If you have a union in the suction piping as many well systems do it may be the culprit. some times tightening the union will fix the problem.
Another place air can enter is through the pump seal,if it is worn it may allow air to be pulled into the pump when it is running. replacing the seal can sometimes fix the problem. If you are losing the prime on your well system the problem is between the check valve and the well,if not the problem is between the check valve and the pump.
check your PCV Valve
Need to check if blow-by valve is working correctly.
Check the valve timing and the ignition timing. If both are good then check the compression in all the cylinders.
nexted oil change get oil filter with a check valve this should help
Thermostat possibly stuck open or heater control valve broken. Now if you mean the blower motor has stopped working, check the fuse, blower motor itself, or the blower motor resistor pack.
A dual check valve is similar to a double check valve in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valve.
check the idle control valve not that hard to replace and inexpensive
replace only when u get a check engine light. the valve is good for the life of the motor(should be)
Check for leaks in vacuum hoses, check for a cracked or gunked up Idle Air Control valve, and check you throttle positioning sensor.
Check the specs for the vehicle you are working on (You can find them on the net if you do not have the manual) and the torque requirements will be there. Do not over torque the bolts as removing a broken valve cover bolt is a pain. Good luck.
You did not say what you are working on. but you can check the plugs, wires, coil, and maybe the fuel injector low engine compresson, broken valve spring.vaccum leak... dist cap. Realey need more info.