1996 Subaru Impreza Engine Idles Low

Hi EttenCO,

Thank you for the donation.

Poor idling can be affected by the following components.

1. Mass Air Flow sensor
2. Throttle position sensor.
3. Crankshaft and camshaft position sensor.
4. Idle Air Control (IAC) valve.
5. EGR valve.

The item most likely to be the cause would be the IAC. When it gets clogged due to dirt accumulation, air passage is restricted and this would reduce the idling speed. Get it and the throttle body cleand and retest. Some carb cleaner sprayed into the throttle body and the IAC port works.

By the way do you have any trouble codes and is the CEL indicating?

Here are the diagnostics for the trouble codes concerned.

DTC P0500 (VSP) - VEHICLE SPEED SENSOR (VSS) MALFUNCTION

1. If speedometer operation is abnormal, check power supply and ground circuit to combination meter and harness connector between combination meter and vehicle speed sensor (VSS). Repair or replace parts as necessary. If speedometer operation is normal, go to next step.

2. Turn ignition off. Disconnect TCM harness connector. Turn ignition on. Using DVOM connected to chassis ground, backprobe ECM harness connector terminal No. 83. If voltage reading is less than 2 volts, go to next step. If voltage reading is 2 volts or greater, check for open in harness between ECM and combination meter connector, poor contact in ECM connector, poor contact in combination meter connector or poor contact in coupling connectors
between ECM and combination meter connector.

3. Turn ignition off. Disconnect ECM harness connector. Check resistance between ECM harness connector terminal No. 83 and chassis ground. If resistance is greater than 10 ohms, repair poor contact in ECM connector. If resistance is 10 ohms or less, repair short in harness between ECM and combination meter connectors.

This is the one causing the low idling

DTC P0505 (ISC) - IDLE CONTROL SYSTEM MALFUNCTION

1. Start and operate engine at idle. Check intake manifold, idle air control solenoid valve and throttle body for loose installation and gaskets for cracks. Check by-pass and vacuum hoses for loose connections and cracks. Repair or replace as necessary. If air intake components are okay, go to next step.

2. Turn ignition on, with engine off. Using DVOM connected to chassis ground, backprobe ECM harness connector terminals No. 13. If voltage reading is 3 volts or greater, go to next step. If voltage reading is less than 3 volts, go to step 8).

3. Backprobe ECM harness terminal No. 14. If voltage reading is 3 volts or greater, go to next step. If voltage reading is less than 3 volts, go to step 8).

4. Turn ignition off. Disconnect idle air control solenoid harness connector. Turn ignition on. Backprobe ECM harness connector terminal No. 13. If voltage reading is 10 volts or greater, repair short in harness between ECM and idle air control solenoid. If circuit is okay, replace ECM. If voltage is less than 10 volts, go to next step.

5. Backprobe ECM harness connector terminal No. 14. If voltage reading is 10 volts or greater, check for short in harness between ECM and idle air control solenoid harness connector. If voltage reading is less than 10 volts, check for poor contact in ECM connector. If contact is poor, repair contact in ECM connector. If contact is okay, go to next step.

6. Turn ignition off. Check resistance between idle air control solenoid harness connector terminals No. 1 and 2, and terminals No. 2 and 3. If resistance is 20 ohms or greater, replace idle air control solenoid. If resistance is 5 ohms or less, replace idle solenoid and ECM. If resistance is greater than 5 ohms, but less than 20 ohms, go to next step.

7. Remove idle air control solenoid. Connect battery voltage across idle air control solenoid terminals No. 1 and 2. If solenoid valve does not open fully, clean idle air solenoid valve. If solenoid valve opens fully, connect battery voltage across terminals No. 2 and 3. If solenoid valve closes fully, go to next step. If solenoid valve does not close fully, clean idle air
control solenoid valve.

8. Turn ignition off. Disconnect idle air control solenoid valve harness connector. Turn ignition on. Check voltage between idle air control solenoid harness connector terminal No. 2 and chassis ground. If voltage reading is 10 volts or greater, go to next step. If voltage reading is less than 10 volts, repair open in harness between idle air control solenoid harness connector and ECM connector.

