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Matt, Engineer
Category: Subaru
Satisfied Customers: 21631
Experience:  Mechanical Engineer with 20 years experience in the auto industry, 8 yrs in formula 1 engine testing
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Subaru Legacy: I have a 1992 Subaru Legacy that is having issues

Customer Question

I have a 1992 Subaru Legacy that is having issues idling when it's cold or damp. I changed the idle control valve with one from a salvage yard. The issue remains. Not sure if it's worth buying a brand new idle control valve on such an old vehicle with 225,000 miles. Could the issue be my oxygen sensor?
Submitted: 2 years ago.
Category: Subaru
Expert:  Matt replied 2 years ago.
I'd nextt check for an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.
Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.
The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine
Expert:  Matt replied 2 years ago.
do you still need help ?
Customer: replied 2 years ago.
Sorry for the delayed response but I haven't had time to work on my car lately. Anyway, your suggested method of looking for vacuum leaks concerns me since I'm simply a typical Subaru owner but who also likes to do a little of his own vehicle work. I didn't employ your method because I don't feel comfortable using propane around my running engine without having the experience. However, I did clean the mass air flow sensor. I wasn't aware of this sensor and after spraying some mass air flow cleaner on the sensor, the car runs much, much, much better. The sensor has never been cleaned because I wasn't aware that this should be cleaned regularly. The car only has a little hesitation once in awhile now, but I don't feel it's the vacuum system because it is so infrequent. The hesitation in the engine may still point to my oxygen sensor since it does appear to be dented a little. Possibly a rock hit it somehow or by running over something in the road?
Expert:  Matt replied 2 years ago.
OK good to hear that you've had some progress
yes its possible that the O2 sensor is damaged- typically you'd find that the gas milage woudl also be worse with a damaged O2 sensor so if it seems poor ( i know its never great on these anyway) then it woudl be worthwhile fitting a new sensor
Expert:  Matt replied 2 years ago.
do you still need help ?