PCM 351 Cracked intake manifold
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=51671
Printed Date: November-21-2024 at 9:22pm
Topic: PCM 351 Cracked intake manifold
Posted By: stevejuststeve
Subject: PCM 351 Cracked intake manifold
Date Posted: July-05-2024 at 1:43pm
My friend inherited 79' MasterCraft S&S and asked me to help get it going. When I opened the motor cover the first thing I noticed is 3 of the 6 freeze plugs were sitting in the bilge  , "that ain't good". Having zero history for the boat I wasn't sure what else to do but pop them back in the best I could, hook up the hose and wait for the milkshake to be made. To my surprise, after running for 20 mins the oil still looks good and the motor ran extremely well. However, there was a leak at the front of the intake manifold where water enters and I found what look to be some cracks. I've never seen freeze plugs actually save a block from being cracked. Is it possible that replacing only the intake will fix it? Also I don't think I was able to fully seat the freeze plugs, as getting a good purchase on them is hard with the motor mounts in the way... Any suggestions are greatly appreciated. Thx!
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Replies:
Posted By: Jonny Quest
Date Posted: July-05-2024 at 1:54pm
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If the intake has cracks that go all the way through...then water intrusion into the lifter valley is very likely. At the very least, the intake should be removed and inspected. If you want to replace the intake, I would go with an aluminum (not cast iron) intake -- such as the Edelbrock Performer manifold. Should even pick up a few HP with a better intake. Also...the term "freeze plug" my be incorrectly used. Those are really core plugs. Core plugs (aka Welch plugs) are used to fill the sand casting core holes found on water-cooled engine blocks. The term "freeze plug" was coined many decades ago out of ignorance of the true purpose of the core plugs - they were never designed to be displaced by water freezing inside the block. Displacement of a core plug is merely coincidental if water freezing inside the block does occur. JQ ------------- Current 2003 Ski Nautique 206 Limited Previous 2001 Ski Nautique Open Bow 1994 Ski Nautique Open Bow Aqua skiing, ergo sum |
Posted By: stevejuststeve
Date Posted: July-05-2024 at 3:05pm
Thank you JQ. I appreciate the clarification on the welch plugs. I was thinking no water was entering the valley because of the condition of oil after running it, but perhaps it's a very small leak and hasn't emulsified the oil yet? The intake 100% needs to be replaced, I just wanted to try and be rather sure that this block was still good before putting any more money/time into it. Do you think that is a reasonable assumption being that the oil is still good after 30 mins of run time? Thanks a ton for your insight.
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Jonny Quest wrote:Posted By: KENO
Date Posted: July-05-2024 at 3:54pm
Whatever Jonny wants to call them.......... core, casting, freeze, frost, welch, multipurpose, butt plugs,.in non technical terms, they may have saved that block from freeze damage  New brass ones are readily available and pretty cheap and will stay in better than a reused one that popped out. The ones with an engine mount or a starter motor in the way are easier to replace if you disconnect the driveshaft coupling halves and lift the front of the engine just enough to remove the mount. Then you've got yourself some working room The one above the starter is a lot easier if the starter is removed. Put the plugs in good and get some time on the engine so it's good and heated up, watching the oil and look for cracks on the exhaust manifolds along the bottom edges, since if somebody didn't drain the engine, maybe they didn't drain the manifolds either. Check the cylinders for any water by pulling the plugs and turning it over with the starter to see if any water comes out of any cylinder and look at the plugs too for water drops/wetness.
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Posted By: MrMcD
Date Posted: July-05-2024 at 11:07pm
| Good advice so far Ken and JQ. As far as the term Freeze Plugs, In the engine aftermarket ALL of the suppliers identify them as freeze plugs. Our old Buddy Pete, who I hope is still tuning in always corrected us on the proper terminology. Sealed Power, Pioneer, Melling , Clevite all call them Freeze plugs. All the engine rebuilders I called on who buillt over 3000 engines per month used the same term so don’t feel like you called them wrong. Technically we are all wrong but in the industry EVERYONE UNDERSTANDS FREEZE PLUGS WHEN YOU ASK FOR ONE. Hope that helps. The new intake will be better looking, weigh much less and perform better. Running 30 minutes with zero oil in the water is a good test. Hope your block is good. If it takes 20 hours use to cause milk in the oil, change the oil and fix it someday when you are bored. Not mid summer when the boat should be in service. Just do not park it over the winter with water in the oil. |
Posted By: stevejuststeve
Date Posted: July-06-2024 at 8:25pm
| Thanks guys! |
Posted By: MourningWood
Date Posted: July-06-2024 at 8:33pm
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With first hand experience I second J Q's recommendation for the Edelbrock Performer intake. It adds a lot of much-needed pep to the bottom end. ------------- 1964 Dunphy X-55 "One 'N Dun" 'I measured twice, cut three times, and it's still too short!" |