“The Smoking Gun “ ~ Interpreting Diesel Exhaust Smoke:
Diesels are simple in concept
and much simpler than gasoline engines, since they have no ignition and associated timing or electrical
issues. They rely on two key principles to operate; they use very high compression
to heat the fuel
air mixture to it's flash point to gain ignition, and use a highly compressed stream of atomized fuel
injected directly into the combustion chamber. Additionally, a major difference between gasoline engines, is that diesels rely on a "governor" to attain and maintain engine
speed. In a marine
, you apply throttle to provide additional power. Doing so adds a specific amount of fuel. In a diesel
however, when you apply "throttle" you are really advancing the governor control. Its job is to maintain the set RPM
as dictated by throttle position. If the load on the engine changes, the governor will automatically call for more fuel (or less) to meet the load requirements. This difference between gasoline and diesel engines may seem subtle but as you see when troubleshooting it will play a key role.
The diesel fuel system consists of a low pressure fuel pump
, which maintains a constant supply to the high pressure (injector) fuel pump
. In a four cycle diesel engine, the high pressure pump is a piston like arrangement that pumps each rotation of the crankshaft. The injectors located at the cylinder head
have a check valve that prevent fuel from being pumped into the cylinder during the compression
cycle and also prevent exploded combustion gases from being forced into the fuel system. Fuel not used during "off cycles" is returned via a return line to the fuel tank
The injector system operates under very high pressure (approximately 3000 PSI). Consequently, only rudimentary testing of these devices can be performed without special equipment
Diesels need only two things to run:
1. Adequate fuel and air (at the right time and quantity)
2. Adequate Compression
Since you have no ignition, the only way to stop a diesel is to eliminate one of the above (typically by shutting off the fuel supply). The only way to start one if you have BOTH these thing!
Let the smoke tell you what's going on in your engine!
Presuming it will run, you can tell a lot about what is going on in the engine by looking at the exhaust
. Many problems, either present or slowly coming on can, be diagnosed by the exhaust smoke.
All diesels will throw a little excess black smoke when accelerating. That is normal as it takes the system a little time to develop the RPM
and power necessary to burn all the fuel being provided. Remember, the governor immediately provides whatever fuel is needed to get to a given RPM. If the engine can't get to that RPM, the governor just keeps throwing more fuel at the engine to try!. If the engine smokes much more than a very faint cloud during operation it means something is probably not right. Here is what to look for:
Black smoke is generally be caused when the fuel does not burn completely, and the partially burned particles are expelled in the exhaust. When this occurs it means the engine cannot reach the RPM that is being called for by the governor ,in one or more of the cylindesr. There are two reasons for this:
1. The load exceed the power capability of the engine
2. A Load within the power capability of the engine cannot be handled because of something wrong in the basic engine system (IE compression or air/fuel mixture)
The first thing to check for is an air inlet restriction such as from a plugged air cleaner. Take off the air filter and make sure the intake port is clear. If the engine still smokes with the port clear and the filter off, the problem is elsewhere.
is also a potential common cause for black smoke as is incorrect engine timing. For example in a two cylinder engine, if you have one bad injector, the system cannot develop the RPM to burn the fuel called for since the properly fueled injector is working against an essentially dead or less than efficient cylinder. A poor cylinder will put out black smoke if it is getting fuel, because compression problems prevent complete combustion. The good cylinder will put out smoke, because it is being supplied more fuel than it can effectively use. You may be advised to remove the injectors and have them checked at a competent shop.
To check compression, you will need a compression gauge that can measure pressures over 500 PSI. Additionally, since diesels have no spark plug
holes through which you can test compression, you must accomplish this be removing the injectors and using those ports
. You will need a special fitting to use the injector port from testing. If you have a compression problem, it could be caused by bad rings/cylinders, valves or valve guides (or unusually, a hole in a piston).
Typically, if you have a problem with valve guides, you will have a lot of "blow by".
Ie: The crank case will be pressurized, and you will see pulsing puffs of exhaust coming out of the engine crankcase breather.
You can determine if you have bad rings/cylinders only by inspection
. However, if you pull the head
and find the valves in good condition, you'll know they are bad. By the way, before you pull the head, check for valve clearance and make sure all of the valves operate.
White smoke usually occurs when there is not enough temperature to burn the fuel. The unburned fuel particles are then exhausted, usually with a rich fuel smell. In cold weather
, it is not uncommon to get white smoke until engine temperature builds up. Low engine cranking speed can also create an excessive amount of white smoke.
If the problem persists, after the engine is up to operating temperature, several other things should be checked. A faulty injector can cause white smoke. Timing is often a factor when white smoke is excessive. Low engine compression can cause the problem and the injection pump can also have problems that result in white smoke. Air in the fuel system can also result in white smoke.
Lastly, steam (caused by a head gasket
or other water
leak into the combustion system) can disguise itself as smoke. Steam will dissipate rapidly once it hits lower temperature ambient air, while smoke will persist in the air.
Blue smoke is a sign of lubricating oil
being burned by your engine. If you have blue smoke, mainly at start up (and it is very minor while running), this points to worn valve guides. When the engine sits for a bit (over night etc), oil
left in the head after running can seep down into the combustion chamber. Upon start the bulk of it is burned quickly.
If you see blue smoke all the time when running, odds are you have a problem with rings or cylinders or both. Although, sometimes a little solvent in the cylinders (left overnight) may free rings stuck up with carbon, usually, you will have to pull the engine and rebuild
it to get things back to normal.