OMC KC 460 overheating

[JTKC]

I have a 96 Four Winns 245 Sundowner with an OMC 460. At rpms below 4000 the engine runs at a solid 140-145 F all day long. The problem is once I run the engine above 4000 rpm, the engine overheats (pegs the temp gauge) and goes into limp mode (idle) until it cools down.
The raw water pump has been replaced and the exhaust risers have been checked for blockage.
Any help would be appreciated!
Thanks

 

[kenny nunez]

Put a length of clear hose where the incoming water connects to the thermostat housing. Have someone watch the hose when you go over 4K rpm. If there are bubbles traveling to the thermostat then there is an air leak on the supply side. If the water seems to disappear in the clear hose then there is a head gasket leak.

 

[HT32BSX115]

I have a 96 Four Winns 245 Sundowner with an OMC 460. At rpms below 4000 the engine runs at a solid 140-145 F all day long. The problem is once I run the engine above 4000 rpm, the engine overheats (pegs the temp gauge) and goes into limp mode (idle) until it cools down.
The raw water pump has been replaced and the exhaust risers have been checked for blockage.
Any help would be appreciated!
Thanks

Howdy,

Welcome aboard!

First of all, it will help a lot if you post your model number so we can familiarize ourselves with your engine and drive combo!

Only the year model of your boat is not very helpful. The last year for the OMC King Kobra 460 was 1990 so if your 1996 boat actually DOES have a 460 inside, someone in the last 20 years pulled out whatever was in there and installed either an interim 1990 460 + Cone clutch drive, OR your boat has a MUCH older 1987-1989 Dog-clutch drive, or you have a "Franken-motor/drive"

In any case, the model number will help a LOT.

If it's unmolested, completely original, and it's really a Big-Block 1996 boat, it would have one of the following engine and drive combinations.
744EPNCA 1996
744FPNCS 1996
74FFPNCA 1996
74FGPNCS 1996

Or

744DPHUB 1995
74FEPHUB 1995

All are GM 7.4L bigblock engines.

If you're not very sure of it, take some pictures and post them here so we can see.

Regards,

Rick

 

[JTKC]

Thanks Kenny and Rick!

Kenny
I've heard of the clear hose trick azad and plan to do that. I would think ithinkad a blown head gasket I would see water in my oil. Oil is clean and compression checks. Thank you!

Rick
Sorry for not posting the engine/drive model #. My boat is approx 160 miles from my house. I will get the #'s and post when I get back in town.
it may have a Franken motor-drive. The broker selling it didn't winterize properly and the block cracked. The engine was replaced with a new crate engine, now approx 50 hrs, so it may be a GM 454.
Anyway I appreciate the help!
Motr to come. Thanks again!

 

[JTKC]

The bummer is it ran like a ***** WOT the first year I owned it (2 yrs). Then last year it started overheating above 4k. Fortunately I typically cruise @ 3500 rpm max. The Lake of the Ozarks is way too rough to run a 24 ft boat @ WOT most of the time. It would be nice to have it when I need it tho.

 

[kenny nunez]

A head gasket leak on a raw water cooled system will not show water in the oil unless it is completely blown out. Just the slightest combustion leak is enough to overcome the water pressure from the sea water pump.

 

[JTKC]

Ok.Wouldn't it show up in a compression test though?

 

[HT32BSX115]

An over heating engine is more frequently a raw water pump problem or an obstruction in the cooling system to include manifolds, risers, and the water (circulating) pump.. Leaking head gaskets don't usually cause over heating.

If the engine is more or less original (type) it's likely a 454. (btw, THANK YOUR LUCKY STARS YOU DON'T have a 460!!!)


You have an oil cooler and a power steering cooler. It's possible that you have either debris from previous raw water pump failures or from the raw water itself that could be blocking flow through them. If you get an over all reduction in cooling water flow, the engine would over heat at higher power settings........but flow enough water at lower power to not overheat.

When I installed my 454 Mercruiser, the previous owner told me that he had recently changed the raw water impeller.

I removed the oil cooler to ensure it was "clear" and found it so crammed with rubber impeller fragments, I had to pull them out with needle nose pliers!

No amount of back-flushing would dislodge them!!

It's possible that the PS cooler wouldn't stop small fragments but many water-to-oil heat exchangers have pretty small passages and will not pass small rubber fragments from a failed impeller or other debris that could be sucked up out of the lake.


Also, if you have a leak in the raw water system after the water pickup but prior to the raw water pump (suction) and there's a small leak, you'll end up with a lot of air bubbles.

If you sucked enough air it would reduce overall water flow enough to cause an overheat at higher power settings.

 

[Lou C]

When my 4.3 finally blew both HGs 3 seasons after a bad overheat, my comp test results were close to normal but I was getting water in one cyl and a mist of water in the other.
Post up a pic of what you have. A '96 245 Sundowner should have come with a 5.7 standard with the V/P drive most likely. I'd be surprised if you have a franken boat but you never know...just would have been so much easier for the shop to replace with what you had.

 

[HT32BSX115]

'96 245 Sundowner should have come with a 5.7 standard with the V/P drive most likely. I'd be surprised if you have a franken boat but you never know...

I can imagine the absolute NIGHTMARE of someone taking a boat like that and installing an OMG 460 King Kobra and then selling it to some unsuspecting new boater!!

