(GDI) vehicles on the...30 December 2015 T he population of gas-oline direct injection (GDI)...

30 December 2015

The population of gas-oline direct injection (GDI) vehicles on the road is increasing every year. Virtually every vehicle manufacturer

has at least one model on the road or one planned for release soon. The latest estimates are that 40% of the gasoline vehicles sold in the U.S. in 2015 were GDI-equipped and ap-proximately 65% are estimated for 2021. Subaru is a recent addition and the Fiat Chrysler Group will be add-ing models soon.

What does this mean for you? If you’re not currently servicing GDI ve-hicles, you will be in the near future. GDI vehicles began showing up in the U.S. around 2004, and many OEMs released models in the 2010-11 time frame, which means those vehicles are four to five years old and beyond most manufacturers’ warranty periods.

For the past two years, Bosch has sponsored mobile training vehicles to educate technicians about this new technology and to test their knowledge in a full-immersion 3D virtual garage. One thing learned during these events is that technicians had heard about GDI, but most were not sure how to diagnose such a system. This article will give a quick overview of a typical GDI fuel system and offer information on how to diagnose the system and about any potential service-related issues.

The illustration on page 32 shows the components associated with a typical GDI fuel system. The majority of the engine management components are identical to a port fuel injected (PFI) engine, including the low-pressure fuel pump, which supplies low-pressure fuel to the high-pressure pump. Typical low-pressure pump range is 50 to 75 psi, depending on the vehicle application. Some manufacturers utilize a pressure sensor to verify the pressure.

The output of the low-pressure pump is typically controlled via the fuel pump control module (FPCM) with input from the engine control module (ECM). At idle and light load, less fuel volume is required, so the duty cycle percentage is lower. The duty cycle will increase when more fuel volume is re-quired for acceleration and heavy loads. Ill

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Service Tips2.indd 1 11/19/15 2:51 PM

31December 2015

SERVICE TIPSFOR GDI ENGINES

If you’re not currently servicing GDI-equipped vehicles, you will be in the near future. Port fuel injection experience is a

solid foundation, but GDI requires specifi c diagnostic techniques.

BY BOB PATTENGALE

Service Tips2.indd 2 11/19/15 2:51 PM

32 December 2015

Service Tips for GDI Engines

The high-pressure pump, which for most vehicle applications is mechanical-ly actuated by a camshaft lobe, boosts the low-pressure fuel from approxi-mately 600 psi at idle to 2900 psi at high loads (40 to 200 bar for metric folks). High-pressure fuel is delivered to the high-pressure injectors via a high-pres-sure rail, which includes a fuel pressure sensor (FPS). The FPS supplies the ECM with the actual pressure in the rail and adjusts the pressure using a pressure control solenoid mounted on the high-pressure pump.

The pressure control solenoid has a variety of names, but for this article I’ll call it a volume control valve (VCV).

Two styles of high-pressure injectors are utilized—solenoid-actuated and piezo crystal-actuated. Both require haz-ardous voltages of up to 120V, so you should always follow manufacturer rec-ommendations for testing and service.

Most four-cylinder vehicles have one GDI pump, but V6 and V8 engines will likely have two pumps, one for each bank of fuel.

The fi rst step in the diagnostic pro-

cess is verifying the condition of the low-side and high-side pressure com-ponents. The initial check is fairly simple and can be performed with an enhanced scan tool and two data pa-rameters (PIDs)—fuel rail pressure de-sired and fuel rail pressure actual. It’s important to check the pressures in all operating ranges to fully validate the fuel system. Issues will normally start to show up at higher engine loads fi rst and get progressively worse over time. The chart on page 34 shows four scan tool screen captures taken under different load conditions on a road test.

The key-on, engine-off (KOEO) capture provides the current pressure from the low-pressure pump. The PID Fuel Rail (P) Des is desired pressure for the ECM; Fuel Rail (P) is the actual pressure. In this case the ECM is re-questing 55 psi and the actual pressure is 55 psi. This capture shows the static pressure without any fuel being used by the engine.

