So I was just doing some research and learned a little more:

When the car is initially started, the ECU will run on open loop mode for a pre-determined time by Subaru. I read in some places that the switch over from open loop to closed loop is based on coolant temperatures, but that seems to be a false statement.

The reason for this initial open loop mode at start is because the O2 sensor is cold and for it to work properly in taking A/F readings, it needs to reach a certain temperature. Once this pre-determined time is reached, the ECU will switch over to closed loop mode. Corrections will be made to my first post as I discovered a few things.

Now this leads to a question... is it possible that the ECU is not switching out of open loop mode?... which is the mode that fuel is dumped into the chamber to make for a rich A/F ratio for safety to the engine.

Andre

 

Your understanding of closed loop and open loop are pretty much accurate.
Closed loop uses the reading from the O2 sensor to constantly tweak
the F/A ratio to keep it at stoichiometric (14.7:1), which is most efficient.
Open loop ignores the O2 sensor and uses the preset fuel maps plus
some long term fuel trims (adjustments for altitude and such).
It goes to Open loop under hard acceleration, often to richen up
the mix a bit to cool the intake charge and prevent detonation.

Another time it runs in open loop is at first cold start of the day.
Coolant temps and Cat temps are both low, so it runs rich to heat
up the Cat faster. Sometimes it does so for a preset period of time, and
sometimes it stays in that mode until it sees those temps both come up.
That means that a bad O2 sensor or bad engine coolant temp sensor
can get you stuck in open loop mode, and it'll suck the fuel down.

Lastly, the Subaru 2.5L motor doesn't have a MAF sensor. I mention it
because on my VWs, unplugging the MAF causes the engine to
run rich (albeit normally in all other respects).
Subaru's have a MAP sensor (they just call it a "pressure sensor"),
and I do not know what happens if they fail, but it's something to look at
because it might switch to fuel maps if it doesn't trust the MAP sensor readings.

Have you scanned for OBDII codes? Generally if you're running
at low efficiency, it'll eventually throw a code.

ian

 

Some changes have been made to my first post above.

Andre

 

Ok... this is really good information... thank you... It's a start.

You brought up a good point about the coolant temperature sensor not being good. That is something that could be causing this low gas mileage problem. I have changed the O2 sensor (thinking that could be the problem), but nothing was corrected. But this now leads to maybe the wiring sending the information not working properly... but I'm assuming that would throw a CEL.

I have gotten the OBDII scanned but came up with no codes at all, which is weird I think. I even got it scanned even though I didn't have a CEL to eliminate that maybe the dash light for the CEL wasn't operating.

Are you sure that Subaru's don't have a MAF sensor? I thought that the sensor that sits in the Air Intake Piping (right after the filter) was the MAF sensor?

Andre

Your understanding of closed loop and open loop are pretty much accurate.
Closed loop uses the reading from the O2 sensor to constantly tweak
the F/A ratio to keep it at stoichiometric (14.7:1), which is most efficient.
Open loop ignores the O2 sensor and uses the preset fuel maps plus
some long term fuel trims (adjustments for altitude and such).
It goes to Open loop under hard acceleration, often to richen up
the mix a bit to cool the intake charge and prevent detonation.

Another time it runs in open loop is at first cold start of the day.
Coolant temps and Cat temps are both low, so it runs rich to heat
up the Cat faster. Sometimes it does so for a preset period of time, and
sometimes it stays in that mode until it sees those temps both come up.
That means that a bad O2 sensor or bad engine coolant temp sensor
can get you stuck in open loop mode, and it'll suck the fuel down.

Lastly, the Subaru 2.5L motor doesn't have a MAF sensor. I mention it
because on my VWs, unplugging the MAF causes the engine to
run rich (albeit normally in all other respects).
Subaru's have a MAP sensor (they just call it a "pressure sensor"),
and I do not know what happens if they fail, but it's something to look at
because it might switch to fuel maps if it doesn't trust the MAP sensor readings.

