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

MODELS AFFECTED: 1986 ELDORADO AND SEVILLE

This builetin supersedes Serviceman Bulletin T-87-76 Group 6E, which should be destroyed.

Some 1986 Eldorado and Seville vehicies may experience two different conditions where the engine may stop running, but will restart immediately.

1. Some vehicles may experience the condition during parking maneuvers with the engine at normal operating temperature and with maximum power steering load applied (steering at full lock).

2. Some vehicies may experience the condition during coast down at low speeds when ambient temperatures are greater than 85 DEG F and after a warm soak, extended car idle, or during stop and go traffic.

During high outside temperatures, the gasoline in the fuel tank transfers a large volume of vapor into the charcoal canister when the engine is off or idling. The canister 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 stop running. This condition is aggravated by fuel that has a high vapor pressure or high alcohol content which allows it to evaporate quickly.

A new service PROM has been released to eliminate both of these symptoms during the above operating conditions. Service PROMs P/N 122588 FED and 1228587 CAL have revised idle speed control to repair this condition during parking maneuvers and revised fuel control to repair the condition during coast down at low speeds.

Before replacing the PROM, perform the following checks:

1. With the engine running, perform ECM switch tests. Repair the cause of any switch test that does not pass using the appropriate trouble tree in Section 8Dl of the 1986 Eldorado and Seville Final Edition Service Information Manual.

2. If the vehicle loses engine power or stops running under any other condition than described above, see the Diagnosis Section in the Service Information Manual, Section 6E, Subsection B, page 6E-B-12.

Check PROM I.D. (parameter ED99). if PROM I.D. is not 306 or greater for vehicles equipped with a Federal emission system or 324 or greater for vehicles equipped with a California emission system and, if all of these system performance checks indicate normal vehicle operation, replace the PROM with P/N 1228588 for Federal emission or 1228587 for California emission system.

If the PROM does not repair this condition, refer to the 1986 Eldorado and Seville Final Edition 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 6E, Subsection C2 in the Service Information Manual.

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, any any sharp edges that could cut or damage the hose. Faiture 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 an oscilloscope check ignition system with the engine under idle load condition in drive or reverse. Check for spark plugs, ignition wires or coil, cap and/or rotor breaking down under load. A condition such as 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 Labor 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.