View Full Version : Kohler CH25 blown head gasket????

04-25-2007, 09:33 AM
Bought a '94 Bunton with a Kohler Command 25hp that misfires from a neighbor who was moving. My neighbor was ignoring the backfiring since it didn't seem to happen under load, and tried to compensate by choking it a little. I don't know how long it has been doing this. This year, when I went to start it up I pulled both plugs. One plug was dry with a a little black deposit, but white on edges, like it had been running a little rich (probably from choking?). The other, however, was completely fouled, wet black hard deposits. The hour meter shows 930 hours.

I cleaned the plugs and re-installed, and just re-inspected them after about 2 hours of use. The "good one" seems to be dry and burning cleanly, with a touch of white on the edges; the "bad one" is a little wet and black, and my guess on its way to being fouled again. The mower appears to consume some oil, but I can't see any smoke from it when it is running.

When I purchased the mower I thought maybe it was a carb issue, but now I wonder if the misfiring isn't cylinder related. I posted these symptoms on another board and someone suggested that I may have a blown head gasket on one side. I don't have a leakdown tester, though that sounds like the right tool to diagnose what is going on. Any advice? Could a blown head gasket cause the misfiring?


04-25-2007, 08:25 PM
If only the one plug is oil fouling it surly sounds like a blown gasket, But with this many hours the leak-down test would tell all.

04-30-2007, 08:59 PM
OK...purchased a leakdown tester at Harbor Freight... says to check with the piston of the cylinder being tested at top dead center of the compression stroke. Is there any way to know if it is at tdc of the compression vs. exhaust stroke other than to take the valve cover off? Or do I need to remove the valve cover, turn the engine manually until the intake valve closes and the piston is comes up to the top, which I can verify by sticking a straw in the spark plug hole?

Sorry about this being a stupid question, but I wondered if there was another way. The engine is not dissassembled whatsoever, and in its location on the mower I can't access the flywheel side; only the clutch side.

04-30-2007, 11:17 PM
One easy way is to remove the plug. Close the hole off with your finger and bumb the engine over. Not a full crank. Just off and on real quick until your finger gets pushed off with the compression. Then stop and rotate the piston to TDC by hand. Then make sure you lock the crank in place before you apply the pressure test.

04-30-2007, 11:18 PM
Pull the plug on the good cylinder then put a wrench on the bolt holding the PTO clutch on and turn the bolt counterclockwise (standing in front of the clutch) until you feel it start to get hard. Thats the beginning of compression stroke, Then take that plug out and stick your straw down the hole and rotate the piston up to TDC.

Now, In the instructions it did say to pin the engine at TDC so when air is introduced into the cylinder it won't push the piston down ?

05-01-2007, 06:26 AM
Thanks! Seems kind of obvious once you explain it.... With regard to pinning the engine, I thought I would clamp the wrench holding that bolt so it wouldn't move in either direction.

If i understand correctly, doing this test I should "see" a leak in the fouling cylinder, and hear air escaping somewhere. If I don't, then I have another problem. I am looking forward to hopefully confirming that this is the head gasket that is causing my issues.

Someone else wrote in another forum that I might want to check the engine timing; doesn't this engine have an ignition module with fixed timing?

05-01-2007, 07:57 AM
doesn't this engine have an ignition module with fixed timing?

If it doesn't have the spark advance module (SAM) it is fixed timing other than the flywheel key.

05-29-2007, 12:18 PM
OK...didn't get to do the leakdown test yet BUT....did a compression test.

The side that fouls: 165psi
The side that doesn't: 135 psi

Is the compression higher on the fouling side because of the oil that gets in the cylinder? If I have a blown head gasket on the bad side, wouldn't it have lower compression?

Also, how can you tell if your Kohler has a Spark Advance Module? If it was a timing issue, wouldn't both cylinders have fouling problems?


05-29-2007, 02:18 PM
A CH25 engine will have the SAM module and variable ignition timing.

To diagnose a head gasket failure you really need to do a leak down test, not just a compression test.

What is the serial number of this engine?

05-29-2007, 05:24 PM
Could Be A Head Gasket B/c You Should Have No More Than A 15% Difference In Compression Between Cylinder 1 And 2

05-29-2007, 06:35 PM
In my experience, blown head gaskets do not show up on compression tests.

05-30-2007, 12:31 PM
This is a CH25S, Spec. No. 68515, Serial No. 2427317357.

The reason I am not sure it is a head gasket is that the "bad" cylinder (the one that fouls) is the one with HIGHER compression! I will do the leak down test, but if this is an engine with variable timing, I am suspicious about the SAM. Is there a way to tell if my engine has the SAM based on the spec and serial number above? I was reading through the engine manual and it looks like I can see if the timing is advancing by using a timing light.

P.S. I just looked at the manual and they have a pic of the SAM, which I now see is mounted on the outside of the engine shrouding. From the pic, it looks like it should be easily visible. I will look tonight to see if I have one.

05-30-2007, 01:03 PM
Depends On Where The Gasket Is Blown At As To If It Will Make A Big Difference On A Compression Test Or Not

Bill Kapaun
05-30-2007, 03:09 PM
"The reason I am not sure it is a head gasket is that the "bad" cylinder (the one that fouls) is the one with HIGHER compression!"

I'm surprised your readings are that high.
It seems to me that the ACR isn't working. Since that is controlled by exhaust valve lash, maybe the valves have too much lash. IF the valves on the "good" cylinder are closer to spec, maybe the ACR is "partially" operating, thus reducing the compression "some".
Pulling the valve covers would allow you to also inspect rocker arms etc. for any problems.

