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I've spent the past several months trying to figure out why my mileage has been dropping to no avail so we'll see if anyone here notices something that I haven't.

Original Configuration:

90 L Sedan AWD 5mt. Has racks and with summer tires used to get about 26-28 mpg ish on the highway and ran perfectly. Total of 205 k miles now. Timing belt, valve cover gaskets, cam seals, etc were all done at the dealer shortly after we bought it.

Problems began:

When we first bought the car (at about 170K about two years ago) it had the usual bouncing cold idle, cleaned out the IAC and worked fine. about 6 months ago the car started having a little occasional misfire at idle and the mileage started to drop (about 24 then). I decided it was time for a little tuneup so did K&N filter, Plugs, Wires, ECT, O2 sensor, check for vacuum leaks. Everything except the O2 sensor was from the dealer. The car ran smoother but mileage did not increase. In fact after a little bit of time the mileage went down further 22 and the warm idle misfire became more noticeable again. After some research on here I decided to put in a more preferred O2 sensor (this is after the mileage had dropped to 20 where it seems to have stabilized). I bought the OEM plug replacement Bosch O2 sensor at napa and no change, still lousy mileage and less than smooth warm idle.

One quick side note, the 20 mpg is currently with winter tires on, however we seem to get 20 with the car empty or with four people and loaded racks on mountain passes. Also power seems fine (for a 200k ej22 of course).

Any thoughts? I'm at a total loss now so any ideas would be better than what I'm working with at this point.

Almost forgot: Trans and diff got the subaru special fluid (well, what the dealer uses anyway) and both front calipers have been replaced.

Do you still have the original o2 sensor?

Also what plugs are you using?

Another biggie, your using a k&n filter which requires oil. The oil from the filter can go on the MAF. You might want to try going back to the oem or any paper filter, and cleaning the MAF with MAF sensor cleaner.

I think I still have the original O2 sensor laying around here somewhere, the plugs are the ones recommended in the owners manual and on here (NGK V-powers I think?). I did clean the sensor a while back, but I like the idea of going back to the paper filter now. Also the first replacement O2 sensor I put in was a walker, also with the OEM style plug if that provides any clues.

Well, id reclean the maf, put back the paper air filter, and put back the original o2 sensor.

Well, the original O2 sensor was toast, heater element burnt out (unusual failure it turns out). I have heard that the Bosch ones work fine, but then again there isn't really any way to test it to find out definitively. I should have just bit the bullet in the beginning and bought the Subaru sensor, but it was hard to justify buying a $300 sensor for a 200k mile car. Oh well, next time...

I hate Bosch products, but I think that's the one I used and had no problems. Is the failure covered under a part warranty?

$300?!!!!!!!!!!! WTF!!!

BTW the oem one is bosch lol.

https://www.subarugenuineparts.com/oe_parts_cat.html

Check out here...

I hate to say this, but I would check over wiring very thoroughly. Mine is doing the same thing, and running slightly off normal intermittently. I discovered a few weeks ago that many sensor wires are very old and brittle, especially the ones under the TB area. The cam sensor was hanging on by only 2-3 copper strands!! Fixing that fixed my intermittent prob, but I don't have a warm place to really fix the rest of that harness area. So I continue to watch my car run open loop on my WBo2 and get 210 miles to a tank.

No waranty, turns out subaru won't warranty parts after 200k miles , I suppose I can live with that. I'll definitely look in to the harness. The guys I talked to at the dealer about the O2 sensor both own same generation cars and they recommended the bosch, though they did say ever once in a while a car just won't work on anything but the subaru branded one for some reason. At this point though I kind of doubt that the O2 sensor itself is the problem.

I meant the replacement O2 sensor. I read it as you had replaced it with a Bosch and that didn't work. My mistake.

Do you have the 10% Ethanol, too? If so, is it seasonal or year-round, like up here in CDA?

You mentioned a 'misfire'...did you check the codes? You might have some info there, even if you didn't get/have a CEL.

Td

We do have the ethanol so I expect lower mileage, though we've had year round down here for about as long as I've had the car. No engine codes and watching the ecu with the bcbf scan tool shows the ecu is running normally. So as far as the computer is concerned the car is running fine. Just cleaned the maf and put in a new paper filter so after a little jaunt down to lewiston tonight I'll see how it runs.

I put the stinker station 93 with ethanol in my tank and before my current troubles I could always get 275-300 miles if I drove without a lead foot to Seattle/Boise.

Cleaned the MAF and replaced the filter, Idle seems smoother without the misfire (but hard to tell for sure). Haven't had a chance to check highway mileage yet so we'll see later if there is any improvement.

Didn't you say you were having problems with your coil pack or igniter back when you sold me that sunroof?

Perhaps that is causing misfire?

I don't have a sunroof.... I did think that could be a suspect but I haven't yet looked in to either of those, though they may be next on the list if this doesn't have an effect. Would coil pack or weak igniter cause poor mileage as well?

Oh shit, sorry. Disregard that post lol. I thought this was BigGreen96's thread because I saw his post above mine. My bad. Wrong person.

No worries, I thought maybe I forgot about something for a minute . Quick question though, Since I have the 90 N/A I have heard that I have the "diamond" coil pack which is supposed to be stronger than others. Anyone care to elaborate on that? I've done some research, but I'm not sure I have my story straight.

Kimo my coilpack/ignitor issue was my cracked and brittle wiring.

Suba I had another idea that I had completely forgot about. I examine every sensor that was associated with timing, spark etc and pulled the cam and crank sensors to inspect. They had accumulated dirt, grime that was easily the thickness of a dime. Wiping them clean was a noticeable thing, and they are damn easy to get at too.

Have you checked your knock sensor? Maybe it's cracked and needs replacing. That or the coolant temp sensor I think controls fuel too.

I'll have to take a look at the cam and crank sensors for sure. The ECT sensor is new (dealer part) as of about 5k miles ago. As far as the knock sensor goes, can it crack without throwing a code? Also assuming it is cracked, what effect does that have on the ecu, such as adjusting timing, intermittent signals, etc?

Yeah it'll be cracked without throwing a code. My first legacy EJ22E had a cracked one and it never had any codes. I don't remember noticing any difference but that was right after I got the car so I didn't know it very well yet. I think the twin turbo motor I picked up also had a cracked knock sensor.

The knock sensor retards timing when it hears knock which hurts efficiency and would hurt MPG.

Anyway, haven't had a chance to check mileage recently, but hopefully soon I can.

First Bosch makes the best o2 sensors both narrowband and wideband. Period. Theres threads on other forums both comparison and testing. Second, Their oil filters are decent for the money (only wix oil filters are better for the price) coming in third below K&N and Mobil 1.

As for the issue, I would also check the knock sensor. Remove it either way and look at the underside to see if the insulation is cracked.

Of all the things, I would say its the Knock sensor, IAC, MAF, ECT, a weak coil pack, dirty injectors (only way to really fix that is to get them cleaned eg: Witch Hunter injector) or worn piston rings which is the least likely.

I would start by checking the knock sensor, and maf. Check your connections, etc. Next the coil pack.