1988 OMC Cobra 5.7 Overheat - Stumped

[Nauti-Escape]

Greets All, My first post here.

I have a 1988 OMC Cobra 5.7 which developed an overheating problem after going on a marine railway, engine off then trailer position. Back in water drive down, maybe 3 minutes in a no-wake canal then overheated and by the time we all noticed the smoke blowing around had toasted the heads and any related gaskets (that was the long term damage). No plastic bags noticed around the intake, water was clear.

After cooling down enough to work on and removing the t-stat as the first suspect, I had revved the engine briefly, and everything seemed normal, water pump hose was pulled off and loads of water coming from the impeller, yea, we carried on after reassembly back to port without further incident. Even went thru some locks and stopped and started fine.. Heads redone and all new t-stat, gaskets and rubbers followed.

Now the twist. Any time after that incident (with or without a t-stat), if I tilt the drive even so much as half way (not running), while it remains completely submerged, when I put the drive down and startup/leave, the engine will repeatedly overheat "until" I give it a burst of throttle. It's like the pump lost prime, but never was out of the water!

New t-stat & water pump kit was installed 40 running hours prior to the initial incident, and the next time I replaced it all again, even though there was never even a score on the blades and the t-stat worked fine in a boiling pot of water. Even ran a flusher thru it with no sign of blockages - full water flow from the drive to the t-stat housing.

So what else is there going on? I've heard others asking the same question but never saw any solution. The drive is off once again and I detect no signs of blockages either in the drive or up into the engine.

Let me add this, to clarify:
Never seen salt.
Engine is a recently built 383 stroker (stroked 350).
Heads are ported, polished, fully cleaned and reground.
Exhaust manifolds are pressure and flow tested fine.


Thanks
Mike

 

[Haut Medoc]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Welcome to iboats....
There are some OMC guys here who can help, so stick around...
I'm not one of them though.......:redface:​

 

[Haut Medoc]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

However, i would look at the hose feed form the bell housing for loose clamps or possibly kinks.....​

 

[Don S]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

I would sure be looking at the raw water pump housing and its seal ring. Even with a new seal ring around the pump, I like to put some sealer.
Even a minor leak (sucking air) will cause the pump to not prime properly.
How old was the impeller when you first put the boat in the water?
Even in fresh water, damage can happen to the impeller over the previous season, and old impellers that have been even slightly oveheated WILL develow a set and not pump as well as a new one with a new, properly sealed housing.

 

[bruceb58]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Since the impeller is mounted on the drive, it is possible that there is an air leak round the housing and tilting the drive up slightly lifts the housing out of the water.

Does it overheat when you are on plane? Although the housing would be out of the water in this scenario, you have a lot more rpms at the pump and might overcome an air leak.

 

[JustJason]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

sounds like your pumping air....
Find the line that comes up from the transom (the line from the impeller) and cut it/splice it/replace it with a section of clear vinyl test line. Should be able to get it at home depot. run it and look to see if your getting air bubbles from the impeller.
After your all done put the original line back in place.

 

[Bt Doctur]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

cancel

 

[Nauti-Escape]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

All good points. However the drive is continuously under water all summer (save going over that railway twice a year). I don't think the pump body ever breaks above the waterline while tilted to trailer position.

Although it appears to loose prime by behaviour, I find it difficult to believe that it's pulling air (unless for some freak reason the exhaust bubbles are causing the pump to cavitate, which are emitted above and behind the pickup).

I had suspected that maybe zebra muscles had grown a cluster inside the drive and may lodge somewhere critical when the drive tilted. I can't see any visible blockages and hate to $$$ to have the thing torn down just for a peek (all seals are holding).

This one needs some serious thought. I launch in a month, will replace the entire Pump/housing, rubber hose and fitting from helmet to transom plate. Failing all else I still have that workaround.

DonS, We know each other from BOC. Would you recommend Sierra parts or OEM?

