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All About Engine Compression Test

WHAT IS ENGINE COMPRESSION TEST?


Engine compression test reveals the health of gas-powered engines. To be more specific, it reveals the health of engine piston rings and valves to ensure they hold the pressure.

A combustion engine needs compression to operate and if the compression is lost, the vehicle loses its power and in worst-case scenarios, you can feel the lack of power and the vehicle does not move much.

As you use the car, engine compression naturally drops over time however it is not the same in all makes and models. It all depends on how the engine has been built and what reliability score it ticks.

Mechanic Performing Engine Compression Test



WHAT IS A GOOD ENGINE COMPRESSION?


A healthy engine should have compression over 130 PSI and not less than 110 at the very most. This compression can go up to 180 PSI or more in new vehicles. At the same time, all cylinders must have a similar ratio with less than 10 PSI differences across the cylinders.

For example, if you have a 4-cylinder petrol car, the engine compression for each cylinder must be over 130 but if let's say three cylinders give you 140 PSI and the fourth one is 115 PSI, you should know that the fourth piston needs attention.


HOW MUCH COMPRESSION DO ENGINES NORMALLY LOSE OVER TIME?


There is no set number to give you as it really depends on the car, the maintenance as well as the usage type. If you have pushed the car and neglected it over time, you can keep on losing compression as you run the engine.

You can have a well-maintained car that holds its engine compression till the day they go to the junkyard. This is very common in Japanese made cars provided that they have not been abused much and their tolerance level is very high.

Different cars are built differently and some lose their piston compression much faster than others. Generally speaking, European cars lose their engine compression a lot faster than American cars and they both lose their compression much faster than Japanese cars.

Chinese cars are also much worse than European cars and every other in the market so do not be fooled by the low prices they offer unless they prove to have improved the materials they use to build their vehicles.

To learn more, please visit the List of Reliable Car Brands in Australia.

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HOW CAN YOU REDUCE ENGINE COMPRESSION LOSS?


Maintenance and regular service are your saviours. At the same time using the right engine oil is the key to minimising natural compression loss over time. Different oils with different thicknesses and different makes might suit a single car but what should you really use?

You can choose a full synthetic oil for your engine or just use the conventional oil, you can also choose between 10-Weight to 60 or more weight. A 30W oil normally suits all engines however as the engine gets old, it is better to use thicker oils to maintain the pistons' lubrications in long run.

Also, fully synthetic oil is always better than mineral oils. Synthetic oils are generally made to perfection and can prolong your service intervals so we always recommend using full synthetic oil for all modern engines.

If you have a classic car or an old one, you should be alright with the conventional engine oils and in fact. It is better to use them as classic car engines are built to use conventional mineral oils and using synthetic oil can backstab you.


HOW TO PERFORM AN ENGINE COMPRESSION TEST?


To do the engine compression test, you need the engine compression tester which is reasonably cheap and anyone can afford it.

You also need to have technical knowledge of what to do. To do the engine compression test you need to remove all spark plugs from the engine to get access to the valves and the top of the piston. Before that, it requires a lot of preparation as well so that you do not damage the engine and do not jeopardise your own safety.

Performing engine compression test is much easier in old school and carburettor cars but it can be very challenging on newer model cars with compact and complex engines.

Bmw Series Engine



WHAT PARTS NEED TO BE REMOVED OR DISABLED?


If you have an old and basic car, here is what you need to do. Remove the coils, remove the spark plugs, disable the mechanical throttle body and disable the battery to avoid ignition. Generally speaking, you can finish the compression test on a basic car within an hour more or less provided that the exhaust manifold does not need disassembly.

If you have a modern car, it might take up to 3 hours or more to finish the job safely. Most modern cars need lots of part disassembly only to grant access to the spark plugs. For example, if you have a Toyota Kluger 2010 (Highlander) or a Lexus RX 350, the exhaust manifold sits right on the engine and covers 3 of all 6 spark plugs so you need to remove the manifold first.

