Motor Age Garage: Double trouble

Jan. 1, 2020
We have to understand the code set criteria, and not just the inferred meaning of the code itself, when tackling drivability problems. We need to look at all available data instead of just focusing on one specific area.

We have to understand the code set criteria, and not just the inferred meaning of the code itself, when tackling drivability problems. We need to look at all available data instead of just focusing on one specific area, starting with a large overview and then narrowing the problem down to a certain area. Looking at a set of data Parameter Identifiers (PIDs), instead of just one individual PID, is also required for a complete understanding of the whole picture.

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On most vehicles, I like to get a group of general PIDs and see if anything looks out of the ordinary to give me a direction. The following are the ones I like to graph, if available, on most every drivability problem I encounter:
• RPM
• MAF
• Long Term Fuel Trim(s)
• Short Term Fuel Trim(s)
• Front O2 Sensor(s)
• Throttle Position
• Load

The Power Balance Test show consistent misfires on all Bank 2 cylinders.

The vehicle in this article is a 2004 Ford Expedition with a 4.6L engine. It came to us with a P0171 (Bank 1 System Lean) code. Now most technicians who are familiar with these vehicles would immediately start looking for a vacuum leak on the Positive Crankcase Ventilation system (PCV) hose to the plenum since it is such a common problem. In fact, this one did have a hole in the rubber hose where it attaches to the back of the plenum. The blowby in the PCV system tends to soften the rubber and cause it to break apart.

The technician replaced the defective hose, cleared the code and test drove the vehicle to verify the routine repair. However, this was anything but a routine problem; by the time he got back he noticed two things. First, the engine did not run smoothly (misfiring) and had poor acceleration on the test drive.

 The Relative Compression Test showed all Bank 2 Cylinders were slightly lower than those on Bank 1.

Second, the DTC P0171 was back again, already set in pending status. He continued his testing but was so focused on the lean condition of Bank 1 he never thought to look at any other data the scan tool had to offer. Sometimes we really can’t see the forest through the trees.

The fact is, Long Term Fuel Trim-Bank One (LTFT_B1) did show the vehicle was significantly lean and when the rpm was increased it came back into a normal range. This would lead most technicians to pursue a vacuum leak. That is, unless they looked at the big picture. He tried both artificial enrichment and smoke testing the intake system, but had not found any signs of a vacuum leak. After spending an hour or so chasing ghosts, the problem was about to have parts tossed at it.

A relative compression test coupled with a First Look Sensor in the intake plenum showed a clearer picture of the lower compression on Bank 2 with some corresponding differences the intake pulls.

Bringing In Reinforcements
I was asked for some help and fresh set of ideas. I set the PIDs on my scan tool in graphing mode so I could watch the trending at different speeds. It is important to observe them at idle and 2,500 rpm to see how load affects them and get an overview of the entire engine.

Something jumped out right away. LTFT_B1 was definitely lean at nearly +23 percent, a fact the other technician had obviously noticed, but LTFT_B2 (Long Term Fuel Trim-Bank Two) was rich at -17 percent at idle. (Negative numbers indicate the engine control module is adding less fuel than it should be while positive numbers indicate the ECM is adding more). Fuel trims improved at higher RPMs, but each bank was still the inverse of each other. This gave me some good direction to diagnose the problem.

With a sync on both cylinders 1 and 5, it is noticeable that ignition is occurring slightly earlier on Bank 2 than on Bank 1. The sync on Bank 1 (Blue Trace) occurs approximately at the center of the peak, where the sync on Bank 2 (Green Trace) occurs before the peak.

When there is a fuel trim split between banks, the bank with the negative fuel trim will usually be the side with the problem. My focus has now narrowed to Bank 2 of the engine. At this point, there were a few suspects; a restricted exhaust, a partially stuck open or leaking injector(s), a poorly sealing valve(s), valve timing or even crossed O2 sensors, but how do we determine which one?

Like most drivability techs, I try to work efficiently and gather as much information as possible before tearing into the engine or swapping components, so I use any automated tests that are available first. This way when I do start going into the engine, I have a very specific focus area and do not waste time by doing more disassembly work than needed.

An in-cylinder running compression test using a pressure transducer was performed on both banks of the engine and the results overlayed on top of each other. The differences in exhaust valve timing can be seen between Bank 1 (Blue Trace) and Bank 2 (Red Trace). Where Bank 1 intersects at the correct time in reference to the 180° mark and Bank 2 intersects earlier indicating it is retarded.

The vehicle definitely had a misfire, not severe enough to cause a flashing MIL, but it was noticeable. I grabbed my Ford IDS and performed a Power Balance Test, which identifies which cylinder(s) are misfiring based on the change in rotational speed of the crankshaft. A misfiring cylinder will cause the crankshaft to slow down and the crankshaft position sensor relates this data to the PCM. The software program in the IDS then displays this data as a graph showing the misfiring cylinders deviating downward from the centerline.

I found that all the cylinders on Bank 2 (Cyl 5, 6, 7 & 8) were dropping out on the Power Balance Test, indicating they were all misfiring. This confirmed that my problem is on Bank 2, and it also verified that it is not just a single cylinder. It would be highly unlikely that each cylinder on one bank would experience a failure of a single component simultaneously. This veered me away from items such as ignition coils, injectors or an input sensor and told me I needed to focus on something all the cylinders on the bad bank shared.

While revving the engine, a change in rpm could be seen, but the MAF still reported 0.00 grams per second of airflow.

Test Two
Another test that is fairly easy to perform on the IDS is a Relative Compression Test. I decided to see if the misfiring cylinders compression had been affected. The cylinders on Bank 2 all show slightly lower relative compression when compared to the rest of the engine. However, all are within 5 percent of one another, so I needed to determine what the cause was. I suspected a mechanical issue and a common test would be to take a capture of the CPK/CMP sensor waveforms and compare it to a known good.

