Mercruiser 888 getting warm

[Cpt D]

I'm asking this question on my cousin's behalf. He has a 1974 Silverline with the Merc. 888. It has the Ford 302. When running the boat at about 4000 RPM's and trimmed out the motor gets warmer than normal very quickly, but not overheating. It will cool back down by simply trimming the outdrive down a little bit without changing the throttle. My cousin thinks it's because the boat is going so fast and riding so high in the water that the water intake on the outdrive is sucking air when trimmed out so far. I don't agree with him and he won't listen to me, so I wanted to ask the experts. I appreciate any input. I forgot to mention that the boat does not have trim tabs or a hydrofoil on the outdrive.

 

[Bondo]

Re: Mercruiser 888 getting warm

Ayuh,... Tell him to Replace the water pump Impeller in the drive...

 

[Don S]

Re: Mercruiser 888 getting warm

My cousin thinks it's because the boat is going so fast and riding so high in the water that the water intake on the outdrive is sucking air when trimmed out so far.

Listen, smile, nod, then disregard completely.

A lot of things, like impeller (#1 cause) thermostats, kinked hoses, bad manifolds or risers, plugged up block, etc. can cause your problem, but likely not too fast while trimmed to high. If you trim it that high, you will slow down because the prop is out of the water.

 

[Cpt D]

Re: Mercruiser 888 getting warm

The trim adjustment can really affect the water pump performance in the drive? Please explain because I know he won't buy it. In fact I'll probably print it out so he can read it.

 

[Don S]

Re: Mercruiser 888 getting warm

When you get the trim high enough for air to be a problem getting into the pump, then the prop is so high up that your speed would be dropping like a rock and your RPM's would be around the 8 to 10K area and about to disassemble itself in your bilge.
Since that is not happening and you are at 4000 rpm (middle of the range) the impeller would be first suspect. At Wide Open Throttle (WOT) you are at max load. If the impeller blades are in a permanant bent over position (common on an impeller over 3 or 4 year old) will not pump as much water under WOT at high load.
Even plugged up cooling systems will cause this because the high load causes more heat.
In Boats, the first thing you chech with an overheat problem of any kind it the impeller.
It's just how it done by the pro's. I know........ I are one

 

[Cpt D]

Re: Mercruiser 888 getting warm

I fully believe that you are a pro and that's why I'm asking at this site. I just wanted to know why his motor would cool back down by simply trimming the outdrive down without changing the throttle or RPM's. Again, I want to thank you for your time. I am also a boat owner and this site has been a great tool for me. All my questions for my boat have been answered in the faq's.

 

[chunckj]

Re: Mercruiser 888 getting warm

Just a thought but you might check the hose going from the drive into the boat it might be siting in such a way if you trim it up it gets kinked just enough.

 

[Don S]

Re: Mercruiser 888 getting warm

If you are at WOT at max trim, then trim down, you will loose rpm. It causes the bow of the boat to drop down and start putting more of the bottom in the water (aka DRAG) with more drag and enough trim down, it will start plowing. Both cause less rpm.
When was the last time the impeller was replaced?????

 

[Cpt D]

Re: Mercruiser 888 getting warm

Thanks for the explanation, but you don't have to trim the drive down so much that the RPM's change to make the engine cool down and it would cool down pretty fast. I also want to add that this would happen at about 4000 RPM's, not at wot. I know you know what you're talking about, I just want to clarify what the boat is doing. Thank you so much for your patience with me.

 

[Cpt D]

Re: Mercruiser 888 getting warm

One other thing, we have no idea how old the impeller is and I by all means think you're right as to what the problem is. I just wanted more details to show my cousin. He's a body man and knows cars inside and out, so he has a hard time listening to other people about motor things.

 

[Don S]

Re: Mercruiser 888 getting warm

He's a body man and knows cars inside and out, so he has a hard time listening to other people about motor things

I have explained it, and not planning on jumping through any more hoops. If the impeller is over 2 years old, replace it, test it and report back. If he's not going to do that, then deal with the overheat and call it what ever he wants.

 

[gadget73]

Re: Mercruiser 888 getting warm

Just a thought but you might check the hose going from the drive into the boat it might be siting in such a way if you trim it up it gets kinked just enough.

This is also a good point. The impeller should definitely be checked, but I'd also have a look at that hose. Trailer the drive up and you'll see what looks like a short piece of heater hose between the drive leg and the transom assembly. If thats cracked, it ought to be replaced. Check out the bellows while you're in there, they also need to not have cracks in them or it could possibly sink the boat.

 

[Cpt D]

Re: Mercruiser 888 getting warm

I thank you all for the replies, especially Bond-o for I know he is a professional. I will print this thread for my cousin and show him so he will hopefully see that he does have some repairing to do on his boat before he overheats it. Thanks again.