Strange overheating problem

Rock Hall

Rock Hall

New Member
Feb 18, 2006
163
Lancaster, PA
Boat Info
1989 Searay 220DA 5.7
Engines
Mercruiser 5.7 Alpha One Gen I
I've got an 89 220DA with a 5.7 Alpha One/Gen I
I noticed a few weeks ago that sometimes when I would accelerate over 3000rpm my alarm would sound. it would immediately stop as soon as I backed off. Thinking maybe my water pump was getting old, I replaced it this weekend. Took it back out and same thing. I run a low temp thermostat. 140 Deg I think. Under low load the gauge reads about 140. When I accelerate or run over 3000 rpm it goes to about 150 (this is a guess) the needle hardly moves. this is when the alarm goes off. It is my understanding that the alarm shouldnt sound till at least 180. is this right?
I notice 2 sending units above the seawater pump on the engine. is one for the gauge and one for the alarm? My next replacement is the sending unit if it is separate. it doesnt make much sense why the gauge reads 150 and the alarm is going off.
Any ideas?
Help?

Mark
 
I've always got a little water in there from the rain. the pumps are mounted up pretty high and I've never seen the bilge totally dry. Are you thinking circulator pump? I did try to look for water dripping when the engine is running, but didnt see anything.
 
Yes... to your question about the two sending units. They should be in the T-stat housing.

When you wrote "water pump" did you mean you replaced the impeller?

When you wrote "sea water pump", do you mean the circulating pump (same looking thing that there is on a car)?

Gauges can be off, sensors can go bad. When you get the overheat (and when you don't), try placing your hand on the two exhaust risers. You should be able to keep them there relatively comfortably.

Was a drive lube monitoring bottle ever added to your engine?

How's your oil pressure?
 
I replaced the entire pump, seals and housing in the outdrive. Seawater pump, yes circulating pump, I have not replaced yet. No Drive lube bottle
I'm thinking sensor because the alarm seems to go off prematurely. Oil pressure is where it always has been. I would expect that low oil pressure would most likely happen at low speeds.
When I try to identify the sensors to buy one, I only find one unit. I visually saw 2, but the parts suppliers sell only one. I think one is variable and the other is on/off? I've been scouring the net for the sensors and all just have "water temp sending unit"
Thanks for your reply. I really appreciate the help. I hate to keep throwing parts at it, but thats what I might have to do. the difficult thing is my boat is 100 miles from me and I only go on weekends. So I get to replace one thing a week. to buy parts down there is outragous.
 
If you know your oil psi is good, feel the risers. If they are comfortable, at least you know that nothing is wrong. You could now take turns disconnecting the oil psi sensor and the water temp sensor to see if one of them is flaky.

"Sending unit", or "senders" go to gauges. "Sensors" go to alarms. Senders will have a brown/tan wire and also have two wires. Sensors have only one wire. I think it was the same back in '89, but double check me on the coloring.

Water Temp sensor is in the T-stat housing. Oil PSI sensor is back by the oil filter (or the filter block-mount adapter if you have a remotely mounted filter). The Oil PSI sensor is much larger than the sender - about the size of a baby food jar.

You may just want to call a local Merc shop with your engine serial number and let them get the right part for you.
 
thanks for the info. Risers are warm but not too hot. I will check again after acceleration.
I went through the factory manual and about 10 websites trying to figure out part numbers. I found a pic of the housing and it pictured both units. took the part numbers and cross referenced them on the site that I usually buy parts from. Found both. They are cheap enough that I may as well buy both rather than have to wait another week to try the other one. one has one lug on it with a nut and the other takes a 2 prong connector. I also ordered an oil pressure sensor because mine has shown 80psi almost all the time since I got the boat. thats before and after the repower. That one might be the gauge, we'll see what happens. But I dont think that is the problem I'm having now. Oil pressure should go up when I accelerate. If the senders/sensors dont solve the problem, I will replace the circulating pump. My neighbor just went through the exact problem except his engine was really overheating. he is down to the manifolds. replaced everything else.
Also, my engine shows no signs of being too hot. no steam, no smell, no strange noises.
going back to the original weird thing. my temp gauge reads about 150 and the alarm is going off. Hopefully, it's the sensor. I'm also going to replace the thermostat while I'm at it.
thanks again for the help!
 
Sounds like you're on the right track.

"One lug" should be the sensor. "Two prong connector" should be your sender (with one of the wires tan).

Good luck - post back when you figure things out or have more questions.
 
I was just thinking about the other alarms on your boat. High water alarm to be specific. When you slow down the water might surge forward setting the alrm off.
 
I hadnt thought of that. I'm not even sure I have a high water alarm. I've got a high water pump, but the water level is no where near that. It's not even enough water to pump out with the pumps on the manual switch. I will look into that though.
According to the diagram I looked at, the single lug unit is called a sender and the 2 prong is called a sensor. they both say to be used with a gauge in the description. I think that is a misprint.
http://www.mercruiserparts.com/Show...r=11667&bnbr=170&bdesc=Thermostat+and+Housing
that is the diagram I am using. I am ordering both to be sure.
 
Rock Hall said:
According to the diagram I looked at, the single lug unit is called a sender and the 2 prong is called a sensor. they both say to be used with a gauge in the description. I think that is a misprint.
http://www.mercruiserparts.com/Show...r=11667&bnbr=170&bdesc=Thermostat+and+Housing
that is the diagram I am using. I am ordering both to be sure.
Click to expand...

Arghhhh!!! I gotta stop typing late at night! You and the website you linked to are correct. Except for the part about both being used for a gauge.

However, the t-stat housing in that picture is from a newer model engine. Has your OEM housing been replaced with this newer style? I don't know if the sensor is different, or not.
 
I have 2 "sensors" just like the diagram. Have both on the way. I only get to the boat on weekendndsfdds
 
Yes, that's correct - you should have 2 things. One is the sender (gauge, one wire) and one is the sensor (alarm, two wire). But, take note to #13. That is a relatively new design by Merc. Because it's newer, and newer engines run hotter, I wonder if the sensor is going to be calibrated correctly for your set-up since you probably also use a lower temp t-stat.

That was a good call ordering a t-stat, too (assuming it's the correct one for your engine).

Before you replace anything, though, take a few spins with the t-stat sensors unplugged (2-wire) and see if the alarm stops sounding. If it still sounds, disconnect the oil PSI sensor and drive the boat some more. This is a good idea to do since it will narrow things down and you'll know for sure.
 
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i think the sensors are set to trip at about 180. not sure where i read that, I have been researching all over the web. When I had my engine installed, they put a 165(?) deg thermostat in. I was having some oil consumption issues and took it to another shop for warranty check. they told me that this engine should have a 140. So I changed to a 140. Now I hear it should have a 165. The 2nd shop told me the higher temp was for EFI. I am carbed. It runs fine at the lower temp. I have heard 50-50 what thermostat I should run.
I figured since the engine seems to run fine at 145, then cooler is better.
 
I thought that might be problem at first. pulled boat out last weekend and put in a new pump. cleaned everything up. still the same problem.
 
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