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Oil: Miles or Hours?

Written by Pat Goss

In any given year, I receive a dozens of requests from fleet administrators and maintenance managers asking for help concerning vehicle purchases and maintenance programs. Interestingly, most of the maintenance questions deal with oil-related engine problems.

Usually the fleet will have received what most people would consider exemplary care, yet they still experience excessive oil consumption and higher than average engine failures in certain vehicles. Exemplary maintenance usually means oil change intervals of 3,000 miles or three months using high-quality oil and commercial grade oil filters. But it doesn’t help.

This begs an important question. Where or what is the flaw in these engine maintenance programs, or are they just bad engines?

Whether it is a small fleet or large fleet, proper maintenance is the most important aspect for reliability, durability, and economy. Unfortunately, neglecting maintenance or, more typically, selecting the wrong maintenance schedule, decreases vehicle life, causes more breakdowns, and increases gas consumption. All this brings us back to that maintenance problem.

Since the advent of odometers in automobiles early in the last century, the rule has been that service should be performed based on the number of miles traveled by the vehicle’s wheels or after a certain number of months. Heading-up that service list is the all-important oil change!

Some maintenance experts, including some carmakers, say that the oil should be changed every 3,000 miles. Others of equal credibility suggest that a change at 5,000 miles, 7,500 miles or even 15,000 miles is adequate. In reality, they are all potentially wrong.

Trying to measure the amount of wear on an engine based solely on the miles covered by the car’s wheels is highly speculative. Strangely, automobiles and light trucks are the only vehicles that use this method. Everything else including boats, airplanes, and heavy equipment, and even lawnmowers, measure use in hours. Perhaps the manufacturers of these vehicles know something that has escaped the auto industry!

This may be the case because with automobile use, one size does not fit all. Due to different driving styles and conditions there is no universal relationship between miles driven and hours of use.

For example, if the vehicle is used primarily in the city with an average speed of 30 MPH, it takes 100 hours to log 3,000 miles. But that same vehicle used on the highway with an average speed of 60 MPH would accumulate 6,000 miles in the same 100 hours. Also, the odometer doesn’t take into consideration all the time the engine idles to keep the driver cool or warm or while just sitting in traffic.

Yep, the odometer is a very inefficient way to judge when to perform engine services. And that’s where the answer has been for many of my fleet maintenance questions.

Because most of the fleets were only using miles as a guide to service all their vehicles they were actually short changing some of them. The worst problems were with police cruisers and emergency vehicles that often idled for many hours every day. Accident investigation, for instance, might leave multiple cruisers on the scene for four, five, or more hours idling to keep their batteries alive. Of course, this never showed up on the cars’ odometers. But the engines were still in use, and use was killing oil and ultimately engines.

To further confuse and confound, there are essentially two categories of parts on a car: parts that are in use all the time the engine is running and parts that are only in use when the wheels are turning. The best way to service parts that are in use whenever the engine is running is by counting hours. That means the engine and all its related fluids should be serviced based on hours.

Conversely, those items that only operate when the wheels are turning should be serviced based on the odometer. For example, the transmission, suspension, and most of the steering system can be serviced based on miles.

This method used to be difficult because cars didn’t have hour meters. Today, many cars have trip computers, which keep track of time when the engine is running and can be reset to zero following a service. For 2006, the Ford CVPI will come with an engine idle hour meter. For 2005, the GM police cars use the logarithm- based, on-board Oil Life System.

The generic number of hours between oil changes ranges from 100 to 150 hours. However, regardless of hours, never exceed the allowable time or mileage limits stated in the owner’s manual. Even though it’s a poor way to gauge use, it is still the vehicle manufacturer’s accepted standard and you don’t want to compromise any warranties.

Finally, for cars that don’t have trip computers, an hour meter can be purchased from your local marine, equipment, or auto parts store for less than $50 and installation should take about 45 minutes. On vehicles that may be subject to a lot of idling time, this $100 installation could save thousands of dollars in future repairs.

Pat Goss is the Resident Master Technician for Motorweek TV (PBS), columnist for the Washington Post, and host of radio and cable shows discussing vehicle maintenance. He was also a consultant to the Prince George’s County, MD Police Department on fleet maintenance. He is the President of Goss’ Garage in Seabrook, MD and can be reached at patgoss@goss-garage.com.

Published in Police Fleet Manager, Sep/Oct 2005

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