7.4 Running hot need help

[Casey]

I have a 1997 SCR 2700 powered by a 7.4 carbureted. I've owned this boat for about 1 year. Last time out on my way back in in noticed my engine temp was near 200 deg f. Normally it runs right around 150. I throttled the engine back to and idle and the temp held at about 200 deg. I then shut off the engine and restarted in less than a minute. Temp at and idle dropped right back off to normal. Started underway again and the temp climbed. Back to and idle no drop in temp. Shut engine down and restarted and the temp dropped to about normal again. Eased my way back and trailered the boat home. I replaced the impeller before my first trip along with other maintenance before going out this season. But I had kicked up a lot of sand a couple different times on this trip. I pulled the impeller and changed it even though it look fine I also changed the thermostat not knowing when it was changed last. When I had the thermostat out inside the intake manifold under the thermostat I pulled a chunk of rust out about the size of a silver dollar and curved in shape. Reinstalled the thermostat and ran the engine on a set of muffs. Idled for about 10 minutes and Temp was good. Brought engine up to about 1300 rpm and then temp climbed to about 195 and I idle back and shut down. Checking the exhaust manifolds the port manifold I could put my hand on no problem. The starboard side did not take long before I removed my hand. Could it be I have a bad exhaust manifold that is causing my overheating issue. And if so can someone explain to me why. I am having a had time rationalizing it. If it is not being caused by a bad exhaust manifold any suggestion would be appreciated.

[mmwjr]

First use an IR temp senser to confirm the engine is really that hot also take readings of the exhaust manifolds and elbows. If she is running this hot stop using the boat or you will damage the engine.

Also check the volume of water coming out of the raw water pump. I am assuming you have a Bravo drive, these are know for the water hose to become blocked at the transon assembly where the hose retainers install. You will need to pull the drive to confirm this.

You mentioned sand, this can block any water passage. In the drive, engine, or exhaust.

[shrew]

I'd start with an impeller replacement. I'd then look at the manifolds and risers. Lastly, I'd look at the intake hose on the transom assembly. While a 1997 transom assembly will be a culprit for corrosion crushing the line running through the transom, I'm not sure if that would occur if the boat spent its life in freshwater. I see your profile say NV. Has the boat spent its entire life in fresheater?

[Casey]

As far as I know this boat has only been in fresh water. I also believe it was kept on the water until I purchased it a year ago. I have changed the impeller and thermostat. Did not seem to make any difference. This is where I need some help. Not sure what I would be looking at on the risers and manifolds. Plugging ?

[mmwjr]

Remove the risers and inspect the 4 water jackets around the inner exhaust port. Also inspect the output water port above the exhaust port on the round portion of the riser that fits into the rubber exhaust hose.

With the riser off you can inspect the exhaust manifold water jackets as well.

Once again and IR sensor is your friend to getting real temps at several points as the gauge is only reading one point by the t-stat.

[Casey]

I am going to try and get it pulled down over the next couple evenings after work. I'll let you know.
Thanks

[mmwjr]

Originally Posted by Casey

I am going to try and get it pulled down over the next couple evenings after work. I'll let you know.
Thanks

I would first get temp readings that I have suggested as they may point to where the issue is.

Also try removing the water inlet hose to the t-stat housing (large hose on the port side) to check for water flow when running, should be a good stream.

[shrew]

If I recall, if you pull the manifolds and risers you'll need a new gasket when you reinstall them. I don't believe you can re-use the gaskets.

[Casey]

Thanks, I have already ordered the exhaust kit. It comes with both manifold, both risers, all the gaskets and bolts as well as an assortment of plugs. It's an aftermarket kit. New Sold by Sierra. I purchased it on eBay for $599.00. My local marine dealer wanted $465 for 1 manifold. I have not pulled it down yet but I'm pretty sure there is going to be an issue on the starboard manifold. With the temp being so much warmer then the port side. Not sure that this will be the only issue I will find but not knowing any history on this boat might be just good preventive maintenance.

[mmwjr]

Typically fresh water exhaust will last longer than the age of yours. you may have sand in the t-stat housing blocking flow to the starboard manifold. Once again take temp reading with an IR sensor, pull hoses to check flow.

The kit you ordered is manifolds and risers which are really elbows. Does your set up have risers also referred too as spacers, 3 inch straight parts between the manifolds and elbows?

[Casey]

Yes I do have the 3 inch spacers.

[mmwjr]

Originally Posted by Casey

Yes I do have the 3 inch spacers.

Did you order replacements for them?

[Casey]

No I did not order risers. When I got it torn down both risers looked really good. the port side manifold and elbow looked really good also. However the starboard side manifold and elbow not so good. At this point I believe somewhere along the way they port side had been replaced. While I had all the manifolds stripped off the engine I pulled all the all the hoses and flushed the block as well as I could using a hose. Also inspected the steering and engine oil coolers. This is where I think I found the real problem. About 2/3 rds of the tubes in the engine oil cooler where blocked with large sand particals. I managed to clean the cooler out carfully with a straight stiff wire. Reassembled with both new manifolds and elbow and ran with a set of muffs in the driveway at about 1200 rpm for almost an hour. The temp on the gauge stayed steady at about 165. I used my ir on the thermostat housing which read about 10 deg cooler than the gauge. Also checked both manifolds in several spots the temps ranged from 95 - 103 deg. Both elbows ran at about 98 deg. I then idled the engine and rechecked temps. They all seemed to raise by about 5 degs. That seemed a little odd to me. Anyway I believe my overheating problem is fixed. Headed to the lake Friday afer work for the weekend. Taking me ir and putting it to the real test.

[shrew]

The spacer between the manifold and the elbow should be the riser. I was always told to replace the manifolds and risers together.

[speedysprocket]

Originally Posted by Casey

I then idled the engine and rechecked temps. They all seemed to raise by about 5 degs. That seemed a little odd to me.

As water flow slows down, so does heat transfer. Not uncommon for temp to decrease slower or even appear to rise at idle, than say at 1200 or 1500 RPM.

[mmwjr]

Plugged cooler sounds like it was the issue, let us know how it goes on the lake.