Whether you notice constant engine stalling or poor performance in your car, one of the common underlying causes could be low compression. But, with so many other engine-related issues sharing similar-ish symptoms, how can you tell if you really have low compression? Thankfully, there are easy ways to diagnose low compression and isolate one diagnosis from the next.
Remember that compression is an important part of the internal combustion process, whereby each cylinder should have a good seal so that compressed air doesn’t escape. All pistons and valves need to be air-tight when it comes to sealing if you want good engine performance. If there is no seal within the cylinder, this will produce something that is known as blowby.
Blowby is actually pretty common on higher mileage engines. This means that some air that is escaping through the piston rings and then enters the crankcase or escapes through the valves. The more blowby an engine has, the poorer its performance will be. Eventually, the engine will stop working or will start to produce low compression symptoms that are annoying to deal with.
Symptoms Of Low Compression
With that being said, if you’re trying to confirm whether or not your car’s engine has low compression, here are some symptoms to be wary of:
1) Check Engine Light
First off, when your car’s engine is experiencing low compression, you’ll notice symptoms like a check engine light (CEL). Modern cars are highly computerized, as the ECU monitors the operation of your engine and adjusts performance based on readings it gets from various sensors around your car. However, there are no sensors that directly measure the compression in the engine.
Instead, if there’s low compression in your car’s engine, there are plenty of other sensors that might get triggered. As a result, it’ll show you the check engine light. You can then use an OBDII scanner to make a thorough diagnosis and figure out what’s exactly wrong.
2) Engine Misfires
Engine misfires are a very common symptom of low compression. Remember that engine compression is its ability to seal. So, whenever there’s no proper seal from the valves, there’ll be unequal combustion inside the cylinders. A bad chain of combustion cycles is what results in engine misfires. The misfire itself has many more symptoms to showcase that it’s there.
For example, you’ll notice how your engine will run rough on a particular cylinder, and that you’ll experience noises coming from the valve cover. Elsewhere, another sign of a misfire is the pops that you can hear from your exhaust. While there are numerous underlying causes of a misfire, low compression in your engine could be one of them.
3) Engine Down On Power
Optimal compression is necessary for your engine to make good power, so when there’s low compression, your engine will be down on power. Without a good, air-tight seal, your engine will quite simply produce a lot less horsepower than before. This can impact the driveability of your car, even when you’re just driving around slowly.
While reduced power is more noticeable with smaller engines – such as inline-4 engines – it can still be noticeable with a large, torquey V8 engine. With the latter, it can be especially tough if you’re using your V8 to haul or tow stuff with your car. So, make sure to pay attention to the symptoms of low compression.
4) Engine Keeps Stalling
Another very common symptom of low engine compression is when your engine keeps stalling when it’s under load. Your car might want to start and work smoothly. But, as soon as you load the car up or accelerate, the engine will shut off itself and stall. This is one clear example of how low compression impacts your engine.
Again, like many other symptoms here, a stalling engine can be caused by many other issues, not just low compression. You might be dealing with ignition issues, fuel delivery problems, or even faults with your crankshaft position sensor. Regardless, if you slowly diagnose one issue at a time, you might conclude that you have low compression.
5) Engine Doesn’t Want To Start
Last but not least, you might be dealing with low compression if your engine doesn’t want to start, but keeps cranking. This can happen when your engine has low compression, and that your engine is in bad health. Typically, you might be dealing with something more specific, such as worn bearings or piston rings that have too much play.
Of course, instances where your engine refuses to start could be caused by other problems, too. You might be looking at a bad camshaft position sensor, failure of the ignition system, fuel-related issues, and so on. These are more common, so it’s wise to start diagnosing from here, and then move on to looking at low compression as a possibility.
What Causes Low Compression Symptoms?
If you’re wondering about the underlying causes behind why you’re seeing low compression with your engine, there are a few places where you can start looking:
- Piston Damage – The pistons are what compresses the fuel and air mixture, thus helping to create the necessary compression for good combustion. However, if your pistons – which are highly-stressed parts of your engine – develop cracks, and sometimes chunks fall into the crankcase, it can cause low compression.
- Piston Ring Damage – Speaking of, there are the piston rings, which seal the pistons and the cylinder walls. With continuous use, your piston rings will wear and tear, and over time, they’ll fail, and you need to replace them. The piston rings play a huge role in keeping compression in your engine in check.
- Worn Cylinder Walls – On that note, your cylinder walls are worth checking if you want to diagnose low compression. Worn cylinder walls are common in cars with bad rod bearings, thus letting it scrub against your cylinder walls. With enough wear and tear, your cylinder walls will make it harder to maintain compression.
- Bad Valves – For your valves to maintain a proper seal, the valve seats need to be clean and kept in good condition. Otherwise, if your valves won’t seal well enough, this will result in low compression. You may also experience low compression if there is sufficient carbon buildup around your valve seats.
- Blown Head Gasket – This thin gasket material is what creates a good seal between the head of the engine and the engine block. So, when your car’s head gasket fails (aka it ‘blows’), that seal has now been compromised. Many other issues can trace their roots back to a blown head gasket, but low compression is one of them.
