My engine is overheating

I need help desperately. I have a 2000 starcraft nexstar 1910 lx w/ a 5.0Gl pefs volvo pinta engine. My engine is overheating when I go over 3000 rpm's and my audible alarm is sounding off. The sensor that is triggering the audible alarm is part # 3850397. It's a sensor just on the housing that is above where the thermostat is. I am attaching a diagram of a section of my engine which displays the sensor that is triggering the audible alarm. the sensor is labeled as #30 on the diagram. I am a bit of a self mechanic but have little experience with boat engines but I have a hunch that there is not enough water circulating through my engine just don't know why. maybe impeller, water pump, thermostat. No idea and no idea what the sensor is supposed to detect. If anyone can help please it will be much appreciated!

Re: My engine is overheating

Have you ran through No.1?

http://forums.iboats.com/mercruiser-i-o-inboard-engines-outdrives/vp-357328.html#post2360521

Re: My engine is overheating

When was the last time you changed your impeller?

What does your temp gauge read when the alarm signals?

It will also sound for low oil pressure.

Re: My engine is overheating

" ran through No. 1" I don't understand. Reads under 180 degrees until I exceed 3000 rpm's then it climbs gradually but somewhat quickly almost to 230 degrees. Oil pressure gauge reads 20 psi idle 30-35 psi under load.

Re: My engine is overheating

If it doesn't overheat untill you get above 3000. It sounds like a blockage.
Fresh or saltwater?Profile,Location?
It's possible the impeller is bad?
The thermostat clogged/blocked.
The risers clogged blocked?Its 13 years old and possible they need replacing?Again saltwater or fresh?
Block full of sand?

Re: My engine is overheating

OK...I will repeat my question...

When was the last time you changed the impeller??????????

Its not your sender. The switch that sounds the alarm and the sender that goes to your gauge are both separate from each other and they both say you are overheating. Changing that would be killing the messenger.

Re: My engine is overheating

Fresh water. Bought the boat on Feb 14. This year. So don't know what had been replaced but when I took the drain plugs off the engine to get water out there did seem to have mud on the drain plug. All four of them.

Re: My engine is overheating

The first thing you need to do is to change the impeller. More than likely, that's what is causing your problem.

As far as you know, has the engine ever been started out of the water(even for 30 seconds) without water to the drive through muffs?

The mud you see on the drain plugs is probably just rust.

Re: My engine is overheating

Check the impeller. Bet when you take it out the vanes are all bent over and won't straighten out. When that happens, it can put out enough water to keep the engine cool at lower speeds, but not at higher speeds.

Impellers should be replaced yearly per Volvo, but most people just run them until there is a problem, and you have a problem.
Besides, on ANY overheat problem on a boat, the first thing you check is the impeller. So many things can cause them to fail.

Re: My engine is overheating

bruceb58 said:

OK...I will repeat my question...

When was the last time you changed the impeller??????????

Its not your sender. The switch that sounds the alarm and the sender that goes to your gauge are both separate from each other and they both say you are overheating. Changing that would be killing the messenger.

Click to expand...

:laugh:

Take heed here circulation pumps rarely fail in boat engine's.... however fresh water or sea pumps are so bad they really need to be checked every yr. Take your's apart and look the the fins...change it no matter what and they if the old one has a few vane's missing well your about to learn how to backflush a marine engine..there scattered thourghout the cooling system. It reads like you have one or two stuck in your thermostat now.

Re: My engine is overheating

Make sure yr new impeller is a snug fit at the ends, don't just assume OEM is correct. My new one turned out to be 1-2mm short and I had a massive overheat as a result which all but knackered the engine.

Re: My engine is overheating

Thx for all the input everyone. Very helpful stuff. I talked to the dealer I purchased it from and their mechanics came to the same conclusion. So they agreed to fix it for free since they missed it before the sell. They're going to change the whole sea water pump.

Re: My engine is overheating

Does the motor have a heat exchanger? My motor had the same symptoms last year even after a new impeller.

First check the raw water strainer and clean if necessary.

Then if you have a heat exchanger, remove the copper block inside. This block acts like a car radiator and has cooling fins that clog. Mix in a 5gallon bucket about a 10% mixture of Muriatic Acid and water. Meaning 10% Muriatic Acid the rest water. Soak the copper block in a 5gallon bucket for about 2minutes at a time, rinsing thoroughly with water in between soaking in the acid solution. You will be amazed how much microscopic film is coating the copper block. You can buy the acid at any hardware store.

Be very cautious with the acid solution IT WILL BURN YOUR EYES AND SKIN if you come in contact with it or splash it on you. Wear thick waterproof gloves and definately eye protection and if possible use a long pair of tongs or pliers to dip the copper block.

If that doesnt work check the raw water pickup and the connections on your outdirve, there is a metal neck that sometimes corrodes and sucks air at speeds over idle.

Re: My engine is overheating

Well repair shop replaced water pump and said my impeller was shredded.

Re: My engine is overheating

Ask them if they took some time to look for and fished out any pieces of the impeller downstream of the pump. When impellers come apart the pices that shred off must be found and removed or there will be a blockage after the pump that will affect flow and an overheat condition could still occur.