 

[Lou C]

That would be a cruel joke for sure! The unavailability of parts is the issue. No manifolds, couplers, certain oddball parts, etc. I was pretty lucky as my '88 4.3 has pretty good parts availability, VP Couplers and Exhaust fits just fine, in fact I replaced my OMC batwings with the exact same center riser style exhaust used by OMC and later VP, for years all the way till VP started using the required dreaded Cat Converters. It cost a bit for the parts needed for the conversion but now I'm glad I did it. It had simple points ignition, no split fire module no one makes, simple Quadrajet carb, no oddball NLA fuel injection computer, etc. Even the ESA module I was able to get in the aftermarket (CDI).
The rest, well the OMC stuff IMHO was good quality. I have had to repair a few things on the drive but really normal repairs for the age of it and salt water moorage for years. And, it did not have trouble with what Mercs often have after being slipped in salt water, leaky/rusted steering arms and worn out pivot pins. The bellows lasts 10 years and the impeller takes 20 min to change. As long as I can get parts, I'm running it.

 

[JTKC]

First off, I was incorrect. It is a 96 with a GM 454 King Cobra. The engine is a duplicate of what was installed when built.
Its definitely pulling air in the raw water side @ 4000 rpm and above. All heat exchangers and exhaust risers have been checked. The raw water pump and gasket is new and impeller is engaged with its drive shaft.
The only thing left that I can think of is the suction hose to the pump from the intake

 

[HT32BSX115]

First off, I was incorrect. It is a 96 with a GM 454 King Cobra. The engine is a duplicate of what was installed when built.
Its definitely pulling air in the raw water side @ 4000 rpm and above. All heat exchangers and exhaust risers have been checked. The raw water pump and gasket is new and impeller is engaged with its drive shaft.
The only thing left that I can think of is the suction hose to the pump from the intake

If you used a clear hose in place of the raw water feed hose, and you saw bubbles, you have a leak somewhere.

It may be any of the following models since they appeared to offer MPFI in 95 and 96


744DPHUB 1995
74FEPHUB 1995

744EPNCA 1996
744FPNCS 1996
74FFPNCA 1996
74FGPNCS 1996

I didn't look at the subtle differences but it might be important when you're looking for parts.

You must use the engine/drive model number. Hull VIN (and year model) doesn't tell you much about the engine. All boat manufacturers frequently use "last years" engine/drives until they "run-out".

Can't remember if you said before, If the raw water pump is on the drive, it's a 95.

If the raw water pump is on the engine, it's a 96. (and again, thank your lucky stars!!)

Regards,

Rick

 

[JTKC]

Thanks Rick!
Yes. The raw water pump is on the drive. New pump impeller and gasket. My mechanic put a clear hose b4 the thermostat and bubbles appear @ 4k rpm.
the current plan is to replace the hose from the raw water inlet on lower drive to the raw water pump.
I hope this takes care of the problem bcuz he's out of ideas. I trust him. He's been my mechanic on every one of my boats. I'll post the engine/drive number after my next trip.
Thanks again for your advice!
Jeff

 

[HT32BSX115]

Thanks Rick!
Yes. The raw water pump is on the drive. New pump impeller and gasket.

Sorry to hear that! That means it's a 1995 engine and drive model

744DPHUB 1995
74FEPHUB 1995

Be VERY gentle with that drive because many hard parts are only available if you buy a used drive to "cannibalize"

OTOH, if you decide to part it out, that drive is somewhat valuable! (for people that break theirs) OMC started building that drive type around 1990 as an "Interim" to finalizing the OMC/Volvo Penta partnership and eventual VP buyout of the sterndrive part of OMC.

It's not the "same" as an SX (or a "Dog" clutch drive either)

It's my understanding you CAN put an SX (or DPS Duoprop) on that boat but you have to use an SX transom mount. (which means everything from the flywheel housing -----> BACK

The VP "GURUs" can comment further on this.


I think your ignition is the infamous (or dreaded) OMC "Spitfire" system. When the module dies, you switch to a Mallory or Mercruiser Thunderbolt or other (complete) ignition system. I am not completely sure but I don't think the module is still available .......Again, the VP people can tell you if a Volvo Penta part is available for it.

Good luck!

Rick

 

[JTKC]

Bummer! New suction hose installed and still overheating! I'm out of ideas. Any help would be appreciated. TIA

 

[Lou C]

OK now on the OMC Cobra (dog clutch) and Volvo SX, a plastic fitting is used in the pivot housing, that the hose from the transom mount connects to. If it cracks, or the hose is not on tight, the cooling system will suck air when the boat is on plane. The King Cobra cone clutch is different in design but there are a few parts that could cause the same problem. Take a look at parts #s 68, 9, and 76 (transom to pivot cooling hose). Make sure parts #68 and 9 are not cracked, and that the hose fits tightly on each end.
https://www.crowleymarine.com/johnson-evinrude/parts/53977.cfm?mdl=PNLH3I

part #9 must seal up against the gear housing, check what the aluminum looks like in that area, if its pitted it could allow air to get sucked in.

https://www.crowleymarine.com/johnson-evinrude/parts/53979.cfm?mdl=PNLH3I

this is a schematic of the upper gear housing, see the circular area near part #6, that's what part # 9 must seal up against...the water gets pushed through the drive from the impeller this way....

 

[HT32BSX115]

Bummer! New suction hose installed

AND, you (still) have bubbles?

Or you don't have bubbles and it's still overheating?

 

[JTKC]

Not sure if still having bubbles, but still overheating.

 

[HT32BSX115]

Not sure if still having bubbles, but still overheating.

In general, overheating with any raw water cooled marine engine is either due to poor flow of water from the raw water pump or blockage anywhere through the engine to include the exhaust manifolds and risers (clogged with rust)

Also, if the block is clogged with sand or other debris you can get over heating.

If you're getting low flow it can manifest as only overheating above a certain power level.

I assume that you have already checked your thermostat for operation. If it's a salt water raw water cooled engine you could also have clogged (with rust) block, heads, and water (circulating) pump.

If there's an oil cooler, it could be clogged with debris if prior to the raw water pump and/or impeller vane fragments if after the raw water pump.