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997 C4S
11,065 Posts
Discussion Starter #1 (Edited)
I'll be creating several threads in the coming weeks, to help all of you with strategy-based diagnostics. This forum generally tends to change parts rather than troubleshooting their problems, so hopefully I can educate some of you into making better informed decisions. This will save you both time and money, which I'm sure all of you can appreciate.

A common misconception amongst consumers, is that they herald their vehicles to be some kind of super diagnostics computer. Most people honestly believe that a scan tool is the only diagnostics tool necessary, but that couldn't be further from the truth. Vacuum readings, engine analyzers, gas analyzers, spark plug condition, and even putting your hand in front of the tailpipe all do their part in diagnosing a vehicle. Today however, I'll start from the very basics and hopefully in time, we'll progress to the more advanced procedures.

So what is strategy-based diagnostics? Well as the name suggests, it's a systematic way in approaching the problems that you may encounter. When you stumble upon a problem in life, do you blindly try to find the solution? No, you analyze the situation and carefully develop a solution. It's no different for motor vehicles.

Step 1: Verify The Complaint
-The conditions under which the problems occur mean everything. Does it only happen when it's cold outside? Does it only happen uphill under load? Does it only happen during idle? Find the exact conditions where you can replicate the problem, and take note of every little detail. Missing something here can mean the difference between a 10-minute or a 10-hour job.

Examples of what to ask yourself:
-What is the vehicle actually doing?
-When did the symptoms begin?
-Under what conditions does it occur?
-How often does it occur?
-Were previous repairs/modifications attempted?

-On a side note, this is why you're sent home if the dealership can't replicate the problem. The technicians need to see, feel, hear, and smell every little symptom in order to have a fighting chance in repairing the problem.

Step 2: Preliminary Checks
-These are the "quick and dirty" tests that most of you may overlook. Simple things such as a low fluid level or a blown fuse, can lead to a myriad of driveability problems. It's all too easy to get ahead of ourselves, so stick to the basics and check for the obvious first.

Examples of things to look for:
-Loose connectors
-Kinked lines and hoses
-Fluid level and quality
-Leaking gaskets or hoses

Step 3: Check For DTCs
-If a DTC is present, take note of it regardless of what it may tell you. If you believe that you may have an electrical problem and the DTC is for the fuel system, write that down anyway! I had stated earlier that the computer is not a super-diagnostics machine so allow me to explain why. A computer functions through logic, and will only do what it's programmed to do. It recieves inputs from sensors, compares them to its logic tables, then sends outputs to actuators (fuel injectors, etc). It does not think for itself, nor is there any artificial intelligence.

So what does this have to do with DTCs? Well if it sees that a certain sensor is anomalous, it's going to trigger a DTC for THAT sensor. It's not going to sit there and think "hmmm, what may have caused this problem?" That's your job as a technician or car enthusiast.

For example, a common DTC on this forum is the P0172 (overly rich). What's the first thing that most people end up doing? They buy a new O2 sensor which is precisely what NOT to do. Think about this for a second: if you have a leaking injector that's dumping fuel into the manifold, wouldn't that cause a rich condition? Yes it will. If the A/F mixture is rich, isn't it the job of the O2 sensor to pick that up? Yes it is. The computer doesn't know about the leaking injector nor does it care, because all it does is follow its logic: "Hey, I see an anomaly at the O2 sensor. It's rich so I'm going to trigger a DTC for it." In other words, DTCs often give false positives, and it's your job to diagnose.

Step 4: Check For TSBs
-TSBs are only developed when pattern failures occur. If a certain model develops a frequent problem (3rd gear grind), a TSB is written to speed up the diagnostics and repair time. You can get this information free from your dealership. Take this with a grain of salt however, because it won't always be the solution. Say you're one of the many with a 3rd gear grind. Yes the solution may lie in the TSB, but it could also be caused by something as simple as low transmission fluid. Then again, if you followed Step 2 and checked for the obvious, then you would've found your problem by now. See how this is all starting to come together?

