Merc 150 Overheating - port side only

[crumudgeon]

My 1988 Mercury 6 cyl 150 hp dropped the #2 cyl due to overheating. After a complete rebuild, the installation of a water pressure gauge, I am still getting a overheat problem on the port side. I have good water pressure and flow appears good. This occurs with thermostats in or out. I previously eliminated the auto oil and mix my own, carb is clean, and timing is correct. Any help would be greatly appreciated.

 

[KCLOST]

Re: Merc 150 Overheating - port side only

Please explain further,,,,<br /><br />Overheat on port side only? How do you know that? The overheat switch is normally on the starboard bank and the temp sender will be on the port side (normally).... Is the guage reading high? Is the overheat alarm going off? And where are your senders located...<br /><br />How did the engine overheat originally? Did you pull off the cylinder cover jackets and inspect for blockages? Since you have good water pressure, that can be the culprit... Expecially if you had a failed impeller. Maybe pieces of the impeller are lodged up in there.<br /><br />Let us know all you can, and well try to isolate the problem...

 

[CDMBANNER]

Re: Merc 150 Overheating - port side only

you might want to check spark plug temp with a thermocouple type of digital thermometer...radio shack sells them, but you want to keep the wires for the thermocouple dry...a bit of silicon works wonders...just rub it on, work it in, and let it set up, but don't cover the tip. Then attach the t.c. to the plug where it meets the head. Ahot cylinder could be caused by many things, invluding debris or damage to the ports.

 

[CDMBANNER]

Re: Merc 150 Overheating - port side only

I don't know about graemlins, but gremlins got to my spelcheck... or my fingers!

 

[crumudgeon]

Re: Merc 150 Overheating - port side only

Don't know what caused the original overheat. I know it is the port side as I have set up separate temperature gauges for each side trying to figure this out. I did this because the horn, sending unit on starboard side was not sounding but the temperature gauge (port side) was in the high zone. When this occurred I could feel the starboard side and it was only warm to the touch but the port side was very hot. With the temp gauges the port side shows High High and starboard is Medium High. This generally occurs at 5000 or more RPM.<br /><br />I inspected the jackets during the overhaul - unless I missed something that was not obvious. <br />Again - thanks for any help.

 

[KCLOST]

Re: Merc 150 Overheating - port side only

Have you checked compression on the port bank (or all)?

 

[crumudgeon]

Re: Merc 150 Overheating - port side only

Yes, and everything is on spec. I went 1/1000th under on the rings after I had the block re-bored just in case and due to age, so I lost just a smidgen of compression. The rebuild was complete, block, pistons, rings, rod bearings, all new pins and rollers etc. The only thing I used over are the rods and they were as shiny as brand new. I believe I am losing #2 again due to heat.

 

[KCLOST]

Re: Merc 150 Overheating - port side only

Well assuming you are correct about the water jackets being clear, and the thermostats working (replace both if you haven't already). I would suggest a few possibilities....<br /><br />First, you could have a lean condition on that cylinder (especially since it's the top cylinder on that bank). You have 3 carbs, I know you said they were clean, but have they been thouroughly cleaned and rebuilt if necessary? Start inspection from the fuel pump to the carbs.<br /><br /> <br />Second, you could have a leak from the exhaust manifold into the crank case causing that cylinder to overheat...<br /><br />What do the spark plugs look like in relation to each other?<br /><br />You had a problem before that caused it to overheat, it's probably still there, so let's get it fixed... My guess is one of the above...

 

[crumudgeon]

Re: Merc 150 Overheating - port side only

Many thanks - both of those are not things I had previously considered and they both make sense. I hope it is one of them, as you are correct, I strongly believe the original problem still exists and not only do I not want to do another re-build but this motor, until now, has been very very good to me. <br /><br />Thanks very much for your help, I'll keep you posted.<br />Sam