Nissan Compression Test Carried Out at Our Walsall Garage

A Nissan came into us recently with the owner concerned about how the engine was running. The car had developed a rough idle, felt sluggish under acceleration and was using more fuel than it should. Those three things together told us the engine needed a closer look before we started replacing parts. The right place to start was a compression test, and that is exactly what we did.

A Compression Test Revealed One Cylinder Producing Significantly Less Pressure Than the Others

A compression test works by removing each spark plug and fitting a pressure gauge into the hole. We then crank the engine over with the starter motor and measure the pressure each cylinder builds up on its own. On a healthy engine you want to see consistent readings across all cylinders, typically somewhere between 150 and 200 psi depending on the engine, with no more than about 10 percent difference between the highest and lowest reading. On this Nissan, three cylinders were reading within a healthy range of each other. The fourth was sitting noticeably lower, around 90 psi, which is a significant drop and not something you can ignore.

A low reading like that on a single cylinder tells us one of three things is likely going on. The piston rings could be worn, which means the cylinder is not sealing properly as the piston moves up and down. The valves could be damaged or not closing fully, which lets pressure escape before it builds. Or there could be a head gasket failure between that cylinder and a neighbouring one, which bleeds pressure away internally. To narrow it down further we carried out a wet compression test, which involves adding a small amount of oil into the cylinder before testing again. When the reading came back higher with oil present, that pointed us towards worn piston rings rather than a valve or gasket issue, because the oil temporarily improves the seal around the piston rings and gives the cylinder a better chance to build pressure.

Leaving a Low Compression Cylinder Unattended Puts the Whole Engine at Risk

It is worth being clear about what happens if a low compression reading is left and the car keeps being driven. The cylinder that is not sealing properly is not doing its share of the work, so the remaining cylinders are under more strain. Fuel that is not being burned properly can wash oil off the cylinder walls, which accelerates wear. Over time, what starts as a worn ring can develop into scoring on the cylinder wall itself, and at that point you are looking at a significantly larger repair, potentially a full engine rebuild or replacement rather than a ring and piston job. The running symptoms get worse too. Fuel economy drops further, the engine becomes harder to start in cold weather, and you may eventually see blue smoke from the exhaust as oil gets past the worn rings and burns in the combustion chamber.

We Carried Out a Full Engine Assessment and Discussed the Right Course of Action with the Owner

Once we had a clear picture of what the compression test was telling us, we talked the owner through the findings in plain terms and explained the options available to them. That is how we work at Platinum Vehicle Services. We do not present you with a bill for parts you have not understood or agreed to. We show you what we have found, explain what it means for your car day to day, and give you the information you need to make a sensible decision. In this case, having the full diagnostic picture meant the owner could weigh up the repair cost against the value of the vehicle and make the right call for their situation.

Watch for These Signs on Your Own Engine and Book in With Us in Walsall

If your car is idling roughly, feels flat when you pull away, is drinking more fuel than normal or you have noticed blue or grey smoke coming from the exhaust, those are all signs worth getting checked. None of them always mean the worst, but all of them are worth investigating before they develop into something more expensive. A compression test is one of the most reliable and affordable ways to get a clear picture of engine health, and it takes far less time than stripping an engine only to find the problem later.

We carry out compression tests and engine diagnostics regularly here at our garage in Walsall, and we cover customers coming to us from Bloxwich, Aldridge, Willenhall and Brownhills as well. If you are concerned about how your engine is running, get in touch with us. You can leave a message any time, 24 hours a day, 7 days a week, on 07970901354 and one of the team will get back to you as soon as possible. You can also find out more about what we do and book in at platinumvehicleservices.co.uk.