1. Look at the appearance of the engine
Before the vehicle is not started, you can first static check the engine. First of all, open the engine gas cap, observe the engine appearance of the state of good and bad, observe the internal cylinder whether there is the accumulation of carbon deposits, to see whether the engine is clean, neat and tidy, it can be said that the maintenance of the vehicle engine is intact, and then pull out the oil gauge to see whether there is a lack of oil. Observe whether there is any oil leakage, water leakage and other faults in various parts as well as the aging of various parts.
Look for engine oil leaks:
If it is the valve cover leakage, it is generally not a big problem, and it is also relatively easy to deal with, but some models of the valve cover is particularly easy to leak oil, and it is difficult to deal with cleanly, so such models should also be avoided as much as possible.
If the crankshaft before and after the oil seal leakage, it is more troublesome to deal with, you need to dismantle the engine front cover or dismantle the gearbox. There are also some engines that have had their cylinder heads removed, which will also leave traces, so check carefully.
Look for parts aging:
Check whether there are fine cracks on the outer wall of the water pipe, and whether its head has cracked, if so, it means that the engine has been used for a long time, or the engine was once high temperature.
Check the aging of the generator belt, if the belt are fine cracks, indicating that the aging has been serious, so the engine will have to be careful.
2. Listen to the engine with or without abnormal sound
Usually, a normal engine in operation, only a slight combustion noise and mechanical friction sound, there will be no mechanical impact sound. In addition to the sound, can be judged as abnormal noise. When we check, we have to check the rattling sound of the engine in the cold start stage and under normal operating temperature respectively, and also check the rattling sound under each speed. If the engine’s speed is not constant and unstable, and appears to go up and down, it means that the vehicle’s engine’s speed is unstable.
An engine rattles may have dozens of hundreds of different parts, different sounds, different temperatures of the rattles, respectively, reflecting different failures, so check the engine rattles is a very professional work, the average person just listen to the sound may be very difficult to distinguish exactly where the problem, so only the maintenance of quite experienced people can distinguish.
3. Observe the state of the exhaust gas
Normal engine exhaust is colorless, odorless, moderate temperature, pulsation uniformity, if there is an abnormality indicates that the engine is working abnormally. So we have to check and judge from the exhaust color, smell, temperature and exhaust pulsation of these aspects.
Look at the exhaust color: when the engine has some faults, the exhaust will show black, blue, white and other abnormal colors. It is generally believed that the exhaust is black is too thick mixture, blue is burning oil, white is the engine combustion chamber into the water.
Smell exhaust odor: If you smell the pungent odor through the exhaust, it means that the engine’s technical condition is abnormal, such as the use of poor fuel quality, the use of gasoline additives, the three-way catalytic converter does not work well, burning oil, cylinder pressure is too low and so on.
Measure the exhaust gas temperature: Normal engine exhaust gas temperature is not high, the hand in the place of the exhaust port will feel hot but not hot. But if you feel the exhaust temperature is very high and hot, it may be too thin mixture, ignition advance angle is too late, part of the cylinder combustion is not good and other faults.
Look at the exhaust pulsation: normal car exhaust emissions are very uniform, the more the number of cylinders, the smaller the pulsation of the exhaust. If the exhaust pulsation is large or there is obvious “muttering” sound, that the engine combustion abnormalities, such as the engine is missing cylinders, the cylinder work is not balanced, the ignition advance angle is not allowed to adjust the mixture is too thin and so on.
Look at the state of the exhaust pipe: the normal exhaust port is dry, clean, and even water discharged when the engine is running. If the inner wall of the exhaust port is black, feel the black carbon smoke, indicating that the engine combustion is not good, the car fuel consumption may be relatively large. If there is a thick carbon on the inner wall of the exhaust port, feel the feeling of oil stains, it means that the engine burns oil seriously, so we have to avoid the car.
4. Look at the ventilation in the engine crankcase
Engine at work, there will be a part of the combustion chamber exhaust from the piston and the gap between the cylinder wall to the crankcase, also known as the lower exhaust. The better the engine seal, the less under the exhaust, and vice versa, the more under the engine exhaust, it shows that the engine wear and tear is serious, the service life is not long, should be overhauled.
5. Observe the engine idle speed
When we buy a used car, we will choose to test the car, do not step on the gas in the driving process, in place to keep the vehicle idling. Vehicle cold start, idle speed is relatively high, usually kept at about 1000 rpm, because the cold start of the vehicle oil temperature and engine temperature has not risen. To make the engine to reach the normal operating temperature, after reaching the required operating temperature, the idle speed speed back to the usual stable speed, maintain about 700-800 rpm, can be stabilized between this speed, indicating that the engine has no problem.
If the engine shakes badly at idle, there are shaking and rattling faults in the speeding process, and there is backfiring when decelerating, etc., it means that the engine is not in good working condition.