Have you scanned for OBDII codes? Generally if you're running
at low efficiency, it'll eventually throw a code.

ian

 

Now my thinking is that there could be a possibility that the ECU (for some reason) is malfunctioning and possibly always running on the open loop setting of the ECU.

A malfunction that keeps the ECM in open-loop mode is probably happening outside the ECM itself (such as a bad sensor). From a cold start, the transition time from open to closed loop is timed, and if the ECM does not go into closed loop mode within that time window, the MIL is set.

Also, most cars sold in the U.S. MY2001+ are using wideband air/fuel ratio ("AFR") sensors. These are not the same as oxygen sensors, but they perform a similar function. The AFR sensor allows the ECM closed-loop fuel control under all engine loads and throttle conditions. Where the O2 sensor output is only precise at stoichiometric, the AFR sensor's output is linear from, say, 9:1 through 16:1 air/fuel ratios.

When using an O2 sensor, the ECM alters fuel delivery from rich to lean over and over again so it can see how many times the O2 sensor's output crosses its precision threshold... they call it "cross counts" in the jargon. If you have an O2 sensor gauge, you can watch it bounce up and down while you drive. It's a hoot.

BTW, 14.7:1 is considered air/fuel stiochiometric, but this is not where engines actually run. Engines are more commonly 12:1 and richer. As you approach stoichiometric, the exhaust gas temp (EGT) increases. At stoichiometric on a common gasoline engine, the exhaust valves can run hot enough to anneal or "burn."

The new direct-injection gasoline engines run on the opposite side of stoichiometric... 16:1 and leaner. This is really only something that can be done with direct injection. I've seen it on port injected engines before, but only $$$$ race engines.

A lot of car guys think EGT gets hotter as the engine runs leaner. This is only true when you consider stoichiometric to be lean, which it is in most cases. EGT is at its peak at stoichiometric, remember that as a golden rule.

 

Re: Re: Quest for better MPG: ECU (Open & Closed Loop)

A malfunction that keeps the ECM in open-loop mode is probably happening outside the ECM itself (such as a bad sensor). From a cold start, the transition time from open to closed loop is timed, and if the ECM does not go into closed loop mode within that time window, the MIL is set.

The reason that I'm not ruling out that the ECU might be malfunctioning is because I remember there being one person that posted that they tried everything with no success until he swapped out the ECU. After that, the car had great gas mileage.

It still could be a sensor though. I still haven't checked all of them. I have replaced the front O2 sensor but there's the MAF sensor, coolant sensor, knock sensor, etc. Not only that but if the sensors are ok, maybe the wiring is not relaying the information to the ECU for whatever reason.

Jeez... there is a lot to check now.

Is there even a way for the dealer to see if the ECU is ok? If it is receiving proper information? Functioning in open or close loop mode?

Andre

 

8kGoodENuff, yeah, you have a MAF, but I think you already know that. 00-04 have a MAP, I dunno what 05+ have...I think they might have switched back to MAF again.

Good luck on your quest. It's an interesting problem, although for you, it's probably just frustrating

 

8kGoodENuff, yeah, you have a MAF, but I think you already know that. 00-04 have a MAP, I dunno what 05+ have...I think they might have switched back to MAF again.

Good luck on your quest. It's an interesting problem, although for you, it's probably just frustrating

Ah... I did not know that. Thank You. I'm just getting annoyed with only getting 17 mpg out of a 4-cylinder car... Eh... especially when I hear others getting around 25 mpg average... lol.

I say it is 50/50 right now as far as interesting and frustrating. I'm actually learning a lot from this cause I've always been interested in engine management tuning but never really had the time to focus my concentration to it. Now it seems that I'm getting forced into it... lol.

Andre

 

If your thermostat is stuck open or sluggish you will get poor gas mileage. Its a simple place to start.

