Excessive Crankcase Pressure in 20 HP Honda GX620 Engine in a Deines 1800 KT ZRT

[schwartzy18510]

Hello all! As a dedicated DIYer, Google has led me here multiple times over the years when dealing with small engine repairs. I finally joined the forums today to ask for input on a situation regarding crankcase pressure which has me stumped. I recently purchased a Deines 1800 KT ZRT 60" front-deck mower with about 1,300 hours on it. The original 18 HP Kohler motor was replaced by the previous owner in 2016 with a 20 HP Honda GX620 knock-off. I'm told the new V-twin OHV engine has less than 20 hours on it. Prior to purchasing the mower, I knew the engine leaked some oil. The previous owner disclosed this and said he thought it possible the engine simply hadn't been broken in yet.

After bringing the mower home, cleaning the engine up, and chasing my tail with attempting to resolve a half-dozen oil leaks, I identified what I believed to be the underlying cause crankcase pressure building in the block. I then verified the operation of the crankcase breather valve and hose and replaced both head gaskets, as inspection revealed that one of them was blown between the oil return passageway from the cylinder head to the combustion chamber. Both pistons looked fine when I had the heads off, and the cross-hatching was still quite visible on the cylinder walls, with no evidence of scoring.

The oil leak issue is now lessened, but two of the original four leaks still remain (oil dipstick tube, crankcase gasket). There is no smoke of any kind coming from the exhaust like you would expect with worn piston rings. And with an engine this new, worn rings would seem somewhat unlikely anyway. My question is whether gasket leaks caused by high crankcase pressure from blow-by originating from a blown head gasket could still be seeping oil even after the underlying blow-by issue was resolved. Basically I'm trying to figure out whether I'll need to pull the engine and perform a piston ring job, or whether replacing the crankcase gasket and oil dipstick is all that's needed.

TROUBLESHOOTING
I knew the tractor had been poorly maintained when I purchased it, and immediately went about bringing its maintenance schedule up-to-date. The largest concern was multiple oil leaks, which I've listed in order of worst first:

1. Oil Dipstick
   
2. Remote Oil Cooler Hose Connections
3. Cylinder #2 Head Gasket
4. Cylinder #1 Head Gasket

These leaks were significant, primarily the first two. All-told, I was losing about a half-quart in 30 minutes of operation. As I repaired each leak, I found that I was chasing my tail as I would fix one leak, it would make the others worse. New leaks would even develop, like from the oil drain plug seals to the block and the crankcase cover gasket. It finally occurred to me that it seemed as though there was significant pressure building in the block. Researching this issue led me to checking the crankcase breather hose and reed valve. I therefore pulled the hose off of the air intake with the engine running and verified that it was pumping air, accompanied by small amounts of oil residue. With the engine off, I then blew through the hose and met resistance, but was able to suck air out easily. I assumed this indicated the valve was working properly. Was this a sufficient test?

I then moved on to suspecting blown head gaskets and/or worn piston rings creating a large amount of blow-by, resulting in increased crankcase pressure. I don't have a cylinder leak-down tester, and since the mating surfaces between the heads and the block were indicating gasket leaks anyway, I simply tore it down to the block to see what I could find. In the course of the tear-down I found that in each case, two of the four bolts for both cylinder heads were only finger-tight. Both spark plugs were a little loose as well. The head gasket on cylinder #1 looked fine, but there was visible oil residue marking the block mating surface between the combustion chamber and the oil return passageway leading back to the crankcase. This happened to be on the side that both head bolts were loose.



The head gasket on cylinder #2 was blown between the oil return passageway to the crankcase and the outside edge of the gasket, with blackened oil residue indicating there was oil leaking into the combustion chamber as well. Again, this was the side on which both head bolts were loose. I was happy to find the blown gasket on cylinder #2, as I was hoping that combustion leaking into the oil return passageway was the cause of my blow-by and not simply a symptom of it. I cleaned up both surfaces of the combustion chambers, installed new head gaskets, and torqued to spec.



In the course of the tear-down I also performed the following:

1. Removed the breather valve cover and inspected / cleaned the breather valve.
2. Reset coil-to-flywheel gap to spec, as it was off by over .010".
3. Rotated engine to TDC and reset valve lash to spec (Intake .015", Exhaust .020").
4. Performed a comprehensive cleaning of the carburetor.
5. Checked the oil it was not overfull, nor did it smell of gasoline.

