Bought this mower with the issue thinking it would be somewhat simple. I underestimated it somewhat... Great thing is I'm getting to learn all about early EFI motors :laughing:
Engine will idle smoothly but when changed to WOT it will occasionally backfire and has very little power when motors or deck is engaged. Temp gun shows that the #2 (passenger side?) exhaust cools significantly at WOT, at idle both exhaust pipes are similar temp. Spark plug on passenger side is black and somewhat wet (carbon or oil fouling?). Other side is nice and white. No codes are coming up on MIL - this is the 35 pin metal ECU from Bosch. I'll list what I've done so far:
-Fresh fuel (with lucas to clean up injectors)
-swapping coils side to side - No change to running, still fouling passenger plug.
-New non-resistor plug wires, new NGK resistor plugs - No change to running, still fouling passenger plug. (spark tool shows spark at idle and WOT)
-swapped injectors side to side - No change to running, still fouling passenger plug.
-New O2 sensor (Exmark dealer did this to fix) - no change to running, still fouling passenger plug.
-Compression test - ~110 psi +/- 10 on both sides
Here's where it gets interesting, watching rocker arms while running the passenger exhaust is barely moving, driver side is moving a little more, but neither move as much as intake rockers. Looking down from top of engine it looks like evidence of burn around exhaust valve opening only on passenger side. So I'm thinking hydraulic lifter is breaking down and not pushing rocker to open valve correctly. Manually depressing valve without rocker installed seems to be free and valve stem height is same as intake valve (no valve seat issue). I pulled both exhaust lifters and couldn't manually depress either on bench. Soaked in gasoline overnight and they eventually freed up. Reinstalled after making sure all gasoline was out of lifter and same issue. Question is, I don't know how much movement is normal from exhaust valve. Is it possible that the camshaft has been worn down until it's not opening valve correctly? I also noticed fair amount of smoke puffing out when valve covers were removed, I read somewhere that's a sign of blown head gasket? But would I get 100+psi compression with blown head gasket? Anyone think I dismissed the electronics prematurely?
Engine will idle smoothly but when changed to WOT it will occasionally backfire and has very little power when motors or deck is engaged. Temp gun shows that the #2 (passenger side?) exhaust cools significantly at WOT, at idle both exhaust pipes are similar temp. Spark plug on passenger side is black and somewhat wet (carbon or oil fouling?). Other side is nice and white. No codes are coming up on MIL - this is the 35 pin metal ECU from Bosch. I'll list what I've done so far:
-Fresh fuel (with lucas to clean up injectors)
-swapping coils side to side - No change to running, still fouling passenger plug.
-New non-resistor plug wires, new NGK resistor plugs - No change to running, still fouling passenger plug. (spark tool shows spark at idle and WOT)
-swapped injectors side to side - No change to running, still fouling passenger plug.
-New O2 sensor (Exmark dealer did this to fix) - no change to running, still fouling passenger plug.
-Compression test - ~110 psi +/- 10 on both sides
Here's where it gets interesting, watching rocker arms while running the passenger exhaust is barely moving, driver side is moving a little more, but neither move as much as intake rockers. Looking down from top of engine it looks like evidence of burn around exhaust valve opening only on passenger side. So I'm thinking hydraulic lifter is breaking down and not pushing rocker to open valve correctly. Manually depressing valve without rocker installed seems to be free and valve stem height is same as intake valve (no valve seat issue). I pulled both exhaust lifters and couldn't manually depress either on bench. Soaked in gasoline overnight and they eventually freed up. Reinstalled after making sure all gasoline was out of lifter and same issue. Question is, I don't know how much movement is normal from exhaust valve. Is it possible that the camshaft has been worn down until it's not opening valve correctly? I also noticed fair amount of smoke puffing out when valve covers were removed, I read somewhere that's a sign of blown head gasket? But would I get 100+psi compression with blown head gasket? Anyone think I dismissed the electronics prematurely?
