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Car Care Advice

8 Reasons For Blue Smoke From Exhaust (Plus How To Diagnose)

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Blue smoke from exhaust — should you be worried?
It means that oil, or another vital fluid, is mixing with the fuel inside your engine. This points to a faulty engine component that needs urgent repairs, or you risk further damage.

Worry not!

This article will explain all the causes of blue exhaust smoke, how to conduct a proper diagnosis, and the potential cost of repairs needed.
As a bonus, we’ll also cover some common FAQs about exhaust smoke.

This Article Contains:

Let’s dive right in.

8 Causes For Blue Smoke From Exhaust

Excessive blue exhaust smoke can present itself for several reasons.
Here’s what to look out for:

1. Blown Head Gasket

Blown head gaskets are famous for leaking excess oil onto hot surfaces. 

Blue smoke indicates that lubricating oil has made its way into the hot exhaust system or other connecting engine parts.

2. Oil Leak

Oil leaks from engine components such as gaskets, worn engine oil seals, cylinder heads, and the engine block are common. Leaks can drip onto hot surfaces, like the cars exhaust and drivetrain, which causes burning oil.

When this happens, you can expect blue smoke and a nasty smell. You’ll likely notice this blue smoke when the car is idling or accelerating.

3. Oil Mixing With Fuel

Unfortunately, engine parts like a piston ring and valve seal don’t last forever.

If you don’t follow a rigorous oil inspection routine, the oil will do a poor job of reducing engine friction. Excessive engine heat or friction can damage a gasket and valve seal over time.

Damaged piston rings and valve stem seals will inevitably leak excess oil into your engine’s combustion chamber, allowing it to mix with the fuel. 

When this mixture burns in the combustion chamber, it turns the normal grey exhaust smoke blue.

4. Stuck PCV Valve

Another likely cause for excessive blue exhaust smoke is a stuck positive crankcase ventilation valve (also called a crankcase breather).

The crankcase breather is responsible for releasing fuel pressure build-up in the crankcase. This fuel pressure is diverted into the intake manifold for fumes to be re-burned.

When the PCV valve gets stuck, it forces lubricating oil to mix with pressurized air and other gases. This fume mixture eventually gets burned, creating blue smoke.                                    

5. Blown Turbocharger

If your car has a blown turbocharger and emits blue smoke, it’s not a coincidence.

You’ll likely notice a large blue cloud of smoke right as your turbo blows. This results from a damaged turbo casing or a broken oil seal inside the turbo itself. Both causes allow oil to seep into the engine’s air intake manifold, causing oil to mix with the fuel.

6. Faulty Transmission Modulator

Commonly found in older vehicle models, a transmission modulator controls shifting in vacuum-controlled automatic transmissions.

A faulty component in the transmission modulator, like a failing diaphragm, can cause the engine block to pull in transmission fluid. 

Here’s the kicker.
Burned transmission fluid creates blue smoke!

7. Bad Glow Plug (Diesel car)

If your diesel car produces blue smoke during start-up, there’s a good chance you have a bad glow plug. Another common sign of a bad glow plug in a diesel engine is longer cranking times than usual.

8. Cylinder Head Valve Guide Issue

If you notice blue smoke happening during deceleration, you probably have a cylinder head valve guide issue.

This means that the valve stem (valve guide) is leaking oil out of the cylinder wall of the combustion chamber. Oil may drip from the valve stem onto the exhaust pipe, and in some cases, even the fuel injector. 

Both instances result in either blue smoke or black exhaust smoke, depending on whether the oil has mixed with fuel or not.

With a firm understanding of the causes of blue exhaust smoke, we can now cover its diagnostics procedures.

Blue Exhaust Smoke Diagnosis And Possible Repairs

The diagnostic procedures for blue exhaust smoke are pretty straightforward, and a qualified technician can complete them in good time.

Here’s what your technician will do:

Step 1: Head Gasket Inspection

A blown head gasket is the most apparent reason for blue exhaust smoke. 

