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PM Based on Fuel Consumption

Written by Kevin Roberts

The goals of preventative maintenance are minimizing down time, eliminating unexpected failures, and prolonging the vehicle’s life in the most cost-effective way. We do this by developing proper maintenance procedures and then establishing proper communication between fleet management, the officer, and the technician.

We also do this by obtaining the information necessary from the technical and operational side of fleet operations. Technical information prevents us from putting the wrong transmission fluid or coolant into a vehicle. Operational information prevents us from doing a Preventive Maintenance Inspection (PMI) at either too long an interval or too short an interval.

Of course, disagreement exists on the best way to determine these maintenance intervals. Some hold to the traditional way, i.e., tracking mileage. Others think that tracking hours is more accurate due to the extended amount of idle time seen by the average police car.

The problem with both of these ways of determining PMI intervals is that idle time can vary widely based on usage. The state patrol would likely be on the low end of the scale, less idle time. The city police would likely be on the high end of the scale, more idle time. And the county sheriff could be anywhere on the scale, either between or even beyond the parameters of the state or city agencies.

Fuel consumption is an alternative to using mileage or hours to schedule the most cost-effective PMI. Whether an engine is idling, cruising, or in pursuit, it is using fuel. And the fuel use occurs at a varying rate depending on the type of use.

At idle, you don’t use much fuel but you don’t go anywhere, either. Since the primary function of the car is transportation, your average fuel cost per mile goes up proportionally to the amount of idle time. You are also operating the vehicle outside its design parameters, which leads to a greater need for maintenance.

At cruise, you operate your vehicle in its most efficient mode. This is true from both a fuel cost per mile standpoint and also from a powertrain wear standpoint due to the fact that vehicles are designed to cruise. Cruising provides excellent fuel economy, maintains design operating temperature, and is less stressful on the power train than most other forms of operation.

Pursuits, or any high speed operation, increases fuel consumption as well as increases stress on the powertrain.

The things that stress your vehicle are the same things that increase fuel consumption. We can use this correlation to more efficiently provide Preventive Maintenance if we can find a simple way to obtain the information on fuel consumption.

It is here where the use of a modified passenger vehicle design rears its ugly head. Your police car was originally designed to take Mom to work and allow Dad to run errands while the kids are in school and take them to soccer afterwards. Occasionally, there will be a trip to the Grandparent’s house or a couple weeks of vacation. The engineering staff didn’t have police usage as their primary design factor. Therefore, there is no provision for setting a maintenance light based on fuel usage.

Medium duty trucks, on the other hand, are essentially all commercial vehicles, some of which see considerable idle time. If you buy an international, you can program the PCM to have the SI (Service Interval) light come on based on fuel consumption. Ideally, this would be an option on the vehicles used in police service.

Until the OEMs add this option, there are alternatives. One is the old fashioned way. Your officer tops off the tank, records the mileage on the receipt or other form, makes sure the office gets the paper work, and you have all the information needed. If your agency can’t get the officer to check the oil when the engine is rattling, then there may be a question about the extra paperwork actually getting done.

A few high tech options exist. There are GPS systems that will track trip mileage, location and speed, and automatically download selected information once the vehicle comes within a certain distance of the home base. Furthermore, there are software programs that will allow fuel consumption to be tracked and then the scheduling can be done.

For a small agency that wants neither the time of doing manual paperwork or the expense of the high tech black boxes or software, another option exists. Determine each vehicle’s fuel mileage at cruise and record this as a mile per gallon number. Then determine each vehicle’s actual miles per gallon over an existing PM interval. Then do the math to determine the interval of the PMI based on fuel consumption.

Let’s say your CVPI gets 18 mpg during a 55 to 60 mph steady cruise. The PMI interval for a vehicle that cruises all the time is 6000 miles. Therefore, use about 335 gallons as a base line for PMI scheduling, i.e., 6000 miles divided by 18 mpg.

You then find out your patrol cars are only getting 12 mpg due to idling on one hand and high speed use on the other. At 12 mpg, it only takes 4000 miles to use 335 gallons of fuel. There is your PMI interval. Your numbers will vary, perhaps dramatically depending on how your vehicles are used, but this shows how the math works.

The drawback of this method is that it uses only long term averages rather than actual fuel usage between each PMI. However, it can be a first step to get the PMI interval to reflect fuel consumption rather than simple miles traveled.

Kevin Roberts is the president of Roberts Repair in Rhinelander, WI. They have specialized in emergency vehicle maintenance since 1989. He is an ASE Certified Master Automobile and Master Truck Technician. He may be reached at robrep@frontiernet.net.

Published in Police Fleet Manager, Jan/Feb 2006

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