Could an additive cause engine problems?

I recently received an e-mail from a gentleman who has an O-235 Lycoming and was experiencing a slight miss under some load conditions. He went on to describe all of the work he had done on the engine and yet the engine continued to miss under the same conditions. I gave the owner some suggestions on what to check, then sent the note to my good friend Paul McBride to get his expert opinion.


I recently received an e-mail from a gentleman who has an O-235 Lycoming and was experiencing a slight miss under some load conditions. He went on to describe all of the work he had done on the engine and yet the engine continued to miss under the same conditions. I gave the owner some suggestions on what to check, then sent the note to my good friend Paul McBride to get his expert opinion.

However, in the note there was this comment: “”Recently I tried an additive (MMO) in the hopes that this may help out long term. It was only in the engine for one or two flights before I started having problems, which may or may not be related. Then at the annual two hours later, it was flushed out with new oil.”"

The questions to ask here are: “”Could the additive cause the problem?”" and “”What should he do now?”"

The answer to the first part is yes, it could have caused the problem. Now whether it did, I cannot answer because I do not know what was in the additive. If it was an automotive type additive, there is a good chance that it contained zinc dithio phosphate, which is a very effective anti-wear agent. It works by chemically attacking the metal and putting a coating on it. Then when a metal part comes in contact with another metal part, the film is sheared off. The mechanism is called sacrificial lubrication. The problem is that the additive works very well on iron or steel, but it also attacks soft metals like copper. Here it tarnishes the surface and can leave a heavy deposit on it. In an aircraft engine, many of the exhaust valve guides are made from a copper alloy. If a zinc additive is used, it can tarnish the valve guides, which can cause sticking and a rough running engine.

The second part of the problem is that since the additives are very surface active, they do not just flush right out. It may take a few oil changes and possible guide reaming to get rid of all of the active ingredients.

The point of all this? Ask yourself some questions when faced with a problem. Is the desire for a quick fix or an easy way out? Second, realize that most of us learn by talking to others in the industry who may have had the same problem. If we talk to 10 people who have had the same problem and an additive fixed their problems, we logically assume that the additive will fix our problem also. But if the other 10 had steel valve guides and you have copper alloy guides, you have a big problem. Unless an additive is approved for your engine, don’t use it. In your automobile, you may be able to get by with it. But in your aircraft, always take the safe way out and find out what the problem is first and then fix it right.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at Visser@GeneralAviationNews.com.

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