The idle speed capture below it shows that desired fuel rail pressure is 600 psi and actual is 550 psi. As a rule, if

the difference between the desired and actual values exceeds ±10% you should make a note of it, but ultimately the ECM will decide when the deviation is too low or too high and set the appro-priate fault code. (Typical fault codes are P0087 – Fuel Pressure Too Low or P0088 – Fuel Pressure Too High.) When performing this test, it’s import-ant to allow the fuel system to stabilize before making a final decision; 20 to 30 seconds should be suffi cient. In this example, the actual pressure is a bit low, but still within the ±10% range.

The light load capture in the upper right shows that desired pressure is 1800 psi and actual is 1440 psi, which is low by 20%, but still not enough to set a fault code. You can clearly see the prob-lem is getting worse—an 8% difference at idle and now a 20% difference at light load.

The heavy load capture shows that desired pressure is 2100 psi and actual is 1500 psi, which is now a 28% differ-ence. So under heavy load is when the code P0087 sets. Monitoring the two data PIDs confirmed the issue, but

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These are the typical components associated with a gasoline direct injection fuel system. The high-pressure fuel pump, fuel rail and fuel injectors are the primary upgrades to the system when compared to a port fuel injection system.

Fuel Pump Control ModuleECM

Fuel Pressure Sensor

50 to 75 psi

Low-Pressure Fuel Sensor

HDPS

Low-Pressure Fuel Pump

Fuel Pressure: 600 to 2900 psi

Fuel RailFuel Filter With Pressure-Limiting Valve

Service Tips.indd 3 11/19/15 8:38 AM


a complete road test was required to duplicate it.

At this point, two additional PIDs will be helpful in diagnosing this fault. In the chart, Fuel VCV % is the VCV described earlier, which the ECM uses to control the amount of low-pressure fuel entering the high-pressure pump. Fuel Pump % is the duty cycle of the low-pressure pump, which controls the speed of the pump and fuel volume available to the high-pressure pump.

The idle speed capture shows per-centages of 15% for the VCV and 24% for the fuel pump. Unfortunately, it’s not easy to fi nd specifi cations for these PIDs, so this is where connecting to known-good vehicles helps provide a baseline. In this example (a Ford Es-cape), known-good values would be ap-proximately 8% for the VCV and 23% for the fuel pump duty cycle. The ECM sees that the actual fuel pressure is a bit low and is attempting to increase the volume of fuel into the high-pressure pump to compensate.

The light load capture shows a big jump in VCV to 45%, while the low-pressure pump duty cycle has increased to 29%. This means that the ECM is requesting a signifi-cant increase in fuel volume from the low-pressure pump to reach the desired value of 1800 psi. The ECM was able to increase fuel pressure, which means the VCV is working, but the desired fuel pressure cannot be reached. The differ-ence between desired and actual is still not enough to set a fault code.

The heavy load capture in the low-er right shows 50% VCV and 40% fu-el pump duty cycle, which in this case represent maximum values for these components as it relates to fault code criteria. You may be thinking, why not continue to increase both components to 100% and see if the desired pres-sure can be achieved? The engineers programming these systems are pret-ty smart and know that once the fuel pressure difference exceeds a predeter-mined point, all hope is lost and a fault code should be set.

In this example, the vehicle is still driveable, but not performing well un-der high-load conditions. At this point the ECM would illuminate the Check

Engine light and set a P0087 low-pres-sure fault code, because the difference between the desired and actual pres-sures is greater than 25%, which was established in the fault code criteria. The good news? The customer would still be able to bring the vehicle in for service prior to being stranded on the side of the road.

At this point you need to determine which portion of the fuel system is at fault—low-pressure side or high-pres-sure side. Looking at the VCV and fuel pump percentages can give you some insight, but cannot confi rm which com-ponent is at fault without further test-ing. The best course of action is to be-gin checking the low-pressure side fi rst and move forward in the system.

One point of caution: You cannot de-pend solely on the pressure sensor and/or pressure gauge to validate the condi-tion of the low-side pressure pump. It’s important to check the output volume of the fuel pump. In the KOEO portion

of the chart, the desired and actual pres-sures matched; however, the vehicle is not running and no fuel volume is being consumed. At idle speed the actual pres-sure is 8% under desired, and the split gets progressively worse with higher load conditions. The low-pressure fuel pump is able to deliver enough volume at low demand, but not enough for high demand. The low-pressure fuel pump needed to be replaced.

