5.7 OMC overheating

[possumkingdom]

I know there are a lot of post about overheating, but I've tried everything and am still stumped. 1987 sea ray, 5.7 liter cobra. had an overheat condition last year and found some leaking manifolds and risers. New manifolds and risers, impeller, thermostat. Correct timing. Checked flow from inlet hose at thermostat housing, good flow. warm water exiting bellows, warm water will drain from petcocks on block. Ran for four hours last weekend bow high, around 2000 RPM's with no over heat, temp gauge around 170. ( have a 145 thermostat, new). planed off for a while around 3500 and no overheat Stopped for about 5 minutes and restarted and temp rose to 220 almost immediately. Definitely not a sender or gauge, you can feel the hot risers and manifolds when the temp rises.

The only item I've neglected are the hoses. I've had the boat for 8 years and never changed them and I don't believe they've ever been changed (26 years?). Now, when the temp started to rise on the muffs yesterday, I squeezed the hoses and the temp started to drop. When it started to rise again I squeeze the hoses and it drops and levels out. I feel like I may be on to something, but I want to make sure I'm not fooling myself into thinking I have a solution when I really don't. The hoses look good externally ( which I why I never changed them) The seem to look good internally also but is it possible that they have broken down and are collapsing with heat? I saw in another post where a collapsed hose was a solution and I'm hoping it may the holy grail for me too. BTW this forum and all the input from the community is why I can keep a 26 year old boat going. Many thanks.

 

[Gilded_Splinters]

Re: 5.7 OMC overheating

I experience this same thing with the same motor/drive, but I have 2 of them.

Mine run so cool at idle, I was beginning to doubt the functionality of my temp. gauges, but at the same 3500 RPM, I got to about 160 degrees.

When you run at that RPM and stop and shut down abruptly, the engine is hot, but the water inside it is no longer moving. Instead of the water removing heat from the engine, the engine heats the standing water. This brings the temp of that trapped water up, which seems natural enough. Next time, do the same thing you've done as described here, but re-start the engine, rev it up to about 1500 RPMs for a few seconds, and you'll probably see the temp. drop rapidly due to the cooling water flowing again.

The raw water pump produces lots of volume, but not much pressure and does't really pump too much more even though the engine RPMs are higher...there's a limit. It follows that when running at higher RPMs you'll run warmer, and at idle - when the motor is working less, but the pump is moving a similar amount of water, you'll run cooler.

The opposite of this can occur when your impeller is going south. At idle you get hot because the pump has lost efficiency, but when it spins faster, it can move enough water to keep things cool.

Next time you go out, come to a stop, but let the motor run for a while and watch the temp. gauge. It'll probably drop right off.

 

[possumkingdom]

Re: 5.7 OMC overheating

Last time I was out and I came to a stop and restarted. My temp was up above 200 and I ran a minute or so and it didn't drop until I shut down and anchored for a few hours. then I restarted and no over heat the rest of the day. You did make me think about water movement though and the impeller again also. I know the impeller is in good shape, but I wondered about it maybe pulling in air. So I tried this experiment. I got some clear hose from Lowes and hooked it up temporarily so I could see water move, air bubbles, anything that might give me a clue. Here's a short vid of the water going from the thermostat housing to the manifold. Some air bubbles are there and I'm not sure how much is too much or what they may indicate. Maybe Don or someone else will be able to make some sort of determination from this video. This is running on the muffs out of the water.

 

[Gilded_Splinters]

Re: 5.7 OMC overheating

I always have the clear hose on there..I like to see what's what.

That looks like too much air to me.

It could have been as simple as weeds blocking the water inlet temporarily. I docked in shallow water last year and would get clogged with silt all the time. If the temp went up too soon, a quick jab of the throttles cleared it out.

You say that you know that the impeller is good. There's more to it than that. Since it's so easily accessible, I would take the pump apart and have a look around in there just so I could rule any problems in there out.

