STALL/LOW SPEED COAST DOWN DIAGNOSIS & NEW PROM P/N

VEHICLES AFFECTED: 1986 ELDORADOS (E) AND SEVILLES (K)

Some 1986 Eldorados and Sevilles may exhibit an engine stall during coast down at low speeds when ambient temperatures are greater than 85 degrees F and after a warm soak, extended car idle, or during stop and go traffic. With high outside temperatures, the gasoline in the fuel tank produces a high amount of vapor into the charcoal canister when the engine is off or idling. The canister then purges this vapor whenever the throttle switch is open (off idle). As the vapor enters the combustion process, the oxygen sensor will detect a rich mixture driving the fuel integrator lower. This compensates for the extra fuel from the canister. If the integrator (parameter ED19) drops to its lower limit of 88 counts and the throttle closes cutting off the canister purge vapor, the engine may stall. This condition is aggravated by fuel that has a high vapor pressure or high alcohol content allowing it to evaporate quickly.

A new PROM has been released to eliminate the "engine stall" symptom during the above operating conditions. The new PROM (P/N 16059154 FED and 16059158 CAL) has revised fuel control to repair the coast down stall at low speeds.

Check PROM I.D. if PROM I.D. (parameter ED99) is not 304 for Federal Emission System or 322 for California Emission System and if all system performance checks indicate normal vehicle operation, replace the PROM with P/N 16059154 for Federal Emission or 16059158 for California Emission System.

If the PROM does not repair the stall condition, refer to the 1986 Eldorado/Seville Service Information Manual Section 6E, Subsection B "Symptoms." Pay particular attention to the following checks:

1. Check the minimum air RPM using the procedure in Section Subsection C2 in the Service Information Manual. 6E,

2. Connect a 0-15 PSI fuel gauge to the fuel pressure test point.

CAUTION:

When taking the following fuel pressure readings, be sure the fuel pressure gauge is securely attached to the pressure test point. Be careful to route the pressure gauge hose away from the exhaust manifold, any other hot surfaces, and any sharp edges that could cut or damage the hose. Failure to do so may result in a fuel leak while diagnosing the car.

Duplicate the condition while observing fuel pressure. If fuel pressure fluctuates during the maneuver, repair cause of fuel pressure fluctuation.

3. Block the drive wheels and using a oscilloscope check the ignition system with the engine under the load condition in drive or reverse. Check for spark plugs, ignition wire or coil, cap and/or rotor breaking down under load. A condition such a cracked plug may cause abnormal engine operation, yet be difficult to detect on a scope. Replace any suspect spark plugs or ignition wires and inspect the cap for arcing before proceeding with diagnosis.

CAUTION:

Brakes must be applied at all times.

For warranty purposes, use Labour Operation J6364 at .5 hours.

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.