Tell-tale signs of a blown head gasket include coolant fluid under the car, engine overheating, milk-colored engine oil, and poor performance.

Aside from the previously mentioned signs, a mechanic will also lift the oil filler cap as a final foolproof check. If the head gasket is damaged, the cap will be covered in a milky brownish-yellow coolant mixture.

Step 2: Check For Leaking Engine Oil

A mechanic will check the car’s oil level with a dipstick. If the engine oil is noticeably low, they’ll check under the vehicle for any visible oil leak. 

If a leak is located, the mechanic will do further inspection to see which parts might need replacing or repairing. If there’s an engine oil leak, it’s most likely worn engine oil seals, piston rings, or valve seals ( valve stem steals.)

Step 3: Valve Seal And Piston Ring Inspection

Both faulty valve seals and faulty piston rings can cause leaking oil in the combustion chamber.

Luckily, it’s straightforward to figure out!
If your exhaust system only expels blue smoke briefly after an engine starts, valve seals are the culprits. 

If the smoke persists long after the engine starts, you have bad piston rings.

Step 4: PCV Valve Inspection

The PCV valve is typically attached to the valve cover and connects to a tube or hose.

All that a mechanic needs to do is remove the PCV valve and give it a stern shake. If they hear a metallic rattle, the valve is still in good working order.

However, if no sound is heard from inside the PCV valve, it has seized and is no longer functioning correctly. You might be able to clean a PCV valve by submerging it in a carb cleaner. However, it’s often best to replace it.

Step 5: Transmission Module Inspection

To test the transmission module, a mechanic will first inspect the car’s transmission fluid level. If the transmission fluid level is noticeably low, it’s cause for further inspection.

The mechanic will lightly accelerate the car from a standstill until it reaches around 25 mph. 

Depending on the engine sound, how easily the transmission shifts gears, and rev speed, they can deduce whether the transmission module has failed.

Step 6: Turbocharger Inspection

A damaged turbo can easily be identified with a visual inspection of its casing. 

Other mechanical signs of a damaged or failing turbo are:

Repair Costs

Some estimated repair costs (inclusive of labor) for all possible fault parts that cause blue exhaust gas are as follows:

Now that you have the ins and outs of blue exhaust smoke under your belt, let’s answer some common exhaust smoke FAQs next.

3 Exhaust Smoke FAQs

Here are some answers for a better understanding of exhaust fumes: 

1. Can I Still Drive With Blue Exhaust Smoke?

We strongly advise you not to drive your vehicle when it emits blue exhaust smoke.
Prolonged driving with blue exhaust gas can cause permanent (and expensive) damage to your engine internals. 

Best case scenario?
You’ll only need to repair an oil leak.

Worst case scenario?
You’ll need to replace your entire engine.

2. Is Black Smoke Or White Exhaust Smoke Normal?

White smoke or grey smoke, which is thin in density, is normal.

Grey smoke and white smoke are caused by water vapor in your exhaust pipe evaporating during start-up. You’ll commonly see white smoke during cold starts in the morning when water vapor commonly collects on cars. 

Black smoke is caused by your engine running too rich (improper air to fuel ratio). If you’re experiencing black smoke, you might have a vacuum leak.

3. Is Exhaust Smoke Harmful?

Short answer —Yes.

Your cars exhaust fumes are essentially carbon monoxide, a poisonous gas. Carbon monoxide can irritate the respiratory tract and eyes and even cause death. Long-term exposure (20 years or more) can increase the risk of developing lung cancer.

It’s no joke.

Ensure you are in a well-ventilated space when working with a running car so exhaust fumes can dissipate and not cause you harm.

Final Thoughts 

Blue exhaust pipe smoke can be worrying at first, but fear not!
Thorough diagnostics, high-quality repairs, and general maintenance will make blue exhaust smoke a thing of the past. 

But who do you contact when you’re experiencing blue exhaust smoke?
RepairSmith!

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Contact RepairSmith for an accurate cost estimate to diagnose the blue exhaust smoke and other auto repairs.