What can be done about exhaust valve sticking?

I received an interesting letter from Kurt Wegge, who flies a VariEze with a TCM O-200 engine, and has experienced numerous incidents of exhaust valve sticking. He was wondering if Marvel Mystery Oil would help reduce this. Kurt also noted documents from the FAA (Special Airworthiness Information Bulletin No. NE 03-45, dated June 27, 2003) and TCM (SIL 99-1 and SIL 99-2A) that give a lot of information on this subject. I strongly recommend that you get a copy of these documents if you have an O-200 with this problem. The information is also applicable to other engines and aircraft that have experienced this problem.


I received an interesting letter from Kurt Wegge, who flies a VariEze with a TCM O-200 engine, and has experienced numerous incidents of exhaust valve sticking. He was wondering if Marvel Mystery Oil would help reduce this. Kurt also noted documents from the FAA (Special Airworthiness Information Bulletin No. NE 03-45, dated June 27, 2003) and TCM (SIL 99-1 and SIL 99-2A) that give a lot of information on this subject. I strongly recommend that you get a copy of these documents if you have an O-200 with this problem. The information is also applicable to other engines and aircraft that have experienced this problem.

Back to the original question: I have never seen any data indicating that Marvel Mystery Oil reduces the incidence of exhaust valve sticking. I have heard from many pilots who claim that the product reduced or eliminated intake valve sticking, but not exhaust valve sticking. Based on the physical properties of the product, I would not expect it to have any significant effect on exhaust valves. Marvel Mystery Oil is a very light or thin mineral oil. Based on past combustion work, I would expect that almost all of the product would be consumed by the combustion process, leaving no product left in the exhaust stream. In addition, I do not believe the product would stand up to the high temperatures of the exhaust valve and guide region. It would evaporate rather than lubricate the valve and guide.

So what to do? It is important to understand that most exhaust valve sticking is caused by excessive amounts of lead being deposited in the exhaust valve/guide area. The above mentioned bulletins outline several steps, such as aggressive leaning and more frequent oil changes, which can reduce the amount of lead by burning it (leaning) or removing it (more frequent oil changes). If all else fails, consider using TCP, which is approved for all certified aircraft engines and has proven to help reduce exhaust valve sticking. It works by reacting with the lead to chemically change it from a tacky deposit to a dry flaky deposit. The mechanical action of the valve will sluff off the deposit rather than allowing a build up of deposits.

Another option is to purchase an auto gas STC and reduce the amount of lead that your engine consumes. (I recommend that you blend a small amount of 100/130 LL, about 25%, with the auto gas to ensure against valve recession, especially during break-in.) If you plan on using this option, it is very, very important that you purchase the appropriate STC and follow the guidelines to the letter. A surprising number of pilots seem to think that if they have any 80/87 engine in their aircraft, they can just go ahead and use auto gas. I have also received letters asking about the use of auto gas in 100/130 certified engines, even some injected turbocharged engines.

I am not sure how to put this strongly enough, but you absolutely, positively can only use auto gas in those engines/airframes that have a proper STC covering the particular aircraft that you are flying. You must also follow all of the guidelines outlined in that STC. There are very definite technical reasons for these guidelines and limitations ?and what you don’t know can hurt you.

The great philosopher who once said that “”a little learning is a dangerous thing”” was probably a pilot.

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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