Merc 500 overheated - no problem?

[dodl]

Hello,<br /><br />First of all, YES i searched and read a lot, but just to be sure - let me post the 256th overheating Thread <br /><br />I was out on the water and wanted to go up to WOT, when suddenly the rpm dropped pretty fast. Went down from 3500 to Zero in about 3 seconds. At the first look at the motor i could see smoke coming out everywhere (rests of oil etc. on the powerhead from winterizing). And only steam at the pee hole.<br /><br />My first panic reaction was to start again, and the motor started immediately. Turned it off again of course. Now i changed the impeller and drove the motor 4 days without the slightest problem. Idles smoothly, and has a lot of punch out of the hole. It ran now for approx. 15 hours. <br /><br />But i still worry, if the overheating situation did any internal damage to the motor. Is there an "easy way" to check for damage, or do i have to open the powerhead? Sometimes at about 4800-5200rpm, i find the engine to run a bit loud. A bit like "screaming". Hard for me to explain in english. The engine is pretty old. A mercury 500 4 Zylinder from 1976. <br /><br />Now my question is, open the powerhead and have a look, or "nevertouch a running system"??<br /><br />many thanks<br />martin

 

[rickdb1boat]

Re: Merc 500 overheated - no problem?

A compression test would tell alot. If you shut it down RIGHT AWAY, I think you are OK. Let us know what the readings are.. Of course, the best way would be to have a look at the cylinders for damage, but I'd still do the compression test first and if all is well, I'd run it, since it is an older engine and a rebuild would be out of the question, in my opinion. ..

 

[dodl]

Re: Merc 500 overheated - no problem?

Hello,<br /><br />Well it wasn't me who shut it down the first time. The engine stopped and after starting again, i shut it down right away, as i saw the smoke <br />Yes, sad but true, a rebuild would be out of question, although i love that engine.<br /><br />I will try to get a look through the spark plug holes. Maybe i can see something there.<br />I do not really believe, that there is a problem with the cylinder walls, since the engine runs still great for 15hours. I'm more worried about the needle bearings at the piston pin. Or are they not the problem?<br />The spark plugs look still good by the way. I thought, if i had no compression on one cylinder the plug would be fouled because of gas from the crankshaft housing? Or am i wrong here?<br /><br />Thanks<br />Martin

 

[rickdb1boat]

Re: Merc 500 overheated - no problem?

The only way to know if you lost compression is to test it. If it were me, as I said, I'd run it till it dies, if ever.. And yes, if damage was done, many things could be damaged. But if it continues to run well, run it.. The "Screaming" could be that the cylinder walls are scored and that's what you are hearing. Is it much more pronounced than before it overheated?

 

[Chris1956]

Re: Merc 500 overheated - no problem?

dodl, Usually the cylinder walls are the first to be damaged during overheating or due to lack of lubrication. If the compression is OK, it is likely the balance of the motor is fine. When your RPM went to zero, it is likely your motor seized(usually piston expansion due to heat). I would be surprized if the compression is still balanced between all cylinders, but stranger things have happened. <br /><br />Since you are not interested in a rebuild, and it seems to run OK, you may as well run it until it dies!

 

[jfholijr]

Re: Merc 500 overheated - no problem?

I realize that dodl might not be in the USA, so the parts availability/cost might be prohibative to him giving his local. But, generally speaking and looking at dodl's speach (it is very clear that he has the wit and mechanical thinking to be able to do it) rebuilding is definately a viable option, if need be, for his motor, or anyone elses for that matter, if they do as much of the work themselves as possible. The parts are still plenty available so I ask why is it usually the first response to advise not to rebuild when alot of these people could do the job themselves, inexpensively and have some great motors for themselves?

 

[dodl]

Re: Merc 500 overheated - no problem?

Hello,<br /><br />Well my problem is, that parts in Austria are extremely expensive. <br /><br />Had the motor running today on the flush device. Ran good, but then i pulled out the plugs.<br />And this does not look good i'm afraid...<br /><br />

<br /><br />cu<br />martin

 

[rickdb1boat]

Re: Merc 500 overheated - no problem?

Sorry, but it's over, unless you rebuild..

 

[dodl]

Re: Merc 500 overheated - no problem?

Hello,<br /><br />Well maybe....<br /><br />I did a compression check today and get between 102 to 108 PSI on the top 3 Cylinders. I cant reach the 4th one, so i hope...<br />The Cylinder on the foto also does 108.<br />This readings with cold engine and carburetor on idle. I cannot run the engine on the parking place and i can't engage the cranking motor at WOT.<br /><br />The deposits which i thought are Aluminium, are salt! Found the reason, the gasket at the exhaust cover is damaged due to overheating. So cooling water could get to the cylinder. And i have a little hole at the fuel pump. Now i do an overhaul on the fuel pump and the carburetor and put new gaskets at the exhaust plate and the plate where the spark plugs are. <br /><br />Since compression seems OK (at least for a '76 motor), i think it's worth the effort?<br /><br />thanks<br />martin