Overheating Possibly Caused by Restricted Thermostat Housing Port

[KM7]

1991 Four Winns 200 with 5.8 L Ford V8 and OMC OD.
I was cruising the harbor on Sunday with friends and noticed overheating at the port exhaust elbow. The engine temp gauge was only slightly above the midpoint, so it was approximately 172 Deg F. The starboard elbow did not seem as hot.

I was expecting to find rusted and blocked passageways in the elbow, so I removed it. It was easy to do, remove 4 bolts and 2 hose clamps and the elbow, along with the round exhaust pipe comes out. See pictures

I think that I found a different cause. The hose from the thermostat housing to the exhaust manifold was very cloged and the outlet of the thermostat was rusted and had narrowed down to only about 1/4 of an inch.

Questions: As you can see on the picture. I have not seperated the elbow from the exhauts pipe. Any suggestions?

Has enyone tried cleaning out the rust with Evapo-Rust?

I'm considering that and replacing the hoses.

Below is a picture of the hose

 

[KM7]

Update 7/29
I was able to separate the round pipes from the elbow. A few hits with a rubber mallet was all it took
After looking at the elbows more closely and checking prices, I have decided to replace both elbows. I can get both with the gaskets for under $250

I will clean and repaint the round pipes. That way I will be sure that I get a good seal with the rubber hose between the pipes and the elbows.

I checked the Exhaust Seals and they are working fine. (item 51 on the Crowley Marine page)

INTAKE MANIFOLD, EXHAUST & COOLING - 1991 Stern Drive 5.8 584APLRGD | Crowley Marine

View parts diagrams and shop online for INTAKE MANIFOLD, EXHAUST & COOLING parts, 1991 Stern Drive 5.8 584APLRGD. Offering discount prices on OEM parts for over 50 years. FAST $4.95 shipping available.

www.crowleymarine.com

 

[KM7]

Some of the gaskets between the exhaust manifold and the elbow have 4 wide open passageways for water. On some of the gaskets, 2 of the sides only have a hole for water flow and not the full area. Why is that?

Did OMC make a revision? Which gasket should I use? BTW, I am using new elbows, which should give very good water flow, and the exhaust manifolds should run cooler.

Which gasket should I use?

 

[Lou C]

I have that style exhaust on mine and I used the gaskets with the 4 open sections. That elbow that you posted a pic of was definitely in need of replacement given the level of clogging seen so that's a good choice.
With this job the issue is making sure that the sealing surfaces are flat and level, and that they are not pitted with rust. Leaks around those gaskets into a cylinder are quite common and cause a lot of damage. How do the cooling passages in the manifolds look? If they are as clogged as the elbow you posted I would at least try to rod them out.
The passages in the thermostat housing also get clogged/necked down over time you can rod them out with a coat hanger wire. That clogged cooling hose was probably the cause, what was clogging it? Might be able to re-use it if you can blast that crud out with a garden hose.
On my OMC 4.3 I did a 2 piece exhaust conversion about 9 years ago using Barr Marine exhaust parts with Volvo Penta 90* down pipes and hoses, it all fit perfect and no leaks. Probably have to replace the elbows at least this fall or next year. Saltwater use for me.

 

[KM7]

Lou, thanks, you have been a great resource for years. I have not been on since the gimbal bearing/ housing/ bellows repair a while ago. She has been running well.

The exhaust manifolds look better than the elbows but I will try to clean them out. I can prevent large pieces of rust from falling into the manifolds by stuffing a rag into them and then vacuuming them out, but I expect some debris will get past. Should I be concerned? How big are the passageways downstream?

I used a drill to open up the passageways in the Thermo housing for the hoses to the exhaust manifolds. Now that the thermo is out I'd also usind Blaster rust remover. I think the debris in the hose may have been the main cause. I did clean them out and will reuse them.

Mine is always in saltwater, Long Beach CA, I rinse after and run Salt Away.

Now I have to find elbows that use the gasket with 4 fully open passageways. It seems most have the one with the 2 small holes.

 

[KM7]

One of the suppliers says his fits a 5.7 but 19192 and on. My boat is a 92 but with a 91 motor:
Ford 351 Cu In Model 584APLRGD Serial No. T01224704

Did something change in 92?

 

[Lou C]

As far as the elbow fitment, I think there was an issue with the engines that had EFI, the elbow for the carbed engines wouldnt work for them probably because of some bracket for an ECU, not sure of other changes.
I'd look at marineengine.com and look at the Barr exhaust catalog. Those gaskets should be readily available.
About the manifolds, yeah you don't want any rust flakes falling in the center exhaust passage for sure. I would probably remove the manifolds to clean em out and re-deck that sealing surface (mill bastard file does a good job, rotary tools are a bit risky).
The other test you can do on the manifolds is the acetone test, fill the water passage with acetone with it propped up level (careful flammable) and see if the level drops in the ports over time, also look at the exhaust ports to see if there are any wet spots. If so then you need new manifolds. Expensive but it's unavoidable in salt water with cast iron exhaust.

Here's another thing that a lot of people miss that should be checked with a raw water cooled cast iron engine....the water passage in the intake manifold, these can rust through and put salt water in the engine oil, I have not had that problem yet but I check the bottom of that water passage every time I have the thermostat housing off. I tap it with an extension and a small hammer to make sure it "rings". If a dull thud it's on the way to rotting through. l have a spare Chevy V6 intake in the garage that I am cleaning up in case I need it. Not as familiar with the Ford intake so not sure if it's the same issue. But good to find out, because salt water in your oil is a BAD thing!

 

[Lou C]

look here:
I like them better than Crowley because they list multiple aftermarket options when there is no OEM (often the case with OMC). Shipping is pretty fast.

https://www.marineengine.com/parts/omc-parts/omc-boat-parts.php?year=1991&hp=5.8&model=584APLRGD&manufacturer=OMC+stern+drive&section=Intake+Manifold%2C+Exhaust+%26amp%3B+Cooling