TechTips
Eyes On Fuel Control
When The PCM Doesn’t See What We Think It Seesby Dave Sill* It has been said that the eyes are the window into the soul. In a closed-loop fuel control system, the oxygen sensors are the eyes of the system, and if one wishes to look into the soul of the system, the best way to do it is through these eyes.
Case in point: I recently worked with a technician named Mike. His customer had a 1993 Ford Ranger pickup with the 4.0L EFI engine. This was a high-mileage vehicle in which the engine had recently been replaced. The new engine had about 10,000 on it. The customer complaint was low power, poor performance and poor fuel economy — ever since the engine was replaced.
The truck’s owner had complained to the engine installer, and had taken it to another shop, but neither had been able to locate the cause.
Mike had already scanned for trouble codes. KOEO and continuous memory tests showed code 111 — system pass. The customer stated that the MIL lamp never came on while driving. All indications from the computer were that there were no problems in the system.
When Mike took the truck for a test drive, he could definitely tell that it was doggy and just didn’t have the power it should. He decided to give us a call. A quick check of the EEC system wiring diagram told me that this was a very unusual system for a Ford. The 49-state version normally uses bank-fired injection rather than sequential injection. But this engine was a variation from Ford’s normal bank-fired fuel injection systems.
On most Ford bank-fired systems, injector banks are not the same arrangement as cylinder banks, and only one oxygen sensor is used for fuel control. For example, the 2.9L EFI engine is typical of most Ford V-6 bank-fired systems. Injectors 1, 4 and 2 are on one bank, and cylinders 5, 3 and 6 are on the other.
The system on this Ranger 4.0L, however, fires all the injectors on the passenger’s side of the engine (1, 2 and 3) together, and all the injectors on the driver’s side (4, 5 and 6) together. It uses two oxygen sensors, one in each exhaust manifold.
I asked Mike if he had checked base timing. He had. I then asked him to also verify that timing was advancing with the SPOUT connector installed. I suggested that if the timing advance checked out, the next logical step was to monitor both oxygen sensors while driving the truck. Since both oxygen sensors were new, I felt we should go straight to using them for information (see sidebar next page).
Mike called me back with his results. Timing advance was working normally. However, watching the oxygen sensors had revealed a strange pattern. Rather than constantly switching rich and lean as would be expected, one oxygen sensor would hang rich for a few seconds and the other would hang lean, then vice versa, in a constantly repeating cycle. I suspected that the computer was trying to control the wrong side of the engine!
Often during the course of removing and installing an engine, wiring harnesses and connectors can get well out of their normal positions. So I asked Mike to verify by wire colors that both oxygen sensors and both injector banks were connected correctly. Shortly thereafter, Mike called me back with the happy ending. Apparently both oxygen sensor harnesses came out of the main harness at the same spot, were the same length, had identical connectors, and had been connected to the wrong sensors after the engine had been installed!
Mike swapped the connectors, and normal engine power and oxygen sensor operation returned. Needless to say, Mike had a happy customer!
Logically, this should have been a problem that was recognized by the computer and there should have been a MIL light on, or at least a trouble code set. In the real world things don’t always work out that way.
By looking into the right window, we can often find problems that the system itself doesn’t tell us about. If you suspect a fuel control problem, the oxygen sensor or sensors provide this window. Lab scopes are best, scan tools are great, but often a lowly voltmeter will provide you with enough information to solve a tough problem.
Test That Oxygen Sensor!
Here’s how to test a Ford oxygen sensor to make sure it’s in good working condition. This can be done using a lab scope (best) or a digital voltmeter. Hook the lab scope to the sensor’s signal wire. Set the voltage scale to around 200 mV/div and the time scale to around 1 second/div. Set the ground one or two divisions up from the bottom of the screen. If the oxygen sensor is heated (has more than one wire), make sure that battery voltage and ground are available for the heater element.Warm the sensor by running the engine at about 2,000 rpm for a couple of minutes. Then create rich and lean conditions, and monitor the sensor’s response. This can be done by adding fuel (propane works well) to cause a rich condition, and creating a large vacuum leak to cause a lean condition. Or, try a series of rapid engine accelerations and decelerations, with short pauses between. Quick acceleration should cause a rich condition, and quick deceleration should induce a lean condition.
Look for the oxygen sensor voltage to go above 800 mV when a rich condition is present, and below 150 mV when lean. Maximum voltage should never be above 1.1 V, and the minimum voltage should never stray into the negative range by more than a few mV. Transition time from lean to rich and rich to lean is also important. The oxygen sensor should be capable of switching from below 250 mV to above 750 mV (and vice versa) in less than 100 mSec. The same tests can be done using a good digital voltmeter, however, a voltmeter can’t tell you transition time. If using a voltmeter, look for transitions from rich to lean and lean to rich to appear instantaneously on the meter. If the sensor fails any one of these tests, it can’t be reliably used for fuel control information.
Note: These tests apply to any Ford oxygen sensor. Other car makes may use other types of oxygen sensors, so be sure to consult your service information before testing them.
*Dave Sill has 10 years experience diagnosing and repairing automotive problems. He is the team leader for the Ford section at AutoLine Telediagnosis fielding calls in all areas, including electrical, driveability and drivetrain. Dave holds an A.A.S. degree in automotive technology and has both his Master and L-1 certifications.
TechTips is a monthly feature in AUTOINC. Each month, specialists who field calls for Autoline Telediagnosis, a technical hotline service, share some of the most common vehicle problems and their solutions. As an ASA member, you’re already signed up to use the AutoLine service at a discounted rate.
AutoLine Telediagnosis: 1-800-288-6220, Monday-Friday 7 a.m. to 7 p.m. CST
ASA Main Page || AutoInc. Main Page
TechTips || Stat Corner || News Briefs
News Briefs Extra || Taking The Hill || Directions
A Team Approach || EPA Proposes || Tech To Tech
Testing Shortcuts || Paint Preparation || Convention Puts Members
Around ASA || More Info About AutoInc. || Chairman’s Message
AutoInc. Magazine ®, Vol. XLIV No. 6, June 1996