Merc 150 EFI Overheat Alarm

[Czarthree]

I have a year 2000 Merc 150 EFI. Yesterday when hammering on the lake a overheat alarm came on. I instantly shut her down and let cool off. 10 minutes later I started off again with out any alarms. Upon hammering another 10 minutes the overheat alarm came back on. Same thing, shut her down instantly. Was able to motor about at lower speeds,(10 to 15 mph) without the alarm going off again. I recently had the impeller changed and am getting a steady stream when the engins warms up. It has 2 thermostats that have never been changed. Could bad thermostats be the culprit? Any help is appreciated. Thanks in advance.

[jpollman]

If you've recently had the impeller changed, my next step would be the thermostat.

One other note though. Are you sure it's an overheat alarm? A couple years ago with my '93 Merc 90 ELPTO I was getting what at first I thought was an overheat alarm. I had just replaced the imepeller too. But by chance I looked at my volt meter and discovered that It was pegged off scale! It turned out that I had a charging system problem. I ended up replacing the stator and regulator and it solved the problem.

John

[Czarthree]

Thanks for the reply, John. The alarm is constant and not intermittent and therefore indicates an overheat problem according to my manual. It seems my voltage is fine as the charging system was putting out near 14 volts according to my bottomline and in dash volt meter. I`m thinking thermostats as they have never been changed. Once again, thanks fo your input.:) Anyone else?

[barryl]

I won't be much help because I have a 1999 optimax 135 that does the same thing. When the waves are up and I try to find that right speed mine overheats also. Mine will overheat just above 1800 rpm but when I can open it up over 4000 rpm I can watch the water and cylinder head temp move right back to normal. I had it to the shop several times and they can't get it to overhead because they try it out on a smaller lake and don't get the pounding. He has changed the impeller (and housing), the thermostats, two different cylinder head gaskets, flushed the whole system many times, even replaced the cooling tube gasket that run up into the power head (many other things that I can't thing of right now). He was in contact the factory many times and as far as I know it still does it. I left it with him at the end of October last year when the wife and I went south and I haven't had it out this year yet, but I will be up to Burt Lake next week and I'll have my fingers crossed (and what ever else I can cross).

If it is fixed I'll let you know and give you the guys number, actually it's Lapeer Boat Service. He has done lots of work for me, is very reasonable and other than this particular problem I'm very happy with the service.

I know this wasn't any help but I just thought I would clue you in what I've went through so far and maybe someone else can chine in if they have anything similar. If you find your problem let me know.

[Czarthree]

Thanks for the reply Barryl. I`m gonna change up the thermostats and take it from there. Anyone else with ideas?

[barryl]

You may also want to check - http://forums.iboats.com/index.php - I have found this forum to be a great resource for anything boating. Good luck!

[jakester]

I had the very same motor on my old boat. I had to change my thermostats every other year. I can almost bet thats your problem. Very simple to chagne out'

[Czarthree]

Thanks jakester. Changi`n them this thursday.