Blue smoke from the exhaust: is my car burning oil?

Every engine burns a small amount of oil, which is why manufacturers publish acceptable consumption figures. Many allow up to a quart every 1,000 to 2,000 miles before they consider it a fault. The trouble starts when consumption becomes visible as smoke, or when the dipstick reads low long before the next scheduled change.

Oil reaches the cylinders by only a few routes: past worn valve stem seals at the top, past worn or stuck piston rings at the bottom, through a crankcase ventilation (PCV) system that is pulling oil mist into the intake, or through a turbocharger whose shaft seals have worn. Each has a different timing pattern and a very different repair bill, so a careful look at when the smoke shows up saves money.

One related problem gets mistaken for burning oil: oil dripping onto a hot exhaust manifold. That produces smoke from under the hood, not from the tailpipe, and it is usually a cheaper gasket repair.

Symptoms

• Blue-gray puff from the tailpipe for a few seconds after a cold start
• Smoke when you accelerate hard or merge onto the highway
• Smoke after coasting downhill or decelerating, then pressing the gas again
• Oil level falling between changes with no drips on the ground
• A sharp, burnt smell that clings to the garage after you park
• Oily black soot inside the tailpipe tip
• Misfire codes or rough running from oil-fouled spark plugs

Likely causes and what they cost

Costs are typical South Florida ranges for parts and labor, for budgeting only.

What you can check yourself

1. 1
   Note exactly when the smoke appears

   Have a friend watch the tailpipe, or check your mirror, at three moments: the first start of the day, a hard acceleration from a stop, and pressing the gas after a long coast. Write down which one produces smoke. That one observation narrows the cause more than anything else you can do at home.

2. 2
   Run a simple oil consumption test

   On level ground with the engine off for 10 minutes, top the oil up to the full mark and write down the odometer reading. Check the dipstick every 500 miles under the same conditions. More than about a quart per 1,000 miles is worth raising with a shop, and the log is exactly what a dealer will ask for if your engine is covered by an extended warranty program.

3. 3
   Rule out an overfill

   If the smoke started right after an oil change, check that the dipstick is not well above the full mark. An overfill of a quart or more is enough to cause smoke on some engines.

4. 4
   Look for under-hood smoke instead

   Open the hood after a drive and look for wisps rising from the engine, and for wet, dark oil around the valve cover edges. Do not touch the exhaust manifold, it can be hotter than 500 F. If the smoke is coming from up there rather than the tailpipe, the oil leak guide is the better place to start.

5. 5
   Check the PCV hose if it is easy to reach

   On older engines the PCV valve sits in a rubber grommet on the valve cover. With the engine off and cool, pull it out and shake it. It should rattle. Hoses that are cracked, collapsed or soaked in oil are a cheap fix worth doing first.

When to take it to a shop

• You are adding a quart or more every 1,000 miles.
• The smoke is visible in normal driving, not only at startup.
• The check engine light comes on with misfire or catalytic converter codes such as P0420.
• Your car is a turbo model and the smoke appeared suddenly with a loss of power or a siren-like whine.
• Before paying for major work, ask for a compression test and a leak-down test so you know whether rings or valves are the source.

The Florida factor

Florida has had no state emissions inspection since 2000, so an oil-burning car can stay on the road here for years without anyone forcing a repair. That is convenient until the burned oil glazes the catalytic converter, which typically costs $1,000-$2,500 to replace on a common sedan. If you are shopping for a used car locally, a blue puff on cold start is worth asking about, because no inspection report will flag it.

Seasonal residents see a specific version of this problem. A car that sits from April to November lets oil slowly seep past tired valve stem seals into the cylinders, and the first start in the fall produces a startling blue cloud that clears in a minute. If that is the only symptom and the oil level holds steady afterward, the seals are aging but the engine is not in crisis.

Long, slow stretches of Glades Road and Palmetto Park Road at rush hour also add up. Idling in 95 F heat with the A/C running keeps under-hood temperatures high and oil thin, and engines with stuck rings tend to burn noticeably more oil in summer traffic than on a winter highway run.

Notes for specific makes

• Toyota: Toyota's warranty enhancement program ZE7 covered excessive oil consumption on certain 2AZ 2.4L engines, including 2007-2009 Camry and 2009-2011 Corolla models among others, with a piston assembly replacement. Secondary coverage ran 10 years from first use or 150,000 miles, so most cars have now aged out, but a dealer can confirm by VIN.
• Subaru: Subaru extended powertrain warranty coverage to 8 years or 100,000 miles for excessive oil consumption on some FB-series engines, including certain 2011-2014 Forester and 2012-2013 Impreza models, after a class action. Eligibility required a dealer oil consumption test, and by now the 8-year limit has passed for every covered model year, so the history mainly matters when you are judging a used one.
• Audi: The 2.0L turbo four used in many 2009-2011 A4, A5 and Q5 models became known for ring-related oil consumption and led to a settlement that extended warranty coverage. A later lawsuit made similar piston ring claims about 2012-2017 Audi 2.0T engines. A PCV valve failure on these engines can mimic ring wear, so it is checked first.
• Honda: V6 Accord, Odyssey and Pilot models with Variable Cylinder Management from roughly 2008 to 2013 drew many complaints of oil consumption and oil-fouled spark plugs, and a class action settlement extended some coverage. Plugs that keep fouling on the same cylinders are the pattern to mention to a shop.

Frequently asked questions

More on this job: Engine Repair in Boca Raton, costs and how to choose a shop