I have scanned through a series of threads in search of an answer. However, everything seems to touch around my issue and not directly on it; So here goes.
Recently purchased a 1988 Wellcraft 18 foot with a Mercruiser 3.7 liter (4 cylinder) and alpha one/ generation one stern drive. The seller advised we needed a new impeller in the seawater intake. Impeller was relaced and I cleaned/flushed out remnants of the old impeller which had been ground up in the lower unit. Looked as though the water pump housing had been through serious heat as well. Disassembled and flushed heat exchanger and back-washed the system to remove any impeller carnage that may have made it deeper into the system. External seawater intake hose has been replaced and new thermostat installed. Riser was removed, flushed (not scraped), and replaced with a new gasket. ( Riser is rusty and is next on the list of projects, but did not appear to be obstructed). The old water shutter was non-existent and I have since installed a new one.
After impeller replacement and heat exchanger flush we took the boat out and still experienced overheat to 220 or so; particularly at increased speeds. That is when we replaced the thermostat and flushed the riser/reservoir.There was also some stalling when the boat was pulled out of gear, so I rebuilt the carburetor. The stalling was eliminated. Now the engine runs smoothly at fairly normal temp now, at idle connected to a garden hose. However, when throttled up the engine makes a tapping noise and begins to emit a white smoke from the exhaust ports. When throttled back the noise and smoke disappear.
With my limited knowledge of automobile engines I suspected that one of the over-heat incidents fouled the head gasket. I understand that the tapping may be a cylinder that has water intruding. But it was difficult to determine if the tapping was coming from the engine or stern-drive. I conducted a compression test on the engine and obtained the following results cyl 1 100 cyl 2 145 cyl 3 120 cyl 4 90 to 100. These readings were obtained as the engine was cold ( i was concerned with heating it up). I repeated the compression check using a small amount of oil in each cylinder to see if the liquid would indicate a piston ring problem. All readings remained unchanged. Therefore, I feel better about the pistons, even though some of the readings are low.
Based on this information does it sound as though I am on the right track with the head gasket assumption? Or is there another possibilty I should investigate?
Thank you in advance for any assistance.
As a side note.......occasionally, when at idle (on the trailer and connected to the garden hose), the boat lower unit will start to rattle and seems to drift into gear. The prop turns gently, but I cannot determine why this happens. It is not a jump or anything severe; just a slight change in status.
Recently purchased a 1988 Wellcraft 18 foot with a Mercruiser 3.7 liter (4 cylinder) and alpha one/ generation one stern drive. The seller advised we needed a new impeller in the seawater intake. Impeller was relaced and I cleaned/flushed out remnants of the old impeller which had been ground up in the lower unit. Looked as though the water pump housing had been through serious heat as well. Disassembled and flushed heat exchanger and back-washed the system to remove any impeller carnage that may have made it deeper into the system. External seawater intake hose has been replaced and new thermostat installed. Riser was removed, flushed (not scraped), and replaced with a new gasket. ( Riser is rusty and is next on the list of projects, but did not appear to be obstructed). The old water shutter was non-existent and I have since installed a new one.
After impeller replacement and heat exchanger flush we took the boat out and still experienced overheat to 220 or so; particularly at increased speeds. That is when we replaced the thermostat and flushed the riser/reservoir.There was also some stalling when the boat was pulled out of gear, so I rebuilt the carburetor. The stalling was eliminated. Now the engine runs smoothly at fairly normal temp now, at idle connected to a garden hose. However, when throttled up the engine makes a tapping noise and begins to emit a white smoke from the exhaust ports. When throttled back the noise and smoke disappear.
With my limited knowledge of automobile engines I suspected that one of the over-heat incidents fouled the head gasket. I understand that the tapping may be a cylinder that has water intruding. But it was difficult to determine if the tapping was coming from the engine or stern-drive. I conducted a compression test on the engine and obtained the following results cyl 1 100 cyl 2 145 cyl 3 120 cyl 4 90 to 100. These readings were obtained as the engine was cold ( i was concerned with heating it up). I repeated the compression check using a small amount of oil in each cylinder to see if the liquid would indicate a piston ring problem. All readings remained unchanged. Therefore, I feel better about the pistons, even though some of the readings are low.
Based on this information does it sound as though I am on the right track with the head gasket assumption? Or is there another possibilty I should investigate?
Thank you in advance for any assistance.
As a side note.......occasionally, when at idle (on the trailer and connected to the garden hose), the boat lower unit will start to rattle and seems to drift into gear. The prop turns gently, but I cannot determine why this happens. It is not a jump or anything severe; just a slight change in status.