If you don't want to take your TDI into the Volkswagen dealer every time there's an issue, it may be beneficial to learn how to do some troubleshooting of your vehicle at home. While there are many features on a VW TDI that can go bad, the TDI has a few common problems. Some of these are small enough to fix on your own. Others will require expert repair. Either way, you'll gain some insight into the problem.
Know what's under the hood and body of your TDI. TDIs have turbocharged diesel engines, but there are different versions. A 1996 or 1997 Passat TDI in the U.S. will have a B4 chassis, A3 engine and Garret GT15 Turbocharger. A 1997 through 1999 Jetta TDI has an A3 engine and probably a KKK K03-006 Turbocharger. The components of your car will be listed in the owner's manual, or check the build sheet that is located in the trunk, near the spare tire. This sheet will tell you the engine code. A "1Z" code stands for an A3 engine with a Garret GT15 Turbocharger.
Test for power if the engine will not start or is sluggish in starting. To check the power supply, put the car in neutral and the clutch to the floor if the car has a manual transmission. Turn the ignition to the "Run" position. Watch the dash for the "check engine" and "glow plug" lights. If they do not come on, you are getting no power to your engine. More than likely, the battery needs to be replaced.
Determine if the engine is cranking. If the "check engine" and "glow plug" lights come on in the power test, the problem is probably not the battery itself. Turn the ignition to the "start" position. If the engine makes a noise like it is trying to start, you may have fractured connections between the engine and the battery. Check the wires connecting your battery to the engine for corrosion. If there is corrosion, the wires should be replaced. It is also possible that the connections have simply come loose. Tighten the connections and try starting your TDI again. This is also a symptom of low fuel, so fill up before you test again.
Check for a "relay 109" failure. If either the "glow plug" or "check engine" light has been coming on for seconds at a time and then going off again, this may indicate a relay 109 failure. This relay provides the power to all of a TDI's engine-based electronics. To diagnose the problem further, you must first replace the relay. If the lights continue to come on, the most common causes are the ignition switch or security system. If the "glow plug" light comes on intermittently by itself, check your brake lights. A burned-out bulb will cause the "glow plug" indicator light to come on.
Pay attention to your fill-ups if your TDI smokes when fully accelerated. This issue is often a problem with fuel quality. Switching to biodiesel and using a diesel fuel injector occasionally at fill-ups can cease the smoking.
Don't worry about every strange noise. If you are new to your TDI, you may have noticed a few noises that you haven't heard in previous cars. Many of these noises are normal. TDIs are known to make a swishing noise when the engine is shut off. This is the anti-shudder valve working. Clicking noises from the engine compartment are the boost pressure control and EGR valves, which switch on and off regularly as part of normal operation. You may also hear some routine clicks in your TDI's steering column. All of these noises mean that your TDI is operating just as it should be.