Open loop is when the car is cold. The engine and O2 sensors have not reached operating temperitures. Once the car reaches operating temperiture (i think 172F but i have to look it up to be sure) the car will switch to closed loop. At that time the car is fully reading and reacting to all the sensors.

The best place to start with a car with poor mpg is a tuneup, plugs wires PCV valve fuel and air filters. This is one thing that doesn not change as long as the car burns gasoline .




nipper

 

If your thermostat is stuck open or sluggish you will get poor gas mileage. Its a simple place to start.

Open loop is when the car is cold. The engine and O2 sensors have not reached operating temperitures. Once the car reaches operating temperiture (i think 172F but i have to look it up to be sure) the car will switch to closed loop. At that time the car is fully reading and reacting to all the sensors.

The best place to start with a car with poor mpg is a tuneup, plugs wires PCV valve fuel and air filters. This is one thing that doesn not change as long as the car burns gasoline .




nipper

I don't know if it matters or not, but the car always runs at it's normal temperature, according to the gauge of course. So I'm assuming the coolant temp sensor and thermostat are working properly, but it is a place to start like you said.

As far as all others... I have done pretty much everything:
-air filter
-PCV valve
-plugs
-plug wires
-O2 sensor
-fuel filter
-ignition coil

I was going to do the EGR valve next but I'd figure to post about this first before going off spending anymore cash.

Are dealerships capable of knowing if an ECU is working properly?

Andre

 

The gauge on the dash at midband is extreemly innacurate.

You need to rent or borrow a scantool. All scan tools can read engine temp from the ECU.

How old is your timing belt?

Also get a cheap vacume gauge and tell us what it reads and what the needle is doing.


nipper

 

PS yes subaru can read your ecu.

A few other thoughts.

Tire pressure can make a huge difference.

Is this a manual or auto?

Lots of short trips will give you poor gas mileage If your trips are under 10 miles with lots of traffic lights and stop signs, then yes i can see 17 mpg. I get that sometimes with lots of short trips.

Take a good highway run someplace then recalculate gas mileage by how many gallons used and let us know.

 

The gauge on the dash at midband is extreemly innacurate.

You need to rent or borrow a scantool. All scan tools can read engine temp from the ECU.

How old is your timing belt?

Also get a cheap vacume gauge and tell us what it reads and what the needle is doing.


nipper

Oh... I did not know that... ok.

Would a scantool be the same as a code reader?

Andre

 

PS yes subaru can read your ecu.

A few other thoughts.

Tire pressure can make a huge difference.

Is this a manual or auto?

Lots of short trips will give you poor gas mileage If your trips are under 10 miles with lots of traffic lights and stop signs, then yes i can see 17 mpg. I get that sometimes with lots of short trips.

Take a good highway run someplace then recalculate gas mileage by how many gallons used and let us know.

I fill up the tires once a month to about 35 psi. When I fill, they are usually no lower than 30-32 psi.

Auto Trans with a fluid replacement at 146,000 miles. I now have 149,800 miles. I'm going to do the diff fluids when the weather warms up again with synthetic.

I have no idea how old the Timing Belt is cause I bought it used last December. So I'm not sure when the previous owner changed it. Car only had one owner but I bought it from a used car dealer, not private.

I do make a lot of short trips everyday (around 5 miles each way), but when I do the longer trips, the mileage is still fairly the same... maybe 19 mpg rather than 17 mpg. Next time I make another long trip (might be this weekend), I'll keep you posted.

Andre

 

Oh... I did not know that... ok.

Would a scantool be the same as a code reader?

Andre

Yes its the same thing. Also a vacum gauge mounted inside the car can be used as a MPG gauge, The higher you keep the needle the better your gas mileage. It can also tell you lost about the internals of the engine and the exhaust system.

nipper

 

I fill up the tires once a month to about 35 psi. When I fill, they are usually no lower than 30-32 psi.

Auto Trans with a fluid replacement at 146,000 miles. I now have 149,800 miles. I'm going to do the diff fluids when the weather warms up again with synthetic.

