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Direct Injection – The good and the bad

The Direct Injection engine, sometimes known as GDI (Gasoline Direct Injection) is seen as a leap forward in the quest for greater economy and lower emissions and has actually been delivering results, with OEMs moving over to GDI in droves.

The success of DI engines means that they are being seen as potential replacements for diesel engines in the small car market as they have high MPG and lower emissions. So, is the GDI engine too good to be true? Well it seems that for the upside of economy and low emissions there is a downside of maintenance issues and loss of performance.

This means there is an opportunity for the independent sector for preventative maintenance and repair. Let’s look at how Direct Injection (DI) works…

By injecting petrol at high pressure directly into the combustion chamber, DI delivers fuel more precisely and efficiently than port fuel-injection systems (PFI). The result is more complete combustion and cooler cylinder temperatures, enabling higher compression ratios for greater efficiency and power. Bosch says that DI can return 15%, or more, gain in fuel economy while boosting torque as much as 50 percent.

Combining DI with turbocharging delivers even greater gains in economy and performance. That enables OEMs to use smaller displacement engines, resulting in even more efficiency.  

Because of the position of the injectors (inside the cylinder) they are in direct contact with the soot released from the burnt fuel. The soot deposits itself on the injector tip and interferes with the accuracy and atomisation of the spray pattern. A second issue is that because the injectors do not spray across the intake valve, the valve is subject to additional deposit formation as the detergents that are in the fuel do not touch the valve.  These build-ups can be exacerbated by oil vapour deposits being recirculated from the PCV valve.

Deposit build-up on the valve
Valve, post-clean

In order to reduce frictional energy loss and improve MPG, GDI engines use lighter, low tension piston rings. Ring spring force in older engines was as high as 200psi. New GDI engines can be 60% less. Also, engine compression ratios have increased from 9:1 to as high as 14:1. This increased pressure creates more force on the low-tension rings. Piston rings should prevent oil seeping up, and fuel and combustion gases from ‘blowing-by’ between the piston and cylinder. This increased crankcase pressure means that more oil vapour is released back into the intake tract and finding itself attaching to the intake valves. The deposits on these valves can build up very quickly and restrict airflow, causing power loss and other running issues.

The blow-by past the rings is exacerbated by restricted piston ring function. A ‘vicious circle’ is created when oil deposits behind the piston ring restrict the piston rings’ function. Degraded engine oil is more susceptible to thickening. When oil thickens it flows less easily and when some oil gets behind the rings it tends to stick. Over time it hardens and restricts ring function, reducing the all-important compression, and allowing even more gases, soot and fuel into the oil. This additional heat, pressure, soot and fuel further adds to the degradation of the oil and so the vicious circle starts. Deposits lead to reduced compression, reduced compression leads to suboptimal combustion, this leads to more soot, reduced compression is caused by reduced sealing by the rings, this allows more blow-by and fuel into the oil, and so on.

The impact on compression is a surprising feature. BG Products tested a Ford 2.0 GDi and it had lost 8% compression in 3 cylinders after only 11,000 miles, and a Mazda3 lost up to 14% in 36,167 miles.

So, while GDI engines are known for efficiency, the constant struggle between low tension piston rings and high operating pressures will eventually erode fuel efficiency and diminish horsepower.

BG Products’ research has shown that a preventative maintenance regime to keep injectors clean of the hard baked deposit, to keep the integrity, and slow down oxidation of the oil and ensuring that the old oil is thoroughly cleaned out of all critical areas during an oil change, will drastically limit the problems that will impact GDI engines.

But what about engines that are already suffering from deposits on the valves?

BG have carried out extensive research on GDi engines and there is much more information and data about causes, progression and solutions for GDi issues at and

The traditional solution has been some heavy labour oriented, stripping down and also using walnut blasting machines to ease away the carbon. BG Products solution is a chemical solution sprayed in via the air intake. This requires minimal labour and has produced very impressive results.

BG Products’ Platinum Fuel System Service tool
About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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