I have a customer of mine who claims that last year a mechanic did a complete engine overhaul on his I-4 170 Mercruiser. It took all spring, summer, and fall for the engine rebuild and he missed the entire season of boating and fishing. He brought the boat to me this week with the complaint of it beginning to backfire at midrange and full throttle. I did the basics and came up with a few observations I would love a hand with since I have never personally experienced this particular situation.
This is a closed cooled engine
Compression check 170ish across the board
Fuel Sample clean and clear
Good crisp spark on all
Timing set to manufacturer guidelines
New spark plugs and tune up kit earlier this year installed by the customer.
The oil is dirty but no water discoloration.
So here is the issue. I removed the spark arrestor and found a ton of milky oily residue in the breather line. I decided to investigate further and remove the valve cover to expose the valve train. On the inside of the valve cover I found it caked with the brown milky oil. Upon inspection I found that each of the intake valves were oozing with the brown milky matter and there was a small drop or two of antifreeze in the valve train. The drops of antifreeze may have been from my dissassembly process and can not be a determining factor in the diagnosis.. None of the exhaust valves show signs of anything out of the normal. I have yet to remove the head and inspect the head gasket until I asked the pro's here for some advice.
With a good compression test, can there still be a head gasket issue or a valve issue? What should be my next step in the diagnosis process? I dont want to tear the engine down any further if it isn't necessary. SO if anyone has experienced this and knows a good area to start I would appreciate the help.
My question would be, why is there only the oily residue around the intake valves? And if it is a valve spring/valve failure, how common is it to only fail on the intake side? Just seems a little too suspicious to me. Any takers?
This is a closed cooled engine
Compression check 170ish across the board
Fuel Sample clean and clear
Good crisp spark on all
Timing set to manufacturer guidelines
New spark plugs and tune up kit earlier this year installed by the customer.
The oil is dirty but no water discoloration.
So here is the issue. I removed the spark arrestor and found a ton of milky oily residue in the breather line. I decided to investigate further and remove the valve cover to expose the valve train. On the inside of the valve cover I found it caked with the brown milky oil. Upon inspection I found that each of the intake valves were oozing with the brown milky matter and there was a small drop or two of antifreeze in the valve train. The drops of antifreeze may have been from my dissassembly process and can not be a determining factor in the diagnosis.. None of the exhaust valves show signs of anything out of the normal. I have yet to remove the head and inspect the head gasket until I asked the pro's here for some advice.
With a good compression test, can there still be a head gasket issue or a valve issue? What should be my next step in the diagnosis process? I dont want to tear the engine down any further if it isn't necessary. SO if anyone has experienced this and knows a good area to start I would appreciate the help.
My question would be, why is there only the oily residue around the intake valves? And if it is a valve spring/valve failure, how common is it to only fail on the intake side? Just seems a little too suspicious to me. Any takers?
