Smoke

(Difference between revisions)

Revision as of 23:08, 24 December 2011

What color smoke do you see?

• Black smoke
• White smoke that dissipates
• Bluish-white smoke that does not dissipate, but blows away with the wind

Black Smoke

Black smoke indicates a tuning problem, either the carburetor needs adjustment or repair OR ignition problems.

White Smoke

White smoke that dissipates is normally seen when first starting the engine on a cold day. All exhaust consists partly of water vapor, and until the exhaust heats up, it will condense into clouds of smoke.

This type of smoke quickly evaporates as soon as it comes out of the tailpipe, even as you watch it.

For white smoke that continues even after the engine is warmed up, the cause could be:

• Automatic transmission? Disconnect and plug the vacuum line to see if the problem stops. If it does, the vacuum modulator is leaking transmission fluid into the engine
• Coolant leak. Give the cooling system a pressure check. If it won't hold the test pressure, it could be a leaking intake manifold gasket (leaking into the engine, not outside) or a compromised head gasket. Or another coolant leak into the intake system.

Bluish-white Smoke

Bluish-white smoke that does not dissipate indicates a worn engine. This smoke does not immediately disappear like water-vapor smoke, but instead will stay in the air, eventually to drift away with the wind.

This indicates internal engine wear, either:

1. Valve stem seals leaking or worn valve guides allowing oil into the engine. This is evidenced by bluish-white smoke on engine startup, but after 30 seconds, no more smoke is evident. It suggest a cylinder head repair may be needed
2. Engine rings are worn or not properly seated. This causes bluish smoke either when you accelerate (compression rings), OR when you decelerate (oil rings). Either way it suggest an engine overhaul is required.

Engine Smoke Tests

When buying a used car, these are the critical mechanical tests:

1. Blow-by test
2. Engine rap test

Blow-by Test

While the engine is running, pull off the valve cover vent hose, and cup your hand around the back side of the opening. The hand serves as a background to see the faint vapors that may come out. A good engine will have no, nada, zero observable opaque vapor come out.

If you see any opaque faint smoke, this indicates engine ring wear.

You can also do this with the oil cap, but some engines spray a lot of oil droplets out the filler hole when the engine is running. In this case, ignore the oil droplets and concentrate on observing opaque vapor (or hopefully, lack of such).

Engine Rap Test

1. With the engine fully or partially warm, and running, set the parking brake 2. Open the car door and step outside 3. Walk around the back of the car and observe the location of the tail pipe 4. Go back and stand by the front door. While watching in the general direction of the tailpipe, reach in with your foot and rap the accelerator briefly three times. Rev it up each time to about 4000 rpm (heavy throttle, but only for as long as it takes to press the pedal, immediately let up). Practice this first on your own car before trying it on someone else car.

NOTE: car owners may get nervous when you do this.
So smile and let them see you are confident.

What are the results?

• Good engine -- no bluish-white smoke should be observed.

• Worn engine -- any amount of bluish with smoke, even small amounts, indicate the engine is worn

Even with severely worn engines rings, an engine may run very well and the car drive perfectly. Looking for the bluish-white smoke will indicate the true condition.