Wanted to share my experience troubleshooting the 4.3L 2bbl v6 in my 1989 Searay Seville mid-cabin.
Purchased the boat used and it ran fine initially and then started losing power and backfiring. The boat had been stored for a couple of years so I expected some carb / fuel issues. Have been troubleshooting for the past couple of weeks.
Rebuilt the Mercarb with rebuild kit including powervalve since I believed the engine was running lean. Filled the tank with fresh fuel. New fuel/water separator. Didn't help the situation.
Removed the spark plugs and later found that the wrong plugs had been installed. They were AC delco plugs but someone had put in the long extended tip ones which I believe are intended for the vortec head which is thicker. Two of the plugs were severely damaged, (see photo) however I was unsure if the damage was due to running lean and not being tight, or contact by the piston. The plugs that were in it were AC MR43LTS (long reach,extended tip). Correct plugs were AC MR43. Replaced them with the correct ones and the engine ran better but still lacked power and backfired.
Checked all ignition components, water in the fuel, anti-siphon valve, fuel pump, etc with negative results.
Checked timing thoroughly including electronic advance. All appeared to be working fine.
Did a compression check and found 4 of 6 cylinders fine with 140-160psi. Two adjacent cylinders had 0 psi compression. Obviously I figured it had blown a head gasket. Pulled the valve cover and confirmed valves weren't stuck, just in case, and they were all operating fine.
Removed the cylinder head today and found head gasket to be fine, no piston damage, but two cracked exhaust valves. I believe that as the spark plugs were disintegrating, bits may have gotten stuck in the exhaust valves as they were closing and cracked them. The two destroyed plugs came out of the two cylinders with cracked valves. There was evidence of contact on one of the other plugs but it depended on which orientation the plug was in when it went tight as none of the others were damaged. Appears as though the piston or valve struck the ground electrode.
Here is what the underside of the head looked like. (see photos). No other damage observed and normal compression on all other cylinders. Tomorrow I'll be ordering two new exhaust valves and hoping there is no damage to the valve seats. I'll post results once it's back together.
Purchased the boat used and it ran fine initially and then started losing power and backfiring. The boat had been stored for a couple of years so I expected some carb / fuel issues. Have been troubleshooting for the past couple of weeks.
Rebuilt the Mercarb with rebuild kit including powervalve since I believed the engine was running lean. Filled the tank with fresh fuel. New fuel/water separator. Didn't help the situation.
Removed the spark plugs and later found that the wrong plugs had been installed. They were AC delco plugs but someone had put in the long extended tip ones which I believe are intended for the vortec head which is thicker. Two of the plugs were severely damaged, (see photo) however I was unsure if the damage was due to running lean and not being tight, or contact by the piston. The plugs that were in it were AC MR43LTS (long reach,extended tip). Correct plugs were AC MR43. Replaced them with the correct ones and the engine ran better but still lacked power and backfired.
Checked all ignition components, water in the fuel, anti-siphon valve, fuel pump, etc with negative results.
Checked timing thoroughly including electronic advance. All appeared to be working fine.
Did a compression check and found 4 of 6 cylinders fine with 140-160psi. Two adjacent cylinders had 0 psi compression. Obviously I figured it had blown a head gasket. Pulled the valve cover and confirmed valves weren't stuck, just in case, and they were all operating fine.
Removed the cylinder head today and found head gasket to be fine, no piston damage, but two cracked exhaust valves. I believe that as the spark plugs were disintegrating, bits may have gotten stuck in the exhaust valves as they were closing and cracked them. The two destroyed plugs came out of the two cylinders with cracked valves. There was evidence of contact on one of the other plugs but it depended on which orientation the plug was in when it went tight as none of the others were damaged. Appears as though the piston or valve struck the ground electrode.
Here is what the underside of the head looked like. (see photos). No other damage observed and normal compression on all other cylinders. Tomorrow I'll be ordering two new exhaust valves and hoping there is no damage to the valve seats. I'll post results once it's back together.
