Open loop at idle Subaru

[MCM73]

Also I texted back & forth with previous owner. He said the guy he bought it from bought the engine, harness & ECM from a company in Colorado that rebuilds them. I have the receipt but no good information on it. So I don't know how I'd ever find the donor info.

[wesitarz]

"A faulty coolant sensor that always reads cold may cause the fuel control system to run rich, pollute and waste fuel. A coolant sensor that always reads hot may cause cold driveability problems such as stalling, hesitation and rough idle."

"Open/closed loop feedback control of the air/fuel mixture. The PCM may ignore the oxygen sensor rich/lean feedback signal until the coolant reaches a certain temperature. While the engine is cold, the PCM will remain in "open loop" and keep the fuel mixture rich to improve idle quality and cold driveability. If the PCM fails to go into "closed loop" once the engine is warm, the fuel mixture will be too rich causing the engine to pollute and waste gas."

"A bad coolant sensor can also cause a noticeable increase in fuel consumption, and it may cause a vehicle to fail an emissions test if it prevents the engine management system from going into closed loop."

"Keep in mind that many coolant sensor problems are more often due to wiring faults and loose or corroded connectors than failure of the sensor itself."

"The coolant sensor's impact on the engine management system, cold driveability, emissions and fuel economy can also be influenced by the thermostat. If the thermostat is stuck open, the engine will be slow to warm up and the coolant sensor will read low. Or, if someone installed the wrong thermostat for the application or removed the thermostat altogether, it will prevent the engine from reaching normal operating temperature and cause the coolant sensor to read low."

[MCM73]

The coolant temp seems to be accurate. Averages 181°-192°depending on the circumstances. Data shows it starting at a normal temp &rising as it should.

[MCM73]

So I've been living with the issue for the last few weeks because of time & the switch from closed to open is so mild. It doesn't die anymore. Obviously the problem still exists. I haven't had luck finding anyone I work with that has the smoke leak detector. I also haven't wanted to drop the $150 to buy one just yet. I did however buy a fuel pressure gauge. I hooked it up this morning & notice about 35psi at idle. I raised rpms & saw a max of about 38psi. When it went from closed to open I didn't see much change. I'm getting conflicting info on what the pressure should be. I did see that Smallcar has an adjustable one that was recommended to be set at 49-51psi. Not even sure if that is the type of direction needed. Thanks!

[mathieux46]

Hi,

Reviving this thread since I have the same issue.

1999 Forester engine with rough warm idle. Cold idle is OK. No check engine code.

Coming to a stop when engine is fully warmed up, STFT goes to +23 % then goes to open loop with STFT & LTFT at 0%, 0V reading from both O2 sensor causing idle to be rough, not stalling but very rough.

Current MAF reading are at 2.63 g/sec to 3.45 g/sec at idle which I believe is low. I have ordered a new MAF from Subaru will report if it solved the problem.

Thanks

Mat

[mathieux46]

Hi,

Happy to report that the new OEM subaru MAF 22794AA000 fixed the open loop idle.

Hope this helps someone in the future.

Thanks

Mat