After getting her all back together and cleaning up the block well to be able to identify any additional leakage, I took her for a test mow and found that two of the original four oil leaks had resolved, but the oil dipstick and lower corner of the crankcase cover gasket are still leaking. Of the two, the oil dipstick leak is the worst, although the leak is not as bad as it was before. I'm now trying to determine if these two leaks were simply caused by the original issue of high crankcase pressure and are only leaking still because the gaskets / seals were already compromised, or whether I still have an issue of excess crankcase pressure in the block, indicating stuck / worn / broken piston rings. Note that I do not have ANY smoke from the exhaust even when the engine is under load as I would expect with bad rings.

This would be a perfect time for a leak-down test, but I don't have a tester and am reticent to spend $40 for a Harbor Freight-quality unit which has mixed reviews. None of the local auto repair stores loan them out. I can attempt to measure blow-by using a home-made manometer and interpreting results, but I don't have factory specs to compare my findings to so I'm not sure how useful that would be.

I did test compression on the #2 cylinder this afternoon, but skipped the #1 cylinder as accessing it requires the removal of the starter and some other assorted engine components. I performed the compression test with the choke and throttle both wide open. Engine specs call for a range of 85-114 PSI at 500 RPM. Compression test results for cylinder #2:

1. Cold - 40 PSI
   
2. Cold w/oil added via spark plug hole - 40 PSI
   
3. Hot - 52 PSI
   
4. Hot w/engine running - 120 PSI

I believe the compression test numbers may be way off due to a compression release mechanism, but found the 120 PSI results from when the engine managed to start on the opposing cylinder only via some left over gasoline fumes in the carb unexpected. Any thoughts on these numbers? Questions for the peanut gallery:

1. Is it likely that the two remaining oil leaks are simply the result of the previous blow-by that has since been resolved?
   
2. I've read that a failed dipstick seal / breather valve can cause high crankcase pressure. Can a bad oil dipstick seal and crankcase cover gasket actually create crankcase pressure?
   
3. I've read that out-of-spec valve lash that is too wide will lead to high compression due to a failure of the compression release feature to work. Does this seem likely?
   
4. Some engines actually intentionally utilize crankcase pressure to, say, activate a vacuum-operated fuel pump. The fuel pump on this engine does indeed operate on vacuum pulses from the crankcase. Is some amount of crankcase pressure normal on these engine types for this reason?
   
5. I believe this engine may have been sitting for up to a year. Is it possible that the rings are simply stuck and are not standing proud off of the pistons? Should I try a treatment with Marvel Mystery Oil to see if it will help?

 

[schwartzy18510]

Bueller? Anyone have any thoughts on this quandary?

 

[ericm979]

> Can a bad oil dipstick seal and crankcase cover gasket actually create crankcase pressure?

No.

> I've read that out-of-spec valve lash that is too wide will lead to high compression due to a failure of the compression release feature to work. Does this seem likely?

You have low compression, so no.

> Is some amount of crankcase pressure normal on these engine types for this reason?

No, the pump works via pressure changes, not absolute pressure above atmospheric. The reed valve should result in a low crankcase pressure, or would if there wasn't so much blowby.

> Should I try a treatment with Marvel Mystery Oil to see if it will help

I'm not a fan of snake oil compounds. Given the low compression reading I'd first double check that the compression release was not active (there should be a way to disable it for testing). Then if the reading is still low, you either have worn rings or leaking head joint or leaking valves. You can put some oil in the cylinders and retest to check for leaking rings. The oil will temporarily improve the ring seal. With loose head bolts and a leaking gasket it's possible that the head(s) are warped or the mating surfaces damaged. You can check flatness with a glass plate and feeler gauges.

Removing the dip stick when it's running should let you feel or hear if there's a lot of crankcase pressure- it will escape through the tube.

Double check that the breather reed valve is not installed backwards. It should allow flow out but not in.

 

[schwartzy18510]

Thanks for weighing in, ericm979! I appreciate your time and thoughts on this issue. I'm eager to get this resolved, as the Deines 1800 ZRT was an unplanned garage sale purchase and unexpected problem which has ended up consuming far more of my summer (and garage) than I had planned. With two young ones under the age of three, time is in short supply these days and the wife is getting impatient at the house projects getting pushed back due to my work on this engine.

>> Can a bad oil dipstick seal and crankcase cover gasket actually create crankcase pressure?

No.

Thanks for clarifying. If I'm recalling correctly, I believe the theory I had stumbled across online involved the thought that compromised gaskets / seals in the block could be allowing air to enter the crankcase on the upward stroke of the piston(s), leading to a higher absolute level of pressure in the crankcase. In effect, negating the action of the one-way breather valve.