Once the low-pressure side has been eliminated, the next component in the system to be tested is the high-pres-sure pump. The most likely cause for a low-pressure fault code would be wear between the high-pressure pump and the camshaft. A high-pressure fault would likely be a faulty VCV or inter-nal high-pressure pump failure. You’ll need to follow manufacturer recom-mendations for inspecting and testing the high-pressure pump and hard parts.

The most common issue with GDI fuel systems is carbon buildup in the

33December 2015

Circle #19

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intake and on the backs of the intake valves. Unfortunately, this is not easy to detect and may be different from manu-facturer to manufacturer and vehicle to vehicle. In the De-cember 2014 edition of Mo-tor, Sam Bell authored an ar-ticle “GDI: Gasoline Deposits Inside?” which is a must-read. I’ll offer a quick review and provide a few more ideas on this subject.

The illustration on the left below shows a typical port fuel injected configuration and the illustration on the right shows a typical GDI configuration, where the fuel is injected directly into the combustion chamber. In this exam-ple, the injector is positioned in the cen-ter of the combustion chamber, which defines this as a spray-guided system. If the injector were positioned at an angle, it would be called a wall-guided system.

The illustration on the left shows the fuel for the next combustion event be-ing injected into the combustion cham-ber, together with the air around the intake valve. The ECM has some flex-ibility to inject the fuel before the next opening of the intake valve, with the ultimate goal of mixing the air and fuel evenly, which should create a quality burnable homogenous air/fuel mixture. As it travels into the combustion cham-ber, the air/fuel mixture offers a built-in self-cleaning process for the intake and intake valves.

The illustration on the right shows the air entering around the intake valve; shortly after, fuel is injected directly into the combustion chamber. The air and fuel mix together while the piston is traveling down on the intake stroke and as the mixture is compressed during the compression stroke. The ECM has a very short window to inject the fuel; there’s not very much time for com-plete air/fuel mixing.

We already mentioned that GDI vehicles suffer from carbon buildup issues, but how does the carbon get there? How does clean air, which is fil-tered to remove dust, passing through a brand new intake system, around clean intake valves and into the combustion

chamber produce carbon? It doesn’t seem to make any sense.

The photos at the top of page 36 were taken from two of the eight in-take valves on a 2010 BMW 750Li F02 N63 with almost 65,000 miles on the clock. All eight intake valves had car-bon buildup, but what surprised me was that the buildup on all of the cylin-ders was not the same. The top photo is cylinder 4, which shows a darker baked carbon; the middle photo is cylinder 6, which shows a dryer-looking, maybe soft carbon.

If you’ve not worked on a GDI vehi-cle, you now know what carbon looks like on the backs of the valves. So let’s go back to the question: How did the carbon get there? The answer is simple and complex at the same time. The sim-ple answer is incomplete combustion. You may be thinking, if we have incom-plete combustion in the cylinder, the carbon matter should simply exit out the tailpipe. How does it get back to the intake side of the engine?

In a perfect world, the cor-rect amount of air and fuel would enter the combustion chamber, perfect combustion would occur and clean emis-sions would exit out the tail-pipe. But the internal com-bustion engine is not perfect and neither is the air/fuel mixture. The carbon formed during incomplete combus-tion can go in a number of directions in the engine. The simple path is out the tailpipe during the exhaust stroke, which creates a different is-sue, which we’ll come back to in a minute.

Some of the carbon makes it around the piston rings into the engine oil, and this is where, most experts agree, the carbon suspended in the engine oil is introduced into the intake, through the positive crankcase ventilation (PCV) system. Use of an incorrect engine oil, extended oil change intervals, dirty air filters and wet PCV systems contribute to the oil and carbon mixture found in the intake area.

Another path for the carbon is around the intake valves during the ex-haust stroke, when both the intake and exhaust valves are open. This is an im-portant point; don’t forget to check for technical service bulletins that might suggest a reprogramming event to re-duce carbon buildup.

