3 Important Checks When Buying a Used Car With a Gasoline Direct Injected Engine

A few simple checks and tips can help ensure maximum peace of mind when buying a used GDI-powered car.

The Gasoline Direct Injection (GDI) engine is marketed under different names by different automakers and GDI engines are more popular than ever. That’s thanks to a unique engine design that boosts efficiency, power output, and fuel mileage. Still, like many new technologies, the GDI engine hasn’t been without its problems.

The most common of these relates to deposits that can build up on the intake valves of a GDI engine. Intake valves are like little round metal doors that allow air into your engine’s cylinders. In most engines, two of these intake valves are used to control the flow of air into each cylinder.

Thing is, the air that passes the intake valves is filthy: full of contaminants like engine oil molecules and unburned fuel. These contaminants can stick to the hot intake valves, burning onto them and forming a buildup called valve gunk. Valve gunk is bad.

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In non-GDI engines, intake valve gunk is hardly a concern. That’s because fuel injectors sprayed gasoline over the surface of the valves, just before it got sucked up into the cylinders. This effectively washed the tops of the intake valves with a continual spray of gasoline, which prevented gunk-causing deposits from building up excessively.

But in a GDI engine, fuel never touches the intake valves since the fuel injector is located inside of the cylinder, beneath them. Translation? Intake valves on a GDI engine are exposed to the same gunk-causing contaminants, but there’s no cleaning spray of gasoline over the valves to keep the contaminants from building up. In some GDI engines, owners experience problems with intake valve deposits and valve gunk, which can result in issues with performance, mileage, and reliability.

Some GDI engines can run for the life of the vehicle with no gunk issues. Others, not so much. Many factors are at play.

That’s why we’ve compiled the following tips and advice to bear in mind if you’ll soon hit the used car market in search of a GDI-powered vehicle. Keep the following tips and checks in mind, and you’re likely to experience longer engine life, more trouble-free operation, and more money staying in your wallet.

Look for Stored Misfire Codes with a Diagnostic Scan: Valve-gunk buildup can cause an engine misfire — that is, a failure of the engine to properly ignite its fuel. Engine misfires may not be detectable to the driver but are recorded by the engine computer if special sensors in the engine detect them. When a misfire is detected, it may or may not cause a CHECK ENGINE light to appear.

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One easy way to protect yourself from buying a used GDI-powered car with a gunked-up engine is to have a diagnostic scan. Quickly and on the cheap, a technician can scan the vehicle’s computer brain to see if any misfire events have been logged. If that’s the case, valve gunk deposits may be to blame and further assessment is required.

Triple-Check the Ignition System: As a used car shopper, remember that many vehicle owners wouldn’t have their vehicle serviced on time for all the tea in China. This has numerous implications and at least one big one that can result in rapidly accelerated valve gunk buildup.

In a GDI engine, on-time spark plug changes are absolutely vital to keeping the engine running clean — which is a great way to keep valve gunk deposits at bay. Most drivers should follow the “severe” spark-plug service interval in the owner’s manual, and change the spark plugs not a moment later than specified. Assume the used GDI-powered car you’re considering is overdue for a new set of plugs until you see service records or receipts that prove otherwise.

Even a slight stretching of spark plug replacement intervals can result in dirtier combustion which creates more gunk-causing contaminants. Dirty spark plugs also cause strain on other ignition system components like ignition coils. This can further dirty the combustion process, creating a rapid snowball effect towards valve-gunk disaster. Do not underestimate the importance of on-time spark-plug changes and a healthy ignition system in the GDI engine.

Fluids Fluids Fluids: You’ll want to confirm that the seller only filled the vehicle you’re considering with top-tier gasoline, available in all octane grades at most major fuel retailers. Top tier gasolines contain extra cleaning additives that indirectly reduce valve gunk buildup by keeping the engine’s fuel delivery system clean.

You’ll also want to seek out service records proving that the seller had all oil changes performed at or before the requested interval. Using quality fuel and oil at all times can help fend off excessive valve gunk buildup.

Tracy Lewis is the owner of Automotive Product Design and Engineering Group (APDEG), in Tampa Bay, Florida. He’s been working in the auto industry for 40 years and has specialized in all things GDI since 2008 — including weekly tear-down and analysis of GDI engines from across the globe.

Lewis advises that shoppers do their homework when it comes to oil selection, and opt to use only a high-quality full-synthetic oil to avoid a process called coking, which can dirty your intake valves.

“Using a full synthetic oil from the start, instead of most dealer-supplied synthetic blends, will result in much less severe coking at as much as a third of the rate. It’s the mineral oil portion that leaves the residue on the valves, which ends up being baked into a hard abrasive crystalline gunk,” he says.

Lewis advises that, for maximum peace of mind, shoppers should seek out a vehicle that’s always run a full synthetic engine oil. Based on years of research, he recommends Amsoil signature series or Mobil One for a lower-priced alternative. Lewis also suggests that shoppers avoid extremely light engine oils, and using a viscosity of at least 10w30 or 10w40.

Consider Cleaning, but do your Homework: Some GDI-powered vehicles can benefit from a special intake system cleaning procedure, carried out at a dealer or private shop. Ask the seller if they’ve ever had this service done, and consider starting it on a roughly annual basis if the GDI-powered vehicle you’re considering is newer and lower mileage.

Do your homework though, as the jury is out on whether or not this type of cleaning is a good idea.

One school of thought suggests that annual cleanings from low mileage will keep the valves clean before deposits build up to harmful levels and turn into gunk.

Lewis has a different take and says that this solvent-based cleaning procedure may dislodge very hard and abrasive particles from the valves, allowing them into the cylinders.

“This can cause scoring to the cylinders, pistons, and ring damage in most every case,” he says.

“Companies that sell these cleaning products protest loudly that they’re safe, and also offer damage guarantees, but we don’t know of anyone who has ever collected on a damage claim. This type of cleaning can loosen up to 20 percent of the gunk and most is expelled out the exhaust, but the particles forced between the pistons and cylinders can cause damage.”

Lewis recommends that only a manual valve gunk cleaning procedure be performed when required. Here, intake valve gunk deposits are blasted away by some abrasive medium or removed manually with hand tools.