Intermittent Problem Detection Strategies

Dec. 12, 2019

Someone once said that a good technician is not expensive but, rather, priceless. Having said that, I note we have all spent way too much time on an intermittent problem to justify the hours spent on these types of problems.

So you drove the vehicle for 20 to 30 minutes without a single stumble or symptom. It happens to all of us. Our best hope is that although the service we’re performing may not turn out to be all that profitable, at least we retain a good customer.

There have been many times in my own experiences where I have finally duplicated the symptom and finally narrowed down the problem and fixed it. Later on I go home and think about the tests I could have done, might have done or should have done that would have cut down the time on my diagnostics.

First things first!

Have you ever documented the symptom based on what the car owner stated only to find out that it was not an accurate description of the actual symptom? In addition, how many times have you needed to speak directly to the car owner to find out how often the symptom occurs and at what temperature and at what speed. In addition, when you have the MIL on, did you take the time to analyze and duplicate the freeze frame conditions?

Many times during my live seminars I compare an automotive technician to a medical doctor. Suppose you have a medical problem and you go to your primary care doctor and you give him a complete and accurate description of your symptom. In the real world he most often is going to write a prescription for you to try and see if you get any improvement. That is what we call practicing medicine.

Are we as technicians any different from the medical doctor? I don’t think so. However, in the customer’s mind they don’t want to pay a technician to guess. But what do we do when we can’t get the vehicle to act up? Is there a pattern failure or a silver bullet?

Can this be honestly communicated to the car owner? I have had to do it many times — with success.

Have you ever addressed an intermittent lean code, and after reading the freeze frame find out that it occurred under steady road load conditions? The MAF sensor has been replaced. The fuel trims are double digit positive at all load conditions. The fuel pressure specs are good and all the sensor values are within specs. Making certain E-85 fuel was not used in a non-flex-fuel vehicle, can you honestly recommend we try to chemically clean the injectors through the rail?

Did you catch the keyword here? We stated that we could try an injector cleaning process. It is certainly worth a try and can be the cure. But in reality can you guarantee a positive result?

Always keep in mind that we are technicians and not magicians. How many times were you forced to make a diagnostic guess based on tech tips or maybe a silver bullet? I have heard some instructors make the statement that they test and don’t guess. In the real world this simply doesn’t always hold water, especially when dealing with a very random and intermittent problem. There are certainly pattern failures on every make and model so we all investigate these pattern failures.

Case studies

The first case study that I want to share with you involves a 2001 Honda Accord with a good crank and intermittent no start. When I arrived at the shop the work order stated exactly that — an intermittent no start.

I was faced with an issue where the shop owner did not get complete information from the car owner.

I successfully started the Accord at least 20 times with no problems found. After doing a complete network test I found no codes at all.

Plan B required a call to the car owner. After interrogating the car owner I found out that the crank and intermittent no start occurred after the car had set in the summer sun with the windows up. The interior heats up and can cause the solid state main relay located under the dash to heat up and expand the solid state board and create an open. The solid state main relay powers up the CKP and the CMP sensors on this car.

Since I could not duplicate the symptom I was forced into making a diagnostic guess that the main relay could be the fault.

Many of you know that this is in fact is a pattern failure on these vehicles.

In addition, the ignition switches can also cause the same symptom but not necessarily after a hot soak in the sun.

Where do we go? Is there anything wrong with being honest with the car owner? We cannot duplicate the symptom. Can we recommend trying a main relay? Have you been down this road before?

In summary, the car owner stated that the crank and no start always occurred after an extended hot soak in the summer sun with the windows up. Do we have enough clues to justify in making a diagnostic guess in replacing the main relay?

In closing, that is exactly what I replaced. With my communication with the car owner he fully understood that since I could not duplicate the problem I simply could not pinpoint the exact cause. Did we jump on the pattern failure band wagon? Not until we obtained the specific info from the car owner who stated that the crank no start always occurred after a prolonged hot sun hot soak condition with the windows up. There are cases where you may find a crack in the circuit board with a magnifier.

Know your limitations

The second rule we must cover is to know the strengths and limits of your diagnostic equipment. In cases of electrical problems the DVOM is our prime weapon. All DVOMs are not created equally, so in this presentation we will use the Fluke series 87 DVOM.

The reason being is that the Peak Detect mode will capture and record a 100 millisecond signal drop out. It can also detect a 1 millisecond signal drop out by pressing the button under the Peak Detect button (see Figure 1).

We all have utilized the wiggle test and by using this meter function, the meter will beep when a signal dropout occurs from a poor connection.

