School Bus Mechanic Maintenance & Repair Tips.

Diesel engine principles are pretty straight forward for the experienced mechanic but what about regular everyday people who have never taken the time to find out more about a diesel engine and how it works?

I am going to explain in layman's terms the basic principles of a diesel engine. WHAT'S THE DIFFERENCE BETWEEN A GASOLINE ENGINE AND A DIESEL ENGINE?...a quick overview.

The first thing you should know is a gasoline engine and a diesel engine are totally different. A gasoline engine is constructed much lighter than the heavier built diesel engine and runs on an air/fuel mixture combined with a high energy spark that is timed to ignite the air/fuel mixture inside each engine cylinder at the precise time creating power and torque [turning force] which drives your vehicle.

A diesel engine uses high compression [intake air that is compressed / squished] into a very small space inside each cylinder causing extreme heat! This is called 'Heat Of Compression' which ignites a very fine high pressure mist of diesel fuel that is injected into the cylinder at the exact time.

So you now know that a gasoline engine needs a high energy spark to run while a diesel uses 'Heat Of Compression' THE FOUR STROKE PRINCIPLE Every engine today runs on FOUR STROKES or FOUR CYCLES- both these terms mean the same.

Here is how the four stroke diesel engine operates. The four strokes are:

INTAKE-COMPRESSION-POWER-EXHAUST.

The pistons, valves and injectors work together in each cylinder in a set sequence over and over.

1} INTAKE STROKE: Intake valves in the cylinder head open allowing pressurized air to enter each cylinder while the piston is travelling downward.{the pressurized air supply is made possible by the TURBOCHARGER which pushes air into the intake system giving the diesel engine a boost of air to keep up with instantaneous injection of fuel}

2} COMPRESSION STROKE: When the piston starts to make it's way back upward the valves close which traps the intake air in the cylinder which allows compression to take place, the HEAT OF COMPRESSION is reached when the piston reaches the top of the cylinder, the diesel fuel is then injected into the cylinder at the precise time.

3} POWER STROKE: After injection takes place an explosion occurs in the cylinder because of the combination of heat and atomized diesel fuel. This causes the piston to be forced downward which produces torque and the horsepower required from a typical diesel engine.

4} EXHAUST STROKE: After the power stroke the piston moves upward once again while the exhaust valves open allowing the previously ignited gases to escape to the atmosphere out the exhaust system.

As mentioned before each cylinder goes through this exact sequence over and over in a set firing order. For instance, a 6 cylinder diesel engine has a firing order 1- 5- 3- 6- 2- 4 This is the order that each cylinder goes by, following the 4 strokes mentioned above.

This sequence has been engineered to allow the diesel engine to run smoothly with no imbalance.

DIESEL ENGINE TRIVIA: Today's high performance diesel engines. Fuel passes through the injector at speeds of nearly 1500 miles per hour - as fast as a jet plane at top speed. Fuel is injected into the combustion chamber in less than 1.5 milliseconds, the same time it takes for a camera flash to go off.

The minimum amount of fuel injected into a diesel engine is one cubic millimetre - about the same volume as the head of a pin. Volkswagen has developed a 1 litre diesel powered car that got 100 kilometres out of .89 litres of fuel {60 miles on approximately 3/4 of a quart of fuel!}

There is much to learn about diesel engines and a ton of information online. If you have ever considered buying a vehicle with a diesel engine you have my blessing!

The main purpose of a school bus maintenance / repair shop is to get students to their classes safely without any mechanical problems. The first and foremost necessity is the safety factor on each and every school bus in our fleet.

When we check buses over during Regular Servicing & Motor Vehicle Inspections there are no exceptions when it comes to exhaust leaks, brake wear, steering, tires, suspension, lighting, glass and safety equipment (hazard kit, first aid kit & fire extinguisher). Every component has to meet rigorous standards according to Department of Transport Regulations.

I always think about people's children riding these buses and we can't let anything go when it comes to safety. A typical school bus is approximately 35 ft. long and can hold up to 84 students. The Mechanics in our shop are very talented and can repair, fabricate & troubleshoot anything that comes along.

Keeping up with these standards require us to have spare buses on hand in case mechanical problems occur. This has enabled us to guarantee that students get to class on time.

There are thousands of used school buses for sale but what should you look for? Well there's one thing you know for sure is they were maintained at a school district bus garage somewhere in North America.

Most garages will have a record of the repairs and maintenance performed throughout the life of the bus. This is very important information that you must acquire to get a really good account of what was done along with major repairs that can tell you the overall condition of the bus.

You may want a conventional or a pusher (rear engine) model bus, it depends what you are going to use it for. Most operators will tell you they prefer the pusher style bus because it's quiet and has a great ride.

The conventionals are more solid and have a firmer suspension. The engine is in front of the driver and much louder than the rear engine in the pusher units. Mind you I would grab a International Conventional bus with a DT 466 anyday because of their great reliability.

I would take into consideration where the bus came from and how severe the weather conditions were every year. This means more wear and tear and body deterioration from road salt and moisture.

Do your due diligence when checking a school bus:

Crawl underneath and look for rust.

Ask for repair and maintenance log books

Road test and check for excessive smoke from the engine,

How does it shift up and down hills?

Check out the ride for vibrations or any steering oddities.

The majority of used school buses for sale will have high mileage but rest assured they will have gone through regular maintenance programs that will give them extra life which is good for the end user.

This ISC electronic Cummins Diesel Motor appears to be in trouble. The camshaft has been removed because of excessive endplay that was effecting the sensors that read the revolutions of the camshaft.

The cam position sensor and the engine speed sensor sends a signal of voltage to the electronic control unit commonly known as the ECU. These signals are supposed to tell the computer the engine speed and cam position.

What happens if these signals are incorrect is an engine code flashes on the dash telling the driver there is a problem. The problem here is the camshaft has too much endplay which is changing the air gap setting between the sensors and the camshaft gear.

The only way to fix this problem is to replace the camshaft and gear since there is noticeable wear on the cam and shoulder where the gear sits against. For more pictures and details go to my school bus mechanic blog

Today's diesel engine troubleshooting procedures have changed since the introduction to computers and higher fuel emission standards.

A laptop is a necessity and software is rampant among all the major diesel engine manufacturers. There is still things you can do to overcome the electronic learning curve a lot of technicians have trouble with.

If I were to think of the best advice to give someone embarking on a diesel engine troubleshooting scenario, it would be one thing.
That one thing is "always check the source".

Don't go to the injectors or valves unless you hear a noise of some sort. If there's mechanical noise coming from the crankcase then you're going to pull the valve cover and inspect the top end. Then you need to drain the oil and pull the oil pan looking for scored cylinders or tidbits/chunks of metal.

For a no start always check the fuel tank. This sounds so easy but it's happened caused embarassing moments. The tank gets overlooked and parts get replaced for no reason because of a cracked or rubbed through fuel line coming out of the tank.

Start at the pressure side [after the second fuel filter or fuel pump] of the fuel system and start cracking lines open. If you've got fuel keep going forward if not go backwards checking for fuel.

If the engine runs and craps out after a minute or so check for air in the system caused by a cracked or rubbed through fuel line. Run the fuel line into a bucket of fuel and see if that makes a difference. If so, start going backwards from there. Hook up a clear line to the return fuel line and check for air that way.

This isn't the technical way of diesel engine troubleshooting but it has worked for me many times finding some really goofy problems that are easily resolved. So check the simple things first and don't get too wrapped up in the high tech side of the engine until you're sure you've covered these basics.