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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 15268
Experience:  Certified on Onan and Generac generators
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Just purchased a 99 31 ft C Class generator is a onan 4000

Customer Question

just purchased a 99 31 ft C Class generator is a onan 4000 that when i ran for first time blew oil over the front of generator cabinet below air filter -- before running i hvd changed oil snd cleaned it up oil in same location leaked out about a quart in 8 hours running is there a line or fitting that has come lose
Submitted: 2 years ago.
Category: Small Engine
Expert:  Hank F. replied 2 years ago.
My name is ***** ***** I am going to assist you with this.
What is the full model number of the genset, exactly as it is written on the data label?
Customer: replied 2 years ago.
Model # ***** oil is only in the area below air filter when I shut it off there was an Oil stream on the ground from gem frame
Expert:  Hank F. replied 2 years ago.
If oil is blowing back out through the intake, first check the oil level and make sure is it not over full. If it is, see if it smells like gas.
If is is not overfull, you most likely have a bad breather reed.
Onan actually had an issue with this, and put out a service bulletin on it.
If you provide me with the serial number, I can check to see if yours is one of the affected units.
Customer: replied 2 years ago.
Serial number is ***** is the reed accessible with generator in vehicle are there drawings where reed located
Expert:  Hank F. replied 2 years ago.
The oil issue not come into play until about 10 years after your unit was built - after an engine redesign.
The reed is located behind the valve cover on your unit.
The oil blow back is caused by too much crankcase pressure. There are only a few things that can cause it:
1. Oil level being too high.
2. Reed is bad, and is too stiff to open.
3. Valves are loose, particularly the exhaust valve.
4. Worn cylinder/piston/rings, allowing exhaust to pass the piston and get into the crankcase.
What you need to do first is check your oil level. Make sure it is not over full.
If the oil is good, remove the valve cover (you will need a new valve cover gasket when you put it back on, so get one). Inside, remove the breather reed and check it. Make sure it is somewhat pliable, and not completely stiff. This is going to be somewhat difficult for you to determine without something to compare it to, as the reed is actually metal. Basically just make sure it can move fairly easily. It does not have to move much - maybe a millimeter or so.
Check the valve adjustment. On this unit, both valves should be lashed to .002". It is not at all uncommon for the exhaust valve to loosen itself up to well over .010".
If this all checks good, you will probably need to do a cylinder leakdown test, to check the integrity of the cylinder and rings.
Expert:  Hank F. replied 2 years ago.
Was this what you wanted to know?