Leaking crankcase leads to overheat condition?

[johnyrude200]

Have an 18hp that I ran for 5 mins WOT out on the boat, also ran it continuously for 60mins +. I brought my laser thermometer out and noticed that after running at WOT, the cylinder head was staying within acceptable temperatures, but the cylinders were running a little hot (about 170°). I have installed a brand new thermostat and impeller, and it is pumping a ton of water.

The thermostat isn’t even getting to 125° after WOT.

I pulled the plugs and found them chalky, and then pulled the cylinder head to find things were basically bone dry with evidence of running hot. I found that after running WOT for 2-3 mins continuously, it would cool down fairly quick.

I noticed oil/gas dribble at the crankcase seam on almost all of both sides, but it isnt leaking from the upper crank seal.

I am tearing the motor apart to reseal the crankcase, but wanted to see if others have seen this happen before. I don’t think the motor is running lean. It’s a 1970 18hp evinrude. The fuel mix is correct (50:1).

[frankr]

I would have an awful hard time believing the crankcase leak would have anything to do with running hot. However, there are two water flow paths through that powerhead. One path flows all the time, while the second path is thermostat controlled. I’d do some inspecting along that line of thinking

[johnyrude200]

I pulled the cylinder head and it looks very clean, same thing with the water jacket around the cylinders. I suppose I could pull the exhaust side to make sure there’s no clog there, but given the fact that the cylinder head isn’t getting hot, but the cylinders are, that is what makes me lean toward the ‘lean fuel-air’ condition.

I was consulting a champion spark plug guide (not saying this is the bible or anything), and it listed air leak at the manifold as a culprit. All the other explanations were things I knew I have gone through already. I’ll pull the exhaust cover because I’m reassembling everything right now so it’s easy. Was getting ready to remount the powerhead after resealing the crankcase – that thing was clearly leaking gas/oil out of it after I split it in half.

If this doesn’t solve the problem and I don’t find anything on the exhaust side, I am thinking maybe it IS running lean at the carb setting. I’ll double check my link n sync job, but I’m pretty sure both are OK. First motor I’ve seen do this, particularly these early 70’s ones which run rich usually, anyway.

[chris-p]

Was the old impeller missing any chunks?

[johnyrude200]

no, it was in good condition. I replaced it as this motor is being used for an emergency vessel, so pretty much everything is brand spankin’ new.

ONE THING OF NOTE. I installed a weedless prop on this motor – is it possible that it is running at too high an RPM at WOT causing this overheat condition?

Everything else about it seems to point to a lean fuel condition.

[fleetwin]

Well, I too am not terribly confident in the crankcase leak theory. But, running lean surely does increase combustion temperatures, so your diagnosis can not be ruled out. Are you sure the water pump is in good shape? Those impeller housings only have three retaining screws, and not much "meat" to seal the SS impeller plate.
In any event, I would consider richening up the high speed jet by .002" to ensure a good mixture for today’s fuels. Your weedless prop may be over revving the engine, which may lead to trouble also.

[johnyrude200]

I just got done running the motor sustained at 5500 RPM in my test tank with test prop for 5 mins on 2 separate trials. This is only bout 3/4 throttle in the tank with the wheel. I found that it was still running a bit hot, not on the temp gun (stayed at 145-150° both cylinders), but by pulling the plugs and examining both those and the pistons. The pistons were somewhat wet on the intake side, but nearly ‘white burnt’ dry on the exhaust side. The plugs were pretty dry too. I put brand new NOS J6C’s in for the 1st of the 2 runs.

The 2nd go around, I put J4C’s in. Motor ran a little cooler after 5 minutes of sustained 5500 RPM. I looked at the ’71 service manual and it called for J4C’s. Pulled the J4’s, and the pistons and plugs had a minute amount of fuel moisture. I am guessing initially that the J6’s are too hot for this motor for sustained WOT or high RPM.

I’m going to conduct the test out on my test boat under load in a few minutes. This is a motor that will probably be run at WOT for long sustained bouts since it will be used in an emergency rescue application.

I tried richening up the low speed end, but doubt that is the culprit since the high speed jet is fixed on this unit.

[jeff-register]

Are you running at the factory recommended tempatures at all speeds? How about the spark plug heat range? Are we running the correct fixed jet for the atmosphere? Timing will cause overheating as well? Correct OMC oil used? Frank brought up two water passages thru the block, does this have full passage flow at all speeds? What was the reason to get your attention in the first place? It is a new motor compaired to what we work on. Have you checked your points gap to see if it remains the same thru the complete throttle range? Points gap may be changing in different throttle positions changing dwell during throttle advancement, worth a check to be sure anyway. You might try bonding the magneto plate to the cylinder head,
I have had intermitent loss of bond causing current floe reversing thru the other set of contact points causing burned contacts. Just a few ideas!

[johnyrude200]

After spending 90 mins on the water with the motor, my explanation for running hot is that it was probably the plugs. The J6’s were 1 range higher than the OEM recommendation of J4’s. Granted, this motor is going to be run at WOT quite a bit due to it’s application, but I guess the J6’s are really just for the put-put trolling fisherman with a few short WOT’s on the day. It was only running hot after sustained, 2-3 min bouts of WOT. And I’m talking 170° not super hot, but it would fall back to 145° reasonably quick. What alarmed me is the J6’s a pulled yesterday after using the motor for 90mins were crusty and had blisters on the tips. The cylinders were bone dry too. That was unusual. I ran a heat gun on it yesterday and today and closely monitors the head, cylinders, and thermostat cover to observe trends.

My boat is not lugging. It’s me on a 14′ aluminum starcraft with 180lbs of ballast at the bow. Gets up on plane quickly and I have the motor trimmed a couple of holes to keep it from sitting too low on the transom (long shaft motor, mini-jacker plate installed, ventilation plate is roughly even with the bottom of the boat give or take an inch).

I did switch to a regular prop and didn’t see a significant change in top end speed or RPM’s. I did remove the thermostat and normally wouldn’t run motors without a thermostat, but in this application, the motor only lost about 10° of running temp without the thermostat (115-135, previously 125-155~170 at sustained WOT). It ran fine at idle around 100-120° while at headway speed. So not too cold.

Maybe I was just chasing a ghost with this one.

Jeff Register: all of those things you brought up I already went through previous to the post.

Side question – the motor did develop a minor issue which I wanted to pick your brains about. The shift lever isn’t engaging the fast start safety lockout either at the rear of the manual starter (the black plastic piece which rotates to stop the manual starter if throttle all the way up – prevents accidental starting at WOT), and the shift lever which runs down through the pan and interacts with the actual shift handle to the gearcase. It isn’t hitting that brass ‘fixture’ that is attached to the mag plate, or just barely rubbing. I had this happen on a 20hp of the same era – any suggestions on how to fix this?

[Mumbles]

If the plugs are blistered, take them out right now and replace them with colder plugs before the pistons start melting! 😯

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