Onan 216 rear cylinder overheats

[Albro 0]

Two years ago I sent my 1996 416H to the dealer I bought it from, to take care of an oil leak near the PTO.  Though the engine had been running good otherwise, I also requested a compression check.  The compression check suggested that the FRONT cylinder was a little weak and could use a new head gasket.  (REAR cylinder compression was very good.)

The work was done, no more oil leak, head-gasket replaced, and the engine sounded great.  BUT, I immediately noticed that air flow blowing out the right side of the engine compartment, which would cool the rear cylinder, was only about a third of what it used to be.  Air flow used to be quite strong.  It didn't concern me at the time because the tractor ran fine plowing my 100 foot driveway after a dozen or so snowfalls last winter.   

Comes spring,  I put the mower on and started cutting my grass.  After about 45 minutes of mowing the engine loses power.

Dealer picks it up again. REAR cylinder gets new head, gasket, valves, and seats.  I tell the dealer and his tech person about the reduced air flow to the rear cylinder (and ask that they check for any obstructions, grass, leaves, or dead animals), but they say that's normal for this model tractor.  I get the tractor back again, and  it runs OK for several months.  

Again it goes back to the dealer because a valve seat came loose.  It gets repaired "as good as possible" says the dealer.  I mention the poor air flow to the rear cylinder before getting the tractor back, but he says can't find anything obstructing the air.  

Again, I got the tractor back, used it very little during this past summer because of a long dry spell that we had here in Connecticut. It ran OK with the minimal use it got.

A few weeks ago I took it out to mulch some left over leaves in my yard (air temp about 65F that day).  After about an hour of use, it loses power...rear cylinder again.  (BTW, the rubber seal around the oil filter is where it's supposed to be.)  Any helpful suggestions, ideas, or diagnosis would be much appreciated.  

[KC9KAS 4,743]

1st of all,

I know the Onans do have a problem with the rear cylinder getting hot due to lack of good air flow. Most on here will tell you to do anything you can to make sure your tins and fins are not clogged with oil/grass...whatever.

Some have drilled holes in the belt guard to assist with air flow, but sounds like it  may be too late for your engine!

Others will be along to give better information, (I hope).

Good Luck!

[woodchuckfarmer 333]

Check the intake manifold. They are made in 2 pieces and can separate. this would make a lean mixture letting the cylinder run hot.

[JackC 617]

"BUT, I immediately noticed that air flow blowing out the right side of the engine compartment, which would cool the rear cylinder, was only about a third of what it used to be. "

 

Interesting. I have several tractors with ONANs but never noticed the amount of air flow blowing out the right side of the engine compartment. Air flow reduced by a third is a big reduction in air flow. Reduced rpm would reduce the air flow. There is no tachometer on that model is there?  You should always run the engine at high rpm between 3,500 and 4,000 rpm when engaging attachments. What rpm does it reach with the throttle wide open? You could use an inexpensive non contact digital tachometer to measure rpm at WOT.  You need the high rpm to get the proper air flow.  You could also use an infrared heat sensor to check the temperature of the two cylinders as they heat up.