When recalling the Min/Max values simply press the Min/Max button once and the Max recorded values will be displayed. Pressing the Min/Max button the second time will display the Min values. This is where you will see the signal drop out. Pressing the Min/Max button the third time will give us the average signal value. The Min/Max mode works on all meter functions.

Without this valuable mode we would be dealing with trying to see a voltage change on the digital readout with a slow 4 times-per-second update rate.

Whenever doing a voltage check we should always make sure the circuit is active and loaded instead of disconnecting the component.

When doing an open circuit voltage a DVOM will not properly load the circuit, say as a normal load from a fuel pump. The meter only requires 22 micro amps of current flow when open circuit testing a component. When doing this test in checking for good dynamic voltage with the component loading the circuit as in the fuel pump circuit we should always use the fuel pump ground wire for our negative meter lead. If both power and ground circuits are OK we should drop the full 12 volts across the fuel pump.

Let’s say the meter shows 7.2 volts. The problem is either weak supply voltage or resistance in the ground circuit. In addition, it may be a good idea to utilize the Min/Max mode of the meter and perform a wiggle test.

Many technicians have turned a blind eye for the need of a DSO (digital storage oscilloscope), see Figure 2. I simply disagree. Let’s take a look at a late model Cadillac with an intermittent no-crank condition.

After performing a network test we found a laundry list of codes from pretty much all of the on-board modules. This vehicle is equipped with the GM Pass Key theft deterrent system which in fact will lock out the starter with a fault. Since multiple modules flagged a network of U series codes, we suspected a corrupted network. With a DSO probing the Class 2 network at pin 2 of the DLC we found this signal in Figure 3. A normal class 2 signal is a 7V to .2V signal. Seven volts means that a module is talking and .2V is at rest. This is what is referred to as a pulse train signal.  Every module on the class 2 network supplies its own 7V bias voltage. Notice where the voltage slightly rises above the .2V rest value during KOEO.

Note Figure 4 during a cranking attempt. See how the voltage floats well above the logic .2V rest value? We have to suspect a bad ground somewhere in the ground circuit.

Note the starting circuit schematic in Figure 5. The first dynamic voltage check was done at V1. This should be battery voltage from the ignition switch in the crank position. The #2 meter reading is where the PCM ground side controls the pull in windings of the starter enable relay.

Since this ground is controlled by a transistor inside the PCM, the voltage should go below .4V which told us that the PCM is seeing a good enable signal from the theft deterrent module.

Meter reading #3 is checking for good dynamic voltage from the power side of the starter relay and showed good dynamic voltage. With good meter reading so far we know that the starter gets its ground thru the block. The block is then grounded to the body from the ground cable shown in Figure 6. With the battery located under the back seat the negative battery cable bolts directly to a body ground.

With a negative jumper cable clamped to the block and routed to the negative battery terminal we ran a redundant ground circuit from the block to the battery negative terminal. The engine cranks over and now starts every time. The ground cable between the engine block and body was at fault.

Many of our really good technicians know the diagnostic value of a low inductive amp probe interfaced with a DSO. The attenuation setting I use most often is every 100 millivolts =1 amp (see Figure 7).

Another problem vehicle was a 2001 Ford Ranger with an intermittent hard start complaint. Upon my arrival at the shop the vehicle was in the bay and started just fine. The technician had attached a fuel pressure gauge which showed a good 35 psi. When the vacuum line was disconnected from the fuel pressure regulator the fuel pressure jumped to 45 psi.

I requested a phone call to the truck owner to obtain specifically when the hard starting problem occurred. He stated it occurred after an overnight cold soak. The injector on-time to start a cold engine at 35 degrees Fahrenheit is over 100 milliseconds. The injector on-time to start up a fully warmed PFI system is around 5 milliseconds. The longer the injector on-time, the more stress on the fuel pump to deliver the fuel pressure and volume needed to start a cold engine.

We explained this to the truck owner. With a current probe attached around one wire at the inertia switch we obtained the amperage waveform indicated in Figure 8. The waveform was captured during a KOEO key cycle. Note the voltage per division is set at 500 millivolts per division. The attenuation factor on the amp probe is set at 100 millivolts = 1 amp. So simple math tells us that every vertical division on the DSO would represent 5 amps per division.

Note that the initial current surge is over 25 amps. Note that after the initial current surge we lost continuity and the amps fell to 0 for a brief period of time. Then the amperage value peaked again at about 20 amps and then finally settled in at over 8 amps with a very unstable waveform.

