CAMPAIGN: ENGINE CAM GEAR MISORIENTATION

Models: 1989 M-Van & 1990 M-Van, C/K Truck & S/T Truck - 4.3L (RPO LB4) V-6 Engine

General Motors of Canada has determined that certain 1989 Chevrolet & GMC M-Vans and 1990 Chevrolet & GMC M-Vans, C/K & S/T Trucks equipped with 4.3L V-6 engines (RPO LB4) may have been built with the engine cam gear misoriented to the crankshaft gear. This mis-indexed cam can result in a lack of performance or backfire. Additionally, the emission performance of these vehicles may be affected.

To correct this condition, an electronic scanning procedure to identify affected engines will be performed. Once identified, dealers will reorient the camshaft.

VEHICLES INVOLVED:

Involved are certain Chevrolet and GMC 1989 and 1990 M-Vans, C/K & S/T Trucks equipped with 4.3L V-6 engines (RPO LB4) and built within the following breakpoints:

MODEL/YEAR PLANT FROM THROUGH ----------- ----- ---- ------- CHEV M-Van 1989 Baltimore KB270078 KB272402 M-Van 1990 Baltimore LB100454 LB103665 C/K 1990 Fort Wayne LZ106046 LZ111244 S/T 1990 Moraine L2100105 L2109405

GMC M-Van 1989 Baltimore KB543956 KB544536 M-Van 1990 Baltimore LB500150 LB500965 C/K 1990 Fort Wayne LZ501569 LZ503594 S/T 1990 Moraine L2501187 L2502189

All affected vehicles have been identified by the VIN listing provided to involved dealers with this bulletin. Any dealer not receiving a listing was not shipped any of the affected vehicles.

DEALER CAMPAIGN RESPONSIBILITY:

Dealers are to service all vehicles subject to this campaign at no charge to owners, regardless of mileage, age of vehicle, or ownership, from this time forward.

Whenever a vehicle subject to this campaign is taken into your new or used vehicle inventory, or it is in your dealership for service in the future, you should take the steps necessary to be sure the campaign correction has been made before reselling or releasing the vehicle.

Owners of vehicles recently sold from your new vehicle inventory are to be contacted by the dealer, and arrangements made to make the required modification according to instructions contained in this bulletin.

CAMPAIGN PROCEDURE:

Refer to Section 4 of the Service Policies and Procedures Manual for the detailed procedure on handling Product Campaigns. Dealers are requested to complete the campaign on all transfers as soon as possible.

OWNER NOTIFICATION:

All owners of record at the time of campaign release are shown on the attached computer listing and have been notified by first class mail from General Motors. The listings provided are for campaign activity only and should not be used for any other purpose.

PARTS INFORMATION:

Parts required to complete this campaign should be ordered through regular channels, as follows:

Part Number Description Quantity/Vehicle ----------- ----------- ----------------- 3754587 Gasket - Water Pump 2

10108435 Gasket - Front Timing Cover 1

14088514 Gasket - Oil Pan 1

PARTS AND LABOUR CLAIM INFORMATION:

Credit for the campaign work performed will be paid upon receipt of a properly completed campaign claim card, RAPID terminal or DCS transmission in accordance with the following:

Repair Code Description Time Allowance ------------ ------------ -------------- 1A Inspect/no repair necessary .4 all models

2A Inspect/realign timing gear 3.1 M-Van

3A Inspect/realign timing gear 3.1 C/K Truck

4A Inspect/realign timing gear 6.0 S-Truck

5A Inspect/realign timing gear 4.6 T-Truck

Time allowance includes 0.1 hour for dealer administrative detail associated with this campaign. Parts credit will be based on dealer net plus 30% to cover parts handling.

SERVICE PROCEDURE:

DIAGNOSTIC PROCEDURE

1. Connect SCAN tool or CAMS machine to vehicle. Make sure scan tool is in ROAD TEST mode.

If using a TECH 1 scan tool, it should be set up to display BLOCK LEARN (BLM) values and CLOSED/OPEN LOOP status simultaneously. This can be done by scrolling through the preprogrammed pairs of parameters with YES or NO key until a pair is found with BLOCK LEARN. Fix BLOCK LEARN by pressing "FO" if BLOCK LEARN is top parameter, "F1" if it is bottom parameter. Then continue scrolling other half of the display with either YES or NO key until CLOSED/OPEN LOOP status parameter is displayed. To "unfix" top parameter, press "F1". Press "FO" to "unfix" lower parameter.