This brings us back to the topic of carbon that’s pushed out around the ex-haust valve after the combustion event. In my research for this article, I found many papers and studies related to par-ticulate matter from GDI vehicles. The table on page 36 shows the results of a

34 December 2015

KEY ON/ENGINE OFFRPM 0 RPM

Vehicle Speed 0 MPH

Fuel Rail (P) 55 psi

Fuel Rail (P) Des 55 psi

Fuel VCV % 0 %

Fuel Pump % 23 %

LIGHT LOADRPM 1800 RPM

Vehicle Speed 30 MPH



Fuel VCV % 45 %

Fuel Pump % 29 %

HEAVY LOADRPM 4000 RPM

Vehicle Speed 60 MPH



Fuel VCV % 50 %

Fuel Pump % 40 %

IDLE SPEEDRPM 700 RPM

Vehicle Speed 0 MPH



Fuel VCV % 15 %

Fuel Pump % 24 %

A GDI fuel system should be tested under a variety of engine load and vehicle operating conditions. These scan tool captures show the results of a complete road test.

The illustration on the left shows a graphic example of a port fuel injected system. The one on the right shows a typical spray guide GDI fuel system.

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California Air Resources Board (CARB) study that was used to understand and develop future emissions standards re-lated to particulate matter (PM). To keep this simple, Phase 1 is basically taken with a cold engine and Phase 3 is taken with the engine warm. The first row is easy to understand, as we all know diesels create PM, which is why they’re equipped with a diesel par-ticulate filter (DPF). Row four shows the results with a DPF. In the second and third rows you can clearly see par-ticulate matter is present from GDI engines, but is less pronounced when the engine is warm and the PFI engine is fairly clean.

This got me thinking about another source for carbon buildup in the intake. Most GDI vehicles have an exhaust gas recirculation (EGR) system, and we have all experienced the results of car-bon buildup in EGR valves. Based on what we see from the table on page 34, it’s entirely possible that some of the PM exiting the engine is being recycled back

into the intake via the EGR system and mixing with the carbon from PCV gases.

The results of the CARB study will bring new PM standards. Current stan-dards are 10mg/mi. now, 3mg/mi. start-ing in 2017 and 1mg/mi. targeted for 2025. How will vehicle manufacturers meet the future PM emissions stan-dards? This is another interesting story and might end up solving some of the carbon buildup issues.

The new Audi 1.8L TFSI GDI engine will incorporate two injection systems—port and direct injection. The PFI system would be used when PM would likely be formed and GDI would be used in other operating modes. The table clearly shows the benefit of both injection options. The new Subaru engine also uses the two-in-jection format, and other manufacturers are considering it.

This will not help the early GDI vehi-cles, of course, but might solve the issue as we move forward. One final thought on the subject of particulate matter: We already know a DPF works wonders on

diesel engines, so adding a gasoline par-ticulate filter might be another solution for gasoline engines.

How do you deal with the carbon buildup in the engine? Sam Bell did a nice job of describing most of the cleaning options available and not much has changed since that article was written. The photos above clearly show that there’s a difference in the carbon formation in various locations in the same engine, cylinder to cylinder, and might require different cleaning techniques per cylinder. The bottom photo shows the results after a walnut shell cleaning process.

My best advice is to start with a re-view of the manufacturer recommen-dations, check for any new service bulletins and, if possible, ask the local dealership what it’s doing. Always keep in mind that whatever you introduce into the engine, intake and combustion chamber will have potential side effects.

Bottom line: GDI vehicles are here to stay. The fuel system components will be fairly easy to diagnose and re-pair. Carbon issues are for real and I’m not aware of any quick fixes, but con-sumer education and preventive main-tenance seem to help avoid the cata-strophic repair bills.

36 December 2015

This article can be found online at www.motormagazine.com.


The top two photos show carbon buildup on a 2010 BMW 750Li with 64,884 miles on the odometer. The photo above shows the results after a walnut shell cleaning process.

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Phase 1 Phase 2 Phase 3

Conventional Diesel

LEV II GDI

LEV II PFI

DPF-EquippedDiesel

This is a visual comparison of particulate matter filter samples for various light-duty vehicles during the FTP driving cycle. This clearly shows that GDI vehicles produce particulate mat-ter, and that future standards are needed. The image is courtesy of CARB, 2011. “LEV III PM Technical Support Document: Appendix P,” California Air Resources Board, Dec. 7, 2011.


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