It Can Make for a Bad Day - October/November - 2017

Driveability Diagnosis: It Couldn’t Be That… Or Could It?

Diagnosing today’s vehicles is a challenge that every shop faces. A quick, correct diagnosis is the difference between the job being a moneymaker or a loser. Most of us have seen vehicles that weren’t only a challenge to diagnose but, in some instances, have made us wonder why we ever got into this profession.

Driveability diagnosis, like transmission diagnosis, is affected by the technological changes vehicles have undergone the past few years. Distributorless ignitions, direct injection fuel systems, diesel particulate filtration, selective catalytic reduction systems, variable cylinder engines, and variable valve timing are just a few of the systems we’re seeing today.

The future holds more changes: variable engine compression ratio systems, high speed flywheel energy storage systems, multistage turbocharger systems, two-way one-way clutches, smart injectors, smart solenoids, advanced friction reduction systems, waste heat recovery systems, and telematics, including V-to-V (vehicle-to-vehicle) communication systems, are very close to reality as we speak.

Today’s technology can make diagnosis even more of a challenge. With that said, let’s look at a change that’s occurred over the past few years that’s having an impact on driveability diagnosis: direct injection fuel delivery systems.

Most of us are aware that direct injection systems have been around forever on diesel engines, but you may not realize that, in the mid 2000s, most manufacturers introduced direct injection systems on gas engines. This design change has led to driveability-related issues that weren’t as common with the older port and throttle body injected systems.

In direct injection systems, the fuel injector is located in the combustion chamber. Like a diesel, the system injects fuel directly into the cylinder at high pressures. Advantages for direct injection include better cold start emissions, improved fuel economy (8%), and increased horsepower and torque.

But, as with many other things in life, there’s always a tradeoff. In this instance, that tradeoff is engine or injector carbon buildup and intake valve coking. Yes, these conditions occur on non-direct injected engines, but not nearly as often as on direct injection engines. These issues can cause several different issues, such as:

  • Extended cranking
  • Engine won’t start
  • Malfunction Indicator Lamp (MIL) or Service Engine Soon (SES) light comes on
  • Diagnostic Trouble Codes (DTCs)
  • Cold start misfire
  • Hesitation on acceleration
  • Lack of power
  • Surge or chuggle
  • Rough idle
  • Light or intermittent misfire
  • Shudder or vibration, which may get worse with the TCC applied

With a direct injected gasoline engine, the fuel is injected directly into the combustion chamber. On older, port fuel injected engines, the injector was never exposed to the combustion chamber. In addition, the fuel was sprayed onto the back of the intake valve, cooling and cleaning the valve as the injection process occurred.

The primary issue with valve and intake coking deposits is due to the PCV and EGR designs used with these applications. Today’s engines use low-tension piston rings and ultra-low viscosity oils, which increase the oil vapors that reach the PCV and EGR systems.

These oil vapors travel into the cylinder head and ultimately into the combustion chambers. Since the intake valves no longer have fuel sprayed onto them, deposits form (figure 1).

Manufacturers are attempting to address this issue with changes in camshaft valve overlap, leading to a slight washing effect on the valves. Some manufacturers have even experimented with an additional injector to dilute the contaminates. In addition, some aftermarket suppliers have introduced oil vapor traps to try to address the issue.

Injector and combustion chamber carbon deposits are also caused by gasoline quality. The old adage, “you get what you pay for” definitely rings true with respect to gasoline quality.

Every automotive manufacturer recommends the use of top-tier detergent gasoline. Top-tier gasoline includes a manufacturer-approved additive package that’s designed to lessen the deposits in the injectors and combustion chambers. To find out which fuel distributors sell top-tier fuels, visit

Most manufacturers have even gone a step further, recommending that you add an approved injector cleaner to your tank at every oil change.

Maintenance is typically the last thing that some people think of, so you’ll likely experience coking and injector flow-related driveability issues in your shop. So how should you diagnose an issue on a direct injected engine?

