Ask The Mechanic


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Q: I have been told that I should never clean the air filter in my newer tractor until the air filter light comes on, indicating it needs cleaning. I guess the old oil bath air cleaner on my Ford 9N didn’t have a sending unit. I never have had a light come on in the dash on that tractor since 1939. But, it never powered down.

A: It is true that the dirtier an air filter gets before the restriction affects engine performance is the time the air filter is working the best to keep the engine air clean. That’s because the opening for the air to go through the filter is smaller as the filter gets dirty. Studies have shown that the most popular time for dirt to get in the engine is when the filter is changed, not because the filter got too dirty. It’s almost impossible to remove the dry air filter without a few pieces of dust staying in the canister. Always wipe the inside of the canister with a damp cloth before installing the clean filter. I guess with your 9N, we could say that oil filters dirt, and with your new tractor, we can say “dirt filters dirt.” Q: I have a John Deere 4240 that has developed a slow oil leak at the back of the head between the block and the head where the head gasket goes. I didn’t know any oil was supposed to be in that area.

A: An oil passage that allows oil to lube the rocker arm assembly is just inside the back deck of the engine. Pressurized oil passes through the engine block oil galley and rear cam bearing (sometimes called bushing), through a passage in the rear cam journal, up through the engine block, through the head gasket and, finally, through the head and into the rocker arm assembly. Your leak is seeping from the area where the head gasket seals the head to the block. Sometimes, torquing the head cap screws to specs after the engine is warm will seal the leak. If this doesn’t work, replacing the head gasket will be necessary. Check with your JD dealer for any updates on head gasket cap screw replacement, torque sequence and final torque after the engine has been run. Another good idea I have found is to spray both sides of the head gasket with a product called Coppercoat before installing.

Q: I have a John Deere 4520 compact tractor that, every so often, I find a small puddle of coolant under the center area of the tractor. I simply have given up finding where the coolant is leaking. I first thought it had something to do with changes in temperature or barometric pressure, but then I realized it was coolant. Have you got any ideas where the phantom coolant is leaking?

A: The coolant is more than likely coming from behind the water pump, where it mounts to the front cover on the engine (we hope). This has been known to be a problem with your tractor and others like it. The factory sealant can leak. This is an easy fix with the replacement gasket that is now available for this tractor. The other place coolant can leak and run down and back is from between the front cover and the engine block. The sealant that seals the cover to the engine block can leak. I believe these leaks in the front of this engine stem from the fact that the block is cast iron, the front cover is aluminum and the water pump is cast iron--unusual. The good news is that the water pump leak is a simple fix: cheap parts. The bad news is that a leak between the front cover and the engine block could be your problem. Deere now offers a kit that seals the front cover to the cylinder block. Again, cheap parts. However, the labor portion is pretty tall because the front mechanical front-wheel-drive (MFWD) assembly, drive shaft, engine oil pan and the pickup tube must be removed to begin repair on this problem. Then, keep digging until you get to the front cover so you can remove the damper and front cover. I hope your little leak is not the big problem.

Safety Tip of the Month:

I love net wrap on round bales of hay, but it grabs everything around me--including me. It tries to trip me as I take it from the bale to the back of the truck. After I get it off the buttons on my sleeves and front of my shirt, it’ll still grab my boots. Down I go into the mud, eye to eye with a cow. And, when you throw it into the back of the truck? Get ready to unload everything in the back of the truck with it.

Write Steve Thompson at Ask The Mechanic, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or email


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