You might try running it without a thermostat as well to eliminate that as an area of concern.

 

[possumkingdom]

Re: 5.7 OMC overheating

I"m pretty sure on the thermostat. It overheated with the old one, and I pulled it and ran it for a short while without a thermostat and it still overheated. Put the now one in and got the same thing. But i"m now wondering about where the air is coming from. Maybe not sealing around the impeller housing? Or maybe some crud got sucked in in shallow water like you said. Thanks to this drought in Texas, all water is shallow water here
. I'm thinking maybe backflush through the water supply hose at the thermostat housing and see if I have a good seal at the impeller housing. Thanks for the input, I'll keep trying.

 

[Gangly]

Re: 5.7 OMC overheating

Gilded Splinters

Do you run that clear hose even on the lake, 24/7, or only when you are testing things?
If you run it 24/7, what is the polymer material and where did you find it?

 

[possumkingdom]

Re: 5.7 OMC overheating

I only put mine on for testing so I could see the water movement. It's just clear vinyl 3/4 hose from Lowes.

 

[jerryjerry05]

Re: 5.7 OMC overheating

When you changed the impeller,did it come out whole? Or in pieces?
What condition was the wear plate in the pump?
The drive is it sealing at the water transfer port? O-ring?
The flappers still in the down tubes?Pull drive and see if the flappers have fallen down.
While the drive is off? Look inside the shift cavity and see if the rubber couplers have melted?There is a copper tube with a rubber gromet on both ends.That allows water to pass to the upper unit.
The holes on the upper gearhousing.The ones covered with the Cobra sticker.Both have a screw in them?

 

[possumkingdom]

Re: 5.7 OMC overheating

Impeller came out whole as did all impellers before.
The wear plate was not grooved or worn
O ring on transfer port is an unknown. I'll have to pull the drive and check there
Flappers? I didn't see any flappers in the exhaust past the risers when I replaced them. I could see the cross bar, but no flappers. I didn't know if they were ever installed. I'll check the exhaust from the transom when the drive is off.

In the prop I found what appeared to be a partially melted rubber grommet about half dollar size. Maybe this is the path to my solution. Either flappers, melted grommet or a combination of both.

Those Cobra stickers are long gone but the screw is still in the gear housing on the port side. No screw hole on the starboard side.


Thanks jerryjerry05. Looks like I'm pulling the drive and hopefully finding the end of the train for this over heating problem.

 

[possumkingdom]

Re: 5.7 OMC overheating

SOLUTION!!! After pulling the outdrive I found melted gunk blocking the exhaust in the drive. The flappers had fallen down, melted and welded themselves over the exhaust port in the drive. A miracle any thing was getting by at all. My overheating problem should be solve now. Thank you jerryjerry05 for pointing me in the right direction. I have replaced impeller, manifolds and risers, checked timing, water flow and it continued to overheat. And yet I still looked right at the problem several times. When replacing risers I looked right at the pins holding the missing flappers and it didn't register that there were missing( if they weren't there, maybe they never were is what I told myself) and if they were missing, where were they?( had to be in the exhaust pipe, right. Makes sense) so I should have explored the exhaust when I saw something not right. Then I saw some melted plastic material in the prop, but it didn't look like flappers, so I thought I might have picked up some trash in the water. Sometime the problem and solution is right there in front of you. Thanks guys for the help, I"m posting some pictures of what I found to hopefully help others with a similar hard to diagnose overheat issue.

 

[Levinz11]

Re: 5.7 OMC overheating

Glad you got her fixed up and kuddos for posting the solution.

 

[possumkingdom]

Re: 5.7 OMC overheating

May be a short lived victory. It still heats up on the muffs, but not as bad it seems like. It will idle around 150 or so then if I rev it, it will run up over 200 and not come back down until I kill it and let it cool off. Putting it in the water for a real water test tomorrow, but either way the melted flappers needed to come out. Now I dont know what to think ( again).

