By Jim Newkirk
'When they are good, they are very, very good. But when they are bad, they are horrid!'
The Volkswagen TDI has gone a long way to change the perception of the diesel passenger car.
Borrowing a page from an old nursery rhyme, the title sums up the feelings of Volkswagen TDI diesel drivers as well as technicians who have developed a love/hate relationship with these vehicles over the years. A little history first ... Volkswagen is one of the two remaining manufacturers offering sale of diesel passenger cars in the United States. The other is Mercedes-Benz (see story on page 12). Never the most popular of vehicles, the diesel passenger car has long suffered in the eyes of consumers from perceived excessive noise, lack of power and diesel smell. On the other hand, owners of diesel passenger cars are usually fiercely loyal to both the idea of diesel power and the brand of vehicle. The Volkswagen TDI has gone a long way to change the perception of the diesel passenger car. Excellent power, acceleration, quiet running and of course excellent fuel economy have placed thousands of Volkswagen TDIs on the roads and sooner or later these vehicles will find their way into your shop.
In this example, a 2000 Jetta 1.9L TDI was brought in with a complaint of loss of power. The following diagnostic procedure uses a full-function Volkswagen factory-compatible scan tool to determine the cause of the lack of power.
When a 1.9L TDI diesel engine exhibits a lack of power and/or poor acceleration, proceed as follows:
- Visually check the intake manifold for excessive carbon buildup and clean as necessary. Remove the exhaust gas recirculation (EGR) valve or the intake boot to gain access and inspect the EGR and intake passages. Vehicles that are primarily used for short-trip driving can produce intake carbon severe enough to cause driveability issues in a surprisingly short time span.
- Using a full-function factory compatible scan tool, verify the mass airflow (MAF) readings as follows: (Note: The following tests must be performed on a warm engine.)
- Start and idle the engine. Using the scan tool, enter Engine Address Word 01, Function 08 Read Measuring Blocks and Data Block 003.
- Data Block 003 will display four fields, observe the second (air mass drawn in specified) and third (air mass drawn in actual) display fields. Both fields should match (typical idle range is 230-370 mg/H).
- Next, enter Engine Address Word 01, Function 08 Read Measuring Blocks and Data Block 010. Observe the first (air mass drawn in), third (manifold absolute pressure (MAP)-boost) and fourth (throttle position) display fields.
- Perform a wide-open throttle (WOT) acceleration from 35 mph in third gear and record the displayed values in field one and field three at 3,000 rpm and display field four showing 100 percent throttle. Field one should show in excess of 810 mg/H and field three should show between 1,700 to 2,080 millibar (mbar) of available boost (optimum readings are 1,850 to 1,950 mbar).
- Lower than normal MAF and boost readings indicate a potential plugged intake or a faulty MAF. (A quick check of the health of the MAF can be performed by unplugging the MAF and running the vehicle a second time to see if driveability has been improved. A vehicle that runs with better power with the MAF unplugged likely has a faulty MAF installed.)
By using data blocks 003 and 010 a technician can get a good idea of the health of the intake system. It just takes a bit of data interpretation. The following examples illustrate that by observing the data available with a scan tool, a conclusion as to the lack of power condition can be quickly reached.
- Data Block 003 shows the actual calculated fuel per stroke entering the engine in milligrams based on the air mass entering the engine. If the readings here are not in specification, a faulty MAF or an intake problem is possible. This single data group provides the first clues as to the health of the intake system. To get a good idea of what's really happening in the intake, use both Blocks 003 and 010.
- Data Block 010 provides even more information. In this Data Block both the MAF values under load and the actual Boost numbers are displayed.
- If the boost numbers displayed in Block 010 are normal but MAF numbers are low, the most likely cause is a faulty MAF. In this case the MAF would be incorrectly displaying the actual air flow into the engine. Air flow would have to be near correct for the boost numbers to be on or close to normal specification.
- If the MAF numbers displayed in Block 010 are correct and the boost numbers are low, the problem is more likely turbo function or a leaking pressure side turbo to intake manifold duct. The live data stream available in a data block makes it easy to compare actual engine values to known good values. Once again, if the MAF indicates correct air flow and boost is low, a loss of available boost - not an air flow problem - is indicated.
- Finally, if the MAF numbers show lower than normal and the boost numbers also show lower than normal, the likely cause is poor air flow through the intake system. If the engine can't breathe, it can't produce MAF or boost readings close to specification.
Jim Newkirk is the Identifix European team leader. He is an ASE master with L1 and Bosch BSC certifications.
This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time.
For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time.
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© 2007 IDENTIFIX. All Rights Reserved.
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