Cheers,
Mike

 

[Don S]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

I thought the name sounded familar.
I never recommend Sierra over OEM. Since I left Alaska where we always used OEM and never even seen Sierra, I always heard how great they were. NOW, that I live in WA and where I work likes Sierra, and I get to compare the parts. Always go OEM and there is a BIG difference. IMHO.

 

[Nauti-Escape]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Thanks Don, those are the only (Sierra) cooling parts that I have used - pump kit and t-stat. Why the sudden change in behavior after good use, no clue. Actually I had and OEM t-stat fail from the get go after stroking the engine, but the manifolds stayed cool, so the bypass ports on the t-stat housing had flow.

Incidentally I am currently using (Sierra) head/intake/exhaust gaskets. So far so good but reservations on the exhaust and riser gaskets, they are like hard asbestos/cardboard compared to the carbon-like coated OEM ones. Actually the last set of intake gaskets I bought (as spare) OEM were GM original parts with OMC label. I have all gaskets on board as spare from one brand or another - try to find anything in the middle of the Great Lakes, well you can guess...

Mike

 

[Don Ayd]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Hi, I had a similar problem with the same type complaint on a 88 bayliner with a 5.7 cobra. What I found was wear/corrosion on the water pump adapter located behind the waterpump impeller plate and gasket. I also replaced the water tube cover underneath the upper housing. I also found corrosion in the water passage housing mounted where the water pump would be on a typical outboard lower unit. I also replaced all related seals and gaskets. This was the solution, and fixed the water starvation/overheat problem. This was a one owner drive and had never been apart before. I used all oem parts but I believe all the parts that I did use are now currently available from Sierra so this tells me that these parts must wear/corrode. frequently. Hope this helps, Don

 

[Nauti-Escape]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Thanks Don, so are you implying that I have to break open the top/bottom housings to see inside? Would this cause any concern with regards to any shimming of gears, or should I just take it into the shop and have them inspect and clean it? I don't have the tools anyways to start replacing seals, however it does not appear to take in any water (sleeping dog).

Mike

 

[Don S]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

I think he was talking about the adapter plate (2) in the picture below. It's aluminum and builds up corrosion under the SS plate (1) and causes it to warp a little. Then the housing for the pump won't seal no matter what you do. The water tube cover is #3 in the picture. The housing in the lower is accessed by seperating the upper and lower. You will need new orings and seals, but you don't have to dissassemble the drive where you would have to get into any of the gears.

 

[Nauti-Escape]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Thanks Don S.

If it's that simple to open/close the halves and service water related parts, I'm in. Will grab necessary gaskets/seals and proceed.

I'm also curious to know if there is something that may be floating around in the water passages within the drive (we do have a small problem with zebra muscles, I hear that one can lay as much as 40K eggs in one shot....just as few in there would choke things up when mature). The year the engine blew it sat in the water as a floating cottage for 3 months, then the problem surfaced later the following season. If that's ever been heard of on an outdrive...

Mike

 

[Don S]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

Sure muscles could be a problem. Have found them clooging many a tube that goes thru the transom shield and other sharp bends.
There just is no easy button when it comes to fixing an overheat. Everyboat is different, and mother nature love to play "Stump the Mechanic"

Can I assume you have an OEM service manual? (No, aftermarket is not good enough)

 

[Nauti-Escape]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

I have the Seloc '85-95 manual, pathetic as it is with clarity. Trying to get that adaptor off is a bugger, it has 2 threaded holes and the rest thru to the main housing. I can't see how to get it off safely using any kind of 2/3-way puller and prying with a flat blade looks near futile...tips welcome!

 

[Don S]

Re: 1988 OMC Cobra 5.7 Overheat - Stumped

You have no reason to remove the adaptor. Just clean it up smooth and install a new water pump plate (1) and the gasket between the plate and the adaptor. I would also install a new WP housing and seal. If it's warped a little, it will never seal.