ECM Trouble shooting

What condition is the battery?

The ECM maybe stuck, disconnect the battery and wait 5 mins then reconnect.

Other things that normal cause high rpm at idle:

• PCV hose to throttle body has a leak
• vacuum line to brake has a leak.
• the throttle plate maybe dirty and is open partly
• the IAC is bad. Which is on the throttle body.
• the throttle plate sensor is bad, just unplug it and see if the rpm goes down. If it does, replace the sensor.
• the egr value maybe stuck
• bad front o2 sensor
• bad coolant senor. You should have this replace since the original sensor where plastic tip and crack. The new ones has a brass tip. the transmission and air( IAT) have the same sensor part.

I would start with the vac leak idea.
And is the check engine light on when it is running ?
If so you should have the computer checked for trouble codes first.
There has to be a source for extra air coming into the intake or throttle body for the engine to run fast at idle, assuming the idle air control valve is working correctly.
That air source has to be found and fixed first.
It could even be the throttle plate not closing completely.

What is a iat sensor ?
A faulty idle air control valve would cause a high idle.

If your vehicle is fuel injected the problem you describe could be result of a few different things: 1) Check your Idle Air Control (IAC) sensor/motor. This sensor is located on your vehicles, throttle body. The purpose of this sensor is to control the car's idle under various operating conditions including ambiant temperature. In order for it to function correctly, the internal components must be clean (meaning, little or no carbon build up) and it must be undamaged. Sometimes the IAC can be cleaned; however, if this cleaning procedure is not properly done, the IAC can become damaged. This sensor is also sometimes called the "Cold Start" sensor. A new IAC will cost $50-$100. 2) Check you Coolant Temperature sensor. This sensor is located on the top of your engine and is screwed into your intake manifold. When this fails your car will experience starting problems when cold outside. A new Coolant Temp sensor will cost about $100. 3) Your battery could be improperly sized for your velicle. If the Cold Cranking Amps (CCA) are not at or above the manufactures standards, your starter will not turn fast enough (when it is cold outdoors) to start your vehicle. Take a look at these three solutions first. My guess is your IAC needs to be cleaned or replaced. Good Luck!

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Check the Idle Control System

Idle speed is controlled by the Idle Air Control Valve (IACV). The IACV changes the amount of air being bypassed to the intake manifold, in response to electric current controlled by the ECM. When the IACV is activated, the valve opens to maintain proper idle speed.

Symptom and Subsystems to Check:

1. Difficult to start engine, when cold--check Fast Idle Thermo Valve.

2. Fast idle out of spec, when cold:
a. Check Fast Idle Thermo Valve.
b. Check IACV.
c. Check idle adjusting screw (see Section C).

3. Rough idle:
a. Check hoses and connections.
b. Check IACV.

4. RPM too high, when warm:
a. Check IACV.
b. Check Fast Idle Thermo Valve.
c. Check hoses and connections, check Power Steering Pressure Switch Signal, and check idle adjusting screw.

5. RPM too low, when warm:
a. Idle speed is below specified rpm, with no load--check IACV and idle adjusting screw.
b. Idle speed doesn't increase after initial start up--check IACV.
c. Idle speed drops in gear (automatic transmission)--check automatic transaxle gear position switch signal.
d. Idle speed drops when AC is on--check air conditioning signal and IACV.
e. Idle speed drops when steering wheel is turned--check power steering pressure switch signal and IACV.
f. Idle speed fluctuates with electrical load--check hoses and connections, IACV, and Alternator FR Signal.

6. Frequent stalling, while warming up--check IACV and idle adjusting screw.

7. Frequent stalling, after warming up--check idle adjusting screw and IACV.

Additional Steps:

. Check Alternator FR Signal. Have alternator inspected, if idle speed fluctuates with electrical load. The FR signal communicates to the ECM how "hard" the alternator is working to meet the electrical demands of the car, including the battery and any loads which aren't monitored by the ELD. This square-wave signal varies in pulse width, according to the load on the alternator. The ECM places, approximately, 5 reference volts on the wire. The voltage regulator will drop this signal to approximately 1.2 volts, in proportion to alternator load. The ECM compares the electrical load (ELD) signal with the FR (Charging Rate) signal from the alternator and uses that information to set the idle speed and turn the alternator on and off. This helps fuel economy.

. Clean main ECM ground on thermostat housing.

. Reset ECM, by removing the 7.5 amp Back Up Fuse, in the under-hood fuse box, for 10 seconds.

. Replace PCV Valve, cleaning hose with brake cleaner spray.

. Substitute a known-good ECM. If symptom goes away, replace original ECM.

Check the ICM (Erratic RPM and PGM-FI System)

When the engine is cold, the air conditioner compressor is on, the transmission is in gear (automatic transmission only) or the alternator is charging, the ECM controls current to the Idle Air Control (IAC) Valve to maintain correct idle speed. Here's an overview of how the PGM-FI System works.