05-30-2007, 06:27 PM

There are many other factors that affect compression such as valves sealing, rings, etc.

Command Twins do not have ACR!

05-31-2007, 09:02 AM

Crankshaft--i have a ch 25 engine from a dingo mini loader which has no SAM

Rallen-- as Crank shaft stated there is no compression release on v twins
--check with Kohler customer service [provide engine numbers ] to investigate if this particular engine in question has the nickle seal bores or cast iron .Although nickle bores r not suppose to be honed there r dedicated hones on the internet veryvery fine grit i imagine . Check 4 pitting and of course measure the bore up 4 taper and ovality .[Alittle piece of extra info if u need to go that far ]
--oil may be entering the cylinder in enough quatity to give u a false reading of compression on that cylinder in question .
--u may have a intake manifold vacumn leak also ?[reasonablly common in kohler twins ][the oil fouling is obviously the prority though ].


05-31-2007, 01:47 PM
Thanks for all the info. I will look at it in more detail this weekend; yes, I did indeed find the SAM on my engine shroud. This weekend I will do the leak down test, and I would also like to put a timing light on it to see what is happening with the spark advance. I will post what I discover.


06-08-2007, 01:12 AM
The adventure continues. Threw a timing light on it last night; I DO have a SAM, and reading the manual understood that I should see the marks move as the rpm increases. How many degrees (approximately) should they move? From what I could tell, it appears that they move on one cylinder but not on the other, and on the one that does move it isn't much.

I looked at the engine manual and see that each plug is fired from its own cd module, controlled by the SAM. Is it possible that the SAM is not doing its job properly and only advancing the timing on one of the cylinders?

The SAM is pretty easy to get to, but of course the connectors to it are under the blower shroud.


06-09-2007, 11:54 PM
Finally did the leakdown test today. Did the good side first; could hear just a little bit of air through the dipstick tube, but the gauge did not move at all.

Did the other side (the bad side; which is the side with the oil fill cap), and could hear and feel alot more air coming out of the dipstick tube. I removed the oil fill cap and could feel the air coming out of that as well. The gauge showed very slight movement but nothing significant in terms of loss.

So....if I understand correctly what this might mean...and considering the compression is good in both cylinders...then it appears that I have a head gasket leak on one cylinder, where oil is able to leak into the cylinder, and it is this oil that is fouling the plug and causing the backfiring. I may or may not have another issue with the ignition timing, but there is a noticeable difference in the leakdown test so I think I will start with the head gaskets.

Does this make sense? Thanks.


06-10-2007, 12:12 AM
A few here stated it sounded like a blown gasket....Go for it !

06-10-2007, 07:17 AM
Hello, although leak down testers r a good indicater it would appear it only takes a very small difference in the reading to have oil consumption .

Without going into the reasons why i will explain ..
On one of my units there is only 2--3% difference in reading . The cylinder with the higher reading has a louder noise coming out of the oil filler .The plug slowly gets build up ,not bad but definitely more than the other which looks perfectly ok
Compressions good 5--10 lbs diff
The difference is one cylinder is machine honed and the other was rehoned using a hand hone . Yes the cylinders still measure up well .

In summary my conclusion is it does not take much to have oil consumption but the main discovery is only a small reading on a leakdown tester means alot.


PS ;; Does any body have experiences with leakdown testers they would like to share

06-10-2007, 08:44 AM
@900 hr you need to power tune the engine and that includes both head gaskets i dont know the kit# but there is one made for this repair new head bolts and all the gaskets to do the top end wile you are at it replace those lifters your there so you may as well do it.that way you can mesure the bore's and do it right the first time.

04-15-2010, 12:31 AM
Talk about procrastinating....

FINALLY this spring (3 years after my original post about this problem) I decided to address the continuing backfire problem on the Kohler engine, and I reviewed my posting here written back in 2007. Convinced by the advice posted back then again that it was a blown head gasket, I bought the kit and decided to replace the one side head gasket.

I elected to just do the one side, since the spark plug on the other side looked perfect, while on the bad side the plug was very oil fouled. When I removed the head I was a little concerned: I couldn't see any obvious damage to the gasket that might allow oil to get sucked into the cylinder. The cylinder and head themselves looked much better than I expected. There was some carbon deposit on the intake valve but that was about it. I checked the breather reed and it looked flat and appeared to be sealing as it should. I replaced the head gasket, used the new head bolts, cleaned everything up and put it back together hoping for the best....


It worked! Apparently it WAS the head gasket that was bad. The repair took me about 8 hours on my Bunton, mostly because I had to deal with a few stripped bolts and broken screws that I needed to drill out. If I did the job again it would take less than 4 hours now that I have gone through it. On the Bunton, you have to physically pull the engine away from the transmission to remove the fan shroud, and once I was that far I just removed it completely to facilitate cleaning it out and accessing all the parts.

Hopefully I won't have to do the same repair for quite a while. Now that I have been through it and have dealt with the nuts and bolts that hadn't been removed in 16 years it wouldn't be a bad job to do if I ever have to do it again.

04-15-2010, 07:47 AM
Just hope the other side doesn't cut loose anytime soon......

04-15-2010, 06:26 PM
The Kohler Command 25hp are built like a tank--good, strong engines, but they do have a tendency to blow head gaskets. I have one on a Scag, blew both head gaskets. Fortunately, this is a fairly easy fix, and will be the 1st thing I do next time it gets sick.

piston slapper
04-16-2010, 02:09 PM
If there was no obvious sign of leakage of the head gasket, it probably wasnt the problem. It sounds like you inadvertently fixed your problem,..Probably an intake gasket was leaking causing a cylinder to drop out.

If your head gasket was bad, it would resemble one of these.