International School Bus Diesel Engine Road Test

Here I am….sitting and idling one of our MaxxForce DT diesel engines after a road test on the highway. It can be frustrating when a driver complains about the engine missing and not accelerating. As usual when the mechanic goes on a road test (more often than not) there is absolutely nothing wrong. There are times when we can not find an issue one of us from the shop will go with a driver on the bus run. The only tool we take is the laptop hooked up to the on board computer ready to pounce on any glitches during the ride.

The advantage to this is the bus is loaded up and going up hills and around corners which will increase the chances of the fault to show up. Usually the driver can warn the mechanic when and where the glitch occurs. Sometimes it’s strange how some problems happen at the same spot on the road at the same time every day. The first question I ask the driver is ….. any codes on the dash? If they say no then the job gets harder pin pointing when things go for a loop.

In the video I do a quick load test on the engine when it’s running at operating temperature (5 seconds max). The parking brake has to be on with my foot on the service brake. The transmission is shifted into Drive only and not low gear (puts too much torque on the drive train). This quick check will detect a power loss and give me feel for how much torque is developed while loading up the engine. It didn’t feel too bad so I concluded that it was fruitless to carry on without any obvious signs of a problem.

The diesel injection system in this bus runs at very high oil pressure (over 4,000 psi) to run the hydraulic injectors. If there are any leaks in the high pressure oil system it effects the injection cycle. Any weak spots are hard to locate quickly. The pressure has to be checked and the oil system tested for leaks. The diagnostic software can read the fuel pressure at the electric fuel transfer pump and the high pressure oil levels at different loads and speeds while on a road test.

Usually all of this is carried out when an engine code comes up. If there is a low reading there may be a leak in the system. A high reading may be a pressure sensor fault. In any case the valve cover would have to be removed to troubleshoot these areas. Most of the time it’s not a 15 minute quick fix so if you own a MaxxForce diesel engine get ready to roll up your sleeves. You’ll also need deep pockets.