Tip #4 Running the Overhead

Welcome back. So far my fuel mileage improvement suggestions have been centered around things that you could do that would not cost you any money. I’m going to go in a different direction with my next few suggestions.
Today we are going to talk about your engine. Specifically, “running the overhead”. You may have heard it referred to as “doing” the overhead as well. When you “run the overhead” you are basically having your shop or mechanic adjust the valves in the top of your engine. Most modern diesel engines (I have a Detroit Series 60) are six cylinder engines and have four valves per cylinder. Two intake valves and two exhaust valves. When a truck is new, the factory sets the clearances on all of those valves to manufacturer specifications. As you drive the truck normal engine wear will take place. This is assuming that you are using a quality engine oil and are changing oil regularly or using a bypass oil filter system. Imagine how many times those four valves in each cylinder are opening and closing as you drive 100,000 to 150,000 miles per year. The wear that takes place when those valves come in contact with the cylinder head eventually increases those clearances that were adjusted initially in the factory. When the clearances increase the valve stays open slightly longer which is what causes the extra burning of fuel. That is why we occasionally have to re-adjust them. Your mechanic or shop will use feeler gauges or clearance tolerance gauges to bring those valves back to the original factory specs so that your engine can perform at optimum levels. It’s kind of like in the old days when you did a tuneup on your car. You changed the spark plugs and made sure the spark plug gap was correct before you installed them. When you changed the points and condenser you had to set the clearance on the ignition points using feeler gauges in order for the car to run correctly. It’s the same principle here except we’re talking about the engine valves.
Depending on who you talk to you will get a different answer as to how often this should be done. When a truck is new you would generally think about running the overhead after the first 75,000 miles or so. By this time the engine has gone through it’s “break in” period. The “break in” period refers to the time it takes for the parts in a new or newly rebuilt engine to settle in to where they are supposed to be. Things like the piston rings and valves etc. If your engine is not new or has not been recently rebuilt the common adjustment period is considered to be about every 100,000 miles.
If you keep detailed fuel mileage records (as I do) the philosophy is often times different. I do not run my overhead unless I notice a significant drop in my fuel mileage. By significant I mean 2/10ths of a m.p.g.or more. I am a strong believer in the “if it ain’t broke don’t fix it” theory. If you don’t keep detailed fuel mileage records (I’m referring to every fill-up) you may want to stick to the every 100,000 miles interval.
Depending on where you have it done, it will cost you $200 to $400 to have this done. That may seem like a lot during tough financial times but remember, 1/10th of a mile per gallon change translates into $1,000 per year in fuel cost. The question will always be whether you want that $1,000 to be a savings or a loss. If your fuel mileage improves even one tenth of a mile per gallon by running your overhead, that $200 to $400 you spent will pay for itself many times over.

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