Dart Block Oil Temps
 

Here's a scenario I'd like to pose on behalf of a friend to see what we can come up with as a possible or probable cause and suggested solution(s):

Dart Tall Deck 632" block when assembled, runs high oil temps (easy 280 and rising). Oil temp goes up quick and continues to rise. Oil is Full Synth 25W/50, raw water cooled (no strainer but clean) with a 3" x 24" combo oil/ps cooler (Hardin I think). Engine oil temp sender located in pan, matched to Livorsi gauge. On tear down, bearings show no discolorations or burns/blueing. All clearances were checked and apparantly correct prior to assembly - Crank ground 0.010" and polished. I believe the motor runs the standard Melling M77 oil pump.

The key issue is that this is one of two engines in the boat, the other engine is the exact same and has no problems with oil temps. Water temps for both engines are the same, as is pressure. The oil cooler on the other motor is the non combo cooler but is slightly smaller (about 20"). Everything is identical between the motors, cooling, pumps, ancilliaries, carbs, jets, set ups etc.

Could some small blockage somewhere cause this temp prob? It's a new block so where could the problem be? Casting error?

All other simple checks and tests have been done. My thoughts were bearing clearances. All oil lines are AN-12, the filter is the HP6 (like KN 3002) on Canton adaptor with the big fittings.

The pan feels hotter than the other motor so I don't think it's just sender location in a hot spot on the pan.

Could it be that the oil pump is not sufficient? I haven't confirmed that the other motor is in fact running the M77 as well but I was told they were built identical so I am assuming it is?

The key issue is that the other motor is identical but perfect and never gets the temp above anywhere but optimum (240 - 270). The other motor has a smaller cooler but apart from that, it is the same and has never had any problems.

Any suggestions or other things to check would be greatly appreciated. The motor is out of the boat at the moment and we want to get it right before it goes back in!

Thanks in advance for any help or advice.

John

Friction of course causes the heat. Sounds like theres some clearance issues somewhere in the engine. I would pull the caps from the bearings rods and mains and take a look to see how wear is doing. Could also be a piston fit issue. Just my 2cts worth.

Check for a missing or stuck open oil pressure release valves in the block at the oil filter flange and such. On some blocks if these are misssing or bad the oil will completely bypass the cooler circut depending on how its plumbed.
could be the problem.

Best Regards,
Ray @ Raylar

Just a thought here. How about if the O ring that goes between the rear main cap and the block was left out. That may cause a flow problem, creating the high temps.
I have strayed away from the standard M77 style pumps. Now, I strickly use M10778 pumps.

Thanks for the input lads!

I've confirmed that the filter pad bypass has been either blocked off or came blocked off. We checked the bearings and they showed no abnormal signs of ware (no discoloration or uneven polishing).

I'm curious to know more about that rear main cap O-Ring, is that specific to a Mk IV engine? or is that on all motors? How easy is it to 'Forget' to put in or replace?

I avoid the M77 pumps too, but I run the 10778C just in case. I think that may be a required upgrade on the cards already but if both motors are already running the same pump and the other one is alright, I'm at a loss as to this cause!

Thanks again so far!

John

With a temp gun shooting at the hose fittings to the oil cooler, do you see hot on one side and cold on the other? Have to be close to the fitting to get a good reading, but you should see a difference.
Can you run the motor on the hose and see high oil temps, or did you, should I say.
I left the oring out on the main cap for a couple of motors, till Vinny mentioned it. And I didnt have high temps because of it. I did have an oil thermostat (KE) and it didnt work.. made oil hotter.
Are you getting good water flow through the drive and pump and oil cooler? Any diff in the water temp on the two motors?
Just kind of tossing thoughts out..
Comes down to oil flow and water flow through the cooler. It should cool it down if it has enough oil and water flowing through it.
Is the oil flow against the water flow on both motors?
Good luck with it.. and keep us posted.
Dick

Thanks Dick, we'll give the temp gun a go when the motor's back in. The flow through the drive is fine, the 4 slot low water pick ups in the sportmaster lowers give plenty and now we need to manage water pressure with a valve but the other motor is plumbed the exact same way with a smaller cooler with the same flow. Water temps, oil pressures and all other checks are the same and are perfect, it's just the oil temp on this one motor that has us stumped! There may be an issue about not getting enough water out of the block through the exhaust due to the size of the outlets on the tailpipes (dry exhaust with custom tails with built in dumps). Could the restriction of flow cause the high water pressure which was relieved with the dump valve but is now causing a hot spot in one of the engines maybe? (That's a mouthfull I know!)

Thanks again,
John

What about swapping out the oil coolers & run the other motor on the hose. Would be an easy test to rule out any issues w/ the coolers.

Another cause of high oil temps is lean fuel mixture. Make sure the fuel system is providing the same amount of fuel to each engine. Alittle bit of detonation will heat the oil fast. Make sure both engines have the same amount of ignition timing and maybe back both off a little. 34 total is usually safe for standard style heads.

While I doubt this is your problem, here is a link that shows the #5 main cap O ring... Check out part 18
I am going with a flow problem, either oil or cooling water.

http://www.mercruiserparts.com/Show_...onnecting+Rods