Background:

Various inputs to the ECM are TDC/CKP/CYP Sensor, MAP Sensor, ECT Sensor, IAT Sensor, TP Sensor, HO2S, VSS, BARO Sensor, EGR Valve Lift Sensor, Starter Signal, Alternator FR Signal, Air Conditioning Signal, Automatic Transmission Shift Position Signal, Battery Voltage (Ignition 1) Brake Switch Signal, PSP Switch Signal, ELD, and VTEC Pressure Switch.

Inputs are received and processed by the ECM's Fuel Injector Timing and Duration, Electronic Idle Control, Other Control Functions, Ignition Timing Control, and ECM Back-up Functions. These are the primary functional areas within the ECM.

Outputs from the ECM control Fuel Injectors, PGM-FI Main Relay (Fuel Pump), MIL (Check Engine Light), Idle Air Control (IAC) Valve, A/C Compressor Clutch Relay, Ignition Control Module (ICM), EVAP Purge Control Solenoid Valve, HO2S Heater, EGR Control Solenoid Valve, Alternator, Lock-up Solenoid Valve A/B (A/T), VTEC Solenoid Valve, and Interlock Control Unit.

Idle RPM:

Once you understand how the PGM-FI system is configured, it's easy to see how the ECM, Idle Air Control Valve, and the Ignition Control Module affect idle rpm. If the ECM's Electronic Idle Control function is not working properly, then it cannot properly control the IAC Valve. Likewise, if the ECM's Ignition Timing Control function is not operating properly, it cannot properly control the ICM (igniter). Obviously, idle rpm will also be affected if there's a problem with the IAC Valve or the ICM. As stated above, the ECM controls current to the Idle Air Control (IAC) Valve to maintain correct idle speed. This cannot happen if the IAC Valve is failing. The same situation exists if the ICM is failing. The ECM will tell the ICM to open and close the primary voltage circuit going to the coil and it won't respond properly. The result will be erratic spark plug firing and erratic rpm.

Conclusion:

If you are experiencing erratic idle rpm, try and isolate whether the problem is caused by the ICM (ignitor), IAC Valve, or the ECM. My experience has been that a failing ICM is usually responsible for the problem. Keep in mind that tachometers are connected directly to the ICM. Therefore, a fluctuating tachometer needle is often a dead giveaway. Heat and poor preventive maintenance (causing high secondary voltage to be discharge on internal distributor components) frequently causes the ICM (and coil) to fail. Besides performance, this is another reason why it's important to regularly replace spark plugs, spark plug wires, rotors, and distributor caps. Electricity will always follow the path of least resistance, even if it isn't the intended one. Our job is to ensure the intended path is the path of least resistance.

Ignitor (ICM) and Coil Replacement:

1. Disconnect negative battery cable.
2. Remove hex head machine screws, securing distributor cap to housing, using an 8 mm nut driver.
3. Move distributor cap and wires off to the side.
4. Remove machine screw securing rotor to shaft, using a #2 Phillips head screwdriver. It may be necessary to "hit" the starter once or twice, in order to rotate rotor for access to mounting screw.
5. Remove rotor and leak cover.
6. Unfasten ignitor wires, remove coil mounting screws, and set coil aside. Note: Removing coil first improves access to igniter.
7. Unfasten screws securing igniter to housing.
8. Remove ignitor from distributor and unfasten screws mounting ignitor to heat sink.
9. Coat back of new ignitor (or old igniter, if reusing) and male connectors with silicone grease. Silicone grease increases heat transfer to heat sink. Failure to apply silicone grease will cause the ignitor to quickly fail.
10. Mount ignitor to heat sink and reinstall ignitor, igniter terminal wires, coil, coil wires, leak cover, rotor, and distributor cap. Ensure female ignitor terminals fit snugly--crimp with pliers, if necessary.

AutoZone can test ICMs and coils for free. If you plan to keep the car, I would replace the ICM due the age of your Civic.

then the normal suspects are...
IAT (intake air temp) sensor
IAC (idle air control) motor
dirty/corroded connections

do you see any specific weather conditions before/during the idle concern like cold and/or rain?

check or clean out throttle body at throttle plate it may have carbon build up on it. also make sure air filter is clean and check or replace the intake air tempurature sensor.(iat) sensor. the sensor is under 20.00, the sensor is after air filter in the big hose to throttle body.

ECM Trouble Codes
12-Distributor signal fault
19-Fuel pump circuit fault (short)
30-Idle speed control (isc) motor rpm out of range
41-Camshaft position sensor (CMP) or circuit fault
52-Electric control module (ECM) reset
91-Park neutral position switch or coicuit fault

BCM Trouble Codes
10-Outside temperature sensor or circuit fault
32-Electronic control module (ECM) to Body control module (BCM) fault loss of communcation

Source: Haynes repair manual

The coolant temp sensor sends signal to the ECU and the ECU adjusts the idle RPM. When the coolant is cold, the engine requires more fuel to sustain idle with thick oil (oil gets thicker & more viscous when cold). This makes the engine to idle at faster RPM but it should settle down to 850 RPM or so after the coolant warms up. The sensor is nothing but a thermistor; a device that changes electrical resistance according to the coolant temperature.