Diagnosis techniques for Driveability Concerns

Avery Technician Kansas City, Missouri Posted   Latest  

One of the key reasons I am on this site, like many of you is to learn more, improve my skillset, and come up with new (to me) ways to do things. Diagnosis is always one of those areas where a lot of techs seem to lose money on due to all the different approaches one can take and how many directions can lead you to not finding an accurate diagnosis.

The purpose of my thread here is to spark discussion on HOW we diagnose issues on these vehicles today. While it's a given a scan tool is almost always involved. I am posting some common issues I have seen and MY personal diagnostic approach. I am by no means a trainer, or a "diagnostician", simply a technician trying to be better and help others... I urge you to do the same.

Any critiques, suggestions, or posting your own diagnostic "pathways" would benefit everyone on this website.

Misfires:

For me when I am diagnosing a misfire, my testing begins with a test drive to duplicate the reported concern. I try to find a hill to accelerate up on and load the coils, and fuel pump. I will bring my scan tool with me and monitor for misfires, and also review my freeze frame data to help me narrow down which cylinder or cylinders are acting up. Mode 6 Data is invaluable when it comes to quickly identifying cylinders as its "raw" data that shows accurate counts. After my test drive, if I will perform my "courtesy inspection" and look at the condition of the vehicle as a whole. One of my trainers has told me to always start with a Relative compression test in which I sync my labscope to cylinder 1 of my coil and put an amp clamp around the battery cable. Removing the fuel pump relay and cranking over the engine for about 10 seconds. This will help me identify whether this is a mechanic issue as I look for cylinders that are not performing as they should. If my relative compression check shows no mechanical issues I proceed to swap coils from the "bad" cylinder to a cylinder that isn't showing any misfire counts. In doing so I look over my coil, coil boot, and wiring inspecting for issues. If my misfire follows after swapping coils. I know I have a defective coil. If that doesn't show me that I have a bad coil, I will pull the spark plug from the "misfiring" cylinder and inspect it. If I still have no diagnosis, I will either use a labscope to check my injector, or my Power Probe 4 (which tests for injector issues).

Wheel Bearings:

Really just a cool tip I learned not that long ago. We all have been on a test drive and cannot accurately pin down (based on the test drive) which wheel bearing is bad. Next time you are in this situation try this: Raise the vehicle about a foot off the ground, Using a throttle depressor run the vehicle to about 30mph. While the wheels are spinning touch the suspension coil springs... The one that is vibrating is your loud wheel bearing. This is one of my new favorite tricks for finding wheel bearing noises.

+7
Albin Diagnostician
Leavenworth, Washington
Albin
 

It is always cool to see techs wanting to learn new things. Whoever said "you can't teach an old dog new tricks" never new anything about old dogs. Misfires are always fun to do, since they can come in all different flavors. Some are under load, some are intermittent, some at idle, and the list goes on an on. Add diesels to the mix, and the process gets more interesting. I always keep in mind…

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Pete Mechanic
Newark, Delaware
Pete
 

Hi Avery, Beings you have a scope, I suggest scoping the coils and injectors -voltage and current- instead of pulling and swapping coils. You can glean more information, faster this way. beings you're starting to use a scope it's a no brainier to incorporate this into your testing strategy, earlier than you currently do. Oh and your wheel bearing trick, I call it "resonance testing" and

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