Now I'm thinking some of those melted parts may have been grommets from the intake tube. Can those get into the exhaust cavity? Either way I guess I'm taking the drive off again and splitting cases.

 

[Lou C]

Re: 5.7 OMC overheating

You should try your test with the clear water hose in the water. The vid you posted showed a Lot of bubbles. But on the muffs, I think it's possible to suck in air if the muffs don't fit perfect. I did the clear hose test (used reinforced hose to prevent a big leak) on the water with the engine hot and got nothing like that.

If you saw that at idle with a hot engine it could be a head gasket starting to leak. But yours shows signs of a bad overheat, the flappers, even the plastic guide plate on the side of the upper unit is melted.
So given that the exhaust got that hot, those grommets could be melted. And by the same token that could cause a head gasket leak/cracked cyl heads. If the grommets are damaged it would cause cooling problems both on the muffs and in the water on plane because the drive is out of the water. So yes that means splitting the drive. I've done it and it can be tough if it has not been done in a long time or it's a salt water boat. I rigged up a way to get it apart using all thread, bolts, and 3 scissor jacks. Not easy took some thought.

BTW about the clear hose test, the right way to do it is to have a clear hose to each manifold, and one for the raw water intake hose. The get a helper to watch what happens as you drive. Engine must be warmed up thermostat open. Bubbles from manifold, leaky head gaskets/cracked heads, bubbles from raw water intake hose the impeller or hose is pulling in air.

You sure you got good flow from the impeller/raw water intake hose (test in water, 2"-4" head of water at idle). You sure the thermostat is opening all the way (test in pot of boiling water with a car radiator thermometer, standard stat opens at 160 fully open at 180).

Here's another test I do when I put in a new impeller in the spring. With the boat out of the water and the impeller housing off, hook up the muffs. Hold them tight to the drive and have someone turn on the water. Water should shoot out of the right hand opening in the impeller mount. Now go up to the engine, take your garden hose and disconnect the impeller intake hose at the thermostat housing. Put the garden hose in the raw water intake hose. Go back to the drive and look at the left hand opening in the impeller mount. Have a helper turn on the water, it should shoot out of the left hand opening in the impeller mount. If test #1 is not good you have bad grommets on the water tube inside the drive, if #2 is not good then you have a blockage in the hose to the transom, the PS cooler or the hose from the cooler to the thermo housing.

 

[possumkingdom]

Re: 5.7 OMC overheating

Well I split the drive yesterday and took a look inside. Not fun but not really all that difficult. intake tube grommets were intact. Nothing out of the ordinary, so I put it all back together and start testing for water flow. Pulled the supply hose off the thermostat housing and had no water coming through. I tested this when I replaced manifold and risers and had good flow, so something has gone wrong after I tested for flow. I checked flow back from the supply hose and it will flow water back through the water pump, so no blockages So now I'm back to looking at the water pump. Impeller is in good shape. Wear plate has a slight groove worn in, but I don't know if that's enough to cause the pump not to push water, maybe someone has had that experience and can tell me yes or no on that. Maybe it's not sealing? Maybe those bubble were the pump pulling in air?

 

[Lou C]

Re: 5.7 OMC overheating

An impeller can 'look good' and not pump good, this has happened to me a few times. And while its easy to replace the Cobra impeller, its easy to get that gasket out of place. I use sticky OMC triple guard grease to hold it in place. Try this, if you replace it and the gasket, hok up the muffs and run it with the impeller cover off, and make sure there are no water leaks round that housing, if you see a water leak then air can get in.

