Get a diagnostic game plan

June 2, 2014
Your game plan needs to start with a route on where and how to proceed in diagnosing the problem at hand. We all have different ways to diagnose a problem vehicle, but there may be a better route to follow.

Before we start to diagnose a vehicle, we need to have a game plan — just like a coach or manager of a major league sports team. Your game plan needs to start with a route on where and how to proceed in diagnosing the problem at hand. We all have different ways to diagnose a problem vehicle, but there may be a better route to follow.

I base my procedure on what’s right and what’s wrong, just like the big box scope analyzers did in the past. The good thing about the old big box analyzers is that they forced us to start at the battery, starter, alternator, engine mechanical condition, emission gas readings, ignition and fuel. Remember: If you miss the basics, you miss the problem.

The relative compression test is a real timesaver. Some OEM scan tools, like the Ford IDS, incorporate the test right in the tool.

If we were going to diagnose a sagging roof, you would not install a new roof without checking the walls and the foundation first. Similarly, in diagnosing an engine performance problem, we need to start with the basics. This is why it’s so important that at least two OE scan tools have a test to make sure that the mechanical condition of the engine is good. The reason why Ford and Toyota install these tests on their scan tools is that they have seen too many parts thrown at a problem by their own technicians, with no solution or fix acheived. The root cause of the problem in many cases stemmed back to something basic, from a bad battery to a mechanical issue. 

Many engines that we work on today have some form of variable valve timing. The use of the wrong oil can cause the variable valve timing system to portray a mechanical problem. The cam phasers will not be able to adjust to the proper settings if the wrong viscosity oil was used, due to improper oil flow. What I will attempt to do in this article is to provide you with a good game plan, along with the tools to use in diagnosing problem vehicles.

Where to Start
The first place to start is by interviewing the driver of the vehicle. A good Q&A session might lead you to looking into an area that you might have not checked without the information they provided. 

The next step is to use the best tools you own — your brain, eyes, ears, nose and hands — to check out the problem. After a preliminary “look-see,” research the problem using your service information (SI) source, followed by investigating Technical Service Bulletins (TSBs) that may be related to the issue. 

With so many variations on even the most routine system, you have to know the particulars of the system you are working on before you even pick up a wrench. Information on iATN, Identifix, ALLDATA, Mitchell, Autodata, MotoLogic or even Google can be very helpful with identification, if the vehicle you are working on needs a reflash or has a silver bullet problem. Remember, when looking at a silver bullet solution, always check and test the components and the system before replacing anything. 

After performing an investigation, connect one of the best tools to use to diagnose problems — the scan tool. It’s easy to use and provides us a great deal of information in the shortest period of time. This should be followed by using either an Associated, MidTronics or other battery/starter/alternator tester that will ensure that you are starting with a good electrical foundation.

A relative compression test can be taken in the car, using any scope and a DLC breakout box.

Most shop-level scan tools today provide for a full system scan that queries the entire vehicle’s computer system and all related control modules (ECUs) to find any system faults. Sometimes a fault in one system can affect another system. For example, our shop once had a Dodge Dakota that had a complaint of a hard first shift. The shifting problem turned out to be caused by a PO136 (O2 Sensor Bank 1 Sensor 2 heater malfunction). After Wayne Colonna (ATSG president and publisher of Motor Age’s POWERTRAIN PRO) turned us on to the correct diagnostic path, we found the connection between the O2 heater to the shifting problem. 

Initial Testing
Some scan tools (such as the Ford IDS, Toyota Techstream, OTC and Autel, to name a few), provide a relative compression test that takes less than a minute to perform. If your scan tool doesn’t offer this option, you will need a different way to perform this test. 

The relative compression test can be performed with any brand digital storage oscilloscope (DSO). The relative compression test can be performed in less than five minutes depending on whether the vehicle has “Clear Flood” capability. You can even perform this test right from the driver’s seat, without even opening the hood of the vehicle:

1. Connect a diagnostic link connector (DLC) breakout box to the connector itself, and then connect your labscope to pins 4 (Vehicle Ground) and pin 16 (Vehicle Power). 

It’s important to check the status of the computer monitors to avoid an unnecessary comeback.

2. With the scope coupled (set) to AC voltage, hold the accelerator pedal to the floor while cranking the engine over for about a minute. The waveform that you capture will provide you with information on whether the engine has a mechanical problem.

This test also can be done from the battery terminals while the labscope is coupled to AC voltage. Another alternative is using a high-current probe that is clamped around either battery cable while cranking the engine over. 

Go Generic
Follow this test with using your scan tool in Generic/Global OBDII mode. I recommend this because on many enhanced (OEM-specific) data lists, the data you see might be comprised of substitute values and not reflect what the sensors are actually reporting to the engine control module (ECM). Data values are displayed all the same way, regardless of whether you are working on a BMW or a GM. Generic/Global allows us to look at diagnostic trouble codes (DTCs), pending DTCs, Freeze Frame, Monitor Status and Mode $06. 

