 160 or 140 degree thermostat? As I said, I tested the thermostat with a digital thermometer. It was starting to open in the 140 range and completely open by 150 so I'm assuming it's a 140. How do you back flush? I saw on another thread that incorrect flushing could flood the valve train. As I was running the engine I was feeling the various hoses and elbows. I noticed that the elbows were cold near the hose (front) and warm/hot at the rear. Is this normal? Both hoses from the bottom of the manifolds were hot as was the hose from the circulating pump. I opened the petcocks and a small amount of rust and debris came out. While trying to remove one of the brass plugs from the bottom of the manifold it snapped off. So now I have a new issue to deal with and won't try that again! It runs great and there are no signs/symptoms (other than the idle overheat) of head gasket issues. Sorry to be so long winded but frustration is really mounting. Did I mention the boat is almost 200 miles from home?