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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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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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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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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 · #17 ·
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'.
55 is the only code
 

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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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Discussion Starter · #24 ·
One thing I think we all missed, and it's basic, is to do a full tune-up. These engines eat ignition rotors, and so they need to be replaced every 15K miles. They also burn the forked tips of the ignition wires inside the cap, and so at 15K miles you can remove the wires from the cap, turn them 180 degrees so that the other side of the fork faces the rotor. Then at 30K miles, the wires need to be replaced.
So, replace plugs and ignition rotor at 15K miles, and at 30K miles, replace the wires and distributor cap. If you haven't followed these intervals, this could cause stumbling at idle. Once I missed the schedule, and at 20K miles, I had a sudden no-start. Took me 2 days to diagnose the burned ignition rotor, and once replaced, it started immediately and was smooth.
Did a tune-up as suggested (spark plugs, plug wires, cap, and rotor). It had normal wear and tear with no surprises. It made the car a little happier (~10% better this and that) but still has the same problem. I've done about a dozen things that made the car run a little better but not the fix.
 

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Discussion Starter · #25 · (Edited)
Need some info on the Neutral Safety Switch (NSS). In the manual it states the NSS is an input to the computer used to control fuel, idle, and timing. I compared diagrams in the original OEM service manuals and Chilton's manual and they are the same. Round connector with three terminals. The neutral safety center terminal is normally closed and shorts to the casing for P and N for the starter relay. For neutral safety it should open on R, D, D1, and D2. The outer terminals (1,3) are for the reverse lights and is normally open unless in R. This is what I summize from the manuals and if tested should read as only switches (open or closed) with no resistive states in between and need to know if I'm correct so far. My old NSS reverse light switch works fine if I press the center tab, but the neutral safety contact is constantly open. What has me confused is, if my old neutral safety contact is constantly open, my car should not start, yet it does. To add to my confusion, the new NSS I bought, TE121, the reverse contacts seem to work if I press the center tab, but the neutral safety contact is constantly open and no matter how I actuate the device (does nothing). This leads me to believe the new NSS is defective right out of the box if it does not change. Unless the NSS is not being used as a neutral safety which means both old and new switches are working. So I need un-confused and expect there was a design change sneaking in the background.
 

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Discussion Starter · #29 ·
I don't think you're interpreting the function and states of the switch correctly.
The center wire goes to ground. One of the outer wires goes to the backup light switch and the other to the starter relay. The switch only matters to starting, and not running. It has no effect once the vehicle is started.
According to the OEM service manual the NSS does several things including idle. The center terminal does go to ground after passing through a normally closed switch. Included info that has me wondering why my NSS is not functioning like the manuals says. I figured someone on allpar knows what's right.


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