Auto Service World
Feature   November 1, 2003   by Rick Cogbill

Drivability vs. Fixability

Tips for better fuel system diagnostics Blue Streak's Doug Doran on efficient, accurate fuel system repair

If the term drivability problem makes you feel queasy inside, you’re not alone. It covers just about anything that prevents the vehicle from running smooth and steady under all driving conditions, and a large number of drivability complaints directly involve the fuel delivery system.

There is no question that fuel systems have come a long way since Henry Ford began mass-producing automobiles. From fuel pumps to injectors, the basic components themselves have undergone massive changes, and on top of that a new element has been introduced – the computer. We have gone from adjusting float levels and idle mixtures to analyzing data.

Although these systems are entering their third decade, there is still a lot of uncertainty when it comes to proper diagnosis and repair. Problems like intermittent misfires to high fuel consumption can be baffling without a good understanding of how the system works and some perfectly good parts might get replaced in the process. In fact, according to Doug Doran, Field Trainer with Blue Streak-Hygrade Motor Products of Mississauga, Ontario, it’s amazing how many technicians are still tempted to throw up their hands in frustration and “replace the black box.”

Traveling the country for the past eight years, Doran teaches technical seminars on drivability problems, emission failures, and oscilloscope diagnostics. “I teach the guys how to read their hand-held scopes better,” says Doran, referring to his favorite tool. “I show them what things to look for, especially with OBDII.”

Even though Doran has observed that on the whole Canadian technicians are better trained than their American counterparts (thanks to our provincial apprenticeship programs), there are still a number who don’t clearly understand how the feedback fuel systems really work. “People are looking not so much for theory at my courses,” he says. “They come looking for quick fixes.” As one student expressed it, “I wish there was a program where I could tell it what was wrong with the car and it could tell me what to fix.” In truth, the information and the tools are there; the key is in knowing how to use them.

Important tips for better fuel system diagnosis


When diagnosing a drivability problem, it’s important to know where to start. “Everything ties in so dramatically to the control systems that it’s really essential to check for trouble codes,” says Doran. “Make sure that the computer hasn’t already found an existing problem with the emission control system.” Of course, checking for basics like compression is important, but pulling spark plugs on some vehicles is a formidable job. Instead, Doran recommends doing a Secondary KV wide-open throttle test. Any low KV’s that show up mean you have a weak cylinder. It’s quick, easy, and allows you to get on with your diagnostics.


If you have no codes and no Check Engine light, Doran recommends making sure the feedback fuel system is responding to the exhaust oxygen content. “Make sure the computer is in charge,” he says, noting that with OBDII systems, input rationality has to equal output rationality. Simply put, an OBDII computer stores sensor data over time and if a sensor fails, the ECM will resort to using sensor readings from its database instead of live data to ensure the emissions stay within limits.

To check for computer control, a snap throttle test at 2000 rpm should show you a couple of things. First, make sure that the oxygen sensor is capable of controlling the fuel delivery system. You should see a range of over 800mV to less than 175mV. Don’t get too hung up on how fast the response is, just ensure that it can cover the range.

Next, find out if the computer is controlling the injector according to the O2 sensor response. At 2000 rpm, snap the throttle and monitor the pulse width. Starting at a base pulse width of around 3 milliseconds, it should first jump up to 5 milliseconds, and then down to around 1.5 milliseconds. “If the injector responds correctly both ways,” says Doran, “then I know there’s nothing wrong with my feedback fuel control system. The computer, the sensors, and the wiring are all working the way they should.”


It’s at this point that Doran would start looking for mechanical problems, such as vacuum leaks, leaking or plugged injectors, etc. The beauty of these tests is that they should only take about two minutes. That’s valuable time when you’re tracing a difficult problem.

Avoiding common mistakes and misconceptions

There are some common statements that Doran runs into during his seminars:

“A rich mixture will always make the O2 sensor read rich.” Not so. Remember that an O2 sensor doesn’t know what gas is; it only reads oxygen. If your rich mixture is the result of a dead miss, then your O2 sensor will be reading lean and instructing the computer to add more fuel.

“A noisy fuel pump is ready to quit.” Actually, this may be true, but do a scope test on the pump to be sure. Look for proper amp ramping (the pattern resembles humps on a camel). Each hump represents the amount of current being drawn by each segment of the rotor on the fuel pump armature. A spike or a flat line will indicate a problem with one or more of the segments.

“It can’t be misfiring because the OBDII misfire monitor doesn’t show anything.” Be aware that if you have cleared codes or cleared the Keep Alive Memory on an OBDII system, then the misfire monitor needs to be reactivated. Ford and Chrysler typically require you to decel from 60mph to 40mph three times with no brake or downshift, whereas GM says to warm the engine to operating temperature, and then take the rpm up to fuel system shutoff, which is between 4,000 – 5,000 rpm depending on the vehicle.

“Oxygen Sensors last forever.” Although many manufacturers recommend replacing the O2 sensor every 100,000km, Doran says you should really test them. “I’ve seen one working beautifully at 230,000 km, and another one fail at just 7,000 km.” Blue Streak lists the following test specs:

– Must be able to read above .800V and below .175V

– Must be able to respond from over .800V to below .175V in less than 100 milliseconds

– Should have an average voltage of around .450V

– Should never show a constant voltage of below .175V

Doran also points out that it’s important to run the scan tool tests at different rpm. For example, if you see a positive number in the Long Term Fuel Trim column at idle, yet it returns to zero at 2500 rpm, then you know you have a vacuum leak (hose, intake, or maybe EGR). Conversely, if Long Term Fuel Trim is zero at idle, yet climbs at higher rpm, then it indicates low fuel pump volume or even a plugged filter. “I had a misfire problem where Long Term Trim was 3.9 at idle, and went up to 14 at 2500 rpm,” says Doran. The problem was a plugged injector. After cleaning, trim readings were zero at both rpm levels. “A plugged injector will add fuel even at idle,” he adds, “but it can’t handle the high level load.”

So don’t let drivability problems leave you behind. Get familiar with your hand-held oscilloscope and you’ll soon be leading the pack when it comes to fuel system diagnostics. SSGM

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