The car has started losing power, burning oil, smoking or struggling to start cold, and the mechanic mentions that the compression should be "measured". A compression test is one of the oldest and most reliable ways to check the condition of an engine, and the numbers it produces speak very clearly: whether it still makes sense to invest in repairs or whether the engine needs serious work. Here is what the procedure means, which values are normal and what most often causes a drop.
What compression is and why it matters
Compression is the cylinder's ability to hold pressure during the compression stroke as the piston moves toward top dead centre. That pressure is the precondition for fuel and air to burn properly: on a petrol engine the spark plug ignites the correctly compressed mixture, while on a diesel only compression itself produces the temperature needed for self-ignition.
When a cylinder cannot hold pressure, part of the energy escapes past the piston rings, through the valves or through the head gasket. The result is less power, poor combustion, increased fuel and oil consumption, and accelerated wear of the remaining engine components. That is why measuring compression is an essential step before any serious repair decision.
Symptoms that point to compression loss
An engine rarely fails overnight. A drop in compression usually announces itself several months in advance, but drivers often write those signs off as "the age of the car". Typical symptoms are:
- loss of power, especially on hills and when overtaking
- rough idle, the engine "shakes" the car at traffic lights
- increased oil consumption between services, with no visible leak
- bluish or white smoke from the exhaust
- hard cold starting, especially in winter
- misfire on one cylinder, check engine light on
If two or more of these symptoms appear at the same time, compression is the first diagnostic step, before replacing spark plugs, ignition coils, injectors or other parts that often get unfairly blamed.
How a compression test is done in the workshop
The procedure sounds simple, but it requires experience and the right equipment, so it is not a DIY job. The mechanic first warms the engine to operating temperature, because a cold engine always reads lower and gives a false picture. Then all the spark plugs (on a petrol) or all the glow plugs (on a diesel) are removed, so the engine cranks freely without compression in the other cylinders.
The compression gauge is screwed into the spark plug or glow plug opening of one cylinder at a time. The throttle must be fully open so air can flow freely into the intake manifold. The starter cranks the engine for 4 to 6 revolutions, enough for the needle on the gauge to reach its highest value. The procedure is repeated for each cylinder and the values are written down, then compared against each other and against factory specifications.
Normal values (petrol and diesel)
Exact factory values depend on the manufacturer and the year of the engine, but there are reference ranges that apply to most series-production engines in good condition:
- healthy petrol: around 11-14 bar per cylinder
- healthy diesel: around 25-35 bar per cylinder
- the difference between cylinders should not exceed 10-15 percent
In other words, if one cylinder reads 12 bar, the others should be in the range of about 10 to 13 bar. If one cylinder drops drastically below the rest, the problem is usually mechanical and localised to that cylinder. If all cylinders are uniformly low, the engine is generally worn and we are looking at overall wear rather than an individual fault.
There is also an additional "wet" test: a teaspoon of motor oil is poured into the suspect cylinder and the measurement is repeated. If compression noticeably jumps, the problem is the piston rings. If it stays the same, the issue is in the valves or the head gasket.
Most common causes of compression loss per cylinder
When one cylinder shows weak compression, the cause is almost always one of a handful of classic faults. A burnt exhaust valve is the most common culprit, especially on engines that have run on poor fuel or been overheated. Worn piston rings appear on high-mileage engines and are recognised by increased oil consumption and blue smoke.
A blown head gasket usually causes compression loss in two adjacent cylinders, and is often accompanied by oil emulsion ("mayonnaise" in the oil), bubbles in the expansion tank or overheating. On modern direct-injection petrol engines, GDI (Gasoline Direct Injection) and TSI (Turbo Stratified Injection), a frequent cause is carbon deposits on the valve seats, which prevent the valve from closing fully.
For more precise fault localisation a leak-down test is used. Compressed air is fed into the cylinder through the spark plug opening and the mechanic listens for where it escapes: if it leaks into the intake manifold, the intake valve is at fault; if it leaks into the exhaust, the exhaust valve; if it hisses from the oil filler, the piston rings; and if bubbles appear in the coolant, the head gasket has gone.
What next, from repair to rebuild
Once the exact cause is identified, the decision on repair depends on the overall condition of the engine. If the other cylinders are healthy and only one valve or the head gasket is at fault, the repair makes sense and the engine will probably go on for many more kilometres after the job. If all cylinders have fallen below 70 percent of factory value, the engine is no longer fit for normal operation and the conversation shifts to a head overhaul, a full engine rebuild, or possibly fitting a sound used engine.
Particular attention is needed on vehicles with an LPG (autogas) system. Gas burns at a slightly higher temperature than petrol and, if the valve seats are not suitable for gas operation, compression drops faster than on pure petrol. That is why in BiH it is standard practice to measure compression before fitting an LPG system - the numbers do not lie and tell us whether the engine can reliably run on gas in the first place.
If you notice any of the symptoms from the start of this article, or the engine simply no longer behaves as it used to, drop in for diagnostics and we will measure the compression. It is better to check now than to spend thousands of marks on parts that will not solve the problem.