98 Mystique acting strange

It sounds like it's something to do with the IAC valve circuit. Have you checked the voltages on that circuit?

ENGINE CONTROLS - TESTS W/CODES - 2.5L -1998 Mercury Mystique
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CIRCUIT TEST KE - IDLE AIR CONTROL (IAC) VALVE

NOTE:
After each service or repair procedure has been completed, reconnect all
components. Clear DTCs and repeat QUICK TEST procedures to ensure
all EEC-V systems are working properly and DTCs are no longer present.
Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the following:
? Throttle linkage.
? Wiring harness circuits (IAC and VPWR). ? Faulty Idle Air Control (IAC) valve.
? Faulty throttle body.
? Faulty Powertrain Control Module (PCM).
Fig. 55: Identifying IAC Circuit & Connector Terminals
1) Check For DTCs

Retrieve and record all Continuous Memory DTCs. Perform KOER ON-DEMAND SELF- TEST. If DTC P0505, P1504 or P1507 is present, go to next step. If DTC P0505 or P1507 is not
present, IAC system is okay and testing is complete. If symptom exists, go to TESTS W/O CODES - EEC-V article.
2) DTC P0505, P1504 & P1507

DTCs P0505 and P1504 indicate IAC system malfunction has been detected. DTC P1507 indicates that IAC system under speed fault has been detected. Possible causes are:
? IAC circuit open or shorted to PWR. ? VPWR circuit open.
? Contaminated IAC valve assembly. ? Damaged throttle body.
? Faulty IAC valve. ? Faulty PCM.
Turn ignition off. Disconnect IAC valve connector. Turn ignition on. Measure voltage between VPWR terminal at IAC valve wiring harness connector and battery ground terminal. If voltage is
more than 10.5 volts, go to next step. If voltage is 10.5 volts or less, repair open in VPWR circuit.
3) Check IAC Valve Resistance

Turn ignition off. Connect DVOM positive lead to VPWR terminal at IAC valve. Connect DVOM negative lead to IAC terminal at IAC valve. If resistance is 6-13 ohms, go to next step. If
resistance is not 6-13 ohms, replace IAC valve assembly.

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ENGINE CONTROLS - TESTS W/CODES - 2.5L -1998 Mercury Mystique
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4) Check IAC Valve Internal Short To Case

Turn ignition off. Measure resistance between each IAC valve terminal and IAC housing. If both resistance readings are more than 10,000 ohms, go to next step. If any resistance reading is 10,000
ohms or less, replace IAC valve assembly.
5) Check Air Inlet System

Turn ignition off. Remove air filter. Inspect air filter, MAF sensor and air inlet system for excessive dirt or contamination. Repair as necessary. If air inlet system is okay, go to next step.
6) Check For Vacuum Leaks

Start engine and allow to idle. Inspect air inlet system any of the following possible faults:? Cracked or punctured air inlet tube.
? Loose inlet air tube or air cleaner housing. ? Loose or damaged throttle body.
? Contaminated or damaged IAC valve assembly. ? Faulty EGR valve or gasket.
? Faulty PCV valve or hose.
Check entire system for vacuum leaks. Repair as necessary. If no vacuum leaks are found, go to next step.
7) Check IAC Circuit Continuity

Turn ignition off. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Install Breakout Box (014-00950), leaving PCM
disconnected. Measure resistance between breakout box test pin No. 83 (IAC) and IAC terminal at IAC valve wiring harness connector. If resistance is 5 ohms or more, repair open in IAC circuit. If
resistance is less than 5 ohms, go to next step.
8) Check IAC Circuit For Short To Power

Turn ignition on. Measure voltage between test pin No. 83 (IAC) at breakout box and chassis ground. If voltage is one volt or more, repair short to power in IAC circuit. If voltage is less than
one volt, go to next step.
9) Check IAC Circuit For Short To Ground

Turn ignition off. Disconnect scan tool from DLC. Measure resistance between test pin No. 83 (IAC) and test pins No. 51 and 103 (PWR GND) at breakout box. If resistance is more than
10,000 ohms, go to next step. If resistance is 10,000 ohms or less, repair short to ground in IAC circuit.
10) Check IAC Signal From PCM

Connect PCM to breakout box. Connect IAC valve to wiring harness connector. Connect DVOM between test pins No. 83 (IAC) and No. 51 (PWR GND) at breakout box. Start engine and slowly

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ENGINE CONTROLS - TESTS W/CODES - 2.5L -1998 Mercury Mystique
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increase speed to 3000 RPM. If voltage is not 3.0-11.5, replace PCM. If voltage is 3.0-11.5 volts, proceed as follows:
? If Continuous Memory DTC P1504 or P1507 is present, go to step 30).
? If Continuous Memory DTC P1504 or P1507 is not present, check throttle body for damage.
Repair as necessary. If throttle body is okay, replace IAC valve.
NOTE:
A break in step numbering sequence occurs at this point. Procedure skips from step 10) to step 20). No test procedures have been
omitted.
20) DTC P1506

