Diesel Diagnostics

By Dave Biegel. Scan tools are only part of the key to the diagnostic process, knowing when to use them is equally important. If a vehicle comes into your shop with a drivability complaint, should plugging in your scan tool be the first step in diagnosing the problem?

Probably not — keep in mind that even though modern engines have sensors and computers that monitor just about every aspect of their operation, deep down they’re still machines that require air, fuel, and oil to run. Remember — a reflash won’t fix everything.

Before plugging in the scan tool and throwing new parts at the engine, let’s take a look at the basics. First we need to start with a plan, or process, to diagnose the problem.

Step One: Talk to the customer. Find out exactly what the complaint is. Ask what the vehicle was doing when the problem first occurred, or was first noticed. For example, was the customer towing a heavy load or driving in stop-and-go traffic with no load? Did the problem come on suddenly or gradually? Next, record what your customer tells you. Not only does that show your customer you care about his problem, but it also gives you something to go back to during your diagnostics.

Step Two: Verify the problem. Test drive the vehicle and try to re-create the reported problem. If possible, take the customer along to get a better idea of the nature of the complaint.

Step Three: Open the hood and do a visual inspection of the maintenance items. Check the oil and air filter. Check the fuel level. Drain some fuel to check for water, gas, dirt, or other obvious signs of contamination. Look for fuel leaks. Inspect the turbo and the air inlet plumbing. Make sure all clamps are tight. Look for loose or broken electrical connectors. Make sure the battery terminals are clean and the connections are tight. If it’s a starting or charging problem, do a battery load test. If you find any problems, correct them first so that you can eliminate them as potential problems later during the diagnostic process.

Step Four: Now it’s time to connect your scan tool. Each scan tool model has unique capabilities, so some tests you can do with one scan tool may not be able to be done with another. Whatever scan tool you’re using, you should once again start with the basics — be sure your scan tool has all the latest updates. Check for, and record, DTCs. Take a look at the trip data log. Is the vehicle’s software up to date? Check fuel pressures, both actual and desired. Do a cylinder contribution test. Check boost pressure and exhaust back pressure. Do any required reflash procedures.

Hopefully by conducting the basics first, you’ll narrow down the possibilities of what is really causing the problem, and you’ll be able to repair the vehicle without installing unnecessary parts. Customers don’t want to pay for parts they don’t need and your shop doesn’t like to pay for them either.

This article written by Dave Biegel of Diesel Injection Service of Windsor WI

This article first appeared in the Spring Issue of ADS Nozzle Chatter