EGR CODE E48 EXPLANATION PROPER DIAGNOSIS INFORMATION

VEHICLES AFFECTED: 1986 Devilles, Eldorados and Sevilles

The 1986 Digital Fuel Injection (DFI) system includes a new diagnostic code, Code E48 (Devilles), code E048 (Eldorados and Sevilles). Code E48 is designed to detect a malfunctioning EGR system, but may also set because of other factors described below. Understanding the operation of code E48 is essential for proper diagnosis and repair of the DFI system.

The test for Code E48 is designed to change EGR flow under certain operating conditions and to monitor the oxygen content change in the exhaust gas stream. The specific operating conditions which this test is performed under are as follows:

1. Coolant between 85 degrees C and 100 degrees C. 2. TPS between 8 degrees and 15 degrees. 3. RPM between 1450 and 1650.

Under the above conditions, the EGR system is normally enabled, allowing exhaust gas flow to the engine intake. To test for an EGR system fault, the ECM turns off EGR flow for five seconds and monitors the closed loop integrator parameter (P.1.3 on FWD-C or ED19 on E/K) for a change. The integrator should increase, indicating that a leaner mixture due to the EGR being temporarily turned off has been detected by the oxygen sensor. If the integrator does not increase more than 14 counts within 5 seconds, the ECM records one 'failure'.

The ECM will perform this test up to 5 times each ignition cycle. Code E48 will set if the ECM records three or more test 'failures' in a given key cycle. The 'Service Soon' light or message will be displayed to indicate a current Code E48.

Proper operation of Code E48 is dependent upon the integrity of the DFI fuel control system, including the oxygen sensor. A slow responding or poisoned oxygen sensor may not react quickly enough to changes in EGR flow, and could possibly cause a Code E48 to set. It is, therefore, essential that the DFI control system check (Section 6E, chart A-1) be performed before attempting any diagnostic procedure.

It is important to note that Code E48 can detect marginal EGR system operation as well as an inoperative EGR system. The EGR valve may appear to be operating properly, but a system problem may exist that causes reduced or restricted EGR flow. Use EGR diagnosis chart in Section 6E, Subsection C, to check the EGR system for proper operation.

Small vacuum leaks in the system will cause insufficient EGR flow and may cause Code E48 to set. Be sure to check the entire vacuum system including connections at the charcoal canister, brake booster, and throttle body. Also check for split, kinked or cracked vacuum hoses at the EGR solenoid and EGR valve.

A restricted intake manifold EGR passage may also cause Code E48 to set due to the reduced exhaust gas supply to the EGR valve. If the vacuum system checks okay, remove the EGR valve and inspect for restrictions in the engine EGR passages or around the EGR pintle. See 'EGR manifold passage - inspect' in Section 6E of the Service Information Manual.

Following these procedures and adhering to proper DFI diagnostic techniques outlined in the Cadillac Service Information Manual, Section 6E will ensure accurate repairs and reduce the possibility of comebacks.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.