I have a '94 Legacy wagon that I have two issues with. The first is an inconsistent idle. The car idles high (about 1500 rpm)sometimes with no rhyme or reason as to why or when it does it. Hot weather or cold. Warm engine or cold. Any time of day. Etc. My shop tells me it can’t diagnose the problem unless it does it when the car is there, and of course it never does it when it’s there. The second issue is poor gas mileage which doesn’t seem to be related to the high idle problem. I can go through a whole tank without it idling high and still get lousy gas mileage. Premium fuel does make it run noticeably better and with a bit better mileage. I keep it well tuned, and I’ve recently put in new plug wires. The compression is good. Any thoughts on either/both of these issues?
Thanks!
The idle problem may be due to a intake manifold leak. The fuel problem may be due to a bad coolant temperature sensor for the ECU. This is not the sensor for the gauge. The sensor will have two leads going to it. If the engine is running rich then the sensor may be the thing to replace.
Please define “poor mileage,” and tell us how you are measuring fuel mileage. We need a true mpg figure, calculated correctly (miles driven divided by gallons used). Tanks per week, or vice versa, doesn’t cut it.
Also please tell us how many miles your Legacy has on its odometer.
There are a number of possibilities as far as the intermittent high idle. Has the “check engine” light ever come on, and has anyone scanned the computer for trouble codes?
The coolant temp sensor could very well explain both, since the ECU uses information from it to decide when to put the engine into high-idle mode.
Of course, the sensor itself could be good, but there could be air in the system. I think the first thing the OP should do is check their coolant level and put on a new radiator cap. I don’t know if there is a specific air-bleed procedure for Subarus.