DETONATION/COLD IDLE STABILITY RELEASE OF NEW PROM

MODELS AFFECTED: 1983 CIMARRON FEDERAL AUTOMATIC

Some 1983 Cimarrons equipped with a 2.0L Electronic Fuel Injected (EFI) engine may experience spark knock or detonation under light to medium acceleration in the 1500-3200 RPM operating range.

A service PROM is available to correct detonation in cases where the condition is not caused by engine mechanical or ignition systems. This PROM is applicable to 1983 Cimarrons equipped with automatic transmissions and Federal emission systems only.

To diagnose and correct detonation on 2.0L engines, the following procedure should be used:

1. Read EFI diagnostic codes and repair any codes before proceeding.

2. Check base spark timing. Timing should be set at 0 DEG BTDC by the "averaging" method described on pages 6D-19 through 6D-20 of the 1983 Cimarron Service Information Manual. Reconnect the set timing connector and verify with a timing light that timing advances occur.

3. Check for proper Thermac operation (door should be wide open once car is fully warmed up).

4. Thorough checking of the EGR system is important for proper diagnosis. Refer to Section 6E2 of the 1983 Cimarron Service Information Manual (page 6E2-33).

5. Check for vacuum leaks and proper routing of vacuum lines.

6. Check for proper control of the closed loop fuel system, as indicated by the oxygen sensor. Use the field service mode as described on page 6E2-23 of the Cimarron Service Information Manual. A persistent lean fuel condition should be corrected. Refer to the trouble tree on page 6E2-50 of the 1983 Cimarron Service Information Manual for diagnostic procedure, even if code 44 is not yet set.

7. Check for the correct spark plugs.

8. Only on vehicles equipped with Federal emission systems and automatic transmissions, and if steps 1-7 have not corrected a detonation condition, install the service PROM P/N 1226789.

If PROM replacement or steps 1 through 8 above do not correct detonation, perform the following additional steps:

A. Check for proper Early Fuel Evaporation (EFE) operation. The EFE coolant temperature switch, located on the L/H side of the valve cover adjacent to the cooling fan switch, is fed by a pink wire and should be open when the engine is warm. A standard test light may be used in checking EFE operation. With the engine running and warm, connect the test light lead to ground and probe the pink wire at the EFE switch. The test light should not come on under these conditions.

B. Check for proper Throttle Position Sensor adjustment utilizing the procedure on page 6E2-63 of the 1983 Cimarron Service Information Manual.

C. Check for a restriction in the MAP hose.

D. Check for proper compression.

E. If an internal carbon build-up is suspected, remove carbon with GM top engine cleaner (or equivalent). Follow instructions on can.

F. Remove engine front cover and check camshaft timing as described on page 6E2-13 of the 1983 Cadillac Service Information Manual.

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.