What are the reasons for increased fuel consumption after an engine overhaul?

[OValeria]

After an engine overhaul, the increased fuel consumption is due to the new parts being more precisely machined and tighter, which increases the engine's workload, leading to higher fuel consumption. This is a normal situation. Reasons for increased fuel consumption after an engine overhaul: The viscosity of the engine oil used increases, and higher viscosity oil is generally better. The primary function of engine oil is lubrication, and higher viscosity indicates better quality. However, if the oil viscosity is too high, the engine will encounter greater resistance during high-load operation, consuming more power and, consequently, significantly increasing fuel consumption compared to before. Precautions after an engine overhaul: Avoid high engine speeds; it's best not to exceed 3,000 RPM. After a cold start, it's advisable to warm up the engine for a while before driving, typically about 5 minutes, until the water temperature begins to change. Adhere to the vehicle's specified load capacity, preferably not exceeding 70% of the maximum load. Avoid sudden acceleration and hard braking whenever possible. Hard braking can cause significant impact damage to the new engine, especially during sudden acceleration, which can severely affect the break-in of new engine components and directly impact the fit between parts. Anticipate sudden situations while driving, maintain a steady speed, and drive smoothly. Keep the speed between 50-80 km/h, but avoid prolonged low-speed driving. Pay attention to gear changes and avoid driving in a single gear for extended periods.

[LeWyatt]

I've been running an auto repair shop for many years and have seen many cases where fuel consumption increases after an engine overhaul. The most common issue is improper assembly of components, such as incorrectly adjusted piston ring gaps or a poorly sealed cylinder head, leading to improper air-fuel mixture and reduced combustion efficiency, which naturally consumes more fuel. Newly replaced parts like pistons and bearings also require a break-in period, and higher fuel consumption in the first few hundred kilometers is normal. However, if it continues to rise, you should check whether the fuel injectors are clogged or the spark plugs are misaligned. Another hidden cause is the ECU (Engine Control Unit) not being reset after repairs—disordered parameters can mess with fuel injection amounts. I recommend observing engine vibrations and exhaust color while driving. A sudden increase in fuel consumption indicates overlooked details during the repair, so take the car back to the shop for a full diagnosis immediately—don’t delay and risk further damage or higher costs.

[MacCarson]

I just had a major engine overhaul recently, and within a few days of driving, I noticed a scary surge in fuel consumption. At first, I thought it was just the normal break-in period, and a friend mentioned that new components like the gearbox and pistons tend to have higher initial wear, which is normal. Later, I started suspecting that the fuel system might not have been handled properly during the repair—perhaps the fuel injectors weren’t thoroughly cleaned or there was air trapped in the fuel lines, causing uneven injection. It could also be that the ECU settings got messed up, and the computer hasn’t adapted to the new state, leading to unstable idling and excessive fuel consumption. Driving habits play a role too—after the overhaul, I kept wanting to test the car by flooring the accelerator, which inadvertently wasted fuel. It’s best to avoid rushing onto highways; instead, drive at low speeds for a few hundred kilometers and observe whether the fuel consumption curve drops back. If the issue persists, it’s better to have a professional shop check the wiring connections to prevent minor problems from turning into major hazards.

[CarolineLynn]

Having played with cars for decades, it's not surprising at all to see slightly higher fuel consumption after an engine overhaul, especially common in older models. The core issue lies in the poor internal break-in of the engine, where new components create greater friction and lower efficiency, naturally consuming more fuel. Other contributing factors include spark plug misalignment during maintenance causing ignition delays, clogged fuel filters, or air intake system leaks—all of which disrupt combustion balance. Occasionally, ECU malfunctions can also increase fuel injection volume. Don't worry too much about the break-in period; fuel consumption will stabilize after normal driving for a month or two. Focus on checking repair quality: loose screws or unevenly applied sealant can cause air leaks. Ensure regular maintenance of the fuel system for better fuel economy and engine protection.