OMC 4.3L Cobra Overheating

[maltz88]

I have a 1987 4.3L Cobra (GM engine) which started overheating weeks ago on the lake. I'm pretty anal about watching my gauges, and noticed withing 2 minutes of launching, that the temp was about 200 degrees and climbing.

We beached the boat, let it sit for about 2 hours, and started it again. It had cooled off (so the temp. sending unit doesn't seem to be the problem), then it heated right back up to 200 or so.

Trailered it, and put it on the muffs at home, and it does the same thing every time. By process of elimination, I've replaced the thermostat, checked the impeller (new at the end of last season), and replaced the engine ciculation pump.

I've drained the block, and it is scalding hot water, but is clean and clear. The temperature on the exhaust manifolds seems to be uniform and luke warm.

The only oddity, I'm not sure if I just never noticed it before, is that most of the water is now coming out just after the transom, near the exhaust releif valve, very little coming out through the prop.

I'm at a complete loss. I've been told that it may be the exhaust manifolds, but is it typical for them just to go all at once like that? I've never had a problem until the last time to the lake, and now it won't do anything but overheat. I would think if the manifold was going out it would start running hotter, or be an intermittent problem.

Any ideas??

 

[HT32BSX115]

Re: OMC 4.3L Cobra Overheating

There ain't no lake in YELM!

(Oh....wait a min.....LAKE Lawrence!!)


Howdy,

There's a few things you can check. Hook up a set of "muffs" and disconnect the hose that goes from the transom to the T-stat housing.

Turn on the water and start the engine and see if you have good flow of water from the hose. (It would be better to do it "in the water" so you could observe the pump actually pumping water out of the lake)

If you have low or no flow, you may have an obstruction between the pump and the t-stat.


Also, there is a plastic fitting that can sometimes break. (#18 below) If it's cracked or broken or the hose that it connected is cracked or split, most of your cooling water will go over the side.

It's not uncommon to have water exiting from the transom area. The exhaust bellows might have a tear in it, or it's a newer one with the slits in the bottom or it may even be missing!!

You can run without it though......
My 460 King Cobra ran with no exhaust bellows for about 10 years before I got it. (when I bought the boat from my brother......I asked him about it and he said "what's an exhaust bellows?")


I'll also add that if you determine that you have "good" water flow "into" the engine then it MUST get out. I.E. if your risers/manifolds are so rusted that water cannot get out of them they must be replaced. The only way to check is to have a look.

Something tells me those manifolds are really bad JUJU!! Doug Russell lists them for something like $1200 EACH

#28 & #33 in the following link.

http://dougrussell.com/partscatalog...0,371,372,373,374,375,376,377,378,379,380,381




Cheers,


Rick

 

[danond]

Re: OMC 4.3L Cobra Overheating

Something tells me those manifolds are really bad JUJU!! Doug Russell lists them for something like $1200 EACH

You can also buy an aftermarket kit from eBay for around $900 that comes with all the parts you need to convert to a later style, non bat-wing setup. I can't speak to the quality of it, but you can buy 4 of the kit manifolds for the price of 1 of the factory manifolds.

Link here

 

[maltz88]

Re: OMC 4.3L Cobra Overheating

Thank you for the tips Rick.

I have read somewhere else about the test to the thermostat connection. I'm going to get on this today or tomorrow.

I didn't see the link to the plastic piece you mentioned as #18 (unless you were talking about the doug russell link which is to a water pump bolt)?

You are correct about not wanting to mess with the manifolds. I've got a $5,000 boat, and the manifolds would be about $1,000 if I buy an aftermarket like danond mentioned.

Any ideas on if I buy new gaskets, and pull the manifolds, clean them out, and get some more life out of them? Or would I be opening a whole new can of worms?

.....also, our lake of choice is Alder. So we have to leave the big city of Yelm for our boating.

 

[Gary H NC]

Re: OMC 4.3L Cobra Overheating

If they are the one piece original manifolds they have well outlived their normal life.

I have installed a couple sets of these with great success..699...

http://bpi.ebasicpower.com/mm5/merchant.mvc?Screen=PROD&Store_Code=ebasicpower&Product_Code=HGE58994

 

[HT32BSX115]

Re: OMC 4.3L Cobra Overheating

Thank you for the tips Rick.

I have read somewhere else about the test to the thermostat connection. I'm going to get on this today or tomorrow.

I didn't see the link to the plastic piece you mentioned as #18 (unless you were talking about the doug russell link which is to a water pump bolt)?

You are correct about not wanting to mess with the manifolds. I've got a $5,000 boat, and the manifolds would be about $1,000 if I buy an aftermarket like danond mentioned.

Any ideas on if I buy new gaskets, and pull the manifolds, clean them out, and get some more life out of them? Or would I be opening a whole new can of worms?

.....also, our lake of choice is Alder. So we have to leave the big city of Yelm for our boating.

Oh yeah....I've been on Alder! STUMPS over in the south end!

Anyway, sorry about that pic... Screwed that up!

see #18 below:

That picture is from Doug Russell's parts site for your year model

Most of those parts are available I think.

I would absolutely get the aftermarket manifolds instead of the OEM OMC ones. (I think I remember someone saying the OEM ones are basically not available any more anyway.)


You could try to "clean out" what you have but if they are truly rusted that bad, you could just be setting yourself up for a failure where you'd get water into the open exhaust valves and hydrolock your engine. If they start leaking when you park it for winter then you have water sitting on top of the pistons all winter and a frozen engine next spring/summer.

You cannot predict when they'll fail completely when they're bad.

If they're really bad, you need to replace them.


The other thing to check is for debris clogging the raw water line.

sometimes remnants of previous failures can get stuck anywhere from the pump to the T-stat housing or even in the t-stat housing. reducing flow into the engine. That way, your t-stat may be opening but flow thru it is being blocked by pieces of a previous impeller failure. or other debris...