Onan P220G compression lost

[Prewarvtwin 1]

Hello, My first post and my first Horse problem.

Engine was acting up on 1990 520H and started running on one cylinder. Turns out rear cylinder stopped firing. Lots of spark though , so checked compression and only came up with 20psi. compared to the front 120psi. So I guess my next move is to tear apart and check my Valves? Am I going in the right direction or should I check something else before I try to get the little rusted screws out on the chrome muffler shroud LOL  Thanks Gents

[Snoopy11 5,717]

Could be the valves or maybe piston rings... can you see anything if you take the spark plug out... rotating the engine over by hand? (talking about valves and seeing if the cylinder wall is scarred...

 

Getting the little rusted screws out might be simpler if you soak them in PB blaster for a couple hours before using an impact to get it out. I would not use a regular wrench...

 

@953 nut... you got anything on this?

 

Don

[Handy Don 13,342]

Welcome to the forum.

 

You don't mention the hours on the engine, but I agree valves are the likeliest culprit. And yes, those muffler shroud screws are a PITA but I thought you could get the exhaust manifold off without removing that shroud?

 

You'll want to read this manual (from our files/manual section under Browse at the top of the page) before diving in...

 

Good luck!

[Snoopy11 5,717]

Are you having any blowby oil? 

 

That would tell if it is rings...

 

Don

[Snoopy11 5,717]

The good thing is, once you take the head off, you can buy new head gaskets which could also be the culprit. If you valve seats look fine, (are sealing properly) I would certainly try a new gasket...

 

Don

Edited November 11, 2021 by Snoopy11

[Achto 28,808]

The only way to know where your compression is going without removing the head would be to preform a leak down test. This test would tell you if the leakage is at the intake or exhaust valve or if it is leaking past the rings.

[Snoopy11 5,717]

1 minute ago, Achto said:

The only way to know where your compression is going without removing the head would be to preform a leak down test. This test would tell you if the leakage is at the intake or exhaust valve or if it is leaking past the rings.

Yeah, I kinda figured 953 nut would say that 

 

Thanks Achto

 

Don

[oliver2-44 10,375]

Looks like they have you headed in the right direction.  Keep us posted of what you find.  

[lynnmor 7,577]

The rear head needs to come off regardless of where the leak is.  You should be able to pop it with only some tin removal, of course removing the belt and engine mounting bolts will allow better access by moving the engine.

 Apparently this came on suddenly and the cause isn’t normal wear, we’ll know more when you report back, hopefully with a photo.

[JoeM 7,942]

1 hour ago, lynnmor said:

The rear head needs to come off regardless of where the leak is

Yes.....but the leak down will show things you can not see. like the rings etc. It will point you in the right direction. 

[Prewarvtwin 1]

Hi Guys, Thanks for all the Replies! Great Site with no shortage of help here, and a downloaded manual to boot. UPDATE--- I went ahead and dug my way down to the valve adjust cover, then rolled engine over so both valves were closed. I put air into plug hole as part of a leakdown test to find all the air blowing out thru the intake valve. Checked with feeler gauge to find no gap at all on intake valve adjuster. Would it be my lucky day that valve will close once I adjust lifter to proper clearance?? My next move to see if compression returns! The valve seats look ok and dont look like they are loose thru intake port.  STAY TUNED    CHEERS

[Achto 28,808]

Definitely try adjusting the valve lash first. You may have gotten lucky.

[Handy Don 13,342]

 

4 hours ago, Prewarvtwin said:

Hi Guys, Thanks for all the Replies! Great Site with no shortage of help here, and a downloaded manual to boot. UPDATE--- I went ahead and dug my way down to the valve adjust cover, then rolled engine over so both valves were closed. I put air into plug hole as part of a leakdown test to find all the air blowing out thru the intake valve. Checked with feeler gauge to find no gap at all on intake valve adjuster. Would it be my lucky day that valve will close once I adjust lifter to proper clearance?? My next move to see if compression returns! The valve seats look ok and dont look like they are loose thru intake port.  STAY TUNED    CHEERS

 

 

That would be terrific!

If you can get the right clearance and the valves seal, then just eyeball to make sure there is enough valve left (wear somewhere caused the loss of clearance-valve stem, valve seat, or valve head).

Also, the exhaust port looks pretty clean and the valve box is clearly getting lubed, so those are good.

 

Edited November 12, 2021 by Handy Don

[Prewarvtwin 1]

Well , i guess it was my lucky day! Compression back up to 120psi in rear cylinder. Local dealer even had new intake gaskets for me. Reassembled and fired right up and never ran sooo good. Why over the years would it slowly open the valve and adjustment be so far out that it wont run? Anyway now when I engage PTO the engine immediately quits.. whole new problem. Any recommendations as to what this is about?? Thanks Guys 

 

[Handy Don 13,342]

14 minutes ago, Prewarvtwin said:

Well , i guess it was my lucky day! Compression back up to 120psi in rear cylinder. Local dealer even had new intake gaskets for me. Reassembled and fired right up and never ran sooo good. Why over the years would it slowly open the valve and adjustment be so far out that it wont run? Anyway now when I engage PTO the engine immediately quits.. whole new problem. Any recommendations as to what this is about?? Thanks Guys 

 

Outstanding!

 

To engage the PTO and have the engine continue to run, you MUST be in the seat. If you are in the seat, then that switch, or its wiring, is the likely culprit!

Do you have the wiring diagram?

 

Enjoy!

[lynnmor 7,577]

1 hour ago, Prewarvtwin said:

Why over the years would it slowly open the valve and adjustment be so far out that it wont run?

 

There is the possibility that the valve seat is loose and pounded into the head taking all the clearance.

[Handy Don 13,342]

32 minutes ago, lynnmor said:

There is the possibility that the valve seat is loose and pounded into the head taking all the clearance.

Something caused the valve clearance to be lost and you still don't know what.

[Snoopy11 5,717]

Wonder if some cam lobe or follower wear would be the cause? 

 

Don

[lynnmor 7,577]

5 minutes ago, Snoopy11 said:

Wonder if some cam lobe or follower wear would be the cause? 

 

Don

That would make more clearance, not less.

[Snoopy11 5,717]

Oh yeah, I did not realize that @lynnmor... so it is negative clearance... 

 

Don

[Snoopy11 5,717]

Are those intake valves titanium, per chance? Anybody have an idea? 

 

Don

[squonk 42,596]

Seat sinking in head. Valve stem stretching can cause the clearance to close.

[Handy Don 13,342]

8 minutes ago, squonk said:

Seat sinking in head

Yep, this would be my first look. How much shoulder is left on that valve. If it is already worn and the shoulder is shot, the next failure could be much worse.

 

[Snoopy11 5,717]

16 minutes ago, squonk said:

Valve stem stretching

Yep, that is why I was wondering if it is titanium. I have heard of this happening before, exactly what you are referring to @squonk... and I know that some manufacturers run titanium on the intake side and steel on the exhaust side... so... that could be the culprit...

 

The Onan P220G engines are notorious for having loose valve seats... and obviously, as the valve train components wear, mainly the valve face and the valve seats, the valve clearance goes away.

 

I am looking over this: 

 

 

Don

Edited November 14, 2021 by Snoopy11

[lynnmor 7,577]

20 minutes ago, Snoopy11 said:

Are those intake valves titanium, per chance? Anybody have an idea? 

 

Don

They are steel.  Onan made a heavy duty engine that had better steel valves and a few other things, but they weren't used in Wheel Horses.  I never understood why Onan just didn't make them all the same, the price would have been only a little more.