How Dirty Fuel Injectors Affect Engine Operation, Fuel Economy and Performance

While this article applies to almost every engine, we’ll be using a GM V8 as our example. The same points would apply to a 4 cylinder, there would just be 1 bank of 4 cylinders instead of 2 banks. Most 4 cylinder engines will use one O2 sensor for exhaust measurement and PCM operation. There is typically a second O2 sensor after the catalyst to measure it’s efficiency and function. Everything here also applies to a V6, it would be 2 banks of 3 cylinders with an O2 sensor on each bank, and one behind each catalyst. Inline engines will sometimes split their manifolds into two banks of 3 with individual sensors per bank, v10’s and v12’s will typically follow the same rules as the v8. We won’t be getting into W8’s, W16’s, etc. but all of the principals below apply. We’ll also be referencing the PCM (Powertrain Control Module) also called the ECU (Engine Control Unit) or just “Computer”. Different vehicle manufacturers call it different things, but for this article we’ll just be using the term PCM which is universal among all engine controllers. We will also mention fuel trimming, fuel trimming is when the PCM adds or removes small amounts of fuel through short and long term strategies by typically +/- 10%

In this article we’ll use this example set of fuel injectors from a 2002 Chevrolet Corvette Z06. It’s equipped with a 385 horsepower LS6 engine with Bosch 12561462 fuel injectors. It’s fuel injectors were sent in for a check up. The car failed it’s emissions inspection, was becoming hard to start, developed a rough idle, loss of power, and a check engine light was on.

This corvette (and almost every other late model car on the road) uses an oxygen sensor in the exhaust system on each side of the engine. By measuring the amount of unburned oxygen left in the exhaust, the PCM can determine the mixture; there are narrow range methods of measurement and wide range methods of measurement which won’t be discussed here. Being a v8 we will talk about each side of the engine as a bank, a bank is a group of 4 cylinders. The PCM uses data from the oxygen sensors and other sensors to trim fueling. With only two sensors, it works by taking an average measurement of the remaining oxygen of all 4 cylinders in each bank. The PCM will then apply corrective trimming of fuel by adding or removing a percentage of fuel to an entire bank of cylinders by the same amount, trimming each individual cylinder would be impossible without an oxygen sensor per cylinder. The PCM assumes(because it has no way of knowing otherwise) that each injector in each bank is flowing the same amount of fuel. If the PCM applies the same pulse width and corrections based on average readings to a poorly flowing set of injectors, some cylinders end up with significantly more or less fuel then the others, only aggravating the PCM’s correction strategy. The rich cylinders tend to consume excessive fuel, foul the spark plug, produce excessive emissions and damage the sensors. Lean cylinders tend to cause a loss of power, idle misfires, and hard starting.

This engine has cylinders 1,3,5,7 down the driver side and 2,4,6,8 down the passenger side. Oxygen sensor 1 is reading cylinders 1,3,5,7 while oxygen sensor 2 reads cylinders 2,4,6,8. Lets say, for example, cylinder 5’s injector is dirty and flowing 30% less then 1,3 and 7. The PCM will read this as a lean condition for all of bank 1 (1,3,5,7) and apply enough additional fuel to all 4 cylinders (1,3,5,7) until the oxygen sensor sees the correct mixture. Unfortunately now 1,3,7 are being over fueled, this will foul the spark plugs, dirty the oil, cause a loss of power in those cylinders, excessive emissions, exhaust smell, and reduced fuel economy in those cylinders. At the same time, cylinder 5 is STILL leaner then it’s supposed to be, which will typically cause a rough idle, a misfire, and a loss of power in that cylinder. This engine makes 385 horsepower (to the flywheel) from GM. Each cylinder is producing around 48 horsepower. Because of the incorrect fueling, and the one dirty injector, all cylinders in that bank could now be making 10-20% less power while using more fuel. Typically this happens gradually and you may not notice if you drive the vehicle regularly until it’s bad enough to set a check engine light.)

The initial static flow test. Dynamic flow testing will be completed when finished. Dynamic and Static flow testing is explained in a separate article. This is just a simple measurement of the flow rate of each injector. Each injector is being fed by the same supply, at the same pressure. This is the actual flow variance in a dirty set of production injectors. [Apologies for the shaky video, we now have a mount for the camera]

Let’s talk about injector leakage. As part of our service, we pressurize each injector to it’s maximum rating, then inspect the body and valve area for leakage. This particular set had a set an injector which was leaking. When this happens, fuel continues to leak through the injector into the engine until no fuel pressure in the line remains, fuel will puddle in the intake manifold, cylinder head, and cylinders if a valve is open. This is the primary cause of hard starting, as one or more cylinders are completely fuel fouled after sitting. Beyond fuel leaking through the injector, the older style injectors (Bosch EV1 style, fat metal body) tend to leak between the top of the injector and the metal base. This is obviously a fire hazard, and we inspect for this.

