“Tear it out, put on carbs.” That statement, made in 1980, reflected the conventional wisdom about the Bosch fuel injection system that caused so much trouble on the ’75 Super Beetle. The “tech” dispensing this rather dubious advice was yours truly, and what I really meant wasn’t that the Bosch system was faulty, but rather that my understanding of injection systems was.
Today, there are few who would dispute that EFI has been a change for the better. It’s doubtful, in fact, that modern vehicles could meet current emissions specs without at least a throttle-body system. A generation of technicians has now serviced EFI vehicles for their entire careers and feel comfortable with OBD codes and scan tools. How many, however, really consider what’s happening inside? It’s worth a closer look.
Working from back to front
If it’s a gasoline engine you’re working on, fuel delivery to the rail depends on two simple, interrelated parameters: pressure and volume. Not enough of either is a problem which can be traced almost anywhere in the system, right back to the fuel filler “neck”. In-tank pumps are more rugged than they look, but rust, dirt or moisture are enemies that can degrade performance in ways that can affect driveability without setting a code. If it sounds like a running pump, most techs move on, and with many systems inviting a pressure test at the fuel rail by way of a convenient Schrader valve, that may be a sensible next step, even before pulling codes. Many experienced diagnosticians, however, change the fuel filter as a first step, before touching diagnostic tools. Why? One reason is to eliminate the filter as a possibility. There are “white box” and even counterfeit parts out there, and if you didn’t install it, there is a chance that the filter media is now corking the system. Another good reason is that a filter change lets the technician peek at a fuel sample, as the pump sees it, contaminants and all. Fuel generally looks good at the “log” but if it’s brown with rust on the tank side, there’s an issue that needs resolution regardless of the cause of the driveability problem. In this case, tank repair and flushing isn’t an upsell, but an essential part of the service. Regardless of the brilliance of your diagnosis, the customer will attribute the inevitable comeback to faulty workmanship.
Clean fuel to the firewall is a given, and once delivered at sufficient pressure and volume, it’s up to the regulator and injectors to follow orders from the ECU. A key conceptual point is that the parts of the fuel system discussed so far have nothing to do with how much fuel is delivered, or at what point in the piston stroke. Injector timing and pulse width is sourced externally to the injector body, unlike other emissions critical valves such as EGR and throttle, which often use position sensors to feed information back to the ECU. Injectors are dumb: they receive the signal, but that doesn’t mean that they either open, hold open, or deliver adequate quantities of properly atomized fuel when they are open. Deny the ECU proper feedback, and it will add fuel in the best way it can with the information provided. By far the most common contributor to ECU-dependent driveablity issues are oxygen sensors, although EGR and TPS, as mechanical systems, have notable wear issues. Removing port injection connectors with the engine running to measure RPM drop is a common practice, but it’s contraindicated for some engines, so check a source before using this procedure. The telltale “click” audible with a stethoscope at idle also confirms a cycling injector, but there’s a lot going on nearby. Noisy valve operation can be a problem, as can vibration from poor ignition performance.
Even if the computer fires the injector with correct timing and pulse width, and even if the injector flows the correct mass of fuel, it may not be atomized properly or directed in a good spray pattern. Unfortunately, technicians haven’t had the luxury of direct observation of spray patterns since the days of throttle body systems, so how can the technician know that an injector is flowing enough fuel in a good pattern? One solution is to sell injector cleaning as both a maintenance procedure and as part of high-end diagnostics. If there’s an improvement, at least the technician knows that fuel quality at the injectors is a primary culprit.
In many cases, however, they simply have to come out. Whether bench cleaning is sensible depends on the vehicle and owner preferences. Fuel injectors are actuators, like EGR valves or door lock solenoids. The fact that they control the delivery of a performance critical fluid doesn’t change an important reality: they’re much more robust electrically than many technicians realize. A good scan tool helps here. Resist the temptation to apply battery voltage to an injector either on the bench or on the car. You’ll never know whether the injector died on the road or in your hands. With aftermarket and reman injectors widely available for popular models, however, the ultimate diagnostic may be to simply replace. Can you sell injector replacement as a diagnostic procedure? Maybe, but one thing is certain: no one ever decreased driveability or performance by replacing a suspect injector.
An OEM viewpoint
Hans Ruschka knows fuel injection. As National Technical Service Manager for Robert Bosch Inc., Ruschka has trained many technicians and has strong opinions about EFI service. “We are doubtful whether an injector flush and/or other procedure will bring any long lasting relief. I hear cases where some people swear by it and others say ‘well, we’ve done it and in six months he’s back’. But where we are very firm is that we do not support any additives. We feel that the gasoline additives being sold in most Western nations should be of such quality, even the low-octane versions, that it will flow. If the gasoline meets the Canadian General Standards Board Regulations, then it is definitely good enough for our injectors. That’s why we are hesitant to even endorse or talk about any possible additives.”
