Peterbilt 389 / Cummins ISX, Paccar MX-13 etc. FAQs

I. Common Causes of DEF Contamination There are several common causes of DEF contamination in your Peterbilt 386, 389, 567, 579 etc. with a Paccar MX-13 engine. Here are a few: Mixing DEF with other fluids: DEF should never be mixed with other fluids, as it can cause contamination. Using incorrect concentration, low-quality or expired DEF: Using DEF that has an incorrect ratio of water to urea, is past its expiration date or of poor quality can lead to contamination. Dirty DEF pumps at a fill station: If you fill up your DEF from a station, watch out for dirty nozzles that can transfer debris to the inside of your DEF tank. Contaminated DEF storage containers: If the DEF is stored in containers that are not clean or are contaminated with other fluids, it can lead to contamination. DEF pump issues: If the DEF pump is not functioning properly, it can lead to contamination of the DEF. DEF filter issues: If the DEF filter is old, dirty, or plugged up, it can cause contamination of the DEF. II. Diagnosing and Fixing DEF Contamination To diagnose and fix DEF contamination in your Peterbilt 386, 389, 567, 579 etc. with a Paccar MX-13 engine, you can follow these steps: Check the DEF level using the gauge or dipstick. If it is low, add DEF fluid as needed to bring it to the proper level. Check the DEF fluid for any signs of contamination, such as a change in color or consistency. If the DEF is contaminated, it will need to be replaced with a new, high-quality batch. Inspect the DEF storage containers to ensure that they are clean and free from contamination. Check the DEF pump for any signs of malfunction, debris or damage, such as leaks or faulty seals. If the DEF pump is found to be faulty, it will need to be repaired or replaced. Check the DEF concentration mixture. DEF should be mixed with deionized water to a concentration of 32.5% +/- 1%. If the DEF concentration is incorrect, it can cause contamination. To troubleshoot this issue, you can have the DEF tested with a test strip or use a diagnostic scan tool to check the DEF concentration. TIP: verify the scan tool parameters with a test strip, don’t rely on what the computer tells you.  If the DEF concentration is incorrect, you will need to drain and discard the contaminated DEF properly. Depending on what the contamination is, you may also need to rinse your DEF tank with distilled water. Afterward, replace the DEF filter. Fill the DEF tank with new high-quality DEF (bottled DEF preferably, not from a bulk pump at a fill station) Again depending on the extent of the contamination, you may need to purge the DEF system to clear the lines and injector of the bad DEF.  Check the DEF filter for any signs of contamination. If the DEF filter is old, dirty, or plugged up, it can cause contamination of the DEF. Clean or replace the DEF filter as needed to resolve the issue. Have the DEF system inspected by a professional to determine the cause of the contamination and have any faulty components repaired or replaced as needed.

Here are some common causes of inaccurate or erratic DEF level gauge readings on your Peterbilt semi truck with a Cummins ISX engine: Actually Low DEF fluid level: This may seem obvious, but if you’re near empty on your tank it can do weird things. This is the most common cause of inaccurate or erratic DEF level gauge readings. If the DEF fluid level is low, the gauge or dipstick may not accurately reflect the actual DEF level. Faulty DEF level sensor aka DEF sending unit: If the DEF level sensor or wiring is not functioning properly, it may not accurately detect the DEF fluid level and the vehicle's computer system may not display the correct DEF level on the dashboard gauge or dipstick. This can cause the DEF level to fluctuate or appear to be erratic. The DEF level sensor is part of a multi-purpose component that also includes the DEF quality sensor, DEF tank temperature sensor, and DEF tank heater. Please note if you are seeing DEF  fault codes for any of these components you may have a malfunctioning DEF sending unit aka header, which is mounted on top of the DEF tank and looks like a cap with a clamp, but has tubes and wiring that goes down into the tank.

Generally if you're a long haul truck driver, roughly about every 4-7 days is a sign of a healthy exhaust aftertreatment system. Local truck drivers should generally need a regen about every 7 days or more possibly. Again this also depends on how often your truck or if it does at all do highway regens.