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#1417283 Sat Jul 17 2021 10:28 PM
Joined: Jul 2021
Posts: 7
S
'Bolter
Hello folks. This is my first post on this site. I have a 250 engine (year unknown) being used in a 20 ft. boat. I replaced the engine that came with the boat because of mechanical problems with its design with the 250 which is what it originally came out of the factory with. The drive mechanism is a mercruiser outdrive configuration. The water pump in the drive was replaced last year and the water pump on the engine was replaced with that for a marinaded engine (brass impeller). While replacing the water pump I opened up the drain plug on the back of the block and didn't see allot of rust scaling while I was flushing out the block. The proper thermostat for raw-water cooling was put in (140 degrees).

I tuned the engine up on the trailer and took it out on the bay for it's first mechanical test. Engine ran as smooth as I ever experienced an engine to run. The temp gauge on the dash is reading normal. Water exiting the manifold is about 113 degrees and going out the riser thru the exhaust and out of the boat is about 107 degrees. Readings on the lower block were scary at around 215 degrees on the exhaust manifold side and 180 degrees on the stater side. Temps where read with a IR sensor gun.

My question for all the knowledgeable people on the forum is, what is the engine block temp suppose to be ? I suspect that the rear of the engine has a blockage toward the rear restricting flow. The only way to fix this from what I know is to remove the head and water pump and rod it and use compressed air. Engines used in marine raw water application don't have radiators and pressure buildup like that of a car. Raw water setup are standard in the marine industry and are still used to this day. Also the oil which is being used is conventional 10w-40. If I'm forced to run this engine at these high temps then I'll probably change the oil the synthetic simply because it won't break down and damage the engine.

Thanks in advance for any help.

Greg


New to Chevy engines. Want to learn more about these old straight inline 6's, for they appear to be bullet proof and smooth.
Joined: Dec 2005
Posts: 554
A
'Bolter
With the cylinder configuration on Straight 6 engines, the first two cylinders tend to run cool, middle 2 just right and the rear tend to get hot. Some one with more knowledge about boat specific 6's may come along and be able to help out more. Do you know the year of your engine? That may help with future questions.


58' 3200 235 3-speed/OD

Joined: Jul 2021
Posts: 7
S
'Bolter
Thanks ApacheFiend for that information. How do determine the engine year ? Are there markings on the block ?


New to Chevy engines. Want to learn more about these old straight inline 6's, for they appear to be bullet proof and smooth.
Joined: Feb 2000
Posts: 4,465
J
'Bolter
My 250 had the temp sender between cylinder 1 and 2, I used to see 210 all the time and tried varies things to cool it down. It never over heated when it was this hot. I moved the sender out of the head to just under the thermostat, no other changes were made, the temp now reads 180 at all speeds. It seems odd the coolant ran out of the block drain, this is where you would expect it to be plugged. Did you happen to check head temp? Water surrounds the cylinders but on the starter side, there is more iron due to the lifters and camshaft, I suspect thats why it reads cooler. The drivers side ( exhaust side ) is pretty thin and not much separates the outside from the cylinders. I am taking my truck out today, I will check and see what the lower block reads.

You may want to look on e-bay at the cheap thermocouple meters, you can get them with one, two, or four sensors, then mount them across the engine so you read temps while under a load.

Out on the highway, my 250's exhaust temp was over 700* when I had a thermocouple on the down pipe, so there is a lot heat on that side of the engine.

Joined: Feb 2004
Posts: 24,575
H
Kettle Custodian (pot stirrer)
Before you do major surgery, here's a way to dissolve a crud buildup in the water jacket without disassembling the engine. Remove the drain petcock from the block and use pipe nipples (1/4" NPT) a funnel, and an elbow to make an "L" shaped standpipe about 3 inches tall. Pour about a gallon of "CLR" calcium/lime/rust remover into the block and let it sit for a few hours. Drain the residue, adapt a garden hose to the drain port, and flush the block thoroughly. The chemical will soften the crud and the flush will remove it. I've also used a solution of "Sani-Flush" toilet bowl cleaner and water for the same procedure. It's a lot less work and expense than pulling the head.
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Joined: Mar 2014
Posts: 2,901
J
'Bolter
Or you might have to let the CLR sit as long as a couple of days, Greg. I had to do that plus remove the water pump and poke a straightened clothes hanger back in there a lot to get mine clean. In addition to CLR I tried some citric acid, strong (25% strength) vinegar and a couple of other things. But it is fine now. Good luck.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Joined: Jul 2021
Posts: 7
S
'Bolter
Update: Took the boat out today to test other repairs. Checked temps on block around drain plug at rear of engine. Was 252 degrees. Moving an IR scanner up towards manifold produced better temperatures around 190. Moving around to the front the block was around 180 and so was the head. The manifold was a cool 120 and the riser was around 115 degrees.