Because all the information is available right there in a format that is the same on every vehicle, it makes it easier to have a good diagnostic game plan to follow. If the monitors are not “Ready/Set,” for example, you might encounter a problem with an illuminated malfunction indicator lamp (MIL) that will not show up until the vehicle has met the enabling criteria. That results in a comeback that could have been easily avoided. The “You Stink Light” is not what the vehicle owner or you want to see. 

Fuel trims are a great diagnostic aid, and this tool maps it out for you.

Remember, the vehicle was brought in to be repaired and keep the light off. So, let’s say the vehicle had an O2 DTC and the exhaust gas recirculation (EGR) and evaporative emissions (EVAP) monitors are not ready yet. You are taking a big chance just repairing the O2 sensor fault without running the EGR and EVAP monitors. When the monitors are not ready, there is a chance the MIL will reappear once the vehicle has met the enabling criteria. Try explaining that to an angry vehicle owner that knows just one thing: He paid you to turn the light off. 

So, you might want to look up the drive cycle for the monitor that is not ready and run them. If you are working on an Audi or VW as displayed on the Ross Tech scan tool, you can run the monitors in your bay. Either method will allow you to confirm the vehicle is repaired before returning the vehicle to the owner. 

You can use Pending DTCs to predict if there is a problem on the horizon, or you can look at Mode $06 data to also predict if there is a problem with a component that will fail and be elevated to a Pending DTC and onto a DTC. Mode $06 can also be useful to see why a monitor is not becoming “ready” or due to fail a test that is being conducted by the ECM. 

Cars may be more complex, but the basics still apply even though they often are overlooked.

Fuel Trim for Success
The next step is to move onto one of the most important data values to look at: the vehicle’s fuel trim. Remember, just because you see a command to add or subtract fuel while monitoring both short-term fuel trim (STFT) and long-term fuel trim (LTFT) does not mean that it’s really happening. If you have a mechanical condition such as a bad pressure regulator, the computer cannot compensate enough to correct the fault. 

If a mechanical condition is present, that will have to be repaired first because the computer assumes all mechanical conditions are good. Just as I mentioned before, the basics are very important: Everything has to be good with the foundation before moving on. 

After fuel trim checks out of the way, I prefer to view the following parameter identifiers (PIDs): O2, air/fuel ratio, load, spark advance, EGR position, engine coolant temperature (ECT), intake air temperature (IAT), rpm, and mass airflow (MAF) in grams per second. 

Depending on the type of problem you are chasing, some PIDs may be more important than others. For example, with a cold engine, the ECT, IAT or battery temperature sensor (BTS) should be in a specific range, typically within 5 to 10 degrees of each other when the engine is cold. The calculated load PID can provide us useful information on whether the engine can breathe properly. At wide open throttle while graphing the data on a scan tool, the reading should be 90 percent or better on a vehicle that has a good air filter, intake manifold, intake and exhaust valves and a clear exhaust system.

Thermal imaging cameras are becoming more affordable, and will see increasing use in automotive diagnostics.

The MAF sensor at idle should provide us data typically showing at least 1 gram of airflow per second per liter of engine size (if you are working on a 4.0 liter engine, you should expect to see a MAF PID reading of 4 gps or so). You can zone in and diagnose a P0420 (Catalytic Converter Efficiency Below Threshold) by graphing the front air fuel and rear O2 sensors on your scan tool at idle, 1,800 and 3,000 rpm to see whether the converter is good or bad. This can be followed up with checking whether the converter is working by using a thermal imager to see the difference between a good and a bad converter. 

Keeping up With New Techniques
Another problem area that has so many technicians beating their heads is a small EVAP leak. A smoke machine along with CO2 is a great helpful tool, but sometimes it’s very difficult finding a 0.020-inch leak or a charcoal canister problem without using a gas analyzer. Fortunately, you are not alone in the challenges you face, and there are some bright guys out there always looking for a new solution.

Recently, I was challenged with a tough EVAP leak problem. I was unable to find the problem looking for smoke, although I knew there was a leak by viewing the ball on the flow gauge of the machine. I used the new BullsEye leak detection system (created by Bernie Thompson of Automotive Test Solutions) and found the area of the leak with the electronic detector. I then could pinpoint the exact spot where the leak was coming after spraying the foam agent that’s part of the system. 

The BullsEye leak detection system uses CO2 and a special foam agent that changes color in reaction to the gas to help find the most difficult leak. Here, the tank is porous and leaking over a large area, which is an unusual case found easily with new techniques.

The foam reacts with the CO2 and changes colors from pink to yellow. The area at the top of the tank where there was no leak stayed pink (see related photo), while the leaking area turned bright yellow. 

In this particular case, the plastic had become porous and was leaking a little bit over a large surface area. A new tank solved this problem, and this new method made it a problem I could be assured of finding. 

While many things continue to change and evolve in our business, many things remain the same. New tools and techniques will continue to make our diagnostic lives easier — but you still have to use the tools God gave you to achieve final success. 

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