Beyond fuel leaking through the injector, the older style injectors (Bosch EV1 style, fat metal body) tend to leak between the top of the injector and the metal base. This is obviously a fire hazard, and we inspect for this. The video above is a severe case but some will slowly seep and create the same hazard.

We also observed poor injector spray pattern. Most fuel injectors are designed to atomize the fuel, spraying towards the back of the intake valve where it further atomizes by evaporation from the heat of the intake valve while being drawn into the cylinder. When the injector pattern is poor and sloppy, fuel is allowed to collect on the port walls instead of where it was designed to go. Just like unequal total flow, the PCM can only assume that all 8 injectors have the same spray pattern, and are operating as designed by the engineers and manufacturer. The fuel collecting on the walls can cause the engine to idle and cruise lean(which the PCM will attempt to correct by trimming in additional fuel) until a large throttle transition (acceleration) occurs and the engine ingests the collected fuel at once causing a rich condition(which the PCM will again intervene by trimming out the additional fuel, creating an unintentional lean condition)

An illustration by Racetronix on what manufacturers and engineers attempt to accomplish with spray pattern. Now imagine a poor spray pattern spraying everywhere else except where the engineer tried to put it. You can see how fuel collects on walls and other places it shouldn’t.

Inspecting spray pattern, and atomization. These are the LS6 Corvette injectors being referenced in this article. The injectors are in order, note 1, 4, 5, 7 and 8. 1,4,5,7,8 will be wetting the port walls creating an artificial lean condition(even if they flow exactly the same as the others) which the PCM will attempt to correct by by incorrectly trimming fuel, compounding the issue. The owner did complain of off idle hesitation and misfires, this is the most likely cause.

Now we’ll inspect the nozzle area of the injector under magnification. The nozzle area is usually contaminated by dirty intake manifold air (which we explain in a separate article). The tip of the injector stays “wet” while dirty intake air flows past it. The oil breathers and crank case ventilation in nearly all OE applications are vented into the intake, some engines also use an EGR system (some newer engines now do the same effect with variable camshaft timing). EGR is “Exhaust Gas Recirculation” (which is also explained in a separate article) For emissions and fuel economy reasons, actual dirty exhaust is allowed in through a valve to the intake manifold (the EGR system can reduce the fuel demands of the engine at lower power requirement conditions by using inert exhaust). Between the EGR, crank case ventilation, oil vapor, and poor quality fuel, deposits and corrosion form in the nozzle of the injector. This is where most of our attention is paid, the spray area and the valve directly above the nozzle spray plate. The below pictures are the spray nozzles of the injectors in the above video and flow pattern check. You can see how some are partially blocked. The area directly behind these plates is the valve, we’re able to clean and service this as well. This is something you would never get clean with an on-vehicle injector service, or commercially available bottle of cleaner, it just won’t happen.

We’ll start by removing the replaceable service parts, seals, o-rings, filters, retainers, and the caps. They’ll then go into an ultrasonic bath to clean the exterior bodies. All of our cleaners are non-toxic, water based and bio degradable.

Since these injectors test okay electrically and mechanically, no repairs are necessary, they’ll only require a thorough cleaning. After the exterior receives a quick bath in a mild detergent, the injectors follow along our process, some of which we don’t show, cleaning the injector inside and out. They’re moved through various cleaning stages to remove varnish, oil, carbon, and corrosion. They’re then sent for a final ultrasonic cleaning which completes the removal of any contaminants we may have left behind, they are then high pressure back flushed with cleaner, rinsed with a mild solvent, and dried. We install new internal filters, seals, o-rings, retainers, clips, and anything else required, and move them back to flow testing. We’ll flow test them dynamically at different rpm’s and pulse widths, we’ll test their overall flow statically, we’ll also pressure test, spray pattern test and leak test them again. Once complete, they’re purged of testing fluid, oiled, sealed and returned to the customer with a final report.

Injectors in the final stages of ultrasonic cleaning. They’re powered and cycling during this process.

At this point they’re completely clean, and free of any contaminants. New filters, o-rings, seals and retainers have been installed. We then put the nozzles back under magnification to inspect. These spray plates were cleaned touchlessly. No wire brushes, sand paper, grinders, etc. We do not anything to alter the flow rate or pattern from the manufacturers original design