Is fuel injection strictly a diagnose and replace system? Injector flushing is a recognized service procedure for original equipment manufacturers, and most OEM’s offer fuel system additives. Many independent service businesses swear by them. Who’s right? It depends who you talk to, but one point is clear: At some point, heavily contaminated or clogged injectors won’t respond to additives or cleaning systems. According to Hans Ruschka, “the trick is this: there are injectors with the straightforward pencil beam; there are also injectors out there now with double or V-type spray patterns. Can you effectively clean those? Possibly not. With some of the more modern injectors, the technology is such that the relationship between pinhole design and orifice size, together with the trigger time, gives you the exact amount of fuel that the engine needs in order to run cleanly and efficiently. So therefore we really like to keep people out of it.”
When troubleshooting injectors, Ruschka emphatically states that technicians must never apply 12-volt potentials across the coils, even with a current limiting resistor in place. Instant failure of the injector coil is a common outcome: “These things will either survive or go in a second if you don’t do the proper maintenance and troubleshooting procedure.”
No conversation with an injection expert could be complete without talking about fuel blends. Winter fuels are often blended with alcohols, and the Bosch injectors common to many European models are compatible to alcohol loadings to as high as 15 percent. Higher percentages are a question mark: “Anything beyond that, you are asking for trouble,” says Ruschka. “Not necessarily in the injectors first, but somewhere down the line in the system with gasket swelling and other problems. Many gas stations, for instance Pioneer, has a sign on their pumps that says ‘may contain up to 10 percent alcohol’, and we have not problem with that from a handling perspective. What it does to the pe
rformance is another question.”
Testing Vehicle Fuel Pressure
Ensuring adequate fuel pressure to injectors is essential to time and cost-efficient troubleshooting. Glenn A. Hunt, Product Development Manager for Mityvac/Prism and an ASE Master Automobile Technician, answers some frequently asked questions about this vital procedure.
Warning: Do not perform these steps when the engine is hot or if there is a presence of smoke, sparks or open flame.
Q: When should the fuel pressure be checked?
A: The fuel pressure should be checked whenever a regular tune-up is performed on the vehicle. If the fuel pressure is too low or too high it will be an indication that further diagnosis or repair is needed to correct the problem. Unfortunately, many times the fuel pressure goes unchecked until the vehicle stalls out on the highway. Indicators of a fuel related problem would be:
Lack of power going uphill or when pulling a load
Backfiring through the intake when accelerating
Poor fuel economy
Fuel mileage that is too good to be true
Q: How is fuel pressure checked?
A: Most domestic and some foreign automobiles are equipped with a pressure test port. This test port is usually located on a steel line of the fuel system and typically located on top of the engine near the center. (Refer to manufacturer’s literature to determine the exact location.) If the vehicle is not equipped with a test port, then an assortment of adapter fittings may be required in order for the tester to work. These adapters are usually available at a local hardware store or from a mobile tool vendor. The fuel injection pressure test kit will test pressure ranging from 0-100 psig; the exact pressure will vary depending on the vehicle manufacturer. Listed below are a few common pressure ranges, but please refer to the manufacturer’s data as listed for your vehicle specific information.
Make/Fuel Injection Systems Pressure Range
GM TBI9-13 psig
GM MPFI40-47 psig
GM SFI60-66 psig
Ford EFI/MPI/SFI 35-45 psig
Jeep MFI39-41 psig
Jeep SFI50 psig +/- 5psig
BMW3.5-4 bar 49-58psig
Honda/Acura MFI38-48 psig
Once you have connected the fuel injection pressure tester to the vehicle, turn the ignition key to the “on” position (with engine not running) and monitor the tester’s gauge reading. If the gauge comes up to the proper reading (pressure range) then the system has passed. If it does not come up to the proper reading here are a few things to check:
1. Ensure that the tester is not leaking and is hooked up correctly. Sometimes the o-rings that are inside the end of the pressure hose need to be tightened enough to allow the push pin to depress the Schrader valve that is located on the vehicle’s fitting.
2. Listen for the pump to activate when the key is turned to the “on” position. This may require a helper to listen closely at the fuel filler door (with the cap removed) for the “whirring or buzzing” noise that the pump will make when in operation. On most vehicles this will only occur for about 10 seconds. If there is no pump activation, the next step is to check all of the fuses and relays that are associated with the fuel system.
3. If the fuel pump does activate but there is low or no pressure at the tester, check for:
a. Clogged fuel filter
b. Restricted or damaged fuel line
c. Defective fuel pump
d. Damaged fuel line at the pump connection
e. Bad fuel pressure regulator
After any repairs are performed, be sure to test the fuel pressure again to ensure that the reading is correct and no further repair is needed. Also be sure to check for any leaks or loose fittings that can cause problems later down the road.