Question about 1984 Oldsmobile Delta 88
1984 Olds Toronado 5.0 Liter Vin Y 140,000 miles.
I have had a perodic rought idle problem for years once engine reaches normal operating termperature. No ECM trouble codes are set.
At one intersection, the engine idles like a sewing machine. At the next red light, the engine is missing and shaking. Problem exists only at idle when normal operating trmperature is reached. Cold temperature operation is flawless.
Just replaced the ECM coolant temperature sensor and TPS after trouble codes were set and diagnosed with DOS based DIACOM OBD 1 diagnostic software running on a 486 100 Megahertz 1995 gateway laptop.
After setting the TPS to .41 volts at 500 RPM in drive as specified in the OLDS factory Chassis Service manual. I put the car in park and watched the engine run smooth then begin faltering. The DIACOM software showed the ECM was switching from closed loop ON to closed loop OFF. When it went into closed loop OFF at idle, the engine shakes and misses.
What would cause the ECM to switch from closed loop on, to closed loop off at idle with a 185 degree engine temperature reading?
I drove the car to work with the PC attached to the ALDL and the ECM never switched to closed loop OFF during a constant 65 mph, 33 mile trip.
The ECM mode switching only occurs at idle.
The 1984 manuals were written before most consumer PC OBDI diagnostic software was available. The 1984 OLDS manual only states that the ECM will go into closed loop ON when the engine heats up. No specific temperature is identified. The manual does not address ECM mode switching after warm up under any condition, and does not list this a problem in the Emmisions diagnostic section.
Need some heavy duty AC/DELCO ECM knowlege to determine the cause and the fix. Buying a replacement ECM may not fix the problem. Must know the cause.
DIACOM software shows all sensors providing input and working properly, including O2, TPS, MAP, BARO, and mixture control solenoid.
This ia a tough one, the intermittant problems are the worst.
Thanks in advance, if you think you can solve this problem.
WHEN ENGINE GOES CLOSED LOOP TO OPEN LOOP.YOU HAVE A RICH FUEL MIXTURE.YOU HAVE A VACUUM LEAK.BY YOU HAVING ROUGH IDLING.AND A HIGH MILEAGE ENGINE I WOULD DO A COMPRESSION TEST OR CYLINDER LEAK DOWN. CHECK FOR LEAKING VALVES AND LOW COMPRESSION .THEY BOTH WILL CAUSE ENGINE TO IDLE ROUGH CAUSE MISFIRE.CAUSE OPEN LOOP CONTAMINATE HO2 SENSOR CAUSE A RICH FUEL MIXTURE.
Posted on Sep 30, 2009
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Posted on Jan 02, 2017
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READING CODES Modes will not switch while the engine is running. The ECM automatically switches to MODE I when the ignition is switchedOFF.
1984-90 Vehicles With Dual LED System
The self-diagnostic system has 5 modes. To switch modes, switch the ignition ON and use a screwdriver to turn the mode select switch through the access port on the ECM. The inspection lights will begin to flash. When the desired mode has been entered, turn the screwdriver fully counterclockwise. The red LED and green LED perform different functions in different modes.
This is the normal vehicle operating mode. The green LED will indicate loop status. If the green LED is not blinking when the engine is running, the vehicle is in open loop, or the HO 2 S, O 2 S, or sensor circuit has a malfunction. If the green LED is blinking, the vehicle is in closed loop. If a malfunction takes place, the red LED and the MIL will glow, indicating a malfunction has occurred.
Both the red LED and green LED are used to monitor mixture ratio feedback control. The green LED will function as described in MODE I. If vehicle is in open loop, the red LED and green LED will remain on or off. In order for results to be valid, vehicle must be in closed loop. In closed loop, the red LED remains off if the system is running rich, comes on if the system is running lean, and blinks at the same time with the green LED if system is within 5% of the exact air/fuel ratio.
-When this mode is accessed, the codes stored in the ECM's memory will be flashed by the green LED and red LED on the side of the ECM. The red LED will light first. This is the first digit of the trouble code; the green LED will flash next, showing the second digit of the code. For example: 2 flashes of the red LED followed by 1 flash of the green LED corresponds to Code 21.
This mode is used for checking the throttle plate (TP) switch, starter, and vehicle speed switch status. When the TP switch or starter switch is activated, the red LED will briefly flash as the status changes. For VSS status, the green LED will remain off when vehicle speed is less than 12 mph (19 km/h) and come on when vehicle speed is greater than 12 mph (19 km/h).
This mode represents a real-time diagnostic test of the crank angle sensor, ignition signal and the mass airflow signal. The malfunction code will be displayed only when the malfunction occurs and will not be stored in memory. The output code will appear as a single or short string of pulses. Count the number of pulses for the malfunction code.
1 pulse-CMP sensor signal lost or noise in signal 2 pulses-MAF Sensor signal abnormal 3 pulses-Fuel pump circuit shorted or open 4 pulses-Ignition signal lost
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