IT’S always said that when buying a used vehicle, one reliable test is to check the exhaust.

As the engine burns fuel and creates exhaust, telltale signs on the state of the vehicle are shown up — things like burning oil, evaporating coolant and unburned fuel.

Paying attention to what’s coming out of the exhaust can give a good idea what problems an engine may be having and greatly aid in the decision whether or not to purchase a vehicle.

According to Internet website About.com, the smoke colour speaks volumes, and gives a clear indication about the nature of the fix.

White smoke or water vapour

If the white smoke or water vapour from the exhaust does not disappear after the car is warmed, there is a problem About.com says.

Possible causes:

Transmission fluid may be entering the intake manifold through vacuum modulator.

The Fix: Replace vacuum modulator

Cylinder head gasket(s) may be bad.

The Fix: Replace cylinder head gasket(s).

Cylinder head(s) may be warped or cracked.

The Fix: Resurface or replace cylinder heads. (Resurfacing is not a DIY job)

The engine block may be cracked.

The Fix: Replace engine block.

Black Smoke

Black smoke from the exhaust when the car is started. The smoke may or may not disappear after the car is warmed. The engine may be running rough or misfiring.

Possible causes:

If there is a carburetor, the carburetor choke may be stuck closed.

The Fix: Repair or replace choke.

Fuel injectors may be leaking.

The Fix: Replace fuel injectors.

You may have a dirty air filter: Replace the air filter.

There may be some other type of ignition problem.

The Fix: For vehicles with a distributor, check distributor cap and rotor. Ignition module may be bad.

Gray or Blue smoke

You notice gray smoke coming from the exhaust when you start the vehicle.

Possible causes:

The engine’s piston rings may be worn.

The Fix: Replace piston rings. (Not a do-it-yourself (DIY) job)

The engine’s valve seals may be worn.

The Fix: Replace valve seals. (Not a DIY job)

Damaged or worn valve guides.

The Fix: Replace valve guides. (Not a DIY job)

Now, armed with the correct diagnosis, the cost of the fix may be checked with a mechanic and a decision made on the wisdom of the purchase.