My friend came to me with his car (2001 Outback 2.5 auto) having the following problem:
When cold, the car idles normally for about 5 minutes. After this "warm up", the idle starts surging... instead of the usual brrrrrrrrrrrrrrrrr, the engine goes brrm, brrm, brrm, brrm, brrm, surging from about 500 to
1000 rpms, once every second.If you could get the car moving, it would run almost normally.
The idle had been getting worse lil by lil but the engine would idle acceptably. The only code the scan tool would show was a P0130, but the front O2 sensor checked normally. The short term fuel trim would go up to +37%.
The problem began after the car sat for the whole weekend unused. It was normal on friday evening.
Checked all sensor inputs with the scan tool and they all were normal and within range.
Measured all the signal inputs at the ECM with a scope meter. All the signals seemed normal and followed the surging in the idle. The sensor power supply is full of positive and negative glitches. This is NORMAL.
Checked for vacuum leaks and found none.
Checked fuel pressure and flow and they were normal. By the way, If you pinch the fuel pressure line with your hand, you can feel it pulsating in this car. This is normal.
Inspected fuel for presence of water. It checked ok.
Discarded exhaust pipe obstruction because the car would run almost normal at higher speeds.
Checked spark for all cylinders with the gap tool sold at the autoparts. Found plenty of spark for each cylinder
Went to the dealer, and the mechanic whom I have known for over 10 years suggested to change the O2 sensor regardless of the tests. Asked to do it before he would help us any further. Friend replaced the $120 O2 Sensor and the problem continued. After doing so, I found a TSB from Subaru that states that if a code like (similar to) P0130 is set, the O2 sensor SHOULD NOT BE REPLACED.
Then the mechanic suggested to replace the $430 idle air control valve. My friend got it on ebay for $55. Replaced it and the car continued the same.
Fed up with the problem, I took out the intake air absolute pressure sensor and temp sensor unit. Plugged the hole in the intake manifold and applied vacuum to the sensor with a vacuum hand pump, the sensor connected to its harness. Started the engine, and after a few trials on how much vacuum to apply, i got the engine running so smooth as if the car were new. Please remember that the sensor would check correctly as per the values in the Subaru manual. Tried cleaning the sensor with throttle body cleaner and the problem got worse.
My friend bought this sensor (intake pressure and temperature sensor) for $60 on ebay and the car now is running like new.
I thought I should share this problem and solution. It took us more than four, 8 hour days to figure this one out.
It is good to see the newsgroup is still alive, and that some of the old timers are still around.