 

[possumkingdom]

Re: 5.7 OMC overheating

Well I split the drive yesterday and took a look inside. Not fun but not really all that difficult. intake tube grommets were intact. Nothing out of the ordinary, so I put it all back together and start testing for water flow. Pulled the supply hose off the thermostat housing and had no water coming through. I tested this when I replaced manifold and risers and had good flow, so something has gone wrong after I tested for flow. I checked flow back from the supply hose and it will flow water back through the water pump, so no blockages So now I'm back to looking at the water pump. Impeller is in good shape. Wear plate has a slight groove worn in, but I don't know if that's enough to cause the pump not to push water, maybe someone has had that experience and can tell me yes or no on that. Maybe it's not sealing? Maybe those bubble were the pump pulling in air?

With the pump off and the muffs pushing water, water enters the starboard side of the housing as it should. With a water hose pushing water through the port side, water gushes out the water supply hose at the thermostat housing. So water comes in correctly and goes where it should correctly. So it has to be the pump right? even if the intake tube seals were bad the muff would push water past and it would still run on the muffs. So I'm down to the pump, but I can't see why the pump isn't working. I put some silicone on the rubber o ring last night and let it sit over night. cut a new gasket for behind the wear plate. bolted it up, let it sit all night and it still didnt pump at all today. Heavily frustrated.

 

[Lou C]

Re: 5.7 OMC overheating

One more thing....the rubber hose that connects to the impeller housing helps the impeller prime, it has to let air out of the housing. Before you re-assemble blow that little hole out with compressed air. When you test it on the muffs leave the rubber hose off and make sure water shoots out of it..

 

[possumkingdom]

Re: 5.7 OMC overheating

I felt some air resisance in the elbow that comes off the housing and connects to the vent tube Lou C. I cleaned it out and will reassemble and give it a try. I almost hope it doesn't work. That means I disassembled an outdrive for a clogged vent tube. I'll update with the results.


UPDATE and SOLVED!!!!

Lou C you got it right. That rubber tube from the pump housing was fine, but the 90 degree elbow on the housing that it connects to was clogged. I had checked the tube and it was clear, but the elbow was clogged. It passed some air but not much. I cleaned it out with a small wire and it flowed air. Reinstalled and water flowed form the 90 degree vent and through the supply hose. Hooked it back up and no more over heat. I had already ordered new impeller and gaskets, etc. But if I had not checked that vent, I would have put on new parts and STILL had the same problem. Read this entire thread and you will see what I've gone through for this relatively simple problem. But thanks to all of you that have offered your help I traced it to the real problem. Thanks to you all. Lou C, if your in my area ever, leave your money at home, drinks are on me.

 

[Lou C]

Re: 5.7 OMC overheating

Really Glad you got it!!! Imagine that little hose causing all that grief. But the work was not wasted cause you know that the grommets etc are ok. Here's the reason for that hose and nipple, impeller is submerged when the drive is in the water. If the housing is full of air when the boat goes in the water, if the air can't get pushed out by the water it can't pump well.

A Cobra with the cooling system in good shape and the standard 160* stat should run between 160-175, 180 at most when coming off plane.

Here's one for ya, mine started with a mind hot running condition 2 seasons ago. I knew I had a few small barnacles growing on that water screen in the lower unit. Boat is moored in salt water 6 months out of the year. In salt we see problems you never see elsewhere. Did all the checks had somewhat low flow. Split drive this spring found barnacles in the area above the water screen blocking the water flow. Pulled out that screen and painted the inside of the water passage with anti fouling paint. All better now. Live n learn....

 

[WesWilliams]

JerryJerry05 please help, I have what I believe to be a similar overheating problem to this old forum. I have a question about your comments about the grommets. I split the lower unit and the rubber bits on the ends of the copper water tube were in good shape.. are these the grommets you are talking about? There was a part on the copper tube that was a bit beat up but not terrible. No idea what it is .. is it the guide you are talking about? My pump is not priming at all.. zero water. The elbow/tube on the impeller housing are fine, i have a new impeller and cover. I just ordered new gaskets/seals for the water pump adapter (that was fun taking off). My main question is on the grommet you talk about.. what is the purpose of this part?

I know this is an OLD thread but I am stuck!

Thanks,
Wes