Opinions Needed

[Gilded_Splinters]

In the interest of not dragging you all through another over heat thread, I'll let you know what I've done and learned and the last things I've boiled it down to.

1989 OMC Cobra 5.7 liter. Over heats at higher RPMs, but does not cool down at idle.

The boat never had thermostats in it. Always ran @ 120. Before launch this year, I put them in. Found an accumulation of rust scale in the recess in the intake manifold under the thermostat housing. Removed rust scale with magnet. Installed thermostats, test ran and overheated. Removed thermostat in over heated engine out on lake. Found more rust in intake, removed with magnet. Temps dropped as I had hoped, but things have never been the same.

A third inspection a week or so later revealed no rust at all, so I thought I had gotten it all. I should add that this rust looked as if it had been bouncing around in there for decades. It was black and soft and easy to crumble. There was no red, fresh looking scale.

1. All raw water tests performed. Plenty of water coming in, no air being pulled in while on plane. No obstructions at all. Again, good water flow, pump is fine.
2. I found risers very clogged. Manifolds only clogged below riser gasket area, once cleaned out, manifold water jackets appeared wide open. Risers replaced, but I neglected to inspect flappers while is was right there. Ugg.
3. Test run post riser install showed abnormal temps considering there are still no thermostats. It ran @ 160 or more and might have over heated, but should not have exceeded 130 as was normal up to this point. It should be noted that before the riser change, it got much hotter, much faster, so this represents progress. The risers were so clogged, that I'm surprised that any water at all got through.

So here's where I'm at:

Either there are rust/mud/gunk obstructions in the block or there are flapper bits clogging up exhaust passages. The flapper's condition (or existence) is something I plan to check on as soon as I can.

Any thoughts or suggestions would be greatly appreciated. I've about had it with this thing.

Thanks.

 

[Gilded_Splinters]

Hmmm, mysterious duplicate post..please delete one.

 

[Bondo]

Hmmm, mysterious duplicate post..please delete one.

Ayuh,.... Have ya pulled the block drain plugs, 'n flushed out there,..??

That's where the crud lays,...

 

[Gilded_Splinters]

If you mean the small block drains, no I haven't, but plan to next time I get down there.

There is one very easily accessible freeze plug that I was going to pop out to see if there was a build up. I guess on the strength of that I'll have to tackle the ones that are literally impossible to get at.

 

[Bondo]

If you mean the small block drains, no I haven't, but plan to next time I get down there.

There is one very easily accessible freeze plug that I was going to pop out to see if there was a build up. I guess on the strength of that I'll have to tackle the ones that are literally impossible to get at.

Ayuh,... Try the drains 1st,...

If it's Soooo bad that ya gotta pop the core plugs, Pull the Motor 1st,.....

 

[HT32BSX115]

Ayuh,... Try the drains 1st,...

If it's Soooo bad that ya gotta pop the core plugs, Pull the Motor 1st,.....

Yes. When I bought my 1997 454 (in 2005 with only 122 hrs since new) I removed all the core plugs and extensively flushed the block.

There was a fair amount of sand and other debris in the block.

Your engine is quite a bit "older" and probably has more hours running with no t-stats which would, IMHO, result in more depositing of sand and other stuff in the block.

Also, were your risers and manifolds rusty because they're old and operating in salt? If so, consider that the block, heads and circulating pump may have as nearly as much rust as the exhaust system.......removing the core plugs and water pump and maybe even the intake manifold will give you a pretty good look into there.....

 

[jerryjerry05]

The manifolds are probably due for a change.
The risers too.
The block drains remove and get a stiff piece of wire and try to break up any crud built up in the system.

Removing the "freeze plugs" can be a B.
Reinstalling them can be a B.
What you can do: install a small magnet under the thermostat or in the thermo housing.
It will collect any "floating/ flowing" pieces of rust or metal.

 

[Gilded_Splinters]

Thanks for all of the responses.

The boat was in salt water for it's first 15 years or so, and the manifolds and risers were changed at some point in it's life. They are/were Osco and Barr Marine parts...the OEM stuff long gone.

I changed the risers as I couldn't really get the old ones as clean and free flowing as I wanted to. I may have them hot tanked and see whats left.

Yesterday I removed the drains and found quite a bit of junk in there. More on the side that was running warmer (no surprise there). Instead of running the engine to pump water through there, I connected a garden hose to the thermostat housing intake. The water pressure at my marina is insane. Over the course of about 4 hours, I poked and cleared until I had 10 times more outflow out of the drains as I had when I started. About a coffee mug full of small, particulate rust came out in all.

I had great flow through the manifolds and out the exhaust. I could hear and feel the water flow in the risers and exhaust pipes and it all got ice cold from the hose water.

Anyway, a test run was successful so far, but there were 4 foot waves and it was hard to really get on it without emptying the galley all over the cabin.

Thanks for your help...I'll post results of calmer water test run.

P.S. I did install a very large rare earth magnet on the top of the thermostat housing that should grab particles as they go by.