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Discussion Starter · #1 · (Edited)
1991 Dodge Spirit 2.5 TBI (155K) bad idle slowly drops to 100 RPM then pops back up to 700 RPM as it warms up. Idles good in park (800 RPM). Idles fair in drive (700 RPM). Idles bad in drive with AC on (oscillates 700 RPM to 100 RPM 20 seconds cycles). Over two years replaced fuel pump, fuel filter, O2, MAP, PCV, Vacuum hoses, plastic T's. Verified and cleaned TB, TB fuel injector, TB pos sensor, and idle speed motor. Checked gas vapor purge hoses. Reseat wiring harness/connectors. Good air, trans, and oil filter. Was getting 23+/- MPG HW/city and it slowly dropped to 15 MPG (with heavy AC use). Gone up to 17 MPG (with AC) and better acceleration after new MAP. All the other fixes made things slightly better but not the idle. Runs good for a few minutes but after 5-10 minutes running good (with AC in drive) it starts to oscillate RPM on idle, acceleration starts to decrease and check gauge light comes on until RPM goes up to 800. Sometimes if I barely press and hold the gas pedal, RPM goes up to 800 and gauge light goes off but has less acceleration. Good charging system. No error codes. Highways are no problem. MAP vacuum line is open but slight restrictive. Upon further study, when at a stop and I crank the steering wheel it also drops in RPM and almost stalls. Added R-12 to AC a few years back and it still blows cold.
 

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1966 Crown Coupe, 2016 200 S AWD, 1962 Lark Daytona V8.
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Welcome to Allpar. What refrigerant is in the A/C? I have had compressors drag down and stall engines from over-oiling and compressor seizure. Does the inner A/C clutch pulley hub turn somewhat freely?

Use of OEM O2, MAP, TPS and PCV might be an important consideration here. I have had issues with some aftermarket sensors and valves.

Does this car have an EGR valve? I have had those leak exhaust into the intake at idle, even with the vacuum hose off. The pintle was burnt off the stem causing full EGR all the time.

The TBI fuel injectors were known to have spray pattern issues. Does it drip while idling? It should have a hollow cone shape that can be seen with an advance timing light used as a strobe. Dial it in until the cone is visible.

Timing is checked with the 2-wire coolant sensor disconnected. It will be way retarded if it is left on while checking timing. Disconnecting it will turn on the 'ck eng' (Power Loss) light, but that can be cleared adter the procedure.
While setting timing with an advance timing light, check the cam sprocket timing mark at 0º. The mark should be centered in the belt cover window.

This is OBD1 with the Chrysler diagnostic connector in the engine compartment. It would be interesting to see a health check of the target idle, O2 sensor switching and injector pulsewidth if you have or know someone who has this tool.
 

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Since it's worst with A/C on at idle, I'd look at 2 things:

1) Charging voltage at idle; and charging system and battery in general.
2) A/C compressor starting to bind and drag. As IC suggested, tell us the history of the A/C system service.

One very rare thing to check is that the cooling fan motor windings are shorting out. With engine off, unplug the cooling fan motor and measure resistance across its contacts. Should be in the range of about 2.4 ohms. If it's about 1 ohm or less, the fan motor is shorting out and dragging the idle down due to the electrical short.
 
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Is it possible with that many miles on it that it's a defective coolant temp sensor?
 

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Discussion Starter · #5 ·
1991 Dodge Spirit 2.5 TBI (155K) bad idle slowly drops to 100 RPM then pops back up to 700 RPM as it warms up. Idles good in park (800 RPM). Idles fair in drive (700 RPM). Idles bad in drive with AC on (oscillates 700 RPM to 100 RPM 20 seconds cycles). Over two years replaced fuel pump, fuel filter, O2, MAP, PCV, Vacuum hoses, plastic T's. Verified and cleaned TB, TB fuel injector, TB pos sensor, and idle speed motor. Checked gas vapor purge hoses. Reseat wiring harness/connectors. Good air, trans, and oil filter. Was getting 23+/- MPG HW/city and it slowly dropped to 15 MPG (with heavy AC use). Gone up to 17 MPG (with AC) and better acceleration after new MAP. All the other fixes made things slightly better but not the idle. Runs good for a few minutes but after 5-10 minutes running good (with AC in drive) it starts to oscillate RPM on idle, acceleration starts to decrease and check gauge light comes on until RPM goes up to 800. Sometimes if I barely press and hold the gas pedal, RPM goes up to 800 and gauge light goes off but has less acceleration. Good charging system. No error codes. Highways are no problem. MAP vacuum line is open but slight restrictive. Upon further study, when at a stop and I crank the steering wheel it also drops in RPM and almost stalls. Added R-12 to AC a few years back and it still blows cold.
Update: Verified MAP vacuum hose is now good after snapping it off when testing if it was leaking, brittle. No noticeable change in idle. For months the oil pressure sensor was dropping way below zero after warming up which caused the "check gauges" light to come on but ignored it as a cause. Replaced the oil sensor and it still has the same problem but to a lesser degree still reading below zero. Tightened alternator belt a little to eliminate a small start up squeak (voltage 13.8). Did a bunch of little things and it runs better but the idle still drops after 10 seconds sitting at a stop then pops back up in a repetitive cycle. Worse when using AC and only a little idle cycling drop in drive no AC but still there. When stopped, using no AC, in drive, and turning wheels also makes it drop idle as if AC is being used. After a few minutes running it gets worse. After warm up I notice a drop in acceleration and temp gauge ~25-40%. Temp gauge still looks functional within range so assume good. So for about 5 minutes idle is fine and acceleration good when cold and slowly becomes worse when hot. Will need to know how to test the Temp sensor to be sure but suspect a few sensors are experiencing a shared problem that causes several gauge problems and bad levels to the MCU.
 

