Overheating at Close Quarters-gctid350435

[Guest]

I have a Bayliner 4387 with twin gas Mercruisers. One is new (Starboard) and one rebuilt. I'm in the SF Bay area and a few months ago I was taking the boat out and I noticed the Starboard engine looked like it was running a little warm, but wasn't overheating while I was underway. As I got to a dock the temperature spiked up and the engine cut off. Not the best time for that but with no wind I was able to get her tied up. About an hour later I started her back up and we left for a spot to anchor and I noticed again, when the boat is moving, she runs on the warm side, but not in the red. Slow her down to close quarter speeds (1,000 rpm or less) and she overheats. We anchored overnight and the next day took her back home (3 hours away) and this time nothing happened at all...worked fine. I assumed the intake under the boat might be clogged a bit (although I have her scrubbed every quarter).

Fast forward to this weekend and took her out and as I was docking her once again she overheated and shut off.

So while moving above 1,000 rpm and maybe 3+ nautical miles per hour...she seems to get enough water to cool...

Any thoughts as to what this might be?

One thing that mystifies me is my coolant overflow tank is low...I fill it up a bit...and she ends up low again. Can't see any leak in the bilge so no idea what's up there.

[Guest]

Check the cooling system (raw water side) first- seacock, sea strainer, hoses, impeller. Next, check your closed cooling system- coolant, heat exchanges, hoses. Odds are it's either a clogged sea strainer or bad inpeller.....

[dmcb]

Impeller would be the first thing I would look at.

And I would quit running it before you need another new engine or head. You are doing great harm to your engine if it overheats to the point of shutting down.

Doug

[Guest]

I just did a test to satisfy my curiosity...

Here in the slip...powered it up and ran the rpm up to about 2500...temp is fine...took it down to 1000 rpm...temperature went right up.

[Chief_Alen]

What drives ?

Without the reply right now i would think a t-stat not opening all the way, a restricted water pickup, a elbow clogged, heat exchanger clogged up, and the 'ole impeller lost pieces.

Me wherever the water enters the motor the first point and if it is a raw water pump mounted on a engine say a bravo system i would pull the hose after the pump and see if you have good water to that point. Say you have good water to the engine you have to follow the system forward to the point you do not have good water.

[dmcb]

With a water pump impeller that is bad it will still pump water. The faster it turns the more it will pump. Coupled with the fins that can still be in the system slowing down the flow, what is happening to you is what happens in these conditions.

Doug

[Guest]

So I started following the hoses from the intake which all had water coming in fine. I then opened the impeller, but I'm not experienced with these so I don't know if this looks good or bad. She's rubber...this look like it's a problem?

[img]/media/kunena/attachments/vb/654295=24659-photo.jpg[/img]

[img]/media/kunena/attachments/vb/654295=24660-photo2.jpg[/img]

[Guest]

I pulled out the impeller and it looks brand new...any other thoughts?

[boatworkfl]

Check the strainer, back flush it if possible in case the pick up strainer is clogged, check the thermostat, it may not be opening at the proper temp.

Check the exhaust at the riser exit for rust.

[Guest]

I would check exhaust manifolds and elbows. When the rust up they can be OK at speed and not let enough water through at idle.

[Guest]

[img]/media/kunena/attachments/vb/654311=24661-photo3.jpg[/img]Feeling around I just found a leak off one of the hoses from the thermostat...this is the antifreeze/coolant side...I said up before that sometimes the overflow would have fluid....and then be empty...so what if the engine is simply low on coolant? I just never noticed this leak before because is was so slow.

[Guest]

Okay I tightened her up, topped off the coolant tank, ran her for 15 minutes...high rpm and low...she never goes up above 150 on the gauge...plenty of room before the red...when I did this the other day the moment I took the rpm down she's spike in heat. So I'm guessing that when I'd head out...I'd run low on coolant...and the raw water at a higher rpm kept the engine cool but at a lower rpm the other coolant wasn't enough...

Does that sound right?

[Guest]

You mentioned there was no coolant in the bilge. If this is in fact a leak, wouldn't you see it in the bilge. My concern would be around where the coolant is actually going. On my previous boat, I discovered one of the walls of the manifold had developed a crack; and the coolant was going into the exhaust stream. I would run the engine for 15 minutes and a third of my coolant would disappear. Check to see whether this is the case. You don't want to be losing coolant, even in small amounts. I eventually replaced the manifold and the problem was solved.

As to whether that happened at slow RPM's, I don't think it made a difference for me.

[Guest]

I found the leak around the elbow joint you see above, where it screws in...I tightened it up...it was loose. I think the reason I may not have seen any coolant is that it's such a slow leak that it slowly ran out (very slowly) and into the bilge and mixed with the other water in the bilge. I also think that the overall coolant levels were really low, but as I'd top it off, it would run fine awhile.

I would hope it isn't a crack though...this is a new engine with only 300 hours on her.

[Guest]

I'm hoping its not a crack either. It took forever to diagnose; and I ended up removing the manifold as a last resort. If you suspect that leak was a slow one and responsible for depleting your coolant, keep a constant eye on coolant levels. Hopefully, you caught the problem.