If using a CAMS machine, select Dynamic Display Option. This will display both BLOCK LEARN (BLM) and OPEN/CLOSED LOOP status.

2. Disconnect canister purge line at the canister, see Figure 1.

3. Apply parking brake.

4. Place automatic transmission equipped vehicles in PARK and manual transmission equipped vehicles in NEUTRAL.

5. Start engine and warm up so that coolant temperature is at least 85 degrees C.

6. Make sure air conditioning system is OFF.

7. Make sure vehicle is in CLOSED LOOP mode.

8. Raise engine speed to 2600 RPM and maintain speed.

9. Monitor BLOCK LEARN (BLM) values and CLOSED LOOP status at 2600 RPM - wait at least 30 seconds for BLM values to stabilize.

10. If BLM value at 2600 RPM is less than 115, the timing chain sprockets are misaligned. Realign timing chain sprockets following REPAIR PROCEDURE section of this bulletin (page 6).

11. If 2600 RPM BLM value is 115 or greater, let engine idle - automatic transmission equipped vehicles in DRIVE and manual transmission equipped vehicles in NEUTRAL.

12. Make sure system is still in CLOSED LOOP before continuing, if not, return to step #7 and rerun testing sequence.

13. Monitor BLM values and CLOSED LOOP status at IDLE - wait at least 30 seconds for BLM value to stabilize.

If system switches to OPEN LOOP too soon, rewarm oxygen sensor by shifting transmission to NEUTRAL or PARK and raise engine speed to 1500 RPM until system switches to CLOSED LOOP STATUS.

14. If idle BLM value is less than 115, the timing chain sprockets are misaligned. Realign timing chain sprockets following REPAIR PROCEDURE section of this bulletin (page 6).

15. If both BLM values are greater than 115, no repairs are required.

16. Reconnect canister purge line to canister.

17. Install Campaign Identification Label.

REPAIR PROCEDURE

NOTICE: Perform repair procedure only if BLM values from diagnosis are less than 115 at 2500 RPM or idle.

a) Remove spark plugs.

b) Remove front timing chain cover - see section 6A3 in appropriate service manual.

NOTICE: It is not necessary to replace front crankshaft seal.

c) Thread a 7/16" x 20 x 1.00" bolt and torsional damper washer into crankshaft.

d) Rotate crankshaft in a clockwise direction (by placing a socket or wrench on previously installed bolt) until timing mark on camshaft sprocket is pointing straight down toward crankshaft.

e) Remove camshaft sprocket and chain.

NOTICE: An assistant may be needed to hold crankshaft in place (with chain still installed) to avoid movement of camshaft sprocket during removal.

f) Rotate crankshaft in a clockwise direction until timing mark on crankshaft is pointing straight up toward camshaft.

g) Reinstall camshaft sprocket and timing chain.

NOTICE: Properly aligned timing marks are shown in Figure 2.

h) Tighten camshaft sprocket bolts to 17 ft. lbs. (23 N.m).

i) Reinstall timing cover with a new gasket (P/N 10108435) - see section 6A3 in appropriate Service Manual for reinstallation details.

NOTICE: Install new oil pan gasket (P/N 14088514) and new water pump gaskets (P/N 375487).

j) Reinstall spark plugs - tighten to 22 ft. lbs. (30 N.m).

k) Verify that repair has corrected BLM values by rerunning diagnostic procedure (steps 1-17).

INSTALLATION OF CAMPAIGN IDENTIFICATION LABEL

Clean surface of radiator upper mounting panel and apply a Campaign Identification Label. Make sure the correct campaign number is inserted on the label. This will indicate that the campaign has been completed.

Dear General Motors Customer:

General Motors of Canada has deterinined that your 1989 or 1990 Chevrolet or GMC vehicle equipped with a 4.3L V6 engine may have been built with the engine cam gear mis-indexed to the crankshaft gear. This misoriented cam gear can result in a lack of engine performance or backfire. Additionally, the emission performance of these vehicles may be affected.

To correct this condition on your vehicle, your GM dealer will use an electronic scanning procedure to identify affected engines. The dealer will then reorient the camshaft on those vehicles determined to have this condition. This service will be provided for you at no charge.

Please contact your GM deater as soon as possible to arrange a service date.

This letter identifies your vehicle. Presentation of this letter to your dealer will assist their Service personnel in completing the necessary correction to your vehicle in the shortest possibte time.

Further assistance, if required, may be obtained from our Customer Assistance Centre, toll free, by calling 1-800-263-3777. We are sorry to cause you this inconvenience; however, we have taken this action in the interest of your cotinued satisfaction with our products.

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.