Common Reasons Why an Engine Smokes

A smoking engine is a serious thing.  It can be caused by quite a few different problems, none of which should be ignored.

 

Engine Smoking Causes

This list of common engine smoking causes is presented in order of likelihood from most to least likely to be the problem.

 

1. Oil Leak From Valve Covers

One of the most common reasons why an engine would smoke is leaking valve covers.  When this happens, oil leaks directly on to the exhaust.  There’ll be oily sludge on the head around the valve covers.

The color of the smoke coming from the engine will likely be blueish black, and burning oil smells bitter.

 This is usually caused by a bad valve cover gasket, and not the valve cover itself.  

It can also be caused by the valve cover bolts not being torqued down enough.  You’ll need to look up the exact torque specs for your valve covers.  This is a great place to start if you notice oil leaking from the valve covers.

 

2. Overheating

If your engine is overheating it’ll smoke as the temperature rises.  Compared to oil, the smell will be sweeter.  The smoke will be a pure white color. Overheating is typically caused by:

• Bad Thermostat
• Leaking Radiator
• Bad Radiator Hose

 

3. Leaking Radiator Cap

When a vehicle overheats, the radiator builds up pressure.  When the pressure gets to a certain level, the radiator cap will allow it to escape.  This saves the radiator from permanent damage.

When a radiator cap goes bad, it relieves pressure too early.  As in, it lets coolant out within the normal operating temperature of the vehicle.  As the pressure is relieved, it’ll spray on the engine and cause a white clear smelling smoke.

 

4. Exhaust Leak

If your exhaust is leaking from the manifold, there will be smoke coming up from under the hood.  An exhaust leak that is large enough to produce smoke coming from under the hood is going to be accompanied by a LOUD tapping sound

 

5. Melting Electrical Wiring

This happens more often that you might think.  Modern engines have large wiring harnesses and if these harnesses rub up against something for long enough it can cause the harness to melt.

Here’s why a wiring harness could melt:

• Rodents can get under the hood (particularly in winter) and cause a lot of damage to the wiring harness
• Damage from road debris
• Wiring harness too close to exhaust

When the wiring harness melts, you’ll notice the smell of melting plastic.  And, you need to be aware that melting plastic can cause an engine fire.  If the wiring harness melts past the point of usability, whatever part that melted may no longer be available to your vehicle, which can leave you stranded even if there is no fire.

 

Engine Smoking Diagnosis

You want to start by making the best observation of the smell and color of the smoke.  This will help you make quick work of figuring out what’s going on.

 

White Smoke, Smells Sweet

If the color of smoke coming from the engine is white, that is engine coolant.  You’ll need to take a look at the cooling system and see where it is leaking from.

Wait for the engine to cool off and see if you can see where the coolant was coming from.  We don’t recommend checking when the engine is running since leaking coolant will often squirt you.

If there is no noticeable leak, you can buy dye that you can put in the radiator to see where the leak is coming from.

 

White Smoke, Smell Like Exhaust

This would indicate that there is an exhaust leak under the hood.  For an exhaust leak to make the amount of smoke that you would be able to visibly see, there would need to a large exhaust leak, and that leak would be loud.

The most common place that exhaust leaks from the engine is where it bolts to the cylinder head.

 

Blue or Black Smoke, Burning Oil Smell

When the smoke is dark, but smells like burnt oil, it indicates an oil leak.  Make sure that the oil cap is twisted all of the way on.  It only takes a moment to check.

If that’s not the cause take a good look to see where it is leaking from.  Oil can leak from any seal on the engine block, but you’ll be looking for a leak from somewhere that can drip onto the exhaust.  

 

Black Smoke, Plastic Smell

This indicates that the wiring harness may be melting somewhere.  It’s usually held in a protective casing.  Follow it and see if you see any points where it could be melting.  Start with looking by the exhaust and radiator.