This DTC indicates IAC system has reached over speed malfunction. Possible causes are:? IAC circuit short to ground.
? IAC assembly stuck open.
? Air intake leaks or restrictions. ? Damaged throttle body.
? Contaminated or damaged IAC valve assembly. ? Faulty Powertrain Control Module (PCM).
Start engine and allow to idle. Inspect air inlet system any of the following possible faults: ? Cracked or punctured air inlet tube.
? Loose inlet air tube or air cleaner housing. ? Loose or damaged throttle body.
? Contaminated or damaged IAC valve assembly. ? Faulty EGR valve or gasket.
? Faulty PCV valve or hose.
Check entire system for vacuum leaks. Repair as necessary. If no vacuum leaks are found, go to next step.
21) Check EVAP System

Turn ignition off. Disconnect hoses from EVAP canister purge valve. Connect a vacuum pump to carbon canister hose port. See Fig. 56. Using vacuum pump, apply 16 in. Hg to port. If vacuum
bleeds off within 20 seconds, replace EVAP canister purge valve. If vacuum holds, go to next step.
Fig. 56: Identifying EVAP Canister Purge Hose Ports
22) Check IAC Valve Function

Start engine and allow to idle. Ensure transmission is in Park or Neutral and engine is warmed to normal operating temperature. Disconnect IAC valve wiring harness connector. If engine speed
drops, go to next step. If engine speed does not drop, check throttle body for damage. If throttle

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ENGINE CONTROLS - TESTS W/CODES - 2.5L -1998 Mercury Mystique
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body is okay, replace IAC valve.
23) Check IAC Circuit For Short To Ground

Turn ignition off. Disconnect scan tool from DLC. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Install Breakout Box
(014-00950), leaving PCM disconnected. Measure resistance between test pin No. 83 (IAC) and test pins No. 51 and 103 (PWR GND) at breakout box. If both resistance readings are more than
10,000 ohms and idle speed is normal, go to step 30). If both resistance readings are more than 10,000 ohms and high idle speed is present, replace PCM. If resistance is 10,000 ohms or less,
repair short to ground in IAC circuit.
NOTE:
A break in step numbering sequence occurs at this point. Procedure
skips from step 23) to step 25). No test procedures have been omitted.
25) DTC P1505: Check Inlet Air Supply

This DTC indicates IAC system has reached the adaptive learning limit. Possible causes are:? Air intake leaks or restrictions.
? Throttle body linkage binding.
? Contaminated or damaged IAC valve assembly. ? Damaged throttle body.
Inspect air intake system for leaks. Repair as necessary. If air intake system is okay, go to next step.
26) Check Air Inlet System

Turn ignition off. Remove air filter. Inspect air filter, MAF sensor and air inlet system for excessive dirt or contamination. Repair as necessary. If air intake system is okay, go to next step.
27) Check Throttle Body & Linkage

Disconnect accelerator cable. Remove air cleaner tube from throttle body. Check cable and
throttle body linkage for binding or interference. If faults are present, go to CIRCUIT TEST HU, step 3). If no faults are present, go to CIRCUIT TEST HU, step 8).
NOTE:
A break in step numbering sequence occurs at this point. Procedure
skips from step 27) to step 30). No test procedures have been omitted.
30) Check IAC System For Intermittent Open Or Short Circuit

Connect scan tool to DLC. Start engine and allow to idle. Ensure all accessories are off and engine is warmed to normal operating temperature. Using scan tool, select IAC and RPM PIDs from
PID/DATA monitor menu. IAC duty cycle should be 20-45 percent. Observe IAC and RPM PIDs for indication of fault while performing the following:

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ENGINE CONTROLS - TESTS W/CODES - 2.5L -1998 Mercury Mystique
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? Wiggle and bend wiring harness starting at IAC valve and work toward PCM. ? Lightly tap on IAC valve to simulate road shock.
Fault will be indicated by sudden change in IAC PID or RPM PID value. If any faults are found, isolate fault and repair as necessary. If no faults are found, problem cannot be duplicated at this
time. Go to CIRCUIT TEST Z.

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Check and test the throttle position sensor and the transmission range sensor-

We'll keep our fingers crossed.

Something's taking a poop when it begins to warm up.

So that's what rasmataz looked like 100 years ago? Heheheh

Maybe, but I'm leaning more towards one of the sensors we've discussed. Is there any way you can get it hooked up to a quality scanner and capture live data?

You can try taking it back and getting another. It isn't unheard of getting defective new parts. When you installed it, was it properly adjusted? Was it firmly connected to the wiring harness? How about the wiring between it and the ECU?

Yep. 3 pieces.