A simple running complaint turns up more than one culprit, and a few more on the way.
It's late on a Friday afternoon, and I've got my hands full. In one bay is a Caravan with its dash peeled back, waiting for an evaporator core. In my other bay is an Eclipse with its block on a stand and its guts on my workbench. Out of the corner of my eye, I see my service writer coming.
"Pete, I know you're slammed, but I have a customer who is heading out of town, and needs his car looked at," the service writer says. "He says it isn't running right, and he's afraid of breaking down on his trip. Can you help me out?"
I've been in this business long enough to know that you are either too busy or not busy enough. I also know it's important to take care of your customers – or someone else will. Of course I'll take a look at it. So I walked up front with my advisor to see what we had.
LET'S TAKE A RIDE
Up front, the customer and his 2001 Ford Explorer were waiting. I asked him to take a ride with me and tell me what the symptoms were. We got in the car, and the rough idle complaint was obvious. It was a little rough, but not severe.
I also noted that there was no Malfunction Indicator Lamp (MIL) on. I asked him when he first noticed the rough running, and he told me it just started that morning after returning home from the local store. Because he was leaving with his family for a long weekend in the morning, he didn't want to take any chances that there was something seriously wrong.
I turned out of the drive onto the main road that runs by our shop. Our location allows an easy test drive route that has city driving on the first half and highway driving on the second half. Normal city driving didn't reveal anything abnormal, but I did notice that the engine smoothed out off idle. The next symptom showed itself as I accelerated onto the on ramp to the freeway: The engine skipped a beat under load. Again, nothing severe, but certainly not normal.
When we returned to the shop, I dropped my customer off at the waiting area and asked him to give me a few minutes to check it out.
IN THE BAY
I pulled in behind the Caravan and grabbed my scanner. Just because the MIL wasn't on didn't mean the Engine Control Module (ECM) didn't have anything to tell me. I went in using Enhanced Ford mode to see what, if any, codes were stored or pending. This check showed nothing.
The next area I checked was current data. When I check current data, I like to take a look at the listed Parameter Identifications (PIDs) to see if anything stands out. With the key on and the engine not running, I saw that the barometric pressure (BARO) PID was off, reading 150 Hz and 27.03 in/Hg. My town is just about sea level, and normal BARO for us is 157 to 159 Hz and 29.90 to 29.98 in/Hg. This may or may not be related, so I just made a mental note of it for now.
The next place I checked was the fuel trims at idle and 2,500 rpm. I wanted to know if the ECM established fuel control and to get an idea if there is anything that is causing the ECM to make corrections outside of normal ranges. Figure 1 illustrates what I saw at idle. Note again that BARO is out of range.
On this model Ford, the BARO PID is inferred by the ECM from data gathered from the Mass Airflow sensor (MAF). This PID is updated under part throttle and wide open throttle conditions and then stored in Keep Alive Memory (KAM). Ford uses a "hot wire" type MAF sensor. The wire is maintained at a constant temperature above ambient (as detected by the "cold" wire). Airflow across the hot wire cools it, and the current required to keep it hot is proportional to the airflow itself.
MORE THAN ONE CULPRIT
VEHICLE: 2001 Ford Explorer Sport
It is not uncommon for the wire elements to become contaminated, causing the sensor to underestimate airflow at higher throttle openings and overestimate airflow at idle and low throttle openings. This underestimation of airflow during high throttle openings leads the ECM to sense that the vehicle is operating at a higher altitude than it actually is, and results in a BARO update that is lower than it should be. This also causes the ECM to add too little fuel at higher throttle openings that results in a lean condition at cruise, and typically results in a code P0171 or P0174 (System lean, bank 1 or 2).
Take a look at the fuel trims: Short Term Fuel Trim (STFT) on both banks were shifting normally at plus/minus 5 percent either side of zero. But the Long Term Fuel Trim (LTFT) had learned a correction for an overly rich condition that was being reported back by the oxygen sensors. This could also point to the MAF. Because a contaminated MAF sensor will overestimate airflow at idle, the ECM is providing more fuel than really needed. The oxygen sensors report this rich condition and the ECM corrects for it by adjusting LTFT. The trim Figures are not quite high enough to trigger a code, but it's an impending problem nonetheless.
To verify this was a MAF issue, I removed the sensor for a closer look at the hot wire. (See Figure 2.) My eyes aren't getting any younger, so I used a magnifying glass to inspect the wires. You can actually see the contamination build-up on a bad MAF. Many techs will use a cleaning agent that is approved for cleaning MAF sensor elements and see if a few Wide Open Throttle (WOT) passes will restore BARO to normal.
Personally, I had all the information I needed to condemn the sensor and add it to my recommendations to my customer. I knew that, even though it wasn't bad enough yet to set a code, it was in this customer's near future, and I didn't want that to happen on his trip.
BUT IS THAT THE CAUSE?
