Diagnostic sound-off

Jan. 1, 2020
I spent one day recently at a clean little garage in the country. The owner, Donnie, and I have known each other for more than 20 years, and we use each other for sounding boards on tough diagnostics. Today was one of those days.

Some parts take a toll on time and your mind.

Motor Age Garage 2003 Ford Expedition vehicle won't start car won't start fixing vehicle repair shop training technician training automotive aftermarket I spent one day recently at a clean little garage in the country with pecan trees shading the edges of the service lot, 100-pound dogs of various colors laying here and there, a refurbished refrigerated Coca-Cola soda box from the 1960s in the corner and homemade barbecue for lunch. The owner, Donnie, and I have known each other for more than 20 years, and we use each other for sounding boards on tough diagnostics. Today was one of those days.

On the lift by the Nissan with which we were engaged was a 1950 model Hudson that, until fairly recently, spent a lot of its time in a barn and was in good shape. Walking under the 60-year-old vehicle, I saw some interesting automotive technology. The brake master cylinder was underneath the driver's feet below the floor pan with some fairly complex linkage designed to mechanically engage the brakes if the hydraulics fail and the pedal goes past a certain point.

Through an inspection port in the bell housing, I saw the gasketed cover of a wet clutch bolted to the flywheel and an external grease fitting, presumably for the throwout bearing. I saw the odd geometry of an antiquated steering system, a very small diameter driveshaft leading to a Rockwell style differential and a toy-like sway bar a third the size of today's large car front sway bars.

As I perused the thrilling technology of yesteryear, it occurred to me that when this Hudson was a contemporary vehicle, there was no problem a factory trained technician couldn't figure out in fairly short order. But today, we live in a totally different world.

The vehicle leading us to sound off on diagnostics was a Nissan Hardbody's 3.0L (OBDI) with a buck/jerk condition that drove the owner to replace every ignition part but the distributor. The engine ran well until it warmed up, and Donnie's diagnosis led to the distributor's replacement, a partially successful repair. The bucking and jerking was still there. Initially, the symptom felt kind of like a spark plug (or two) dropping out, but after a couple of test-drives it was evident there was more.

A quick inspection of the spark plugs revealed the owner painted the threads with anti-seize compound when he installed them.

One plug looked different from the rest of them in that it had a brown stain on its ceramic. But measuring the center electrode resistance produced readings from 4,000 to 6,000 ohms on each spark plug. Ford recommends that test (less than 8,000 ohms is the spec), but center electrode measurement is not an exhaustive test. That means a plug can measure good and still be bad. We replaced the brown-stained spark plug and connected the DRBIII in a way as to use its PEP module scope function with a wide timestamp.

When we experienced the concern using that window, we saw a peculiar trace that looked like the skinny and fat computer wave file of a voice recording when the problem was present. It turned out a second replacement distributor was what the doctor ordered.

The Fords

One interesting vehicle in question came from Spanky, a guy I know who is working at a nearby Ford dealer. He had been fighting a 2006 F150 with 3 valve 5.4L with a low power concern and, working with the Ford hotline, had ruled out just about everything looking for the problem. The end of the diagnostic process was based on the fact that the ignition timing seemed to default to zero during maneuvers that produced the low power complaint, and the prognosis of field service engineers was that it needed a new engine.

Well, their idea was that the knock sensor was picking up some faint noises that the PCM was interpreting as labor knock and retarding the timing in response. Because there are known concerns of crankshaft thrust bearing wear causing this sort of thing, that diagnosis isn't too outlandish. But who would have expected to be told they need an $8,000 engine for a low power concern?

That manufacturer-sanctioned engine replacement situation reminds me of a 2007 Escape Hybrid I saw in Massachusetts. It was a vehicle Ford's field service engineers had given up on and recommended that the internal combustion engine (ICE) be replaced. The engine wouldn't start, even though it had compression (145 psi per cylinder), spark, fuel and seemed to be in time.

One team I worked with took on the job of troubleshooting it and used every diagnostic apparatus they could. By using a dual trace scope and a transducer, the team finally determined that the compression trace was on its down slope by the time the PCM triggered the spark, thus preventing the engine from starting.

