Why is Smoke Coming From My Generator’s Dipstick?

It’s gut wrenching, isn’t it?

You noticed your generator wasn’t running right and you turn it off to check all of the basics like the oil and

Smoke is coming from your crankcase when you pull out the dipstick!

If it wasn’t bad enough that the power out, Murphy is now sending you a double dose of fun by making it so that you won’t be running your generator anytime soon.

Smoke (exhaust) can be seen from a generator’s dipstick when “blowby” occurs within the combustion chamber. A blown head gasket, damaged piston rings, or a gouged cylinder can all lead to exhaust being pushed down into the oil crankcase.

I’ll get into how each of these things happen and what you should do about them.

Internally Blown Head Gasket will Cause Smoke (Exhaust)

Your combustion chamber is where — you guessed it — the gas and the air mixture combusts under pressure as the piston pushes up and a spark plug sets off the reaction.

This chamber is held together by the bottom half which is the engine block itself, and then a top portion which is called the cylinder head.

image of generator's combustion chamber and push rod housing
Combustion chamber above (half circle shown), head gasket is the dotted silver color, and the push rod housing is below which is normally oozing with oil for lubrication and it drains through the small hole in the bottom. If the gasket blows out in between, the oil will be sucked into the combustion chamber and exhaust will be shot back through and go into the crankcase which will cause you to see “smoke” when you pull the dipstick.

You can’t have metal against metal and expect an airtight seal that will contain enormous amounts of pressure, so a gasket known as the “head gasket” is placed in between and this serves to seal the combustion chamber from the outside air and to seal the combustion chamber off from galleries (small passages) that send oil from the crankcase to moving parts in the combustion process (namely valves and pushrods in this case).

UUnfortunately, sometimes the gasket gets weak over time and gives in to the pressures of the combustion chamber. Sometimes these blowouts happen externally, so that you could see the hot exhaust blowing out of a point other than your actual exhaust, or it can blowout internally between the combustion chamber itself and a parallel passageway (gallery) which serves to lubricate the pushrods or the valves.

An generator’s engine cycles at 3,600 RMP’s and a piston head is about the diameter of a coffee mug and travels up and down about the height of the same mug. That means that everytime the piston drops down it expands the surface area of the cocombustion chamber by the capacity of that mug.

Like a syringe valve stem being pulled down, it creates a huge vacuum and generally tries to suck that air (and gas) rapidly through the jets of the carburetor. If, however, you have an internally blown head gasket, it will suck in air (and oil) from the passageways that normally are used for lubrication purposes.

image showing the inside cylinder walls of a generator's combustion chamber
Every time the piston descends it sucks in air (or liquid) to fill the combustion chamber within a millisecond. It’s a very powerful pump when you think about it.

Once this oil gets sucked in, it is thrown in the combustion process. The result is a white smoke. It will be pushed out the exhaust like normal (and you’ll see plumes of it), but it is also pushed through the ruptured head gasket and back into the passageways that serve lubrication processes.

Since these passageways (galleries) carry oil, they necessarily originate from the crankcase where the oil is located. The compressed exhaust is therefore pushed into those passageways and down into the crankcase which each complete cycle of the 4-stroke engine.

When you go to pull the dipstick, you’ll be greeted by hot exhaust (smoke) and a crankcase that is running low on oil since it has been burning it up.

Before attempting to fix, shut off the fuel petcock, remove the spark plug boot, and turn off the engine start switch as a redundancy.

To fix your head gasket, you’ll need to separate the cylinder head from the engine and this may involve removing your gas tank, exhaust, carburetor, and other covers.

Then, take a few clean shop rags and place them in the galleries that lead to the crankcase so debris doesn’t fall into your oil.

You’ll then need to use a metal putty knife to separate the damaged head gasket from the engine or the head (whichever it has stuck to).

Then, I like to take a Dremel and a wire brush attachment and clean up any of the remaining gasket or carbon build up in the combustion chamber. Have the piston at the top of its stroke if you do this.

I then spray a rag with some carb cleaner and spray the areas that I cleaned up and wipe them down.

Place the new gasket on the engine and align it properly.

WARNING! Ensure that there is no debris (wire bristles, carbon flakes) on top of the piston or where the piston borders the cylinder wall. triple check this or you’ll soon have the next issue that we’re about to talk about!

image showing excessive carbon build up inside a generator's combustion chamber
Hard carbon buildup on the inside of the cylinder head. Over time pieces can break free and score a gouge in your cylinder wall if they get wedged between the wall and the piston.

Damaged Piston Rings or a Gouged Cylinder Wall Can Lead to Exhaust in the Crankcase

Then there is the issue of excessive “blowby” which is compressed exhaust from the engine firing and having some of that exhaust not blowing out the exhaust but rather blowing by the piston rings and down into the combustion chamber with the engine oil.

If you have a piston ring that is damaged, or the set of piston rings that are all aligned in the same phase of rotation (they all have a tiny gap since they don’t make a complete circle), then oil can more easily glide by and exhaust can do the same.

Make sure to run your finger along the inside of the cylinder wall to feel for any scored areas or gouges. If the head gasket looks good, and the cylinder wall looks good but you still have smoke from the dipstick, it’s time to look at the piston rings. I recommend having it serviced at that point unless you really love tearing into engines.

You will need to really tear into the engine to remove a piston and change the rings. It isn’t a job if you’re new to engine repair or unsure about what you’re doing. I recommend getting a small engine mechanic to give it a go if the cost doesn’t exceed the value of your generator.

If the piston rings are fine but you have a scored or gouged cylinder wall, then your piston rings will never be able to seal that up. Oil and exhaust will always have a clear pathway from the combustion chamber to the crankcase.

The cylinder wall can get damaged from the aforementioned reasons from the previous section. If someone attempted to clean the combustion chamber before with a wire brush and lost a bristle, or if a carbon chunk fell down (often by itself), then they will become wedged between the piston and the cylinder wall. At 3,600 RPM’s, it will quickly cut into the cylinder wall.

Once a cylinder wall is damaged in such a way, it is generally cheaper to scrap the unit and buy a new one. Check with a trusted small engine mechanic and get their input.

They might be able to get a new engine or bore out the cylinder and refit it with a new piston, but the cost will likely be more than what you could sell the generator for. It might be best to sell it to the mechanic for parts or you could do that yourself online.

Either way, once you have a damaged cylinder wall or piston rings, you shouldn’t be surprised to see exhaust or smoke coming out of the engine when you pull the dipstick.

Robert Van Nuck

Robert lives in central Michigan and enjoys running, woodworking, and fixing up small engines.

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