OMC Cobra blowing exhaust

[180horizon]

Gday everyone
i recently bought a 1990 Four winns 180 horizon with a 302 windsor and a OMC Cobra #985 687 leg.
When i first bought the boat, the previous owner had an exhaust rubber blown out and replaced, about the 3rd time i took the boat out, the exhaust rubber blew again.
This convinced me it must be a overheating problem so i put in a new thermostat (old one was missing) and a new temperature sender unit (old one broken) cleaned out thermostat housing and some hoses to make sure everything was flowing alright, i then replaced both the exhaust rubbers with some better quality ones which has been fine for about 5 days out, but this recently blew as well, it looks like they are being heat damaged and slowly burning away.

Something else worth mentioning at this point, when i was replacing the rubbers i noticed one of my exhaust flappers sitting in the bottom of the exhaust pipe, as for the second one, cant be found?
I did a little research about what these do, and it seems they just stop water re entering the cylinders and stalling/causing a hydro lock, when out on the water without the flappers installed everything still seems too be fine.

From what i have researched it seems blowing exhaust rubbers is common with engine overheating, my engine does not seem to get too hot, never goes past 90 degrees celsius.
I have ran the boat with the exhaust rubbers off and a reasonable amount of water seems to be passing through the manifolds for cooling, and they have never been too hot to touch.
I am now wondering if maybe the problem lies in the leg, maybe a blocked exhaust? possibly caused by the missing flapper?
I did notice that when running the boat out of the water, about 90% of the exhaust comes out of the flexible hose rather than the prop.

I am a automotive mechanic by trade, and thought i would find a inboard v8 quite familar for me to work on but i definately need a little more educating before i can figure this one out, any help/advice greatly appreciated

 

[bruceb58]

Re: OMC Cobra blowing exhaust

From what i have researched it seems that this is common with engine overheating, my engine does not seem to get too hot, never goes past 90 degrees celsius.

That is way too hot. Normal temp is no more than 160F or around 70C. You need to change your raw water pump or you have restrictions in your manifolds.

 

[180horizon]

Re: OMC Cobra blowing exhaust

The thermostat i bought for it was 82 degrees celsius, is that too hot?
After having a closer look at my gauge it usually reads around the 82 mark
(180 f)

 

[bruceb58]

Re: OMC Cobra blowing exhaust

I am assuming you have raw water cooling. If so, that is too hot of a thermostat. Regardless, you should not be burning up your exhaust hoses even with that temperature thermostat. You have low water flow. When is the last time the raw water pump impellor was changed? It should be done every two years.

You also need to locate those missing exhaust flappers. They could also be causing a problem.

 

[180horizon]

Re: OMC Cobra blowing exhaust

Yeah its raw water cooling
Previous owner had the impeller replaced when he got the rubber fixed the first time it broke.
Would be about 1 year ago now, the boat hasnt been used much since then, but i will check the condition of it, from the outside it looks quite new

 

[JSMoore]

Re: OMC Cobra blowing exhaust

First, have a look at your raw water pump. Starting the motor without being in the water, or on "the muffs", can easily destroy a brand new impeller in just seconds. And, unless you know for a fact the motor has NEVER been started dry, opening the pump for a look-see is quick and easy. If the impeller even looks funny, replace it.

Check all of the water lines and fittings. If you, or the previous owner has ever blown a raw water impeller bad enough for blades to separate from the thing, they could be stuck somewhere in the system - causing a restriction in the flow of water. Since you found that there was no thermostat installed when you got the boat, there could even be this type of restriction somewhere inside the motor it's self.

Change the thermostat to a 160* thermostat. 180* is probably not going to cause the problem you are having. But, it sure isn't helping prevent it. And, a check of the specs for your motor will probably show that a 160* thermostat is suggested.

Remove your exhaust manifolds and check for rust and corrosion build up that could possibly restrict the flow of cooling water through the manifolds and out through the exhaust. Unrestricted flow is important! Just a little restriction may set up a situation where it stays cool enough until you get on the throttle and the motor is making power at higher RPM. Under power, the cooling system needs LOTS of water to do it's job.

I think I would also want to know what happened to my "flappers". Could one, or parts of one, be stuck downstream in the exhaust someplace, restricting the flow of exhaust and cooling water out through the prop? Just a thought....
Cheers!
JSM

 

[sethjon]

Re: OMC Cobra blowing exhaust

The thermostat i bought for it was 82 degrees celsius, is that too hot?
After having a closer look at my gauge it usually reads around the 82 mark
(180 f)

Way too hot? No, but too hot. Keep it at 160/70 or expect problems.

 

[a70eliminator]

Re: OMC Cobra blowing exhaust

If an exhaust flapper has broken off (very common) laying loose it can certainly kick up and block the exit resulting in a backpressure that can easily blow the rubber boot.
Exhaust exiting out the bellows running muffs is totally normal, there are holes on the bottom of it, but it should also be exiting the prop too.

 

[180horizon]

Re: OMC Cobra blowing exhaust

Im now tending to think its a heat problem rather than a locked exhaust because the rubbers seem to have slowly been burnt away
i pulled out the impeller today, it looks to be rather new but one of the blades is broken about 3/4 off, is that enough to cause the exhausts to blow?

 

[JSMoore]

Re: OMC Cobra blowing exhaust

Im now tending to think its a heat problem rather than a locked exhaust because the rubbers seem to have slowly been burnt away
i pulled out the impeller today, it looks to be rather new but one of the blades is broken about 3/4 off, is that enough to cause the exhausts to blow?

Absolutely!!!

Replace it. It ain't that expensive, or hard to do!

Also, pull your exhaust manifolds and check for corrosion restricted, or blocked water passages. All it costs you to check is some time and a set of cheap gaskets. And, it could save your motor!
JSM

 

[Lodgepole]

Re: OMC Cobra blowing exhaust

I agree with all the above. By all means replace the impeller and check in lines for broken off impeller pieces. I had an OMC flapper burn/fall off and block the passage in the drive. this heats things up real fast. Low water flow because of a borderline impeller will drasticly raise exhaust temps and destroy flappers and rubber exhaust parts.