TUL: 5746 E Apache St. Tulsa, OK 74115
FXE: 1072 NW 53rd St. Fort Lauderdale, FL 33309

Tempest Tech-Tip

Changing Spark Plugs


Although complaints from the field about Tempest spark plugs rarely occur, most take place immediately after their installation, reporting that the engine ran rough. If requested, we expeditiously send replacement plugs to get the customer back in the air ASAP and arrange to have the suspect plug(s) returned to us for analysis. Upon receipt, we inspect and test each one. In virtually all cases, after cleaning, the plugs test ok. So, what’s going on?

Carbon and Combustion By-Products Accumulation

In most engines a crust of carbon and other combustion by-products accumulates inside the cylinder head. This crust bridges from the head onto the shell of the spark plug. When the spark plug is turned to remove it, this crust is broken and bits of debris fall free in the cylinder. Subsequent movement of the piston sweeps the debris to the upper end of the cylinder. Then it falls into the recess around the bottom spark plug where it takes but one flake across a spark plug’s gap to short the spark plug out. The flake may become a glowing ‘hot spot’ in the cylinder once the engine starts. If this occurs, it can cause pre-ignition. An indicator of pre-ignition is popping and snapping noises originating from the running engine.

Typically, the debris is blown from the cylinder within a few seconds after the engine starts. But, sometimes a flake sticks in place, shorting a spark plug and causing a rough running engine with a large mag drop. (Similar problems may occur with the initial start after an engine overhaul. In that case, it’s usually oil or grease from the engine buildup process that’s the culprit.) Of course, once the plugs are removed and cleaned or replaced with new ones, the problem vanishes because the offending material has been removed.

Tempest Recommendation

Tempest recommends the following; for safety purposes, remove all spark plugs from the engine, use shop air to blow back and forth through the cylinder, down through the top hole, out through the bottom, and vise versa. Turn the prop so the pistons chase loose particles to the upper end of the cylinder where they’re easier to blow out. Then install the spark plugs per Tempests’ recommended installation procedures (please go to www.tempestplus.com/literature). This will help to remove the particles that could cause problems.

If you do have excessive mag drop, here’s a helpful way to isolate the problem to a specific cylinder. Buy a 600 to 700 F degree ‘Temp Stick’ (manufactured by Tempil®) at your local welding supply store. Start the engine and let it warm up. Shut the engine down and let the exhaust pipes cool down until you can safely touch them. Make marks, starting at the cylinder head and going down the side of the pipe, 12 to 18 inches long, so that observers standing on each side of the airplane can see them.

Crank the engine. Immediately turn off the good mag. Operate only on the rough mag and immediately increase rpm to the rough running speed. The observers will see the marks quickly burn off of the properly running cylinders’ pipes. The mark will remain on non-firing cylinder’s pipe. Remove and clean the fouled spark plug. Check the resistor value. If it’s more than 5000 ohms, replace the spark plug. If resistance is ok, reinstall the spark plug. In the vast majority of cases, these steps will resolve your problem!

Oil Filter Micron Rating

So why not use finer filter media to screen the fines out? It’s been tried; and rejected because it causes more problems than it solves. Airplane engines have high volume oil pumps and use thick oil. While a 15 or 20 micron or finer filter might work for a car, it will quickly plug up on an aircraft engine. That will force the oil system into filter by-pass mode. Then you’ll have dirty oil circulating continuously. It’s a subject for another article, but fine media isn’t the answer. 

By-Pass Valve Effect

Differential pressure (delta pressure) across the media inside the filter occurs when oil can’t get through the filter media fast enough to prevent excessive back pressure in the filter. This can happen because of an excessively dirty filter element or because of super-cold, thick oil at start up. When the delta pressure in a Continental engine filter exceeds about 12 to 14 psi, a spring loaded valve (the by-pass valve) located in the filter cracks open. It bleeds some of the oil around the filter element and straight back to the engine. That prevents crushing the filter element or bursting the filter case. As the oil warms, the valve re-closes. (Obviously, if the valve is open because of an excessively dirty oil filter it will not re-close.) In Lycoming type engines this valve is usually in the engine accessory case, not in the filter. In remote or aftermarket filter adapters it may be in the adapter.

Anti-magnet naysayers suggest that when a filter mounted by-pass valve opens, fine particles might be swept off the magnet in the filter and go back into circulation in the oil. 

Let’s think about that for a minute and, for argument’s sake, assume it’s true. Say that after 40 hours of operation an amount of fines the size of a BB (.06 cc) has accumulated on the magnet. Then, one extremely cold morning, the engine is started and the rpm gets a little high, forcing the filter’s by-pass valve open. Remember, were it not for the magnet 100%, of the fine particles in the oil at any given moment would be circulating through the engine. Now, say that 2 percent of the particles on the magnet are temporarily (hypothetically) swept free and re-enter the oil stream. That’s still only one fiftieth (1/50 th ) of the particles that, were it not for the magnet, would have been circulating through the engine continuously! Within a few minutes, as the oil warms up and the by-pass valve closes, the few particles that got back in circulation should be trapped by the magnet again. 

What would you think is better for your engine? To have 100% of the particles going round and round through your engine, turbocharger, controllers, prop governor and so on, hour after hour; or for one-fiftieth of the particles to go through your engine for a minute or two until the by-pass valve re-closes and the magnet traps them again? Note that this anti-magnet / by-pass valve argument hinges on extremely cold weather engine starts forcing the by-pass valve open. But, in reality, during millions and millions of engine starts at average temperatures the conditions necessary for this argument to even flap its wings, much less fly, simply don’t exist. And, in the cases of Lycoming type engines and some remote installations where the by-pass valves aren’t in the filter, the anti-magnet argument can’t be made at all!


For additional information on this tech tip and other Tempest products, please go to www.tempestplus.comor call (800) 822-3200.