On that particular engine, neither the balancer, the crank gear nor either cam gear is keyed to any shaft. Like the camshafts, the crankshaft has to be positioned with a pin that screws through the block and into a hole in the crank throw counterweight, and the balancer has to be pinned to the front cover as well, with the torque of the bolts holding everything in time. The spark happening after the peak of the compression cycle would explain why the engine wouldn't start, especially since that particular engine is a Late Intake Valve Closing engine (Atkinson Cycle).

To verify the theory, they moved to the Ford Escape on which my team was working and performed the same test. On the Escape Hybrid that would actually start, the spark was happening as the compression was rising rather than after it was falling.

Upon removing the sealing bolt that provides a threaded port in the block for the crankshaft locator tool, it became evident that the crankshaft was slightly out of time, which would account for the engine killing mismatch between the spark and the compression. The way that engine is set up, the timing chain gear can be out of place on the crankshaft while the crank sensor trigger wheel (part of the balancer) is perfectly in time with the camshafts.

A Stumbling Expedition

Finally, we have the 2003 Expedition with 99,700 miles and a stumble that ranged from a mild hesitation to a snorting induction backfire and stall. It had to idle for a couple of minutes before the symptom would occur. On snap accelerations performed within two minutes or so, the stumble would be much less severe. The diagnostic trouble code (DTC) that had been present from the beginning was a P0320, which led an earlier technician to TSB number 04-14-09.

That TSB brought that tech to the point of replacing the wire harness that leads from the alternator to the battery with a couple of branches leading to other places along with a PCM reflash. But none of that did any good. Nor did replacing the PCM solve the problem, but it seemed not quite so pronounced.

At that point, my man Jimmy was handed the ticket. The first thing I noticed was a lean induction backfire that almost always happened along with the stumble. It was great to have a symptom that was so predictable — it enabled us to compare readings on a normal throttle snap and a hesitating snap, which we did. Fuel pressure was an on-target 55 pounds and mildly bouncy, which was normal, but what about fuel quality? With E85 gasoline available (85 percent alcohol), it's possible to have quite a lot of that stuff in a tank that shouldn't see more than 10 percent alcohol.

With a straight-walled plastic vessel, a tape measure and a Sharpie, we made a testing container, filled it with 10 percent water, added 10 times as much gasoline, shook it up and let it settle. If we saw the dividing line between the gas and the water move up the scale more than a mark or so, we'd know how much alcohol the gas contained, because alcohol likes to mix with water more than it does with gas. As it was we saw about 8 percent alcohol in this mix, which is OK.

With the IDS (Ford's laptop based scan tool) connected, we opened our window to snoop on the TP sensor, ignition timing, O2 sensors (1/1 and 2/1), Mass Airflow and Injector pulse graphs. With the IDS is that you can change the histograph to a horizontal bar graph, and with CAN bus speeds, screen response is practically real time.

We noticed when the engine took its nasty stumble, the TP sensor was smooth all the way up, the ignition timing would drop back to -1.5 degrees (normal), the O2 sensors would go lean (not normal) and the and short fuel trim numbers would dive into the negative (not normal). Pulse width during a stumble was about 16 milliseconds vs. 23 milliseconds during a normal throttle snap, and the fuel trims went positive on a normal acceleration. If the fuel trims went positive, the pulse increased and the O2 sensors went lean, we'd know a misfire was the cause not the effect.

The PCM was making bad decisions with its fuel trim when under duress. Jimmy retrieved As Built data from Ford and reflashed the PCM to no avail, so a new PCM was obtained. But the real problem with the Expedition wasn't a bad PCM, it was interference from the No. 1 ignition coil! With that coil disconnected, all the Expedition's problems went away and a new coil fixed it.


So much of our work today is done with components that can't be analyzed except by comparison with a known good part. Thousands of labor hours are lost yearly in pursuit of problems created or sustained by faulty replacement parts. We certainly live in a changing world.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail Richard at [email protected].

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