7.3 The radiator fluid should be clean and contaminant-free. Remove the cap and check its underside for foreign substances such as a light-brown, foamy residue. This mousse-like coating is a sure sign that oil has contaminated the coolant system and indicates a possible blown head gasket or, even worse, a cracked or warped cylinder head.
8.1 Listen for any unusual noises emitted from the engine while the vehicle is running. There shouldn't be strong noises coming from the engine under any condition-no matter its temperature. A constant knocking or tapping sound during cold starts indicates poor engine maintenance, unless the engine is using a set of high-silicone-content aftermarket pistons. Knocking, tapping or rattling noises indicate excessive internal wear, while whistling may be caused by a loose belt. If the engine makes too many noises, avoid buying it.
8.2 Turbocharged vehicles are plagued with numerous problems if regular maintenance isn't performed. Access the turbine housing and give the turbine wheel a quick spin with your finger. If the wheel feels frozen in place, move on and look for another vehicle. If you push in and pull out the input shaft, does it wobble or produce excessive play? Chances are the input-shaft bearing has been worn-down or damaged. If unattended, this poses a threat to any engine. When you pull the intake hose off the compressor housing, do you see a liberal amount of oil coating the inside of the compressor inlet and intake hose? The turbo oil seal might be damaged due to issues such as contaminated oil or oil buildup from the owner not cooling off the oil and turbo before shutdown.
8.3 The most common and affordable way to check the engine compression is to use a compression gauge or compression checker, which is sold at any auto parts store for $30 to $50. Testing should display identical compression numbers across the board for each cylinder. If one cylinder displays significantly lower than the rest, use the oil method as a test. Pour a capful of oil into the spark plug hole, re-insert the compression checker and retest the cylinder. If the number increases more than 30 psi, the piston rings are worn-out. If you want a more thorough engine analysis, use a leak-down tester to pinpoint any engine problems that lie ahead. Unlike a compression tester, the engine doesn't need to be cranked over; instead, the leak-down tester pressurizes the cylinder to a certain pounds-per-square-inch, measuring (blow-by) the amount of cylinder pressure lost through worn piston rings, faulty valves or a blown head gasket. If you lack the skills to perform this test, take the vehicle to any local mechanic who can test it for a reasonable price.
8.4 You glance at your rearview mirror and notice plumes of smoke bellowing from the exhaust pipe as you let the vehicle idle for five to 10 minutes. Ask a friend or the seller to keep the rpm consistent from 4500 to 5000 rpm for 2 to 3 seconds while you study the engine.
8.4a Blue smoke is caused by engine oil entering the cylinder area and burning along with the fuel/air mixture. This problem is associated with higher-mileage vehicles or faulty piston and oil rings from excessive wear.
8.4b White smoke is caused by water and/or antifreeze entering the cylinder and the engine trying to burn it with the fuel. If white smoke is present, check to see if the proper amount of antifreeze is inside the radiator and the overflow bottle. Also check to see if antifreeze has contaminated the engine oil.
8.4c Black smoke billowing out the tailpipe means the engine either consumes too much gas and is running rich, or has a faulty O2 sensor or possible problems with the fuel injection. If the vehicle is running an aftermarket engine management system or upgraded fuel system, ask the seller when, if ever, the car was properly tuned.
8.5 Test-drive the vehicle on the local streets and highway. If the car shakes, smokes or hesitates while attempting to return home, simply thank the seller and move on to the next vehicle. Don't buy a vehicle knowing you'd have to drop a load of cash to get it back to its normal condition.
9.1 Does the oil on the dipstick have a watery look? Smell the dipstick to determine if the culprit is coolant or gasoline mixed with the oil-either way you're in for a serious engine overhaul. The mixture of gasoline and oil exists because the engine flooded or cylinder walls in the block cracked. Fuel contamination dilutes oil. Because the gasoline breaks down the oil, it loses its viscosity and causes the cylinder walls to scorch.