Engine trouble isn't something you should ignore. Repair is sometimes possible if engine troubles are addressed early, but more severe issues may still require an engine rebuild. These are some of the most common signs that there is engine trouble under your car's hood. 

1. Knocking Noises  

Once you hear the engine knocking it may be too late to avoid all damage. Knocking that is a result of low engine oil levels or issues with the oil pressure in the engine can cause irreversible engine damage thanks to a lack of lubrication. Other causes of knocking include using the wrong octane of fuel and worn-out engine bearings. 

2. Burning Engine Oil

Does it seem like you are constantly adding oil to your car? If so, there could be engine trouble in the making. Usually, cars don't require the addition of more oil between oil regularly scheduled oil changes. An oil leak in the engine may not always be immediately visible, as the oil often burns off on the engine's surface before it can drip onto the ground to leave a telltale puddle. Engine leaks are often around hoses or valves that can be replaced, but if you wait too long more severe damage can occur.

3. Dirty Spark Plugs

Pulling the spark plugs for a quick inspection can provide a clue that engine trouble is developing. Black, wet deposits on a spark plug's threads indicate that there is oil burning off in the combustion chamber, and oil shouldn't be leaking into the combustion chamber. The culprit is likely worn piston rings or failing seals around the engine valves, which can be replaced in order to solve the problem.

4. Coolant Condition

Engine failure affects other components of the car, and one of the first to suffer issues if an engine's head gasket is having problems is the engine coolant system. Cylinder pressure from the engine can cause pressure issues in the radiator that lead to coolant leaks. Overheating is an issue if you don't top off the coolant levels constantly. If the problem isn't in the coolant system, then look to the engine for the cause.

5. Engine Test Failure

If you have some car knowledge, you can do a DIY compression test using a special gauge to look for pressure differences between engine cylinders. Excessively varying pressures indicate engine trouble. Another option is to take your car to a trusted mechanic for a full leakdown test, which will find any engine leaks and trace them to their cause.

Contact an engine repair service if you suspect major engine trouble is the problem with your car.