Mercruiser SBC overheating... Ideas?

[cadunkle]

Originally boat was an '89 Mercruiser Chev 350/Alpha 1. Blew motor and picked up a low miles 305 to get my by for a season or two. Swapped my marine parts over to it, water pump, manifolds, distributor, brass plugs, etc. Engine ran great in the car ('87 Firebird), good power, no overheating (180-190), ran well. It is a GM reman with about 20,000 miles.

Problem in the boat now is that temp just keeps creeping up. I shut it down around 200*. Tried replacing thermostat, no dice. Impeller is new, only 6 hours or so on it with the old motor, no dry starts. Decent water flow out of exhaust ports on gimbal housing.

Thermostat housing has 6 ports. Inlet from drive/hose is cool. Outlet to water pump (engine mounted) is hot, the two ports to the manifolds are hot, and the ports to therisers are cool. Risers are cool to the touch (can leave my hand on them plenty long, warm but not hot). Back side of thermostat housing is hot, can only hold hand on it for a couple seconds.

I believe thermostat is opening because the high ports above thermostat (to manifolds) are hot, and I get same conditions with old thermostat and new thermostat (140*).

Set timing at 10* initial, tried advancing it a bit and no change in temp. Not sure how I should proceed. Any ideas?

 

[cadunkle]

Re: Mercruiser SBC overheating... Ideas?

Any thoughts? I have a new mechanical temp gauge laying around that I can try tonight to verify consistency and that it's actually overheating. I'm using the original senders and gauge that were on the old engine. There are two senders though, one in the intake adn one in the thermostat housing. I don't think I got those wires mixed up but anything is possible. Why are there two senders? Is this normal or is this someone's jerry-rig using a sender as a plug? Original engine was a Mercruiser 350/260HP in an '89 Marlin.

 

[HT32BSX115]

Re: Mercruiser SBC overheating... Ideas?

Howdy,

(If it is REALLY overheating) It almost like you have some of the myriad of hoses connected wrong.

A good way to check the temp would be to get yourself an infrared temp "gun" from Harbor Freight or equiv....and point it directly at the t-stat housing.

Cheers,


Rick

 

[cadunkle]

Re: Mercruiser SBC overheating... Ideas?

Hoses are connected as per Merc service manual...

Lower port: inlet from outdrive
Mid port: outlet to port riser
Upper/rear port: outlet to port manifold
Lower starboard: outlet to circulating/water pump
Mid starboard: outlet to starboard riser
Upper/rear starboard: outlet to starboard manifold

Shortly I'll get to work on the boat. Will verify temp sender wiring and also use my mechanical gauge to verify temp. Also will remove inlet hose from drive to verify good flow from water pump/impeller in the drive.

 

[cadunkle]

Re: Mercruiser SBC overheating... Ideas?

Solved. Problem was a stuck cehck ball valve that allows water to risers... Was letting most go to risers and starving engine of water. Idled 15 mins on hose and temp stayed about 140*, even holding 15000 RPM and revving to 2000 it didn't climb. I think I'm good on temp now.

 

[HT32BSX115]

Re: Mercruiser SBC overheating... Ideas?

Good deal,

I don't have those check balls in my 454. I do seem to remember a discussion in the Mercury Service Manual about those check balls and how to "adjust" them in the SBC's.


If you don't have a Service manual and you plan to do all your own work it might be a good idea to get one.

The following also works pretty good.

http://www.boatfix.com/merc/Techbk/techcov.pdf



Regards,


Rick