1989 Plymouth Reliant

The automatic idle speed motor you referred to controls idle speed, nothing more. It can cause stalling at stop signs from the idle speed being too low, but that doesn't agree with the rest of the symptoms. Also, the oil filter has nothing to do with the problem. It would be more likely something got bumped or knocked loose during the oil change.

The big clue is the Check Engine light. Once it turns on, there is at least one diagnostic fault code stored in the Engine Computer. Many auto parts stores can read them for you for free. You can also read them yourself. Chrysler makes it real easy compared to other car brands. Cycle the ignition switch from "off" to "run" three times within five seconds, then watch the Check Engine light. Expect it to flash once, have a short pause, flash twice, have a longer pause, (that's code 12 and just means the ignition switch was turned off. That can be disregarded). Count the next series of flashes. They all represent two-digit codes. The last code will be five flashes, a short pause, then five more flashes. Code 55 just means "end of message". What you want to record is the code(s) in between. They will lead you to the circuit or system with the problem, not necessarily the defective part.

Caradiodoc

The code and radiator fan are related. In fact, everything is likely related. Code 22 means the voltage from the coolant temperature sensor is too low or too high. The Engine Computer sends 5.0 volts to it and the sensor draws it down to somewhere between 0.5 and 4.5 volts depending on temperature. Higher temperature equals lower voltage. Anything outside that range will trigger a fault code and turn on the Check Engine light.

The first place to look is at the coolant temperature sensor itself. Chrysler has extremely little trouble with them but the plug could have been knocked loose during the recent oil change.

When making adjustments after performing a tuneup, it is common practice to unplug that sensor. That's a real fast way to put the Engine Computer into what's called "base timing mode" so the distributor can be adjusted. That will set the code 22 and turn the Check Engine light on. Once the distributor is adjusted the sensor is plugged back in, then the computer adjusts ignition timing as necessary. The light will turn off either right away or once the engine is turned off and restarted. The code will stay in memory but will erase automatically after starting the engine 50 times.

Here's the clinker. When the coolant sensor is unplugged, the computer has know way of knowing if the engine is overheating so it turns on the radiator fan to be safe. To test the system, that's what we do. We unplug the coolant sensor and expect to see the fan kick in about a second later.

Check that plug to be sure it's tightly connected. The sensor is in the cylinder head just to the side or above the thermostat housing which is above the oil filter. You can also follow the upper radiator hose to the thermostat housing. The sensor will have two wires in the connector. If you also see a sensor with a single purple wire, that is for the temperature gauge on the dash and isn't related to this issue.

If the connector is tight, grab an inexpensive digital voltmeter and measure the voltages on the two wires in the sensor's connector while the ignition switch is turned on, then holler back with those readings. You'll have to back-probe the wires through the rubber seal because the readings are only relevant when the sensor is plugged in.

As for finding a service manual, I buy a lot of them from school and dealership auctions and I resell them at the nation's second largest old car show in WI. I have the electrical book for your car but I'd have to look for the rest of the set. You can look on eBay too. The factory manuals are way better than any Haynes or Chiltons manual.

To tell which engine you have, look at the emissions sticker under the hood. It will be under the middle of the hood, on the right strut tower, or near the center of the radiator. Near the center along the right side will be the spark plug number and the engine size but as I recall, it won't be listed directly. I think it will just be a "2.2" or a "2.5" in a line of numbers. Next, look at the bottom of the engine's oil pan. The 2.2L will be perfectly flat all the way across. The 2.5L has a large bump in the middle, sort of like an upside-down flabby camel hump. If you still can't tell, copy the first 10 digits of the vehicle ID number. I can look the engine size up by one of those letters or digits.

Caradiodoc

Don't have an opinion on the cap and wires. I just put them on my '88 Grand Caravan a few months ago. Old ones were 12 years old! Fuel mileage and power went up. I used the cheapest hardware store products I could find. Champion spark plugs work well and are about the least expensive you'll find.

Idle speed variation is usually the result of a vacuum leak. When the engine is still cold and running, use a spray bottle to spray water on the vacuum hoses and intake manifold gasket. If you see water getting sucked in or the engine speed slows down, look in that area for the leak. The MAP sensor can cause this symptom too.

Since the Check Engine light is on, recheck the fault codes.

Code 22 is "Coolant temperature sensor voltage high". That is usually the result of unplugging it while the ignition switch is turned on. If you don't think that happened, check the connector for corroded or stretched pins.