More Overheating Fun!

[seattle2855]

1989 Bayliber 2855
OMC 460 King Cobra
Half Closed Cooling system (Closed for the engine, but raw water goes out the heat exchange into the manifolds and out the risers)

Problem:

+ At slow cruise (1500 RPM) risers seem too hot (170 - 195+ degrees), engine temp is normal (175)
+ Under load, engine coolant overheats. Risers too hot also (probably -- forgot to check in all the excitement).

Checked / completed:

+ New manifolds / risers.
+ Raw water supply to heat exchange **seems** ok (what is the test?)
+ Hoses from heat exchanger to manifolds are good.

The story:

A weeks or so ago I posted my troubles with an overheated exhaust on the port side. I suspected the manifolds and risers were clogged. I took them off and replaced them this week. They were bad (rust flakes), but not awful.

Today I took the boat out for a test spin. There's about a 2 mile cruise to get to open water. I ran at 7 kts (1500 RPM) to get out there. Once out in open water, I used a infrared temp gun and checked the risers after the slow cruise. One riser was around 170 degrees and the other 195. Engine temp was right at 175 where it should be.

I then opened it up to get on plane. After about 45 seconds, I could see the engine temp rising quickly. I immediately stopped and checked the engine. The exhaust risers were 210+ degrees but not smoking/melting like before. Some antifreeze was coming out of the overflow valve on the heat exchanger cap.

I let it cool down and came back to my slip at 7 knts and the engine temp stayed normal.

I know I need to check/replace the impeller (last done in 2007). I had checked the water flow coming into the heat exchanger by removing the supply hose and running the engine for a few seconds and it seemed ok.

If the impeller isn't the issue (I have doubts) what else could I look for? The risers still seem to be running a little hot even with the new parts and I'm overheating quickly when run under any load.

 

[HT32BSX115]

Re: More Overheating Fun!

You NEED to replace the impeller and maybe the pump housing. (Make sure you locate all the remnants of this or any previous failures.....I think we talked about this before?)

(I forgot you're closed cooled) You may also have a clogged heat exchanger......either from raw water debris or rust in the raw water side, or rust in the engine clogging the engine block side....OR BOTH.

If the engine doesn't overheat at low power settings but does at higher power, the current water flow thru the exchanger is insufficient... either due to a bad pump or a clogged heat exchanger or both.

It's all about water/coolant flow.

If your risers are getting too hot, they're not getting enough water flow thru them.

You should have an oil cooler and a power steering cooler. They usually get cool raw water from the transom and are between the transom water line and the heat exchanger. They may be clogged with debris or raw water pump fragments....


Cheers,

Rick

 

[seattle2855]

Re: More Overheating Fun!

I will definitely be having the impeller changed as soon as I can. But with the boat being moored in water and having no trailer, it's taking a bit to get hauled out.

I want to make sure I have a complete list of things for them to check because you can't really replicate them problem out of water. I don't want them to just change the impeller and say "all done."

One thing that I'm having trouble with is gauging my current supply of raw water. One manual says I should be able to remove the raw water supply line to the heat exchanger, run at idle, and see the water come up and out of the hose to a height of 2 - 4" (water hose pointing upward). I don't get 2 - 4 inches. Maybe I get one. This is after the raw water has gone through the oil and power steering coolers. I didn't check supply at the metal supply line out of the transom.

But another manual (Seloc) says I should be able to get a gallon of water every 45 seconds or so on the house that supplies the manifold. I seems like I am getting that much, but I haven't measured it exactly.

I did take off the cap on the heat exchanger and it looked pretty clean.

 

[HT32BSX115]

Re: More Overheating Fun!

The temp of your risers is DIRECTLY related to how much water you "flow" thru them.

Just about all pleasure boat V-8 marine engines have all the cooling water exit thru the riser/elbows. They'll get extremely hot with insufficient water flow.

Something is restricting the flow of raw water thru your engine be it the raw pump or something in between like the oil/ps coolers associated lines etc.

It's also enough of a restriction to cause your heat exchanger to not be able to cool the engine at high power settings.

It could also be something as simple as your sea water pickup being plugged with seaweed or other debris.

 

[seattle2855]

Re: More Overheating Fun!

Thanks for the help.

I know I've asked this before, but what is the appropriate temperature for the risers on a half closed system? I think you said 170 - 180 before. Is that the max temperature? I'm having trouble finding any documentation on that other than the "can you put you hand on it" test.

 

[gypsysoul]

Re: More Overheating Fun!

I had this very same problem with my 29' safari. the starboard engine would run up to 1500 rpm all day, but as soon as I started to stand on the throttles that engine would overheat. I had replaced the impellar (twice just to make sure). at the start of this season I pulled the end cap off the exchanger and found the remnants of manny old impellars vanes. I cleaned them out, and was also able to run a rod through the tubes on the exchanger (with the discharge hose off so I could wash out the debris). and now no heating problems. anyway I hope this helps and you cure your problem.

 

[HT32BSX115]

Re: More Overheating Fun!

Thanks for the help.

I know I've asked this before, but what is the appropriate temperature for the risers on a half closed system? I think you said 170 - 180 before. Is that the max temperature? I'm having trouble finding any documentation on that other than the "can you put you hand on it" test.

Yeah,

I'm not really sure that there is "documentation" on how hot riser/elbows should get. They certainly wouldn't get hotter than about 210*F or so or they would be pumping out a lot of steam. There's usually quite a bit of water flowing out the exhaust.

My 460 was raw water cooled and the risers did not get so hot that I couldn't hold my hand on the top.

My 454 is no different.

You can see though, if you restrict the flow even a little bit every part of the system gets hot very quickly.


I'll bet your heat exchanger and/or oil/PS coolers are partially plugged.

 

[cantaris]

Re: More Overheating Fun!

Just a shot in the dark here but you said new risers were put on right? was the correct gaskets used. If I am not mistaken there are 2 different gaskets used, one for full system and one for engine only. Like I said maybe a shot in the dark

 

[seattle2855]

Re: More Overheating Fun!

Just a quick update on this problem...

Turns out the metal sleeve in the impeller housing had spun, so the inlet and outlet holes were no longer where they should be. Somehow it was still pumping some water, just not enough to keep it cool under load.

The old risers/manifold just made the problem worse.

 

[HT32BSX115]

Re: More Overheating Fun!

Just a quick update on this problem...

Turns out the metal sleeve in the impeller housing had spun, so the inlet and outlet holes were no longer where they should be. Somehow it was still pumping some water, just not enough to keep it cool under load.

The old risers/manifold just made the problem worse.

Hey! Glad you finally checked on the Raw pump!

This is why some people suggest changing the raw pump every season if they're used/moored in salt water..

They're pretty easy to change on a Cobra while in the water..... Just don't drop the bolts!!


Cheers,

Rick