Engine Maintenance

This section primarily covers the diesel motors. As usual, the information is as accurate as possible but double check with your owner's manual and service manual. If you have extra information, please pass it along.

I recommend that all diesel owners check out Jim Bigley's "The 6.2L/6.5L Diesel Page". He has a wonderful amount of information and keeps getting better.

Oil and Filter

The most common task is obviously the oil and oil filter. This should be done every 3000 for the diesel and gasoline motors. This task is quite straight forward. Some people like to drain the oil hot, some like to do it cold. I prefer it cold because the oil has drained into the pan and is easy to handle. Of course it takes longer to drain. Be sure to check the drainplug washer. I replaced mine a long time ago with a washer like type with a rubber inner ring. These seal much better than the crush type. Don't over torque it.

Next I remove the filter. It is horizontal on all years. Note that it will drain all over the place as you loosen it so be sure the pan is positioned correctly. Which filter to use is kind of a religious thing; personally I like Wix filters. They cost a little more but are well made.

6.2, 6.5, 6.5TD - Wix 51060, Fram PH5, Napa Gold 1060
Note that the Wix 51061 and NG 1061 do not have the antidrain back vavle that we need. Fram lists the PH13 as the proper filter but it too lacks the antidrainback valve.

For the diesels, the best all around oil to use is a good 15W40. Check your manual to see the grade needed but should be CF or CG-4. Note that 10W40 is not appropriate. 15W40 are always a heavy duty grade. I use Shell Rotella T with XLA. It comes in gallon jugs. Synthetic oil can be used as long as it meets the grading. Mobil 1 does not meet CG-4. However, Mobil Delvac synthetic (the HD version of Mobil 1) does. Since we change the oil quite frequently, I don't know if the syn. is really worth the extra cost. For a turbo diesel or operation in cold climates, I would strongly consider it. Only in extreme temps should a lighter weight oil be used. Consult the owner's manual.

For the gasoline motors, a 10W30 is called for. Mobil 1 synthetic is also a good choice. Some gasoline engine owners have reported improved reving when running a synthetic.

Air filter

The air filter is a HUMMER specific part. For the diesels, K&N makes a replacement filter. For the gasoline engines, there is a Fram filter that fits close enough I am told.

People like or dislike the K&N filter. I decided to try it with my otherwise stock motor. I did not notice any significant changes. However, I feel that it breathes slightly better at freeway speeds. However, the change is so slight that it may be purely psycological.

Glow Plugs and Controller

A gasoline motor is a spark ignition device. The air and fuel are drawn into the cylinder together as a mixture and are compressed together. As peak compression approaches, the spark plug produces a spark, igniting the air-fuel mixture, creating high pressure gases, and forcing the piston down for the power stroke.

A diesel motor, on the other hand, is a compression ignition device and does not use spark plugs. It only pulls air into the cylinder. This air is compressed from 2 to 3 times as much as in a gasoline motor. The air gets quite hot from the compression. Diesel fuel is injected at high pressure into the already compressed cylinder via the precombustion chamber. The fuel burns as it hits the hot air, producing the high pressure gases to force the piston down.

The air in the combustion chamber must be hot enough to allow the diesel to burn. When the motor is cold, the glow plugs turn on and heat the air enough to allow the fuel to burn. Each plug consumes about 14 amps and glows red hot. During a cold start, they are on for about 5 seconds when the WAIT light is on. The engine is then cranked and the motor starts. The plugs may cycle on and off to help sustain the combustion process. Soon after, everything is warm enough and the process is sustainable on its own.

The glow plug controller is the brains of the system. It is basically a relay that controls the current to the plugs. It senses the air temperature and block temperature. It gets a signal when the key is in the run position and the start position. The controller operates in basically two modes: one before it sees the start signal and one after it sees the start signal.

Assuming the temperatures are low enough, when the key is first turned to ON the controller turns the plugs on (illuminating the WAIT light) for a maximum of 6 seconds. After this initial cycle, the motor is usually cranked and the controller would see the start signal. If it does not see the signal, the controller waits 4-5 seconds and energizes the plugs for another 1-2 seconds. This process repeats until it sees the start signal or for about 20 seconds.

After seeing the start signal, the controller tries to help smooth out combustion by cycling the plugs while the motor is running. Note that it does not actually know the motor is running, just that a start signal was received. In this mode, it turns the plugs on for 1-2 seconds then waits for 4.5 seconds. This repeats until the temperatures are high enough or after around 20 seconds.

Note that no matter what happens, there is a finite glow plug cycling time. Even if the motor does not start, the glow plugs will stop cycling. I believe older Mercedes did not do this. The actual on times and number of cycles depends on the temperatures.

Glow plugs get quite hot and operate in a harsh environment. After many cycles, they can start to operate at less than ideal performance. They can also totally cease to function. Finally, their resistance can decrease and draw too much current. All of these will lead to harder starts. To check operation, current to a plug or banks of plugs can be measured. Each plug should draw 14 amps and each bank of 4 around 55 amps. The tip of the glow plug can also break off, most common when old or overheated. The tip has to be removed (using a magnet) from the precombusion chamber and the plug replaced.

We use a standard glow plug for a GM diesel motor. Older trucks all called for a 9G glow plug. However, this has been superceeded by the 11G plug and should be used as a replacement.

The controller should function as described above. It makes quite an audible click as it turns the plugs on or off. It should deliver steady current to the plugs, the WAIT light should not flicker. Be sure to check the electrical connections at the controller. There is a big cable from the batteries attached to a lug on the controller. This also feeds power to the entire truck. To access the controller, the engine access cover must be removed. The controller is a standard GM device.