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Sounds like you have tried just about everything.
Pull the vacuum hose off of the EGR valve and put a plug in it.
The EGR system on your vehicle had a filter near the EGR solenoid.
Drive the car with the EGR out of the picture. By your description, it's pulling too much EGR and causing your misfire.
This is a quick and easy test. If the misfire at low speed is gone, you now have a system to focus upon.
During Cold operation the PCM should block EGR operation but if it receives the wrong input from a sensor, it could pull EGR all the time.
EGR VALVE This vehicle uses a positive backpressure EGR valve. A positive backpressure EGR valve requires exhaust backpressure (proportional to engine flow) to open and allow exhaust gas to flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, the engine might misfire. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle. Careful diagnosis of the system is important so that outside conditions (such as an exhaust tube connected to the vehicle causing reduced exhaust backpressure) will not lead to misdiagnosis.The EGR valve is usually open under the following conditions:
EGR SOLENOID OPERATION The PCM controls the vacuum to the EGR valve with a solenoid valve. A constant 12 volts is applied to the positive terminal of the EGR solenoid. The vacuum supply to the EGR valve is regulated by the PCM controlling the ground to the EGR solenoid.The percentage of time that the PCM grounds the EGR solenoid is called the solenoid duty cycle. The duty cycle is the time the solenoid is "ON" divided by the time it is "OFF". A duty cycle of 0% will enable EGR (turn EGR full "ON") since a de-energized solenoid allows vacuum to pass to the EGR valve. A duty cycle of 100% will turn EGR full "OFF" since the solenoid will be energized and will not allow vacuum to pass to the EGR valve.The EGR pulse width is regulated by the PCM depending on engine load conditions. When the engine is cold, within a specified load range and above a specified RPM, the solenoid valve sends 100% duty cycle to the solenoid and blocks vacuum to the EGR valve. When the engine is warm, the PCM sends a duty cycle to the solenoid to allow EGR operation.SENSORS AFFECTING COMPUTER CONTROL OF EGR OPERATION The PCM uses the following sensors to control the EGR solenoid:
DRIVEABILITY PROBLEMS WITH TOO MUCH EGR During cold operation and at idle, the solenoid circuit is grounded by the PCM. This blocks vacuum to the EGR valve. Too much EGR flow tends to weaken combustion, causing the engine to run roughly or stop. With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may occur:
DRIVEABILITY PROBLEMS WITH TOO LITTLE EGR If the EGR valve should stay open due to a stuck open valve all of the time, the engine may not run. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:
Just a weak spring in the EGR valve can cause what you are describing.
Let me know how it goes
Great to know that we've isolated the cause so soon.
Now that we know what's causing it, you should check to be sure the PCM isn't being falsely told to give the EGR valve vacuum.
Make sure the Items below are being correctly interpreted by the PCM.
If the PCM is correctly interpreting the above information, then the problem is in the solenoid or in the EGR valve itself.
In order to monitor the parameters, it would be preferable to have a scan tool capable of connecting to the ALDL underdash connector.
The Cadillac onboard systems are relatively limited as to what they will tell you. If there are no Trouble Codes showing up, then we most likely have sensors providing correct info the the Computer. Be sure you check for any trouble codes.
Is the PCM telling the EGR to Open, when it shouldn't be, or is the command correct, and the Solenoid and EGR valve are simply opening to soon, too far, or too fast.
If the EGR Valve or EGR Solenoid have been on the engine for a long time or for many miles, then yes, I would simply replace them.
I don't want to see anyone spend money that they don't need to be spending.
If I recall, the Solenoids had foam filters on them that clogged up and caused many EGR problems. Not sure if your's had that little foam filter or not.
But honestly, if you are not seeing any Trouble Codes being Stored for any of the systems listed above, it's the EGR Valve and or EGR Solenoid.
Take Care and please let me know how it goes.