Long Idling Can Cause Diesel Engine Problems
Preventive maintenance for diesel engines in the fire service does not depend entirely on the correct selection of lubricating oil, quality of fuel oil, and regularity in changes of oil filters, air cleaners and fuel filters. It includes, so far as possible, avoidance of one of the practices that contribute to increased maintenance and production of some of the problems discussed in previous columns.
This is the practice of permitting long idling periods at the scene of an emergency. We are aware of the reasons usually given for such practice, which, in most cases, is not actually necessary. Certainly the cumulative effect of long idling periods with the resulting increase in maintenance requirements should be weighed against this practice as a general procedure.
Idling problems: Some of the problems created by long idle periods are:
- Crankcase sludge
- Crankcase oil dilution
- Excessive lube oil consumption
- Gum and varnish deposits on valves, pistons and rings
- Engine misses fire
- Excessive smoking of the exhaust
- Hard starting
- Loss of power
This adds up to trouble, time out of service and a good dent in the maintenance budget.
“It idles so nicely, and we want it to be warm when we need it.”
But is it warm? The answer is no. That is what causes the trouble. The normal operating coolant temperature of the automotive diesel engine is 165°-195° F. Remember, the diesel has a high thermal efficiency and heat rejection to the cooling system is appreciably less than for the gasoline engine, so as it idles, the coolant temperature drops below 140° F, and unburned fuel and cold cylinder walls start the trouble.
If it is essential to idle the engine, due to lower temperature conditions while on standby, make sure the throttle is set for a fast idle, 800-1,000 rpm. This will normally keep the coolant temperature sufficiently high to minimize the result of long idling. If the radiator is not equipped with a shutter and atmospheric temperatures are in the O° F and below range, a good practice is to partially cover the radiator core with cardboard or corrugated board as a temporary means of keeping coolant and air intake temperatures at a level for proper operation.
Engine misfiring: If an engine misses fire on one or more cylinders, the most probable causes are one or more of the following:
- Poor quality fuel
- Water in fuel
- Restriction in fuel fine
- Air leak in fuel suction line, fuel tank to pump
- Long idle period at low speed
- Plugged injector spray nozzles
- Injectors need adjustment
- Valves not seating properly (exhaust or intake)
- Poor (low) compression
- Injection pump rack stuck
- Fuel tank vent partly or completely plugged
- Fuel supply pump not operating properly
The first four items have been discussed in detail in this column in the December 1969 issue of FIRE ENGINEERING. Discussion of the fifth item preceded this listing of probable causes for the diesel engine missing fire at idle or high speeds.
There are some probable causes that apply only when the engine misses fire at idle speed. One is improper setting of the governor idle speed. A sticking fuel delivery valve can also cause missing at idle speed.
Before any dismantling is started, and after making sure that items 1, 2, 3, 4, 11 and 12 are not probable causes, the valve covers can be removed to start checking probable causes 7, 8, 9 and 10. The first check would be for proper valve clearance with the rocker arm. This will usually locate any sticking valve or one not seating properly because the clearance is excessive. The injection pump rack can be checked for free movement and correct timing. If the trouble is not located for these items, the injectors should be checked and any faulty ones replaced with new or properly reconditioned injectors.
The final test is a compression test to find a leaky cylinder head gasket causing loss in compression pressure. If torquing the cylinder head bolts to specification does not stop the leak, a new cylinder head gasket must be installed. Worn or scored pistons and rings can also be a probable cause of low compression. With the limited mileage and use in the fire service, we do not anticipate this cause of low compression, providing the engine has been given proper maintenance during the years of use.
Diesel engines with an injection pump have two possible sources of trouble: first, injection pump plunger stuck, and second, worn injection pump plungers. These have not been listed above as, like worn or scored pistons and rings, we do not anticipate such wear during at least 20 years of fire service.
If there is a color to the exhaust when the condition of missing fire occurs, the probable causes of the smoking condition listed in this column in the December 1969 issue of FIRE ENGINEERING can perhaps pinpoint the cause quickly.
Low oil pressure: If the lubricating oil pressure is low, the most probable causes are:
- Restriction in oil pump suction line
- Defective oil pressure regulator
- Defective oil pressure gage
- Crankcase low on oil
- Wrong grade of oil for weather conditions
- Oil needs changing (diluted)
- Restriction in engine oil passages (sludge)
- Engine too hot, insufficient coolant in system
- Faulty thermostats in cooling system
- Loose fan belts
- Clogged water passages
- Clogged oil cooler
- Clogged oil filter
- Worn main or connecting rod bearings
If the engine has had good maintenance with the proper grade oil used and changed at three-month intervals, items 13, 16, 17, 18, 19, 24, 2o and 26 can be bypassed as probable causes. Items 20, 21, 22 and 23 concern the cooling system and if maintenance has been good, these items would not be suspect. This leaves only items 14 and 15 needing a careful check.
Fuel knocks: If there are fuel knocks, the most probable causes are:
- Poor quality fuel
- Air leaks in fuel suction line, fuel tank to supply pump
- Engine overload (lugging)
- Incorrect valve timing
- Incorrect injector timing
- Injectors need adjustment
With this problem, item 27 is the usual cause. The time delay for ignition after injection in the cylinder with the heated compressed air is the important factor which is controlled largely by the cetane number of the fuel used. This point has been covered in detail and so will not be repeated. Item 28 is a physical check on the fuel supply line fittings to be sure they are tight.
Item 29 is usually a cause only when the driver is using a transmission gear position that does not allow the engine to operate at a higher speed when climbing a hill or accelerating. Down-shifting will eliminate this cause. Items 30 and 31 will not usually be a cause unless the engine has been disassembled for overhaul and incorrect timing has resulted. Item 32 will not be a cause for several years after placing the engine in service if it gets proper maintenance.
Not all the trouble-shooting problems have been given to date in this series, but we believe the ones most likely to occur in the fire service use of the diesel engine have been listed. If you have any questions which have not been answered, we will provide the information you require on your request. A letter to FIRE ENGINEERING will get you all available information on your problem.
Technical data is through the courtesy of Mack Trucks, Inc.; Cummins Engine Company, Inc.; and Detroit Diesel Engine Division, General Motors Corporation.