Condition Inspection - Low compression on one jug

[Weatherpilot]

Doing the condition inspection on my new to me Sonex with an Aerovee 2.1 non-turbo. Looking back in the engine log the compressions have always been 76-77 on all cylinders. During our compression check today #3 came in a 4psi. Now the engine hasn't run in about 10 months while I waited to get some documentation squared away. I'm thinking it needs to be run to get that cylinder back to acceptable pressures. What do you think? Engine has 158hrs on it since new in 2012 and the logbook history shows good pressures and normal routine maintenance.

If I do have to replace a jug - what is the best source? Should I just replace one if needed or go for all four and do a top overhaul?

Any recommendations welcomed.

Thanks all.

[Murray Parr]

Compression values are usually done hot (after an engine run). I would make sure the valve clearances are ok and while you are at it, make sure a valve isn't sticking. Then I would run the engine up to operating temperature and do the compression checks again.

Others with Aerovee experience might chime in here too.

[Bryan Cotton]

A jump to 4psi is not wear, something is wrong. I agree with Murray, check valve clearances and if they are sticking.

Did you do a leak down test or use a compression gauge? If you did a leak down you could listen to the intake, exhaust, and breather to see where the air is going.

If it was my engine I'd run it and check again, after looking at valve clearance.

Sonex sells the parts you need.

[daleandee]

If it's reading 4/80 something is well out of whack. It's simple to pull the valve cover and rotate the engine by hand (fuel & ignitions off please!) and watch the valves open and close. Admittedly 158 hours in 11 years isn't much run time and engines that sit tend to have more concerns than those that are run often. I like to fly every few days but at least once a week. But there are times when I don't get up as often as I should. I really try to not let the plane sit without being flown.

You used the term "jug" but on the VW the head (where the valves are) and the cylinder/piston assembly are separate. Pulling the head is really easy and something that you should get acquainted with as, at least from my personal experience, it is a fairly regular maintenance item on VW engines (I have a 74 VW beetle that I drive).

In one instance I had a valve that wasn't opening because of a failed rocker arm and the compression reading was very low. So as others have noted, whatever is wrong should be obvious and fairly easy to find. VW engines are cheap to repair ... that's the great news!

Dale
3.0 Corvair/Tailwheel

[BRS]

If it's not been run in 10 yrs and you did a dry compression reading I would expect poor results. Before getting out your wrenches I'd do a few things. First add a bit of oil to each cylinder, via removed sparkplug, Then pull it through several rotations. Put your tester back on the suspect cylinder and add some pressure (valves closed) Listen at the Exhaust; Intake (open throttle); and Crank-Case (via oil fill location). Listen for hissing to see if you can determine what is leaking (Exhaust/Intake valve or Rings). While you are at it, you might want to check the head torques and valve clearances.

Run the engine for a few minutes and get it warmed up. Recheck compressions. At least this is how I would treat a Lycoming engine that has sat for many years. Sometimes these things will clear after an hour or so of running.

[Weatherpilot]

If it's not been run in 10 yrs and you did a dry compression reading I would expect poor results.

Thanks for the advice. Will try the suggestion. BTW - 10 Months...

Best regards,

Greg.

[mike.smith]

As others have suggested, you can run it to get it warm and see if it improves. Check the rocker arm clearances to make sure a valve isn't being held open. Is the air escaping through the carb (intake valve) or the exhaust pipe (exhaust valve)? If you have a borescope you can have a look at the valves through the spark plug holes. It's pretty easy to take the head off a VW, so that would be the next thing to try. Simply lapping the valve might help return things to more normal. If it all worked 10 months ago, there shouldn't be anything catastrophic that happened while sitting there.

[Prince]

If running the engine as suggested doesn’t help, then check the (Rhino) rocker gear is not contacting the inside of the rocker cover and holding a valve open. This can occur if a cork gasket has been reused and compressed thereby reducing clearance, or if an incorrectly cut push rod requires the tappet adjustment screw to be backed-off too far.

[Weatherpilot]

If running the engine as suggested doesn’t help, then check the (Rhino) rocker gear is not contacting the inside of the rocker cover and holding a valve open. This can occur if a cork gasket has been reused and compressed thereby reducing clearance

Purchased some new rocker covers and corks from EMPI just in case. The original covers are definitely compressed and were RTV'd on so I just bought a new kit with corks and covers for less than $50 online.

Thanks for the suggestion.

[tps8903]

If running the engine as suggested doesn’t help, then check the (Rhino) rocker gear is not contacting the inside of the rocker cover and holding a valve open. This can occur if a cork gasket has been reused and compressed thereby reducing clearance

Purchased some new rocker covers and corks from EMPI just in case. The original covers are definitely compressed and were RTV'd on so I just bought a new kit with corks and covers for less than $50 online.

Thanks for the suggestion.

As other have said. Run it up. Do some taxiing. Corrosion some times builds up on the valve seats.

After a run I would also consider changing the oil in case you do shear any Corrosion off into it.

I saw a similar situation on a Cessna 185 that sat for a decade plus before purchase. Ran it hard. Compression readings all came back. Changed the oil, clearly saw some corossion that flaked off. Ran it regularly for years after before a rebuild was required. Oil was always clean after that first oil change.