Here’s an example from real life: A direct-injected engine comes in with the check engine light on. The engine has a surge under light load. Like any other diagnostic process, start with the basics:

  • Battery and battery cable condition
  • Open fuses
  • Grounds clean and tight
  • Aftermarket devices, such as insurance company dongles plugged into the DLC
  • Bulletins related to driveability issues
  • Updated calibrations that may address the driveability issue

A scan reveals code P0300: random misfire. When dealing with a random misfire DTC, you need to understand how the ECM calculates misfire on most vehicles. Typically it monitors the crankshaft and camshaft position sensors to determine misfire.

Between cylinder firings, the crankshaft slows until the next cylinder fires. The ECM monitors how much change in crankshaft speed occurs between cylinder firings. If it sees an excessive change in speed, the misfire counter will record it. The ECM then uses the camshaft sensor position to calculate which cylinder is misfiring.

Since we have a P0300 random misfire, the problem may be that multiple cylinders are misfiring, or maybe the counters are being erased before the misfire is bad enough to flag a specific cylinder. Mode 06 and misfire data available on some scan tools can help you locate the cylinders exhibiting problems.

Problems outside the engine, fuel, or injection system can also cause random misfire and P0300 issues. Keep in mind, anything that can cause a variation in crankshaft speed can lead to this code, such as:

  • A damaged or faulty accessory or accessory drive belt
  • Wheel, tire, or driveshaft balance or runout
  • Driving on rough roads
  • Transmission issues

You may also have related system issues, such as:

  • Active fuel management (AFM)/ displacement on demand (DOD) valve lifters sticking. On engines equipped with variable-lift rocker arms, check for proper rocker arm operation.
  • High resistance in the injectors or wiring can lead to misfire codes without setting an injector electrical DTC.
  • O2 sensor heater circuits have been known to set a P0300.
  • Other input issues, such as MAF, MAP, TP, APP, or CTS, can cause the misfire DTC to set.
  • Crankshaft learn position no longer stored in the ECM memory. A loose crankshaft reluctor or trigger wheel has also been known to cause a P0300.
  • Contaminated fuel
  • Fuel pressure or fuel pump issues
  • Vacuum leaks
  • Restricted exhaust
  • Engine consuming excessive amounts of oil. Several known problems and solutions are available for various engines regarding PCV and piston ring issues.

In this example, we have a couple choices that can help isolate a misfire DTC related to a fuel, ignition, or mechanical issue. Check the misfire counters or mode 06 data to help isolate which cylinder is causing the issue or use the cylinder deactivation feature on your scan tool to help isolate the cause if the problem is present at the time.

Most scan tools can deactivate cylinders, which allows you to run what used to be called a power balance test. A cylinder that doesn’t exhibit an RPM change is a likely place to start your diagnosis. On engines with misfire counters, cylinders with high misfire counts could be misfiring.

Separating an injector problem from an engine mechanical issue is a little more complicated. Some manufacturers allow you to conduct pressure drop or CC volume tests for the injectors, which helps isolate a restricted injector. Other manufacturers don’t offer this feature, so you need to isolate the engine side first to separate the injector from engine mechanical issues.

A dynamic (running) compression test is a good way to isolate an engine mechanical issue, such as carbon holding a valve open. Compare the running compression for the suspect cylinder with other cylinders. Generally, running compression on most engines will be in the 50–80 PSI range.

During the running compression test, snap the throttle: You should see about 80% of the results you’d see during a static compression test. Release the compression stored and monitor the repeated build time for the compression.

If compression seems slow to build, it indicates likely issues with the valve train (DOD lifters or variable rocker arm systems), a restricted exhaust, a flat camshaft, or carbon possibly holding the valve open.


