302 overtemp at high RPM.....

Hello all, New to the forum and need some assistance. I have a 77 nova with twin 302s in it. I have a problematic starboard engine. Here are the details of whats going on. I have no problems with the motor under 3000rpm. It will stay cool but over that is a different story. When I bought the boat I did some routine maintenance and replaced the impellers and drive oil. The port impeller was alright but the starboard was wasted. Since then I have replaced the water pump, taken off the risers and looked in the manifolds for water...none was found and actually they looked good. I'm stumped what to do next. I thought maybe I should disconnect the power steering cooler and see if there was a blockage but was told this is a long shot if its not overheating at idle. the temp kinda bounces up, not a steady rise and when its gets around 200 I shut it down. The thermostat is out and there are not any obstructions in the cooling lines. She runs like a champ and isn't knocking or anything. Just have no clue what could be wrong. There is no water in the oil, so a crack in the block or manifold is unlikely. Anyone have any suggestions? Thanks everyone!!

Re: 302 overtemp at high RPM.....

jtyphoon1 said:

Hello all, New to the forum and need some assistance. I have a 77 nova with twin 302s in it. I have a problematic starboard engine. Here are the details of whats going on. I have no problems with the motor under 3000rpm. It will stay cool but over that is a different story. When I bought the boat I did some routine maintenance and replaced the impellers and drive oil. The port impeller was alright but the starboard was wasted. Since then I have replaced the water pump, taken off the risers and looked in the manifolds for water...none was found and actually they looked good. I'm stumped what to do next. I thought maybe I should disconnect the power steering cooler and see if there was a blockage but was told this is a long shot if its not overheating at idle. the temp kinda bounces up, not a steady rise and when its gets around 200 I shut it down. The thermostat is out and there are not any obstructions in the cooling lines. She runs like a champ and isn't knocking or anything. Just have no clue what could be wrong. There is no water in the oil, so a crack in the block or manifold is unlikely. Anyone have any suggestions? Thanks everyone!!

Click to expand...

Ayuh,... With the lower unit off, backflush from the t-stat housing to the drive...
'n put the T-stat back in...

I'm guessin' the remnants of that impeller is bouncin' around in the P/S cooler area too...

Re: 302 overtemp at high RPM.....

yeah im thinking the same just was hoping to not pull the lower.... but at this point just hoping its not something more sinister.

Re: 302 overtemp at high RPM.....

I have a 77 nova250 with the same engine set-up. I had replaced everything chasing an overheating problem. new 160 degree thermostat, new raw water pump on the front of the engine, new impeller.

If you can get your boat out of the water, check the hose that supplies water from the outdrive to the outer gimbal housing. It could be collapsing or kinked or have a split/crack. I found mine was cracked when I ran the motor on muffs and saw all the water going to the wrong places.

Re: 302 overtemp at high RPM.....

Thanks for the ideas guys....I have the boat out of the water and im trying to locate the problem on land so I will take a look at that tube before I drop the drive. Its just really weird that the water flow coming out of the engine looks good. Did you have the same problem as in only at high rpm or did it manifest itself at other times also? Additionally, does anyone have a diagram of the cooling system for a 302 I know its a little funky in operation? Thanks again!

Re: 302 overtemp at high RPM.....

UPDATE.... I have not pulled off the lower just yet. Wanted to inspect the tube coming from the upper to the outer gimbal housing. Those lines are new. I don't believe that they are a mercruiser part however they are pretty thick and look to be of good quality. They are strait and have no kinks or really any wear marks. Goodyear's marked on the tube. Additionally, the port side also has the same tube and is not overheating. I ran it on the hose again today and had a laser temp gun at my disposal. Between the two manifolds there is a 20 degree split the right manifolds a little hotter. At shutdown I noticed an audible boiling sound coming from the engine I actually sounded like to was originating from the right side, but could not confirm this. When the motor is up at temp there is a lot of steam coming from the outdrive exhaust ports and propeller. Again, When it gets to 200 or so i just shut it down. Check of the oil shows no signs of water intrusion. The only thing I can think of is a block forward of the power steering cooler or a blockage inside the block or manifolds. Still a little stumped. Thought this info may help diagnosis. Thanks everyone.

Re: 302 overtemp at high RPM.....

Just a side note. A marine engine's horsepower output is linear with RPM. As prop RPM goes up, load goes up, horsepower required up, horsepower developed goes up, and . . . heat goes up. Soooo, if there is anything limiting flow, the overheat would and should be as RPM rises. Below 3000 you have anough flow or capaicty to cool say 100 some odd horsepower. Above that you don't . . .

Re: 302 overtemp at high RPM.....

sometimes gas can ie exhaust enter the water through loose connections at the impeller unit. I'm not sure if your sea pump is in the lower unit or like a bravo 2 in the eng well. If it's in the lower unit the exhaust gas blows by the impeller unit and can be picked up at high speed and over heat the water. If as was the case with a freind of mine the plastic tube leading to the upper unit is a loose fit it will allow gas to enter . In my case the screws were loose on my impeler casing and that allowed the exhaust gas to enter the case and heat the eng up at high rpm .

Re: 302 overtemp at high RPM.....

At this point I have no option but to drop the lower and see about an obstruction or possibly a bad impeller or housing. I'm guessing I've got something stuck in the cooler. Can imagine that an impeller with less then an hours bad but I guess anything possible. I'll check in tomorrow with the results.

Re: 302 overtemp at high RPM.....

That cooling hose I talked about from the outdrive to the outer Gimbal housing. Look at it with the drive raised up and then look at it with the drive lowered. It could still be kinking when the drive is down and restricting water flow.

Re: 302 overtemp at high RPM.....

Mike thanks again for the suggestions....I did Actually check in the up and down position for an obstruction and didn't see anything wrong. So here's the skinny after the drop of the lower. I looked at the housing and impeller and they are both in great condition. I hooked muffs up to the lower and turned the water on to see if there was any leak in the housing. There was a bit of water that manifested itself on the top of the housing were the rubber seal was. other then that the housing didn't leak. I back flushed the P/S cooler and there was no obstructions that came back out of the upper drive. At this point I think I'm gonna just replace the impeller housing and gaskets. Run it after that and if the problems still there I'll take off the manifolds and take a better look inside. Anyone have any other suggestions?