Interesting problem... Great compression, perfect PCV operation, but I have blow by.

[Joe West]

Hi Gang,

I have an interesting problem. I have had blow by issues ever since I purchased this Scrambler. The engine sat for 10 years before I bought it, so I just assumed that this was a ring problem. The problem seemed to be getting worse (or perhaps I am paying more attention to it) so today, I did a compression check. All cylinders were 160 except 3 and 4 which were 151 and 155 respectively. These numbers surprised me. The PVC system appears to be fully functional, good vacuum source, new PVC valve that tests just fine.

The blow by issues are:

Slight oil leak out of the dipstick tube.
Light dusting of vapor out of the oil filler when I remove the cap.
Oil saturated air filter

I am soaking the cylinder tops each in 3/4 cup of Marvel Mystery Oil just in the off chance that the rings are stuck but somehow giving me good compression. I will also do a cylinder leakdown test, but I have absolutely no indication of leaking valves (no telltale puff of blue smoke in the morning after sitting overnight). I actually expect the leakdown test to be good.

By the way... all of the plugs look perfectly normal. No carbon, no oil fouling... just a nice tan coat of soot on them.

One note: The service manual shows normal compression should be 120-140, yet I am at 155 on an engine with 135,000 miles on it. I find this odd.

I am posting this just in case someone can think of something else I can look at before I resort to doing a rebuild... which by the way, I think I need since my oil pressure hot is low at 15-20 psi at cruising speeds. Oil pressure is 40-ish when cold, but as the engine warms up, it drops to the 15-20 range virtually independent of RPM and loading.

Thanks so much for any insights you may be able to offer.

Joe

 

[LetchcoreCJ7]

Not sure on the blow by but a good 258 will be between 140 - 160lbs compression.

 

[mysunnshine]

If the leakdown is good then your PCV system is restricted or not working well enough.

 

[FLCJ8]

What year?
My '83 had a similar problem. PO had run a line from the valve cover to behind the block as a vent. There was a solenoid valve (for emissions?) in the vacuum line that was melted. I don't remember the exact physical location, but it was well hidden in the maze of wiring and tubing for the emission controls.
I reconfigured the emission controls to the factory '81 layout, problem solved.

 

[Joe West]

And the answer is...

Leakdown test on all cylinders show 25 +- 3 % leakdown. No noise on the intake or exhaust, lots of crankcase noise. Looks like the rings are shot even though my compression is good. I assume this can happen because the compression test is a one-way pressure test, it simply pumps up the compression gauge but does not allow for any flow back through the cylinder.

So... I am soaking the top end in MMO to see if it may improve things until I can rebuild the engine in NC. I need this engine to be useful until next summer when I can dedicate time to rebuilding it. Till then, I will keep my fingers crossed that the worst problem I have is blowby :-)

Joe

 

[spankrjs]

My green '83 Scrambler, with 125,000 miles, soaks the little filter inside the air cleaner with oil, too, yep, blow-by

Mine makes a strange, intermittent knocking sound at idle, too, so I might be rebuilding mine before you get to yours, so I will post plenty of pictures

I haven't ran a compression/leak down test yet, but I plan on it, mainly out of curiosity

 

[LetchcoreCJ7]

That doesn't make sense to me. How can a piston ring build pressure but not hold it?

 

[Joe West]

Apparently the rings seal enough to pulse air into a compression gauge, but are not sealed well enough to actually hold the pressure.

That doesn't make sense to me. How can a piston ring build pressure but not hold it?

 

[Rosco1974]

I have always used brake cleaner then oil on rings that have sat for a long period. If you plan to keep running it I would consider changing the oil to Valvoline Premium Blue 15W-40 (my preference or some other diesel oil) and add a can of Kano Kreen. You can use the 10W-30, but Kreen thins the oil so starting a bit heavier is fine. It has worked for me on several occasions.

 

[Joe West]

I will give the brake clean a shot at the next oil change. Thanks for the idea. I have put a little over 30,000 miles on this jeep since I purchased it. I am trying to baby it till I can rebuild it next summer.

 

[bobbysjeep]

I had an inline four cylinder in a boat with the same symptoms that you are describing as far as blow by. The problem ended up being a lobed cam. Along with the excessive crankcase pressure I was experiencing exhaust coming out of the carburetor. Apparently it was not allowing the lifter to raise enough and the exhaust to escape out of the cylinder.