|
Mercruiser 496 HO problem
Hello! Im about to buy a Chris Craft 260 Stinger with a Mercruiser 496 HO. The engine runs fine when its cold, but when it gets warm the rpm changes up and down. Its like one cylinder is not running right. There is also coolant in the oil. Can it be a leaking head gasket or crack in the cylinder head causing this problem? |
Maybe all them, but best to have a knowledgeable marine tech check it over before purchase.
|
Intake gasket, cracked block
|
I would not trust the block nor the engine bearings with coolant in the motor oil. Expect to discount the boat a BARE min of 12.5 K usa dollars for an engine long block replacement Prob will need a new heat exchanger and many more related items - this could be up to 25 K of making the boat 100 % turn key / water ready. The used boat market - the odds are no matter what you find will need something. One has to keep a mind set when looking ---- ridden hard put away wet is basically for the used boat ,market. Also storm boats seem to be an issues as well.
|
Originally Posted by BUP
(Post 4705283)
I would not trust the block nor the engine bearings with coolant in the motor oil. Expect to discount the boat a BARE min of 12.5 K usa dollars for an engine long block replacement Prob will need a new heat exchanger and many more related items - this could be up to 25 K of making the boat 100 % turn key / water ready. The used boat market - the odds are no matter what you find will need something. One has to keep a mind set when looking ---- ridden hard put away wet is basically for the used boat ,market. Also storm boats seem to be an issues as well.
The price are around 15k usd for the whole boat + engine/drive. I read about the heat exhanger problems but non of that was a problem right now on this engine I think. Just the antifreeze in the oil and the ruff idle when engine is warm... |
I do have a complete short block if you’re interested pm me, never ran since rebuilt,total of 18 hours on engine,kept it for a spare because I have a twin engine 496 boat |
Originally Posted by JohanSweden
(Post 4705352)
I think a overhaul of the engine is in its place. The price are around 15k usd for the whole boat + engine/drive. I read about the heat exhanger problems but non of that was a problem right now on this engine I think. Just the antifreeze in the oil and the ruff idle when engine is warm... |
Originally Posted by boatnt
(Post 4705372)
That’s the problem, the problem is the antifreeze in the oil, the antifreeze destroys all the bearings, main ,rod ,cam bearings and along with that the bearings destroy the crankshaft, camshaft,etc I have many speculations about the problem, but I dont know if the two problems with the engine are related. Can the engine rpm rev up and down because of antifreeze leaking in to one of the cylinder/pistons when it gets warm due to broken head gasket and making the sparkplug and fuel misfire?? If thats the problem, it can be a easy fix. But if the problems are not related I dont know what exactly making the engine going bad when its warm.. Any speculations? |
No one can speculate a problem over the internet. You need to start doing so testing to find the cause.
|
I'd plan/budget accordingly for a full engine replacement. I see that you are in Europe which makes things of that sort a bit more difficult. But you should be able to buy and ship a 496HO fairly easily.
|
I have checked for leaks in the oil cooler and headers because its the only way for water to get in to the oil. No leakage, so it cant be seawater in the oil. I have done a leak down test with cold engine. Good results with 5-7% leakage on all cylinders. I have changed the oil in the engine and started it up. It spins like a cat after cold start. But when the engine is getting warm (about 60’C) the RPM is starting to go up and down. Its also like a air sucking sound when the rpm goes up. I read about it and it could be the IAC muffler? Can the IAC muffler be the problem with the bad idle when it gets warm? I did a leak down test after when the engine was warm. Good results again, even better results with 4-5% leakage on all cylinders. Shouldnt the test show if a head gasket is bad? Because now I havent a clue wear the coolant is getting in to the oil.. Does anyone know what the problem with the bad idle can be? And before I take the head gaskets of, can it be a leakage without any signs on the leak down test? /Johan |
The coolant could be coming in from the intake leak, either a crack or gasket. If there is a intake problem this can also impact idle and running.
|
Originally Posted by AllDodge
(Post 4709680)
The coolant could be coming in from the intake leak, either a crack or gasket. If there is a intake problem this can also impact idle and running.
If the coolant is coming from the intake, can it pass trough the leak down test? Because the test showed no leakage. |
Coolant does not flow through the intake manifold |
You can do a leak down test with the intake off the motor. The test determines leaks in valves and rings
Coolant does not flow through the intake manifold |
Originally Posted by boatnt
(Post 4709684)
Coolant does not flow through the intake manifold There is no IAC muffler in my throttle body. Its missing. |
Originally Posted by boatnt
(Post 4709684)
Coolant does not flow through the intake manifold To the OP, use an automotive coolant system tester and pressurize your closed coolant side. If you have coolant in the engine oil, you have a cracked head/block or headgasket. Don't worry about the idle problem now. You have MUCH bigger concerns with coolant in the oil. https://cimg5.ibsrv.net/gimg/www.off...75f926ee31.jpg |
Originally Posted by Ryan00TJ
(Post 4709713)
Correct!! The gen 7 8.1 utilizes a dry intake like the LS platform.
To the OP, use an automotive coolant system tester and pressurize your closed coolant side. If you have coolant in the engine oil, you have a cracked head/block or headgasket. Don't worry about the idle problem now. You have MUCH bigger concerns with coolant in the oil. https://cimg5.ibsrv.net/gimg/www.off...75f926ee31.jpg I also change the head bolts and intake manifold gasket at the same time. |
| All times are GMT -5. The time now is 11:02 AM. |
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.