9. Turn ignition off. Disconnect ECM harness connector. Check resistance between ECM harness connector terminal No. 14 and idle air solenoid harness connector terminal No. 1, and ECM harness connector terminal No. 13 and idle air solenoid harness connector terminal No. 3. If resistance is greater than one ohm, repair open in harness between ECM and idle air control solenoid harness connectors. If resistance is one ohm or less, go to next step.

10. Check resistance between ground ECM harness connector terminals No. 13 and 14. If resistance is 10 ohms or greater, ensure idle air control solenoid valve harness connector terminals are okay. Repair connector terminals as necessary. If resistance is less than 10 ohms, repair short in harness between ECM and idle air control solenoid valve harness connector.

DTC P0400 (EGR) - EGR FLOW MALFUNCTION

1. Connect SSM. Retrieve DTC(s). Check if DTC P0105, P0106, P0403 or P1102 is present. If DTC(s) is present, diagnose affected DTC(s) first. Manually check EGR valve diaphragm for sticking. After checking, go to CONFIRMATION OF ACTUAL DRIVING PATTERN, clear DTCs and retest. If DTC(s) does not reset, go to next step.

WARNING:
Use extreme caution when checking EGR valve. Valve will be extremely hot and can cause severe burns.

2. Check vacuum hoses for leaks, cracks, clogging or other damage. Check and repair as necessary:

"Â Pipes and hoses running between throttle body and Backpressure Transducer (BPT)
"Â Pipe and hose line connecting BPT and EGR solenoid valve
"Â Hose between EGR solenoid valve and EGR valve
"Â BPT pressure hose

After checking and repairing, go to CONFIRMATION OF ACTUAL DRIVING PATTERN. If leakage is not found, go to next step.

3. Turn ignition off. Connect SSM. Set SSM to function mode FD05, to check EGR solenoid operation. EGR solenoid can also be checked by disconnecting EGR solenoid valve harness
connector, then connecting an external 12-volt source to EGR solenoid valve.
.
CAUTION:DO NOT use vehicle battery as 12-volt source. Vehicle electrical system may be damaged.

Start engine. Open throttle valve 5-10 degrees and visually check EGR valve operation. If EGR valve operates, malfunction may be due to freezing or clogging by foreign matter. DO NOT replace EGR valve at this point since it may not be faulty. Go to CONFIRMATION OF ACTUAL DRIVING PATTERN. If malfunction is again detected in CONFIRMATION OF ACTUAL DRIVING PATTERN, check for clogging in gas outlets of intake manifold or cylinder head (check by breathing into outlets). If clogged, repair or replace intake manifold or cylinder head and return to CONFIRMATION OF ACTUAL DRIVING PATTERN. If not clogged, replace EGR valve and go to OCNFIRMATION OF ACTUAL DRIVING PATTERN.

Confirmation Of Actual Driving Pattern

"Â "a" - Conduct CLEAR MEMORY and INSPECTION MODES.
"Â "b" - Connect SSM to DLC.
"Â "c" - Start and warm engine until radiator fan completes one cycle (all accessories OFF).
"Â "d" - Turn SSM on and select function mode FA4.
"Â "e" - Drive vehicle at 52-58 MPH until LED No. 2 is illuminated.

NOTE:
Keep throttle valve opening steady. Diagnosis will be
interrupted if throttle opening varies. Diagnosis starts in 190 seconds after starting engine and requires 4 seconds to run. Transmission should be in Drive range for diagnosis.

"Â "f" - On SSM, select function mode FB0. Confirm "No Trouble" indication on SSM.

DTC P0133 (FO2_R) - FRONT O2S CIRCUIT SLOW RESPONSE

If DTC P0130 is also present, diagnose DTC P0130 first. Clear DTC and retest. If DTC P0133 resets, check exhaust system and repair as necessary. If exhaust system is okay, replace front O2S.