One Question about overheating

[natwales]

1988 5.7 merc w/ alpha 1. Raw Water. 140 deg thermostat. New water impeller in outdrive over winter. New exhaust system and water pump last season.<br /><br />Idling on the muffs my engine overheats. When the engine is running, I get a steady strong flow from the exhaust relief ports. The exhaust water and manifolds never feel warm when the engine is overheating. Tested thermostat and it's fine. Overheat still occurs without thermostat.<br /><br />Can I rule out the water pump in the outdrive? If not, what could be wrong? Where might I look next?<br />thanks<br /><br />nat

 

[Don S]

Re: One Question about overheating

So, you're saying the manifolds and risers are cool to the touch (or at least only warm) and the engine itself is overheating (even with thermostat out). Did you verify this with a temp gun or even feel the engine to see if it was hot?<br />What temp is it going to?

 

[Bt Doctur]

Re: One Question about overheating

Assuming everything else is up to snuff ,nothing clogged, new inpeller, good water exaust, Id check the engine circ. pump for a broken shaft. best way is to remove the pump and shake it. if it sounds like a morocco ,the brass impeller is broken off the shaft.

 

[natwales]

Re: One Question about overheating

As the engine warmed I saw the coolant temp needle steadily rise to 195 before I shut it down. I've never seen it go much above 160. Usually right at 140. I felt the engine block and it sure was hot. Not sure if I can say it was 195 vs 140, but certainly much hotter in relation to the risers that weren't even warm.<br /><br />Circulation pump, eh? I put a new one in last season and it did the job fine for the last few weekends of last season. <br /><br />The only thing that has really changed at all is the water impleller and the bellows/water tube that I just replaced a few weeks ago. If low water flow were the culprit though, I'd have hot risers, right?

 

[Don S]

Re: One Question about overheating

Look and see if there is a kink in that water hose from the outdrive to the transom shield. If you tried to use heater hose and it was too long (even by a little bit) that may cause the problem.<br />You may have enough water to cool the manifolds and risers, but not the engine when the stat opens.

 

[natwales]

Re: One Question about overheating

Sorry, did not answer all questions.<br /><br />Yes, the risers and manifolds were cool to touch. Maybe only a hint of warmth.<br /><br />The exhaust stream from the relief ports was cool to luke warm. No real heat.<br /><br />I wasn't timing anything, but I figure I was running the engine for about 10 mins before I had to shut it down. <br /><br />I winterized by draining and pouring pink antifreeze into engine block. I drained a bit out of the block after running it and clear water from the hose came out. No pink stuff.

 

[natwales]

Re: One Question about overheating

Thanks don. As always.<br /><br />I did check the hose and it was the one supplied with the mercruiser transom seal kit. Exactly the same length as old one, and not kinked at all. <br /><br />Also, my understanding is that if it were overheating due to lack of water flow, wouldn't the exhaust that's there at least be hot? Am I wrong with that assumption?

 

[Don S]

Re: One Question about overheating

Also, my understanding is that if it were overheating due to lack of water flow, wouldn't the exhaust that's there at least be hot? Am I wrong with that assumption?

In general, you are correct. As always, there are exceptions to the rule.<br />I have found some raw water cooled engines that will keep the raw water side cool "FOR Awhile" until the stat opens then it slowly starts heating up along with the engine. I was just throwing out some ideas and fishing for information.<br />You did mention you "Checked" the thermostat...... Just replace it, if it still overheats, pull the circulation pump and check the shaft and impeller like BT Doc mentioned. Even if quiet, I have seen the impeller loose on the shaft.

 

[natwales]

Re: One Question about overheating

Thanks. That's what I am going to do. Replace thermostat. Then pull circ pump. IF that doesn't work I will drop the outdrive and make sure impeller is all good (should be, it's brand new).<br /><br />IF that doesn't do the job I will call in professional help, but I hate to give up like that.<br /><br />Checking thermostat = pulling off termostat housing checking for any clogs and removing thermostat. Tested thermostat on stove and worked perfectly.<br /><br />I will keep you posted when I solve the problem.

 

[natwales]

Re: One Question about overheating

Here is another theory that I found. I am not sure it's specifically my problem because..umm.. I checked and my balls look fine, but it sure fits my symptoms and it may be useful to someone else searching a simmilar problem someday. My understanding is that the worn or loose balls allow water to bypass tstat housing and not provide enough cooling. <br /><br />--- edited down from another post---<br /><br />When raw water cooled the ball valves in the thermostat housing can cause it to run pretty hot at idle. If it is obvious you have enough water with cool risers then consider those nylon balls. They may be worn or or the spring broke. Worth checking out, if badly worn you can turn them around on the shaft putting the good side toward the housing. The spring tension should not be set too high or too light, if the springs look real nice and the balls are very round not tappered adjust with about 1/8 to 3/16 of thread showing outside the lock nuts.