Q: Hi Steve,
I'm the proud owner of a 1949 Piper PA-11. The engine is a Continental C-90-8.
I don't fly as often as I should. Due to some work obligations and some family stuff I had to deal with, last year I only flew 55 hours.
When I was talking to another local pilot he recommended I add some stuff he called Marvel Mystery Oil (MMO) to the oil and to the gas tanks. He said it would keep my engine healthy because it would help keep it clean inside.
I've seen cans of the stuff at the auto parts store; it doesn't cost much. What's the story? Should I add it to the oil and the gas? Just the oil? How much and how often?
A: Dear Cub,
This question has been debated by expert and rookie alike for decades before the internet, and that debate continues to create long threads on airplane owner forum pages near and far.
Will adding MMO to the engine oil hurt the engine? Not unless you add so much you seriously dilute the engine lubricating oil. Will it hurt if you add it to the gas? No, there's no evidence I've been able to find that convinces me it will.
However, MMO is not approved by the FAA for use. Does that stop believers from using it? No. There are posts stating that airlines bought the stuff in 55-gallon drums. There's also a story about it in "Winging It," Jack Jefford's engaging story of early flying days in Alaska.
Jeffords was flying the FAA DC-3 into remote landing strips. One engine was banging and popping during runup one day. HQ was called and a gruff old mechanic was sent out with some MMO—although I think it was referred to as Tiger's Milk in the story. The mechanic dribbled the MMO into the carburetor intake at low power settings and within minutes the engine was purring again.
The Material Safety Data Sheet (MSDS) for MMO lists naphthenic hydrocarbons (70 to 80 percent by weight), mineral spirits (20 to 30 percent by weight), and chlorinated hydrocarbons (0 to 1 percent by weight) as the ingredients. In simple terms, MMO is a light machine oil with some solvent, some coloring and a bit of wintergreen scent mixed in.
Some believers claim that they add some to the engine oil five hours before an oil change in the belief that it will clean carbon deposits prior to dumping the oil. Others claim that adding it to the oil on a regular basis prevents carbon buildup in the piston ring lands, thereby preventing stuck rings; and softens any carbon between the valves and valve guides, thereby preventing stuck rings.
Naysayers claim that today's oils are much better than the oils that were available in the round engine days and that the cleansing action provided by MMO is no longer needed. They contend that the key to keeping today's engines clean is flying often and changing oil at regular intervals. The installation of a full-flow oil filter (if not already installed) is also an important clean oil step.
MMO seems to be something owners and some mechanics feel strongly about. Those that believe in it, swear by it. If you want to hear stories of how MMO freed stuck valves, and resurrected tired, neglected engines you can find them.
As I mentioned earlier, there's no approval that would make it legal to use, nor is there any scientific study showing how using MMO increases compression and prolongs TBOs that believers can point to. But that doesn't seem to lessen the mystique surrounding it.
I have never added it to any engine or fuel tank during my wrenching career—but that doesn't mean I won't if I think an otherwise healthy engine is exhibiting woes associated with the buildup of carbon such as premature low compression after a borescope inspection shows that valves are in good shape, or if there's evidence of a (slightly) stuck valve.
If I ever decide to add MMO to the engine oil of a gummed-up engine, I would dump the oil within a couple of hours since I believe the carbon-loosening action could release a lot of contaminants.
I contacted one of the best engine shops in the country and asked if it had ever used MMO and was told yes, in isolated instances. So even the best have used it, which to me says at least one expert believes it's not going to hurt an engine.
Aside from accidently swigging down a slug of this bright red brew, owners can rest assured that there isn't anything in MMO that will hurt the operation of an engine—if used in moderation.
Q: Hi Steve,
I fly a 1960 PA-24 Comanche. My mechanic says I need to do this repetitive inspection of the exhaust system because of AD is 68-05-01. This Airworthiness Directive applies to the exhaust systems of hundreds of Piper singles ranging from the J-3 up through every model except the PA-24-400 and a few PA-24-260s.
I don't have any problem with doing this inspection since I know that any exhaust leaks are very dangerous and that leaks can also allow carbon monoxide (CO) into the cabin; the stuff is super toxic. I want my exhaust system to be in tip-top shape.
The bone of contention is not whether my exhaust system needs to be inspected, it's whether it has to be removed to be inspected. I don't think it does.
As I read the AD, paragraph 3 (i) says the repetitive inspection doesn't even have be done if the muffler is hollow; in other words, if it's a new one without a flame tube.
I can look up the exhaust down pipe and see that there's no flame tube, yet my mechanic says he has to remove it to do the repetitive inspections because he can't determine if the muffler is the exact part number as the AD refers to in paragraph 3 (i), and because he can't visually inspect every nook and cranny of the muffler/heat exchanger with a mirror and flashlight.
A: Dear Exhausted,
I understand your frustration. My interpretation of the inspection requirements lead me to believe the purpose of this inspection is to locate cracks in the heat exchanger to guard against carbon monoxide entry into the cabin. CO is invisible, odorless and extremely harmful.
Paragraph g (2) of the AD suggests an alternative method—that does not require muffler removal—of determining whether the heat exchanger portion of your exhaust system is leak-free. This method involves the use of a carbon monoxide detector.
The procedure requires you to turn the aircraft to face the wind, then run the engine up to full static rpm with the cabin heater valves full open. Position a carbon monoxide detector at the heater vents (including a rear seat vent, if installed).
If the CO concentration is greater than 0.005 percent—or in the danger level, if the detector doesn't read in percent—the muffler/heat exchanger must be removed and pressure checked at 10 psi to locate the leaks that must be repaired.
Your mechanic may still argue that by the letter of the AD he is right since he can't determine the part number of the muffler/heat exchanger, but I believe the spirit of the AD is complied with by using the CO detector method.
I carry a carbon monoxide detector in the cabin of my airplane at all times. I recently bought what I consider to be one of the best (small, long battery life, self-testing provisions) from AeroMedix, but there are many less expensive battery-powered ones now on the market.
Know your FAR/AIM and check with your mechanic before starting any work.
Steve Ells has been an A&P/IA for 39 years and is a commercial pilot with instrument and multi-engine ratings. Ells also loves utility and bush-style airplanes and operations. He's a former tech rep and editor for Cessna Pilots Association and served as associate editor for AOPA Pilot until 2008. Ells is the owner of Ells Aviation (EllsAviation.com) and the proud owner of a 1960 Piper Comanche. He lives in Paso Robles, Calif. with his wife Audrey. Send questions and comments to .
Marvel Mystery Oil
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