I still had some investigating to do. Because the STFT readings were normal, I didn't think the MAF was the cause of the running complaints. Reviewing the current data, I noticed that the Delta Pressure Feedback (DPFE) sensor was reading 1.74 volts at idle.
This sensor is part of the Exhaust Gas Recirculation (EGR) system, and it measures the flow of exhaust gases across a fixed orifice in the EGR feed tube and reports those findings to the ECM. The difference in pressure between the upstream hose and the downstream hose is the "delta" pressure.
Faulty readings from this sensor are not uncommon, and many Ford models had a recall on these sensors. But I didn't think this was a faulty reading. With the key on and engine off, the DPFE sensor was reading 0.87 volts, which is OK. Hmmm, could the EGR be stuck open? It would explain the rough idle.
Checking the EGR is easy because it is readily accessible. (See Figure 3.) If there is EGR flow at idle, the manifold side of the valve is going to be hot. I used an infrared thermometer to check the temperature and was rewarded with a reading of 313°F with the engine idling. I decided to take it one more step by disconnecting the feed tube and blocking off the opening to the EGR valve. I restarted the engine and was rewarded with a normal idle. No more roughness.
I removed the EGR valve and found carbon stuck between the pintle and the seat, so I opened up the valve and blew it out with some shop air. I reinstalled it with a new gasket and verified the repair. I felt pretty good that this was the rough idle cause, but I didn't think that it was also responsible for the stumble I felt under acceleration.
TIME FOR THE SCOPE
I hadn't noticed anything else unusual in the data provided by my scanner, so I hooked up my scope to the ignition secondary. I don't pretend to be a diagnostic or scope guru, but I do like to use whatever resources I can to help me when troubleshooting. Adept scope users can tell a lot from what they see in a secondary ignition pattern. I'm still working at it.
The Ford uses a Distributorless Ignition System (DIS), which means that each coil fires two spark plugs simultaneously. I hooked up my secondary lead to the Number 4 cylinder and got a pattern similar to the one shown in Figure 4.
While the pattern looks normal at first glance, the firing line of the pattern was higher than normal. This portion of the pattern represents the voltage required to ionize the gap(s) in the secondary and allow spark to occur. Typically, this is affected by the size of the gaps, the pressure the gaps are under and the amount of hydrocarbons available in the gaps promoting conductivity.
The high firing lines were common to all cylinders. The engine had more than 60,000 miles on it, and I decided to check the spark plug gaps for wear. The specification for plug gap on this engine is 0.052 to 0.056 inches. I measured gaps on all the left bank plugs at roughly 0.070 (a 0.060 wire gauge fit easily while a 0.080 gauge was a no-go). OK, if firing voltage is high at idle, what would I expect it to be under load, when cylinder pressures are highest and demand on the system greatest? A stumble.
MODE $06 HELP?
There was one more source of information I thought might be helpful in telling me if I was on the right track, and that was the Mode $06 information stored in Global OBD2. I reconnected the scanner, accessed Mode $06 and looked for Test ID (TID) $53.
Mode $06 lists the individual test results that make up the non-continuous system monitors; CAN systems also include test results for continuous monitors. These tests are identified generically by a hexadecimal number that some newer scan tools automatically translate into English. Ford is kind enough to already list a misfire counter of sorts in pre-CAN models and is probably the most commonly used Mode $06 feature. It is identified as TID $53. Each TID also has a Component ID (CID), and the component IDs with this test correspond to the individual cylinders. The CID in Figure 5 shows test results for cylinder Number 1.
Specifically, this is the test result for the cylinder identified while monitoring for catalyst-damaging misfires. The test numbers do not reflect the actual number of misfires; these have to be converted to a percentage by multiplying by 0.000015. The result represents the percentage of misfires counted during the test cycle, and the maximum percentage for that given rpm and load that would have resulted in a catalyst damaging misfire. If the maximum percentage is exceeded, the MIL flashes and a code is set.
Even though this test "passed," you can see that there have been misfires in this cylinder, and the other cylinders I checked also showed some counts as well. Like the MAF results, the problem wasn't yet severe enough for the ECM to set a code, but there was reason to suspect that it was certainly coming and also helped strengthen my belief that the worn plugs were the cause of the stumble.
I went back to the customer to inform him and my service writer what I'd found. I recommended new plugs and an EGR valve. I also told him what I'd found with the MAF sensor and told him that, while it wasn't causing his MIL to come on right now, I believed it would soon.
The customer approved the repairs, and after the plugs were installed, I rechecked the secondary pattern and got one similar to that shown in Figure 6. Another test drive with my customer a few hours later, and he was ready to take his family on the road with peace of mind.
PETE MEIER is an ASE CMAT, a member of iATN, and a full-time tech with CarMax in Tampa, FL. He started doing oil changes and minor repairs more than 30 years ago and brings a variety of experience to bear. His current job involves all manufacturers' lines, and, as Pete says, "provides me constant opportunity to learn something new." Diagnosing electrical and driveability problems are his favorite challenges.
