Surging Stallion

VEHICLE: 2002 Mustang

MILEAGE: 11008 miles

DRIVETRAIN: 4.6L engine, 4R70W transmission

COMPLAINT:  Bad surge while driving.

Sometimes a tried and true troubleshooting procedure can send us in the wrong direction…

Spring break has come and gone, and I spent four of my five days off working in a service bay at the Ford dealer where I spent so many years as a professional technician.  Work orders were flying thick and fast this week, and I encountered a few intriguing situations I thought readers might want to absorb.

            Any person who has worked in or around the field of automotive service and repair knows that a technician without a personal library of fact-finding and problem-solving skills is prone to continual frustration and possible failure.  The best and most capable technicians are the ones that are willing to tangle with the tough-to-find concerns and hang in there tooth and nail until they unravel the mystery and win the fight. These techs must be willing to exhaust everything the TSB’s and hotline engineers have to offer and go it out alone until they eventually slay the dragon.   In a word, good technicians must have real experience with unusual problems, along with the smarts to go beyond the obvious when looking for the answers.  And we all know tough problems will inevitably come to a serious technician. The difference is made with the right attitude, necessary aptitude, and downright grit, and those factors usually determine who will prevail and who will fail.

            Some concerns naturally lead to a particular troubleshooting practice, and each tech generally has his or her own way of finding the cause of a particular concern.  We’re all familiar with just how fast the world of automotive service is changing.  Last month’s 6.0L Power Stroke situation is a good example of how necessary it is to develop new techniques.

When multi-point fuel injection became the order of the day on so many engine platforms of the early eighties, we had to come up with new procedures in order to pinpoint the new problems that came with that particular change.  Almost overnight (it seemed) we moved from dealing with old fashioned carburetor concerns to the point where we were struggling with high pressure fuel pump failures, dirty injector nozzles, and octane-absorbing carbon/olefin deposits on intake valves.

Complex electronics infiltrated transmissions, suspension, steering, and braking systems. Simple electrical systems evolved into a maze of modules and multiplexing, and a technician without a scan tool and a multimeter could be compared to a soldier without weapons. Developing an understanding of the strengths and weaknesses of new technology in the field has become as necessary as understanding the basics of how the technology operates.

             
Aggravating EGR

            One of the most problem-causing subsystems on emission-friendly engines has been the EGR system.  Some smaller powerplants have been able to satisfy NOx emissions standards without recirculating exhaust gas; for example, some of the old 2.9L and 3.0L Ford V6’s were produced without EGR systems.  GM went to electronically operated EGR valves years ago, but problems with carbon clogging can still produce driveability concerns and MIL lights.  The earliest EGR concerns were related to stalling and surging.  

This month’s title concern outlines an aggravating surge at road speed that would totally evaporate with the EGR disabled.  The Mustang was nearly new, with a little more than 11,000 miles showing, and the surge was instantly noticeable.  The standard troubleshooting technique for a surge is one of the simplest; disable the EGR and resume the test drive.  Re-enabling the EGR and studying the scan tool, we studied the EGR system PIDs.

The EGR system on this vehicle is basically the same system Ford has used since 1991.  The three-wire feedback sensor (DPFE) is a pressure-sensitive device that measures a differential on each side of a fixed orifice in the EGR exhaust supply tube.  At rest, the DPFE voltage signal will rest at about 1 volt on the newest units (0.60 on the older ones).  The EGR valve itself is vacuum operated by the PCM through an electrical solenoid called an EGR Vacuum Regulator (EVR).  By monitoring the duty cycle of the EVR and the corresponding feedback signal from the DPFE, we hoped to gain an understanding of why the vehicle was surging.  An improper feedback signal could have been the root cause, but in this case, it wasn’t.   EGR should be totally inactive at idle and at Wide Open Throttle, and one anomaly we noticed was that the PCM indicated that it was commanding a 44 percent duty cycle with the engine idling, in spite of the fact that the engine idled smoothly (EGR will usually stall an engine if it’s flowing at idle).  Replacing the PCM removed the confusing duty cycle concern, but the surge was still present.  Removing the foam filter from the EVR (it sometimes clogs and can cause excessive EGR) made no difference.

            “Donnie,” I said, as we were returning from a test drive, “remember how the old 3.8 liter engines would develop a surge that could be cured by disabling the EGR?  Remember how sometimes the spark plugs would be loaded with puffy brown oil consumption deposits from the valve stem seals?  The problem was the spark plugs, not the EGR.”

            Since EGR feeds inert gas back into the mix, a cylinder with a weak spark due to spark plug problems might fire just fine without EGR.

Fueled and Fouled by Additives

            My brother once owned a beautiful silver 1971 Thunderbird that really liked high octane gas.  In order to save money, he started buying regular gas and pouring octane booster into the fuel tank at each fillup.  After about three tanks of this vile concoction, the 429 ran like a sick joke, and when I pulled the plugs, they were coated with a fine layer of strange red dust that was playing havoc with the spark.  A new set of plugs and a fresh tank of gasoline reawakened the 375 horses in that tough old engine. My brother never used octane booster again.

            Removing the spark plugs from this 4.6 Mustang GT revealed two obvious problems.  One problem was the same type of fine red dust I had seen on my brother’s plugs, and the other was that the spark had found an alternate path to ground by firing from the center electrode across the ceramic to the side of the spark plug shell.  Heavy carbon tracking was evident, actually having painted the ceramic around the center electrode a glossy black; these strange little sparks would light the mix unless EGR was flowing, but adding inert exhaust gas would cause multiple misfires and an EGR-related surge.

            The tried and true troubleshooting procedure of disabling EGR usually points to an EGR problem, but not in this case.  It turns out that the owner was an elderly man who had added at least one bottle of injector cleaner to his fuel on his last fill up. Whether or not he had been using it on several tanks like my brother did is possible, but unknown. 

Don’t misinterpret what I’m saying here; not all injector cleaners are bad, but the manufacturer of this particular injector cleaning additive apparently added some octane booster to the product in hopes that the customer would feel an immediate difference in the way the vehicle ran, and any additive, no matter how good, can be overused.  By the time Donnie drew the work order, the plugs had been coated with red stuff, and the surging condition had taken hold.  A new set of spark plugs and a Keep Alive Memory (KAM) reset produced a smooth test drive that was extremely satisfying.