I communicated my findings with the truck owner and informed him he has a very bad fuel pump and it could easily be the cause of the cold starting problem.

The truth is we cannot continue with any more testing until we have the OK to replace the pump. Given the OK from the truck owner, the fuel pump was replaced with happy results.

Whenever faced with a potential fuel pump problem we initially decide we need to check fuel pressure.

GM is the only manufacturer that still gives us a fuel pressure test port. The amperage value obtained by current-ramping the fuel pump will show us the electrical and mechanical integrity of the fuel pump. Normally on most PFI systems the on-line amperage values will range between 4 and 6 amps with good uniform oscillations (see Figure 9).

Access to the fuel pump circuit can be obtained at the fuel pump fuse by using a fusible jumper or by removing the fuel pump relay and jumping across the power terminal.

Another intermittent problem was on a GM Express Van with an intermittent P0316 crank sensor code. An intermittent miss and stall symptom followed by an intermittent no-start condition also existed.

The vehicle had been towed in twice before I arrived at the shop. The crank sensor had been replaced two times before with no good results. Notice the secondary ignition wave form in Figure 11. Did you notice the missing point of primary turn on? With that in mind let’s scope check the DREF signal (rpm signal —distributor reference). This is one sensor input that cannot be substituted or the one we allow to disallow.

Looking at the DREF signal in Figure 12 we see the problem. The DREF signal is breaking up. There are pattern failures here where the main bearings wear which destroys the air gap between the tip of the crank sensor and the reluctor. This mechanical interference can cause an erratic signal from the crank sensor.

A visual inspection on the tip of the crank sensor indicated no physical interference between the crank sensor and the reluctor wheel.

Now note the schematic in Figure 13. The signal circuit from the crank sensor to the PCM is color coded yellow. We simply ran a redundant circuit between the crank sensor signal circuit to the PCM. Another happy ending.  Circuit access to a component has always been a trial. When back-probing a connector, I have found that the T-shaped pins available in the sewing department from your local discount stores work great (see Figure 10).

As we stated earlier, running a redundant circuit is usually a good exercise. In addition, there have been many cases where terminal fretting occurs causing an intermittent bad connection of a sensor or output device. Terminal fretting is caused by vibration between the male and female terminal, causing the metal to flake off resulting in an intermittent poor connection. The key is to clean the terminals and to apply a dab of stabilant 22A which is a contact enhancer and will pretty much guarantee conductivity. This product is available from your local parts store. The BWD part number is CL85 (see ­Figure 14).

Many times the round female terminals lose their tension to their male counter parts resulting in a loss of connection. A set of wire size drill bits are available from the folks at Granger. Wire size drill bits from 57 thru 60 pretty much covers most automotive applications (see Figure 15). By running the shank end of the drill into the female terminal you are basically doing a drag test in checking the female terminal tension. In addition you are most certainly cleaning the inners of the female terminal.

In closing, I can’t say enough of the importance of questioning the car owner as to what is the exact symptom and when does it most often occur. Is it temperature related and does it occur at a certain speed? Since most car owners don’t speak our language help them out by asking specific questions in the king’s language.

In the real world we can’t over-emphasize that all too often we must experience the symptom on our own. You simply can’t say enough about what you feel through the seat of your pants during a test drive. When the symptom occurs while under road load conditions be sure to take the scan tool and utilize the record function.

Keep your eyes on the road and your finger on the record button. Upon returning to the shop to review the recorded data, be sure to use the graphic mode of the scan tool to view several seconds of data. When looking at data on a digital readout you are looking at one small moment in time. This may give you a good direction to begin your diagnostics.

Until next time, good luck in your diagnostic dilemmas.  ■

Bill Fulton is the author of Mitchell 1’s Advanced Engine Performance Diagnostics and Advanced Engine Diagnostics manuals. He is also the author of several lab scope and drivability manuals such as Ford, Toyota, GM, and Chrysler OBD I and OBD II systems, Fuel System Testing, many other training manuals in addition to his own 101 Lab Scope Testing Tips. He is a certified Master Technician with over 30 years of training and R&D experience. He was rated in the top three nationally in Motor Service Magazine’s Top Technical Trainer Award and has instructed for Mitchell 1, Precision Tune, OTC, O’Reilly Auto Parts, BWD, JD Byrider, Snap-on Vetronix and Standard Ignition programs. You may have also seen Fulton in many Lightning Bolt Training videos and DVDs and read his articles in many auto service magazines. He currently owns and operates Ohio Automotive Technology, which is an automotive repair and research development center.