If you find an engine that seems to suffer from carbon buildup, you may be able to correct the problem by cleaning the injectors. There are several different injection cleaning service procedures on the market today: Always use a reputable, professional injector cleaning system, and always follow their directions for cleaning the injectors on a direct injection gas engine.

How effective are these processes? In most cases, I’ve found them to be about 50% effective. In other words, if you have an injector flow issue, 50% of the time cleaning takes care of the issue; the rest of the time, you end up replacing the injector.

For valve coking, it’s about the same: 50% of the time the cleaning process addresses the issue and the other 50% require you to remove the intake and clean the valves and ports manually.

As with all other forms of service, if your customer allows you to perform the cleaning process prior to an issue developing, you’ll likely avoid all of these issues. It’s the old “pay me now or pay me a lot more later” scenario. It’s hard for some customers to accept that, even today, vehicle maintenance is still necessary to avoid problems.

As always, it’s up to you to educate the customer. In this regard, a little education can improve your relationship with your customer, as well as your bottom line.

Intake/Injector Cleaning Process

Most manufacturers utilize procedures that allow you to clean suspect injector tips and intake ports. In general, the following process is performed on a direct injected engine to address valve and port coking issues:

  • Load the cleaning equipment with the correct amount of top end fuel injector cleaner (figures A, B & C).
  • Pre-charge the cleaning equipment with the correct air charge.
  • Bring the engine to full operating temperature.
  • Remove the air cleaner inlet tube from the throttle body. Insert the injector cleaner nozzle at the center of the inlet to the throttle plate. Start the engine and raise the engine speed to approximately 2000 RPM. Slowly open the valve on your cleaning equipment to allow the cleaner to be drawn past the throttle plates and into the engine (figure D).
    If you do not have the required cleaning equipment you can use a container with a vacuum hose attached. The vacuum hose must utilize a restrictor to limit the amount of cleaner flowing into the engine or a hydrostatic lock condition could occur which would make for a very bad day. Connect the vacuum hose to a vacuum port and start the engine.
  • Once the cleaner has been consumed, shut off the engine to allow the cleaner to soak into the deposits. Manufactures recommend that you leave the engine off for 2 1/2 hours but not longer than 3 hours since the cleaner can evaporate if the engine is left sitting too long.
  • Put a bottle of fuel system treatment in the tank and reevaluate the concern during your test drive.

To clean the injectors, some engines are equipped with a Schrader valve located in the fuel rail which allows you to check fuel pressure for the low-pressure fuel system. The Schrader valve is usually located close to the high-pressure pump for the system. The Schrader valve can also be used as a point to inject cleaner into the fuel injector.

On those applications that do not use a Schrader valve, you will need to tap into the low-pressure side of the system with some fittings and lines. Several tool manufactures make kits to enable you to perform this procedure. To clean a directed injection system injector, you will need to:

  • Disable the low-pressure fuel pump by removing the fuse, disconnecting the fuel pump control module connector or by using your scan tool to turn off the pump. Typically, a direct injected system does not have a return line so you will likely not have to be concerned with the cleaner getting back into the tank as was the case with previous injection systems.
  • Install your injector cleaning equipment and mix the correct ratio of injector cleaner and gasoline.
  • Pre-charge your cleaning equipment tank with the recommended amount of air pressure.
  • Open the valve on the cleaning equipment.
  • Allow the engine to run for at least 20 minutes or until the engine stalls due to a lack of fuel.
  • Put a bottle of fuel system treatment in the tank and reevaluate the concern during your test drive.

In both scenarios, after cleaning you will need to drive the vehicle to complete the process. “Drive it like you stole it” is how most driveability technicians describe operating the vehicle after cleaning. High engine rpm and high intake/exhaust flow rates tends to expel large amounts of carbon. One would always want to make sure that he/she follows all the appropriate speed limits and traffic laws as I would doubt that the patrolman that stops you will accept your explanation. In addition, the engine must be in good mechanical condition as you do not want to create a problem for yourself by damaging the engine.