The Ultimate 'Machine'

Jan. 1, 2020
I'm not the first automotive technician who has put words on paper about many of our customers' pie-in-the sky ideas about that mythical, all-powerful 'Machine' we have stashed in the service bay...

The all-powerful 'Machine' is a great tool, but it's no match for experience.

I'm not the first automotive technician who has put words on paper about many of our customers' pie-in-the sky ideas about that mythical, all-powerful 'Machine' we have stashed in the service bay that will tell us which bolts are loose and why a vehicle is only getting 19 mpg when it should be getting 20. No sir, and I won't be the last.

How many times a month does a customer ask you about that non-existent 'Machine'? I'm asked about it so regularly that I almost cringe when I hear the question coming. I got so tired of it when I worked at the dealership that I started telling customers I was the "Machine."

Even when a service technician is able to glance at a liquid crystal handheld computer screen and pinpoint a problem, in most cases it's because the tech has seen a similar problem before on a similar vehicle. In that regard, a competent technician's mental database is worth more than most employers care to admit. A large part of the money we earn as technicians comes as a result of our personal experience and the fact that we understand the way modern day automobiles work.

In spite of my opening point, I'll be the first one to admit that our 'Machines' – scan tools – with their computerized diagnostic programs, are rapidly becoming indispensable in the service bay. I won't even attempt to tackle most electronic or drivability diagnostic jobs without a window into the system. And to give the 'Machine' its due, there are times when it actually repairs a problem by reflashing the PCM and replacing a bad operational strategy with a better one. But it is a simple fact is that there is only so much any computerized device can do, and no 'Machine' will consistently tell anybody what's wrong with a vehicle.

This month, we'll look at two similar, yet different, diagnostic situations, both of which required the use of a scan tool to diagnose the problem. Both of these concerns had odd Fuel Trim readings, so let's start our discussion on that subject.

PID picking

Again, from the service tech's point of view, knowing the weaknesses prevalent on a particular vehicle in question is imperative when it comes to getting the job done quickly. That's the knowledge we as technicians buy and pay for on the battlefield of automotive electronics. That, at the end of the day, is what makes a competent technician the ultimate 'Machine.' And while many customers don't understand it, the fact is that being adept at fixing most cars quickly is what enables us to make a good enough living not to lock up our tools and go to work stocking shelves.

For instance, if I see a P0401 Diagnostic Trouble Code (DTC) on a late-model Ford, I instantly jump headfirst into the datastream and have a look at the Ford Delta Pressure Feedback EGR (DPFE) PID. On the older vehicles, the offset voltage on the DPFE was supposed to be about 0.60 volts. On the newer ones, 1.0 volt is the norm, and the DPFE sensors routinely fail. When one fails, the voltage will either be much lower or much higher than the norm. And in less than 30 minutes I've earned fair compensation for a job well done, primarily because my experience tells me where to look. And if I manage to up-sell a serpentine belt, a cooling system flush or an oil change, the whole shop benefits.

Some chain store parts retailers are cashing in on the more routine DTCs by plugging in their infernal 'Machine,' reading the customer's trouble codes and then selling the customer parts to boost their sales. This practice works well until a parts guy pulls a code that he or she doesn't know what to do with, and more than one customer has driven away angry and frustrated after having thrown money away on parts that didn't solve the problem.

The fact is that there are some DTCs that can be hard to figure out. For instance, if we retrieve a P0172 (system rich) or a P0173 (fuel system malfunction), or any of a half-dozen other fuel trim-related codes, does it necessarily mean the O2 sensor is faulty? About 50 percent of the time it does, and because O2 sensors on some vehicles are almost as wear-prone as spark plugs, most techs will toss an O2 sensor at one of these codes with the idea that the O2 might be dropping out of service and flat- lining when nobody but the PCM happens to be watching. The problem with that approach is that it's very easy to get your hindquarters nipped if you don't do your homework before routinely swapping the O2 sensor.

I became involved with a 2000 Jeep Cherokee a few weeks ago that illustrates just how quickly a competent tech can be blindsided by what seems to be a routine repair.

The Jeep

The technician had seen lots of O2 sensor failures. When he first investigated this MIL, the lean code he got was all that he thought he needed to see. A quick look at the PIDs followed by a routine sensor swap, and we were test-driving together to make sure the Long-Term Fuel Trim (LTFT) figures normalized.

Most of us know that Short-Term Fuel Trim (STFT) is the immediate adjustment of air-fuel in response to what the PCM sees in the exhaust stream at the O2 sensor. LTFT is a more accurate indicator of a mixture problem that the PCM is striving to solve. Both STFT and LTFT figures run on a number line, with zero at the mid-point and numbers as high as 35 percent in either direction. The number depends on whether fuel is being added to the mix to correct for a lean condition or subtracted to correct for a rich condition.

