Mercruiser Overheats (351CI, 233HP)

[JackH]

I have a 25' cuddy cruiser with the original 351CI, 233HP Mercruiser that has been rebuilt professionally 85 hours ago. The engine runs as smooth as silk and starts at the touch of the key. It has had all routine service done and has been tuned to specs. Total hours on the engine are 835. The water pump in the outdrive was replaced just days ago to correct an overheating problem. The boat has plenty of power, gets on the step easily but the temp gauge begins to climb shortly after planing. Below 1800 RPM, and just before planing, the temp stays in the safe range. There appears to be ample water coming from the engine coolant exhaust ports and, using an enclosure with the outdrive submerged, the temp never reaches an unsafe level. The thermostat has been removed and discarded and a new 190 degree thermostat installed. This seemed to stabilize the gauge slightly, but under a load the engine still gets over 220 degrees if left loaded. Reducing the throttle will allow the engine to cool properly within a very short time. Manifolds and Risers appear to be in good condition and there are no cracks or leaks apparent. There is no water intrusion into the oil and the compression on each cylinder is within 5% and averages 158. Any suggestion will be greatly appreciated, as we are at our wit's end with it. Please offer even the obvious suggestions, as we may have overlooked the obvious.

 

[Kuni]

Re: Mercruiser Overheats (351CI, 233HP)

have you looked at the flexible connection between the water intake on the sternleg and the transom(back of the boat) have had similar problem once before the water would go trough the intake and squirt out the flexing hose which had split it looked like water from the exhaust when run up on earmuffs but caused slow overheating.<br />another way to check would be to fit a clear piece of hose in the line and see if any bubbles come through when your engines going this will indicate an airleak somewhere.<br />and lastly is your guage faulty?

 

[Trent]

Re: Mercruiser Overheats (351CI, 233HP)

Just to add to Kuni,<br /><br />Make sure you run the boat at cruise with the clear hose on it..If it has ANY bubbles your problem is still in the outdrive. If not, your manfolds and risers need replacement if I had to guess...Is this a saltwater boat? If so they need to be replaced every four to six years.

 

[Gold Bear]

Re: Mercruiser Overheats (351CI, 233HP)

JackH,<br /><br />I used to have a Boss 351C Mustang. I love that Cleveland engine - a great design.<br /><br />I do remember one quirk in particular. The head gaskets have a cooling water passage at both ends. However one end is open while the other end has about an 1/8" hole. If you flip the head gaskets the wrong way the engine will overheat.<br /><br />I admit I was "in to" Clevelands years ago and my memory is not what it used to be = but this is something you might want to check in to.<br /><br />Good Luck, Gold Bear

 

[DJ]

Re: Mercruiser Overheats (351CI, 233HP)

Gold Bear has a point, you can flip the gaskets on a 351. They look VERY similar, yet they are not.

 

[DJ]

Re: Mercruiser Overheats (351CI, 233HP)

Gold Bear has a point, you can flip the gaskets on a 351. They look VERY similar, yet they are not.

 

[JackH]

Re: Mercruiser Overheats (351CI, 233HP)

I have more information now after reviewing the great suggestions on this board. Thanks.<br />We started the engine again today in hopes of identifying whether the problem is in the outdrive or engine, manifolds, risers. This time, rather than use the tank which sloshes water out badly, we used the rabbit ears (flusher). The engine started as normal and water was being expelled from the hub and the coolant exhaust ports at the top of the drive. Oddly enough, the water coming from either place never got more than mildly warm. The temp gauge rose to about 120 deg and stabilized there. I gunned it past 2500RPM several times and the water expulsion increased as it should, but the temp never rose. I could feel water circulating in every hose that I could gain access to and the cold one was cold and the hot ones were warm. The hoses to and from both manifolds were flowing water and reflected a cool 125 deg. I was scratching my head for two reasons; first why didn't the brand new thermostat allow the engine to heat to 190 degrees? second, is it possible that the garden hose and rabbit ears provided the needed pressure to circulate the water more efficiently than when the water pump has to suck the water from a lake or a tank? I seem to have ample water flow through the intake hose that attaches to the circulating pump either in the tank or with the flush kit. I didn't try the bubble trick as I am not clear on exactly which hose to do that with. Is it the main water feed from the stern unit to the circulating pump on the engine? Please advise there. <br />Again, thanks for all the help. I've eliminated the reversed head gasket theory because of today's performance. I would think that if that condition were present, it would overheat regardless. I'm not quite ready to eliminate the risers and manifolds just yet, but they seem to flow water in abundance. I don't know what else I can ask of them. Thanks and please keep the suggestions coming.