Step 5: Analyze Symptoms And Gather Data
-This may be a little difficult, because many of you will lack the necessary tools to perform diagnostics at this point. However if you have a multimeter, vacuum gauge, etc then gather as much information as humanly possible to give yourself a fighting chance. Heck, pulling a spark plug tells you more in 10 seconds than a scan tool will in 10 minutes, so this step is actually very advanced and extremely in-depth. As a result, be on the lookout for a future thread dedicated to this step alone.

-Finding a good starting point is key to any diagnostics. Return to Step 1 and think about how you've answered your questions. Lets say that your car misfires everytime you have a passenger, or whenever you drive up a hill. Analyze the symptoms! What do these two have in common? Engine load. What happens when you load the engine? You step on the throttle, this draws air into the manifold, more fuel is injected, a larger spark is required, valve timing changes, valve overlap changes, etc. etc. etc. Any problem in these related systems can cause a misfire - not just the ignition, which is actually where most people will look. So where should you start? Well that's where the other thread will come in, because I could easily write a 5000-page book on this step alone. If you have the Helm's manual, look through it and you'll see that it's actually one giant book about this very step: symptoms and gathering data.

Step 6: Narrow Down The Problem To A System Or Cylinder(s)
-This depends heavily on Step 5, because it relies on the tests that you'll have to perform. For example, lets say that your complaint was a no-start condition. The car cranks fine, but it just doesn't start (Step 1). If you performed Step 2, then you already know that it's not the fuse or battery. During Step 3, you found a DTC for a misfire which may or may not be related. You then called the dealer and found no TSBs (Step 4). You sat down, analyzed the situation, and begin to gather data (Step 5) starting with the ignition system. It turns out that the spark is strong, and that the ignition system is working fine afterall. Now you can eliminate that section, and narrow down your search to the other systems such as fueling (Step 6).

Step 7: Determine The Root Cause And Perform Repair
-This is generally the easy step, where you've narrowed down all of your possibilities. Now you have all of the data in front of you, and you're looking at a single component, rather than an entire system. Lets use the same no-start example from Step 6 above. It turns out that you have a kinked fuel line which caused a restriction, which prevented most of the fuel from being delivered. Most people would stop here and fix the line, but lets take it one step further.

-Fuel lines aren't supposed to kink. Something happened and caused it to occur. On top of fixing the problem, you need to figure out what had caused it in the first place. A kinked line is pretty straight forward. You may have bottomed out on a speed bump prior to parking the car. But what happens if you blew a fuse? If you install a new fuse without fixing the cause, chances are that the new fuse will blow too. What happens if the problem turned out to be a friend ECU? That's a $600 part and do you really want to keep frying computers?

Step 8: Verify That Problem Is Repaired And No Additional Problems Exist
-Believe it or not, fixing one problem may cause another to arise, or maybe you just didn't put things back correctly. Catch those problems now while you can. If everything is running as it should, congratulations! You've become a novice technician... you're no longer a grease monkey.

Keith On His Soap Box: :soapbox:
I know this is redundant but remember, do not change parts without any supporting data. Yes you'll get lucky with pattern failures (ie. P1166), but without diagnosing, there's no way you would've found that kinked line in the example above. Most people will say "My car doesn't start, so I'll change the battery." Then when they realize that it's not the battery, they'll buy a new starter motor. Basically what I'm trying to say is this: do a little troubleshooting and you'll go far. Can you see how the 8 steps above lead to a $50 fuel line opposed to a $100 battery and $250 starter?

All you had to do was approach this problem like any other roadblock in life. Think about it, learn how things are supposed to work, then develop a plan of attack.

For the techs out there, feel free to add or correct any misinformation. It's 4 AM right now so if none of that made any sense, then let me know so that I can re-word it later as well. I'm not the best at simplifying things (and I ramble a lot) so bear with me.

353 Posts
Good info! Hopefully people will take the time to read it. This is how a true mechanic works.

I find that the biggest problem in this particular forum is that people don't search as much as they should.
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