Unfortunately, this vehicle only utilizes one camshaft position (CMP) sensor, so a bank-to-bank comparison between camshafts in reference to the relative crankshaft position (CKP) is not possible. Newer Ford vehicles with variable valve timing utilize a cam sensor on each bank.

Backprobing the Mass Airflow Signal at the sensor showed a correct output was being generated.

I decided to do my own relative compression test using my scope with a 600 amp current clamp on the positive battery cable to the starter. I synced to the cylinder No. 1 ignition coil and cranked the engine. (To prevent the engine from starting, I depressed the accelerator pedal to the floor and held it there to put the vehicle in clear flood mode, which shuts off the fuel injectors). I also added a First Look Sensor attached to the plenum by the throttle body.

I noticed that all cylinders on Bank 2 were significantly lower, unlike the IDS capture where there was only a 5 percent different between the best and worst cylinder. In fact, some of the Bank 2 cylinders on the IDS relative compression test only showed a 1 or 2 percent difference. I also noticed that the sync on cylinder No. 1 was centered with respect to the peak of the compression rise, which made sense since all engines retard ignition timing to pretty near TDC when starting.

The next step was to verify that the PCM was also receiving the same signal.

Wait a Minute
I wonder if I could use that as a way to check cam timing on Bank 2?

I decided to add a second sync to the other bank on the cylinder No. 5 ignition coil and perform another cranking compression test. During this test I noticed that the sync on the Bank 2 coil was not centered on the cylinder No. 5 peak, the bank that I suspected to have a valve timing issue.

Now it appeared as though the sync on cylinder No. 5 was occurring earlier than it did in cylinder No. 1, possibly indicating a timing issue. But remember, I want to do as little disassembly as possible and besides who wants to spend hours removing a timing cover only to find everything lines up perfectly?

Backprobing the PCM connector showed that the signal was reaching its destination. This ruled out any wiring problems between the MAF sensor and the PCM

A couple of the forward spark plugs on each bank are fairly easy to access and, truth be told, I wanted to use my pressure transducers on an actual problem vehicle instead of just the “known goods” that I had been practicing on. I performed a running compression test in one cylinder on each bank with the engine idling at approximately 750 rpm. The 180° mark, which is BDC (Bottom Dead Center), should intersect at the halfway point of the rise in the exhaust pocket on my pattern. It does on the blue trace, which is cylinder No. 1. However, on the red trace (which is cylinder No. 5), I found it to the bottom or left of the 180° point, indicating the valve timing on Bank 2 is retarded. A couple other notable features that indicate a problem in the cylinder in my capture are a lower running compression and a rounded exhaust pocket on Bank 2.

This vehicle was sent back to the tech for a new timing chain and guides. I was expecting to hear the Ford run smoothly after he completed the repair, but the vehicle ran horribly. The tech that performed the repair attached a scan tool and found a new problem; DTC P0102 (Mass Air Flow (MAF) Circuit Low Input). He inspected the connector at the MAF sensor (which looked fine) and then installed a new sensor, followed by a code clear.

Guess what? That still didn’t fix the problem.

Back At It
The technician who performed the work assured me that the timing was correct and even had it double checked by another technician. He again inspected his work and verified there was nothing left unplugged or misrouted. I grabbed my scan tool and graphed some PIDs. The first thing I noticed was the code was accurate; there is no signal from the MAF sensor showing up on the tool.

After inspecting the PCM side of the connector, the culprit was found.

Driving the vehicle into the bay with the scan tool I found that regardless of the engine RPM, the MAF sensor PID would stay at 0.00 grams per second. I used my Vantage Pro to verify the scan tool data reading of no MAF sensor output. First I wanted to verify power and ground to the sensor, as a lack of either one these would also cause this type of reading. With the Key Off Engine Off (KOEO) and the MAF unplugged, I verified the correct signals were present. I reconnected the sensor and back probed the connector for the sensor signal with the engine running. I was surprised to find a reading of over 1 volt at idle and 4.01 volts when snapping the throttle wide open. Just to be sure, I grabbed another scan tool and verified the readings of the first one. However, the second scan tool also reported the MAF sensor flow at 0.00 grams per second at all rpms.

So how could the MAF sensor signal wire show the correct readings being outputted but the Powertrain Control Module (PCM) not see them? There could be a break in the wire between the MAF sensor and the PCM, so I back probed the MAF sensor signal wire at the PCM. Again, I found the exact same reading as when I checked it at the MAF sensor itself. Now I suspected a PCM, but I needed to verify powers and grounds to it. I also wanted to perform a pin-drag test on the MAF sensor signal pin. Since the PCM needed to be removed during the repairs, it is possible that the female side of the pin may have opened up and is now making poor contact. The pins on the harness side are female and the MAF sensor signal one had a slight drag when inserting the back probe pin.

The male pin for MAF sensor signal had been pushed into PCM circuit board when the connector was reinstalled during the repair causing an open circuit.

But there are two sides to a connector, aren’t there? When I went to look for the male pin side on the PCM itself, I found why there was no reading from the MAF sensor on the scan data. When reinstalling the wiring harness connectors for the PCM, the male pin for the MAF signal had gotten bent and was pushed into the circuit board. Unfortunately it broke the solder joint and dropped into the PCM, creating the open circuit. I tried to gently pull the pin back out, but it would just fall back into the circuit board so a new PCM needed to be ordered.

The replacement powertrain control module came with the latest software already installed so all that was needed was a parameter reset and the vehicle was ready to go. The vehicle ran smoothly and was test-driven to verify the repair. 

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