Those temps were taken with the rpm's at 3k. Boat and airplane engines are alike. They don't shift gears and lower rpm's. The cooling is either done by air or raw water.

HotRod Lincoln, I like your idea of CLR. I'll try that this week. I suspect that by taking off the thermostat housing top and removing the thermostat I can fill the block with CLR. If that doesn't work then it's remove engine time. Not hard. Remove the outdrive, then it's 4 bolts holding the engine down. Disconnect the wiring harness and a few cables and drive the boat to a friends shop. He'll use his fork lift to pull and put the engine on a trailer. To pull would take 15 minutes to put on a trailer. Over the weekend my son and I would remove the water pump and head and start rodding the engine out, along with allot of compressed air.


New to Chevy engines. Want to learn more about these old straight inline 6's, for they appear to be bullet proof and smooth.
Joined: Feb 2004
Posts: 24,575
H
Kettle Custodian (pot stirrer)
I've done a considerable amount of marine engine building for boaters on the TVA lake system in middle Tennessee. One of my mentors many years ago explained it this way- - - -"A boat engine is always running uphill"- - - -back off the throttle on a car, and it coasts down the road. Do the same thing on a boat and it stops almost immediately. The boat is trying to climb out of a hole all the time, which puts a lot more stress on the engine than if it was in a car or truck. That's why we have to build marine engines almost like they were running on a race track- - - -blueprinted, balanced, high volume oil pumps, and extra clearance on the bearings. It's also impossible to run a street engine full throttle for very long without having to slow down, stop, or get a speeding ticket. I can run for hours on any of several rivers and lakes around home with the throttle firewalled. Fun stuff!
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
Joined: Jul 2021
Posts: 7
S
'Bolter
Hotrod (Jerry) you hit the nail on the head....... I love that analogy of running uphill ! Some engines are tight and others are not. The boating experience for you is the same everywhere. Hit the throttle and go. And yes it is fun. Where I boated almost exclusively is on the Chesapeake Bay. I live a mile from there. Now what people don't know, the average depth of the bay is 18 feet. Yes we have major shipping channels which ocean tankers come up, they are dredged quite often. The bay depth is due to sentiment deposited from the ocean with the tides. That's where the heating problem is from.
I had a long talk with the mechanic a bought the engine from. He's been a marine mechanic on the eastern shore of Maryland for over 40 years. I mentioned about rust be the conversation turned quickly to the sentiment issue due to the fact of reading a block temp of 252 degrees at the back drain plug on the manifold side of the engine. Another friend in Detroit who is a car freak about old Cadillac's and Bentley's said the same thing. Due to the age, there is sentiment build up. Remove the freeze (core) plugs, use both a power washer, compressed air. Remove the water pump and the head and rod out the block. On the bay there is mud and sand which is pumped into the engine thru the pump in the outdrive. Over years this will build up causing heating issues. Now, I'n not the sharpest tool in the shed, but I've learned that you don't want an engine locking up on you on the Bay much less the ocean (150 miles away). Don't ask me how I know this........

So the plan is to run the boat on the bay, and remove the drain plug on the block. Yes the bilge pump does work! While my son drives the boat I'll attempt to rod out the back cylinder(s) via the drain plug opening so as to get the temps to drop. If successful, I'll be able to finish out the season. If not I'll remove the engine, water pump, head and proceed remove the freeze plugs to rod out both the head and the block with a power washer and compressed air.


New to Chevy engines. Want to learn more about these old straight inline 6's, for they appear to be bullet proof and smooth.
Joined: Feb 2004
Posts: 24,575
H
Kettle Custodian (pot stirrer)
A lot of the newer boats I've worked on (especially big houseboats) are running water-to-water heat exchangers and running antifreeze in the engine's cooling system year round. It's a lot easier to flush out a heat exchanger than it is to rod out a water jacket! Good luck!
Jerry


"Freedom's just another word for nothing left to lose"
Kris Kristofferson

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!

WAG MORE- - - - - -BARK LESS!
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