>> I've read that out-of-spec valve lash that is too wide will lead to high compression due to a failure of the compression release feature to work. Does this seem likely?

You have low compression, so no.

On a second read, my original question seems a bit daft — poor phrasing on my part. My question was whether valve lash set too loose could have resulted in overly high compression, possibly causing the original head gasket failures. I'm still looking for a root cause of the original oil leaks. It's the old chicken and the egg argument — did high crankcase pressure cause the original head gasket failures, or was head gasket failure the cause of the high crankcase pressure?

>> Is some amount of crankcase pressure normal on these engine types for this reason?

No, the pump works via pressure changes, not absolute pressure above atmospheric. The reed valve should result in a low crankcase pressure, or would if there wasn't so much blowby.

This makes perfect sense. Thanks for the explanation.

>> Should I try a treatment with Marvel Mystery Oil to see if it will help

I'm not a fan of snake oil compounds. Given the low compression reading I'd first double check that the compression release was not active (there should be a way to disable it for testing). Then if the reading is still low, you either have worn rings or leaking head joint or leaking valves. You can put some oil in the cylinders and retest to check for leaking rings. The oil will temporarily improve the ring seal. With loose head bolts and a leaking gasket it's possible that the head(s) are warped or the mating surfaces damaged. You can check flatness with a glass plate and feeler gauges.

I too am skeptical of most additives which claim to fix major issues. That said, MMO seems to have garnered significant praise from what I've gathered online. In this case, it seems potentially worth a shot since pulling this engine to replace rings which are only sticking instead of broken would not be worth it. Pulling it will be a royal pain due to poor access, as the engine was retrofitted into the Deines 1800 KT frame and clearance is a major problem.

I did some reading on how to disable the compression release mechanism, and it sounds like a bit of a crap shoot. Main ideas I came up with were to loosen up the valve lash excessively on the rocker associated with the compression release on the camshaft, which my documentation unfortunately gives me no clues as to.

Note that I did already add oil to the #2 cylinder during my original compression test, and found no change from the original (cold) reading. Combined with the fact that I'm not seeing blue smoke out of the exhaust and that the engine doesn't seem to lack for power or performance, I'm questioning whether the crankcase gasket and oil dipstick are simply leaking from having been compromised from the high crankcase pressure in the block prior to head gasket replacement.

>>Removing the dip stick when it's running should let you feel or hear if there's a lot of crankcase pressure- it will escape through the tube.

Pulling the dipstick with the engine at low idle results in some slight oil spatter about 1-2" above the dipstick seat. I'm unclear whether that's to be expected, or whether there should be no oil escaping whatsoever.

>Double check that the breather reed valve is not installed backwards. It should allow flow out but not in.

I did pull the crankcase breather tube off of the air intake with the engine at low idle again today. The tube is definitely pushing rapid-fire bursts of air out, as one would expect. However, it also seemed to have an intermittent vacuum-like effect as well between pulses — I could actually get a Scott towel to pop on and off the end of the tube with each pulse / vacuum pull. I think it's a bit strange, but this is the first time I've ever tested a breather so it's hard to know what constitutes normal behavior.

In terms of checking to verify that the breather valve isn't installed backwards, that's easier said than done. Requires removing the fan cover, engine fan, timing gear, intake manifold, carb, fuel pump. I tore it down to this point once to verify that the breather valve reed was intact (which it was) and took care to re-install it in the same position that I found it. It had no markings indicating which way was to face in and which way was to face out, so I simply trusted that it was installed correctly from the factory.

I ended up putting an order in just today for an economy cylinder leak-down kit from eBay. Should be here later this week. Hopefully it sheds some light on the issue at hand. Any ideas on the best way to prevent the crankshaft from turning once I have the piston at TDC prior to applying air pressure to the head?

 

[radios1]

there should not be any vacuum on the breather tube. get a new one, and face the reed valve out, so that it only opens when crankcase pressure is applied to it. it'll stay closed under vacuum.. if it's working properly. those Honda knock offs are made in China, so Iwouldn't count on it being properly made or assembled!..

 

[5030]

Easy answer, failed rings, excessive blow by. Get a Predator motor and be happy.

 

[radios1]

!!I forgot to mention, there was a guy on another forum that had this same problem!. it turned out that the dipstick installed on it was too short, so it got overfilled everytime oil was added, getting the correct dipstick fixed this problem. if you remove the dipstick tube, and it's filled to the top with oil, this is the problem!. it should be about 1/2" below the top of the casting, just like on a lawnmower without a dipstick!. filling it too high creates foaming and leakage!..