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[Datsun 1200 encyclopedia]


From Datsun 1200 Club

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Categories: Engine Mechanical | Diagnoses and Corrections

What color smoke do you see?

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



A smokey motor is typically a result of worn piston rings (poor compression is the big indicator of this) or worn valve guides. Also if the valve stem seals are worn you may also see an increase in white smoke.

Worn piston rings lead to the engine losing power in the compression stroke as much of the combusted power is lost via leakage past the rings, oil will also find its way into the combustion process resulting in 1) smokey exhaust and 2) excessive crankcase blowby (PCV) which also recirculates back into the induction (smokey, oil laden vapors get recycled back into the induction cycle).

Some engines do not belch a blue cloud all the time. The biggest oil cloud may occur after slowing via the transmission (5th, 4th, 3rd, 2nd, 1st) to a stop. Then taking off... cough... cough. The high engine vacuum will suck oil past the guides into the combustion chamber if the guides are worn or if the seals are bad.

Generally, if if it is continuous blue smoke at steady rpm it will be from the rings. Off idle and cruise it can be valve stem seals. Black smoke indicates fuel which can also cause oil to burn in extreme cases.

By "blue" smoke people mean white smoke with a slightly bluish tinge -- that's a sign of oil burning. Pure white smoke is normal when warming up the car on a cold day.

worn valve guides typically will not cause smoke while running the engine, but rather a small or large puff on starting the engine. If it blows blue smoke while driving this is usually worn piston rings . Do a wet & dry compression test to make sure.

Oil smoke could be caused by other things, even an overfilled crankcase. The A-series only takes 3 litres of oil.

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. If a lot of vapor comes out it means either extreme ring wear, or improperly seated rings which sometimes occurs after a rebuild.

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.

Coolant System Test

For white smoke, give the cooling system a pressure test. A test includes:

See main article: Cooling System Pressure Test

Cylinder Leakdown Test

If you have an air compressor, you can attach the air line to the spark plug hole and see if the cylinder holds pressure well.

  1. Remove radiator cap
  2. Remove oil cap
  3. Remove distributor cap
  4. Turn engine by hand until the rotor point points to a particular plug wire, and the point just barely open (or for Electronic Ignition, until the teeth line up)
  5. Remove that particular spark plug, screw in the air adapter and pressurize the cylinder

If the pressure holds, repeat the steps for the next cylinder.

If the pressure does not hold, using a long hose, put one end to your ear, and move the other to various spots, listening for the air leak:

  • If air bubbles appear in the radiator, it indicates a Blown Head Gasket
  • If air is leaking out of the intake manifold (from carburetor throat), there may be a burnt or stuck intake valve
  • If air is leaking out of the exhaust manifold or tailpipe, it suggest a burnt or stuck exhaust valve
  • If air is leaking out the oil cap hole, there is an internal leak, perhaps a pinhole leak in the timing chain cover/water pump casting 

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