Because most, but not all, O2 sensors generally fail on the lean side, it's not unusual to see a system with an intermittently dead O2 sensor ramp up the STFT readings first. The system then continues to ramp the LTFT up in an attempt to keep exhaust emissions within acceptable standards. In some cases, the O2 sensor will suddenly wake up and come back online, and the PCM will toss the opposite code while it scrambles to retrace its steps in response to the suddenly active O2 sensor. This scenario is frequently the cause of conflicting (rich and lean) DTCs stored at the same time.

The quickest way to determine whether or not a lean or rich concern has been repaired is to yank the battery cable for a few minutes and let all the Fuel Trim numbers go to zero. After opening the datastream again and restarting the engine, if we see STFT readings rapidly climbing or descending away from zero, we still have a problem. Such was the case with this Jeep Cherokee.

With the Fuel Trims zeroed by disconnecting and reconnecting the battery, we began our test-drive, only to see the STFT numbers max out immediately. That was followed by deliberately ramping Long Fuel Trim numbers that topped out at +27 percent. Something was obviously making the 4.0L run lean, but we couldn't find any vacuum leaks. Carefully analyzing the scan tool readings as we drove, I noticed that BARO (Barometric Pressure reading generated by the MAP sensor) was at 24 inches of mercury. Since our altitude is only about 300 feet, that reading was far too low. The Jeep PCM thought we were at a much higher altitude and had adjusted injector pulse width accordingly. A replacement MAP sensor brought the Fuel Trims back in line with what they should have been. Case closed.

Never having seen that particular problem on a Jeep before, we were blindsided while performing an otherwise familiar repair. Both of us knew better; we had seen lots of BARO problems like this on other cars.

The conclusion? The 'Machine' told us what the problem was, we just had to know how to interpret the numbers to determine that the concern stemmed from a faulty electronic part. Careful probing with a voltmeter and a breakout box might have found it, but it was far simpler to read the numbers in the scan tool window and compare them with what we knew to be normal at our altitude. Give the 'Machine' a pat on the back for this one, but 'Joe Tech' still had to turn the screws and replace the part.

The Dodge Stratus

The second Fuel Trim story concerns our featured vehicle. This 1998 Dodge Stratus originally had a nasty oil leak that had been diagnosed at a repair shop as a rear main engine oil seal. The repair would cost about $1,000, and because the owner's father works with me, he asked us to look at it before the work was done. The entire bottom side of the engine was painted with oil, and we cleaned it with solvent and brake parts cleaner. We added some dye and found that the oil was coming from the left rear corner of the cylinder head. There was no rear main engine oil leak, only runoff oil from the corner of the cylinder head that looked like a rear main leak to a casual observer.

The head gasket and timing belt were replaced. Incidentally, we found that the pressurized feed to the valve train had been the source of the leak, and we were happy to note that the aftermarket head gasket had a more robust seal for the oil passage than the OEM gasket did. A new timing belt and an engine cleaning brought the Dodge up to speed. The oil leak was gone, the adaptive idle learning was burned in and the smoothly running car was to be driven to Florida that weekend. But it never left town. On Monday, the Stratus was back at the shop with a MIL and a rough idle, but it ran well on the highway.

The 'Machine' indicated that the MIL was a P0172: a rich exhaust indication. A quick scan of the datastream showed the O2 sensor reading 0.84 volts, with LTFT readings steady at –24 percent. Engine vacuum was an interesting 12 inches at idle. Checking the temperature of the EGR supply tube (EGR flow at idle can lower engine vacuum considerably), I found it cool. Thus, there was no unauthorized EGR flow. The low vacuum – once again, detected by the MAP sensor – would fool the PCM into thinking the engine was under a load, which would account for the rich condition.

"Fellas," I told the students who had done the head gasket. "I know that the timing belt was properly installed and the tension was right; I checked it myself. But right now we need to get the covers and the balancer off and have another look."

We found the camshaft sprockets several teeth out of time. It's astounding that the vehicle performed so well on the highway. The strange thing was that the timing belt was still tensioned properly. The only possibility I could think of was that a piece of gasket material or some other flotsam might have worked its way onto the belt and then ridden down between the crank pulley and the belt just long enough for the pulley to spin a couple of teeth out of time. I don't know of anything else that could have caused the problem we found. The students timed it again, double-checked everything and the car ran like a top on multiple test-drives.

The conclusion? The 'Machine' gave us some useful information, but it took a working knowledge of basic engine principles to pinpoint this second concern. All the 'Machine' could do in this case was finger some of the indicators, such as low vacuum, expanded injector pulse width and rich exhaust, one of which caused the other two.

The point of my story is that while the 'Machine' gave us some vital information that directed us to check for mechanical problems, in the end, the ultimate 'Machine' was the technician who got under the hood with his hands and put the engine back in time. That's what makes Joe Tech the "Ultimate Machine."

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