To remove this manifold, you need to remove the wipers with the wiper motor assembly in whole and every other part that is on the way. Then you need to disable the battery which can affect the ECU and reset the radio in some cases so ensure you have the radio activation code just in case.

Toyota Kluger does not have a mechanical throttle body, but if it had, that needed to be removed as well for safety and then all the parts need to be put back after finishing the test so it can be a hell of a job so do not attempt to do so unless you are sure that the engine lacks power.


WHAT TOOLS DO YOU NEED?


To perform an engine compression test, you need a compression tester or better say a compression gauge. You also need your basic mechanical tools such as screwdrivers, wrenches as well as a spark plug socket.

Ensure that you have a socket extension if the spark plugs are deep inside or if you do not have good access. Carrying electric tools such as torque wrenches can make your job significantly easier so it is always good to perform the test in a full setup workshop.

Engine Compression Tester



IS IT SAFE TO PERFORM THE TEST ON THE ROADSIDE?


Absolutely not. You need a full setup workshop with all safety equipment and all the tools to perform the compression test.

If the vehicle is modern and requires extensive part disassembly, you must also have a safe place to put the parts aside in good order and a diagnostic tool if the test triggers the engine light after you put the parts back together.

It can however be done provided that you are willing to risk but you should know that you are taking a risk and can get into legal trouble in Australia if the worst-case scenario happens.


DO I GET AN ENGINE COMPRESSION TEST ON PRE-PURCHASE CAR INSPECTION?


The answer is no. Engine compression test requires part disassembly and can damage the engine if it is not done right and we never do such a thing on pre purchase inspections. Performing the engine compression test can also affect the vehicle especially if it is modern as you need to disable the battery, remove the electric fuel pump fuse and much more which are all against our policy.

We only perform visual inspections and are committed not to touching the third party’s vehicle in any way. We never disassemble any parts and we are 100% committed to that. It is our commitment to all sellers that we return the vehicle back to them untouched.

Imagine you are selling a car and someone tells you, I want to bring my inspector to inspect the car first and perform an engine compression test too which requires at least removing all the spark plugs. Would you ever say yes to such a thing?

Rarely you can find anyone to say yes to that provided that nothing happens either and if it does, you need to pay for it. To learn more please visit our Terms & Conditions.

Roadside Pre Purchase Car Inspection In Sydney Australia



IS THERE ANY OTHER WAY TO TEST THE ENGINE COMPRESSION?


The answer is yes. Although none will give you the numeric results, you can get a good idea if the compression is low.

The first factor is blow-by which we check on every engine when performing pre purchase inspection. Blow-by is the smoke that comes out of the engine oil filler when the engine is running and can be an indication of a poor engine.

You however need to be a professional to know that. For example, some blow-by is completely normal for diesel engines and some light one can be considered normal in petrol engines but it is always best that the car gets inspected by a specialist to know what is normal and what is not.

In worst-case scenarios, you also have cylinder misfiring which as well we can detect using our advanced computers. For more information, please visit Advanced Original Computer Diagnosis.


DO I REALLY NEED ENGINE COMPRESSION TEST IN MODERN CARS?


Not really. We can know a lot about modern cars using our advanced computers and the ECU fault indicators which all modern cars are equipped with. Any single engine issue these days can trigger the engine light including cylinder misfires and O2 sensor warning.

You only need to perform a compression test if you are sure that the car is significantly lacking power or has an unusual blowby. As mentioned above, you need to be a professional to know what blowby is normal for what model car and you should never panic if you see a bit of smoke exiting the filler.

Pre Purchase Inspection Of Audi A4 In Sydney Australia



HOW TO BOOK A PRE PURCHASE VEHICLE INSPECTION?


You can book any pre purchase vehicle inspection 24/7 online or by calling us during business hours.

We offer a large number of vehicle inspection services including end of warranty inspections, post purchase inspections, finance approval inspections, truck inspections, bus inspections and much more. To learn more, please visit our Services page.




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