I have a 1990 Honda Accord that is fluctuating between 900 and 1400 rpms while idling and after the car has warmed up. What is going on?

txgreasemonkey

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 Accord.

ntorrs

I have the same car a 1990 Honda Accord. My car idles from about 800 to 2300 rpms. I have already had the distributer replaced (three times, then again with a dealership part) but that was when the car wouldn't start at all before at that time it would randomly not start right away but then it would after trying to start it about three or four times, waiting a few seconds between tries. I took my car to a different car care place and paid them to tell me why it is idling this way and they told me it was because I need to replace the throttle body or they could try and fix it for me. So I just replaced the throttle with one from a wrecking yard, and at the same time replaced the fast idle thermo valve but the car is still idling the same. I don't really understand how the whole ECM, IAC, ICM system works but I have a manual from autozone, a vacuum tester pump, and an new ohm meter. Does anyone have any suggestions of where to start or should I just take my car to a shop and have them fix it for me and let them charge me a swearword amount of money to do it and probably not even fix the cause of the problem?

82-3870

Sounds like you have a vacuum leak. Check all vacuum hoses for leaks. If you can’t find any leaks remove the 2 bolts that hold the IACV on and pull it away and twist it enough that you can get into the back side and clean the ports with carb or brake cleaner & a toothbrush & flush it real good & put it all back together. If that doesn’t work replace the IACV.

If you are still having a problem with randomly not starting but works fine otherwise, replace the main relay located above the control box for the cruise control up under the dash. When these relays get old they cause the random won’t start problems like you described.

graywhale

I had the exact same behaviour on my 1989 Honda Accord LXi - rev up and down when warm. Very annoying. But the fix was easy:

What you most likely have is a dirty IACV a.k.a. Idle Air Control
Valve that causes this. The sensors there can get dirty sending erratic signals. So I cleaned it without even uninstalling the thing, just loosening the two bolts and using a straw from the spray cleaner to get inside and in between the crack to clean the sensors and and viola, the engine runs like new.

Just be prepared for the rubber gasket to fall out. You'll have to fish it out and place it back as best you can before tightening the two bolts again.

Here's more on this issue:
How To: Clean Your IACV - Team Integra

jtenny

Well, I appreciate all of the answers and they have helped me understand how my car works. Nothing was working so I took my car into a mechanic that I trust and come to find out the idle screw was sticking all the way out and so all he had to do was adjust the idle screw. I was trying to understand how this could have happened and realized that I had taken my car into the dealer a couple of weeks prior to the fluctuating idle for a different problem. It seems to me that this was created by the dealer. I have not liked their service and would not recommend taking a car into the dealer if you can help it.

Again, thanks for the responses.

builtontherock

to my experince,on the top of the intake towards the back of the engin more to the drivers sid there is a lil silver plate with two phillip screws take them off and they have a lil plastic sleeve inside take two screw drivers and turn them down all the way.it will stop ideling up and down.