DETONATION/COLD IDLE STABILITY RELEASE OF NEW PROM

1983 2.0L CIMARRON ENGINE DETONATION

Some 1983 Cimarrons equipped with a 2.0L Electronic Fuel Injected (EFI) engine may experience spark knock or detonation under certain operating conditions. Spark knock may occur under light to medium throttle operation.

A production change consisting of a recalibrated PROM has been made at GMAD Janesville, effective at B.C. No. 410423. The new PROM calibration reduces detonation in the 2400-3200 RPM operating range for Federal automatic, California automatic, and California manual vehicles. Light detonation for a short period of time in the 1500-2000 RPM range, just prior to TCC disengagement, is unaffected by this PROM change. Owners experiencing detonation in this range should be referred to pacre 15 of the Owner's Manual which explains the conditions under which light spark knock may occur. The following chart details the new production and GMWDD service PROM part numbers:

PROM GMWDD Service Service Vehicle Part Number PROM I.D. ------- ----------- --------- Federal Automatic 1226472 3956 CDT California Automatic 1226476 3980 CDZ California Manual 1226477 3986 CFA

To diagnose and correct detonation on 2.OL 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. Check EGR operation per operational system check on page 6E2-20 of the 1983 Cimarron Service Information Manual. Tee in a vacuum gage between the EGR valve and the EGR solenoid. Start the engine and increase engine speed to 2000 RPM while observing the gage for vacuum response. If vacuum response is delayed and requires more than one second, replace the EGR solenoid.

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

6. Check for proper oxygen sensor operation in closed loop, using the field service mode as described on page 6E2-23 of the Cimarron Service Information Manual. A persistent lean fuel condition sbould 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 before B.C. No. 410423, and if steps 1-7 have not corrected a detonation condition, install the PROM kit listed on the previous page.

If PROM replacement or steps 1 through 7 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 proper MAP sensor operation utilizing the diagnostic trouble tree on page 6E2-47 of the 1983 Cimarron Service Information Manual, even if code 33 is not yet set.

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 Cimarron Service Information Manual.

For warranty purposes, use labor Operation No. T6069 at .4 hour when performing steps 1-2. If steps 3-8 are required, add .8 hour.

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.