 

[DJ]

Re: Mercruiser Overheats (351CI, 233HP)

JackH,<br /><br />Don't discount the head gaskets yet. You were NOT under a load on the muffs. You previously stated that the engine goes to 220 Under Load. By having the head gaskets wrong, you still have flow, it is just restricted and cannot keep up with a WORKING engine.<br /><br />The hose you described is the right one to be looking for bubbles in the intake side of the system.

 

[johnkc]

Re: Mercruiser Overheats (351CI, 233HP)

jack! the thermostat for this engine is supposed to be a 143 degree thermostat.

 

[ODDD1]

Re: Mercruiser Overheats (351CI, 233HP)

Jack, after reading all the above posts, I think there might be a easily remedied problem here...one that the clear hose on the input trick would verify...you said a new water pump was recently installed...was the base assembly of the pump removed from the gearhousing? I have seen debris [or a chunk of the old gasket] lodged under the base allow air to be sucked into the intake water for the pump...It will work as you describe,and overheat consistently when boat is on plane.<br />by the way..the 233 is a windsor motor, not a cleveland...

 

[JackH]

Re: Mercruiser Overheats (351CI, 233HP)

I want to thank everyone for their help with this. I am confused just a tad about the bubble check. The hose that connects to the circulating pump on the engine from the stern leg appears to be a 2" or 1.5" hose. I think I can get plastic tubing that size but where and how would I connect it? I'm afraid the clear tubing would melt on the waterpump and I'm not sure of another way to do it. We did replace the thermostat with a 140 deg Quicksilver. We ran it again with the earmuffs and had the same result, the engine would not heat up to even the thermostat's rated temperature after running it for nearly an hour. I want to believe that debris under the housing may be the problem, but would like to run the bubble test first. I've cleaned any loose rust and scales from the insides of the manifolds and risers and they actually look pretty good inside. <br />I hate sounding like a bubble head, but if you can provide some tips on exactly how to perform the bubble test and where to connect the hose, I am going to do it this weekend under a load. I just sold this boat to my son who is looking to me for guidance and I am passing that look along to you. Thank you again and I will let all know what happens with the weekend test run.

 

[Trent]

Re: Mercruiser Overheats (351CI, 233HP)

Dont know just what setup you have...So help me? <br /><br />Look for a 3/4 (looks like a heater hose) inch hose comming from the transom plate (inside the boat, runs from the transom to the thermostat housing) This is the hose you want to put the clear line on.<br /><br />Year and serial numbers would really help!

 

[JackH]

Re: Mercruiser Overheats (351CI, 233HP)

Thanks to everyone for all the good information. It's FIXED. Turns out there was a small, almost undetectable, crack in the waterpump housing. It pumped water, just not enough to keep it cool when under a heavy load. The bubble check worked pretty good, but there were bubbles in it even when it was cooling OK. I replaced the thermostat with the 140 deg. one and the pump housing. It cools fine now and the thermostat opens at around 135 or so. Close enough for me. It would never go above 150 even when I forced it to hang on plane a little and the engine was strained. Ran it at 4/5 throttle for several minutes and it still stayed cool. Now that we can run it under full power we discovered that the engine has a highspeed miss in it that isn't consistant. It behaves like an ignition problem and most probably the coil. Not difficult to trace and fix. Thanks again for all your help and I'll ask again if (when) I run into something that stumps me.