Question about 1984 Oldsmobile Delta 88

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ECM switches from closed loop to open loop at idle after warmup

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.

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Posted on Sep 30, 2009


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