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Any stored fault codes? These may have had a red 'Power Loss' light instead of a yellow 'ck eng' icon. Does the 'Power Loss' warning lamp come on for a bulb check at start-up?
The key dance allows you to count-pause-count lamp blinks for a 2-digit code. 55 means 'end of test'.
 

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You mentioned replacing the idle speed motor. Is that the same thing as the Idle control valve? If so, maybe you need to check the wiring in that harness connector. IDK, just trying to help.
 

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is the battery in good shape?
- chrysler engines seems to be able to start on a bad battery but the computer forget its settings during the start and starts to relearn.
Loss of idle is a typical sign of this, the idle stabilises after a minute or two if the engines havent stopped and been restarted.
 

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Discussion Starter · #10 ·
Since it's worst with A/C on at idle, I'd look at 2 things:

1) Charging voltage at idle; and charging system and battery in general.
2) A/C compressor starting to bind and drag. As IC suggested, tell us the history of the A/C system service.

One very rare thing to check is that the cooling fan motor windings are shorting out. With engine off, unplug the cooling fan motor and measure resistance across its contacts. Should be in the range of about 2.4 ohms. If it's about 1 ohm or less, the fan motor is shorting out and dragging the idle down due to the electrical short.
Charging voltage is 13.8V
Disconnected fan and cycling still happens at idle.
AC cooling at 48 F
 

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The PCM is supposed to control idle speed. When you first start the car, the idle speed should 'flare' to nearly 1000 rpm beforw settling down to around 750 rpm. Does it have the start-up flare?

The PCM keeps a target idle which it tries to maintain by slightly opening or closing the IAC (idle air control) motor/valve. An IAC is a 'stepper' motor and basically a computer-controlled vacuum leak. It opens and admits air to raise speed and closes to reduce speed. It tries to maintain target idle.
A tug on the steering wheel, A/C compressor engagement or putting it in 'R' or 'D' will drop the idle and the PCM will compensate by opening the IAC enough to maintain target idle.

There is no manual idle speed adjustment. The minimum air screw is adjusted at the factory and should never need adjustment. The screw head is sealed with a plug. If the plug is missing to expose the screw head, someone has likely tampered the adjustment. There is a special tool and procedure for setting minimum air (again, it is not an idle speed adjustment).
The IAC is a 4-wire motor, 2 wires drive the motor in or out and 2 wires feed back to the PCM where the motor position is (in steps).

This car is OBD1. If you can borrow a scan tool with the Chrysler diagnostic cable, it may be easier to diagnose the low idle. The wiring and hoses behind the valve cover were a trouble spot as the heat from the exhaust manifold would cook and harden the plastic and rubber after some years.

 

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You say you replaced the oil pressure sensor and it still reads zero. Are there one or two wires to the sensor? If one, then the ground for the sensor is through the sensor threads to the engine block. If you used teflon tape or any similar sealant, then you have electrically insulated the ground from the block and it will read incorrectly. No teflon tape should be used.
 
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Discussion Starter · #16 ·
The PCM is supposed to control idle speed. When you first start the car, the idle speed should 'flare' to nearly 1000 rpm beforw settling down to around 750 rpm. Does it have the start-up flare?

The PCM keeps a target idle which it tries to maintain by slightly opening or closing the IAC (idle air control) motor/valve. An IAC is a 'stepper' motor and basically a computer-controlled vacuum leak. It opens and admits air to raise speed and closes to reduce speed. It tries to maintain target idle.
A tug on the steering wheel, A/C compressor engagement or putting it in 'R' or 'D' will drop the idle and the PCM will compensate by opening the IAC enough to maintain target idle.

There is no manual idle speed adjustment. The minimum air screw is adjusted at the factory and should never need adjustment. The screw head is sealed with a plug. If the plug is missing to expose the screw head, someone has likely tampered the adjustment. There is a special tool and procedure for setting minimum air (again, it is not an idle speed adjustment).
The IAC is a 4-wire motor, 2 wires drive the motor in or out and 2 wires feed back to the PCM where the motor position is (in steps).

This car is OBD1. If you can borrow a scan tool with the Chrysler diagnostic cable, it may be easier to diagnose the low idle. The wiring and hoses behind the valve cover were a trouble spot as the heat from the exhaust manifold would cook and harden the plastic and rubber after some years.

Yes, I get the idle flare to 1000 and the drop to 750 RPM
 

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Discussion Starter · #18 ·
You say you replaced the oil pressure sensor and it still reads zero. Are there one or two wires to the sensor? If one, then the ground for the sensor is through the sensor threads to the engine block. If you used teflon tape or any similar sealant, then you have electrically insulated the ground from the block and it will read incorrectly. No teflon tape should be used.
No thread sealant. I am looking at pin 8 from the MCU which drives several sensors to see if the voltage is correct or low
 

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Discussion Starter · #19 ·
You mentioned replacing the idle speed motor. Is that the same thing as the Idle control valve? If so, maybe you need to check the wiring in that harness connector. IDK, just trying to help.
Done that and believe it is functioning. The idle moves around trying to reach 750 RPM but after a few seconds at 500 it drops to ~100 then pops back up to 750. it is worse at operating temperature.
 

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Discussion Starter · #20 ·
Since it's worst with A/C on at idle, I'd look at 2 things:

1) Charging voltage at idle; and charging system and battery in general.
2) A/C compressor starting to bind and drag. As IC suggested, tell us the history of the A/C system service.

One very rare thing to check is that the cooling fan motor windings are shorting out. With engine off, unplug the cooling fan motor and measure resistance across its contacts. Should be in the range of about 2.4 ohms. If it's about 1 ohm or less, the fan motor is shorting out and dragging the idle down due to the electrical short.
disconnected fan and nothing changed
 
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