Check engine code 50 4 stroke

[Balaguatemala]

Have older 50 4 stroke. Get check engine signal after running for about 10 minutes. No oil or temp signal. Oil and filter new. Top of block over thermostat is not hot. Take off cover and idle for 30 seconds. Code goes away and engine runs fine rest of day as long as you leave the cover off.
Changed impeller in water pump. No difference. Water discharge on back of engine warm, not hot.
Any ideas?

[noelm007]

If the engine runs Ok with the cover off, changing the water pump is not your problem... you MAY have an exhaust leak?

[Balaguatemala]

What about a bad intake valve that is blowing back into the intake runner?

[Balaguatemala]

I don't have manual. What sensors are connected to the check engine idiot light?