I have no idea how old the Timing Belt is cause I bought it used last December. So I'm not sure when the previous owner changed it. Car only had one owner but I bought it from a used car dealer, not private.

I do make a lot of short trips everyday (around 5 miles each way), but when I do the longer trips, the mileage is still fairly the same... maybe 19 mpg rather than 17 mpg. Next time I make another long trip (might be this weekend), I'll keep you posted.

Andre

SPend the ten 15 bucks for a t-stat and new cap. Lest see what happens. This is not throwing money away, your due for them anyway

nipper

 

SPend the ten 15 bucks for a t-stat and new cap. Lest see what happens. This is not throwing money away, your due for them anyway

nipper

I'll probably head out there this weekend and pick them up.

Andre

 

T-stat should be OE

nipper

 

A ScanGauge (brand name) can tell you if your car is in open or closed loop, plus lots of other things. I think there are other scan tools that will do it too, but that's the only one I can think of offhand (very popular).

 

yeah those short trips will kill your average mileage, at 5 miles you barely got the car up to temp and running more efficiently. stop and go with a small engine also kills the mileage, takes a lot of power to get you up to speed and the 4 has to work harder.

you really need to do a quick 50 mile highway run to get a better idea of what your top mileage is. keep it under 70 and if there's traffic, scrap those results. next time you need to go somewhere fill it up, get on the highway, when you get close to your destination get off and fill her up, check that mileage.

like nipper said, toss a vac gauge on that motor. i know we discussed checking the valves awhile ago, minor valve seepage can result in loss of compression and engine efficiency without coding. if it gets too bad you'll get a misfire code. also a minor intake leak can cause the engine to compensate by adding fuel, again, if minor it can effect mileage without coding.

most of the other things mentioned like the o2's and other sensors should code if there are wiring issues or they are not functioning withing preset parameters expected by the ecu. there are the exceptions to the rule and a sensor performing on the edge of acceptability can affect mileage but be "good enough" for the ecu to ignore.

 

We are in the same boat as your car with our 97.
My wife is a Home visit nurse and when we got the OB it has just been overhauled by a master subie tech due to head gaskets going. It was getting 15mpg around town.
I did the filters plugs and wires and it is up to about 19 around town. On a few long trips (like 4-6 hours) it is all over the place. it did 25mpg on one leg of the trip, 20 the next, and on the way home with the roof bag it dropped to 16! (and I can understand the drop)
Kind of sad, as my 05 FXT gets 19 in town with me beating on it a bit, and does 26 on the same long trips!

 

yeah those short trips will kill your average mileage, at 5 miles you barely got the car up to temp and running more efficiently. stop and go with a small engine also kills the mileage, takes a lot of power to get you up to speed and the 4 has to work harder.

you really need to do a quick 50 mile highway run to get a better idea of what your top mileage is. keep it under 70 and if there's traffic, scrap those results. next time you need to go somewhere fill it up, get on the highway, when you get close to your destination get off and fill her up, check that mileage.

like nipper said, toss a vac gauge on that motor. i know we discussed checking the valves awhile ago, minor valve seepage can result in loss of compression and engine efficiency without coding. if it gets too bad you'll get a misfire code. also a minor intake leak can cause the engine to compensate by adding fuel, again, if minor it can effect mileage without coding.

most of the other things mentioned like the o2's and other sensors should code if there are wiring issues or they are not functioning withing preset parameters expected by the ecu. there are the exceptions to the rule and a sensor performing on the edge of acceptability can affect mileage but be "good enough" for the ecu to ignore.

Yeah... I know there's a lot of work that the poor little 4-banger needs to do to get this wagon + AWD moving, but there were times where I seriously tried just getting on the gas really easy so that it would accelerate very slowly (reasonably) and it would still get bad gas mileage.

I actually just did a trip up to CT and back (around 150 miles round trip) and I ended up getting around 18.5 MPG, even though that's an improvement, it's not the kind of improvement that I would like to see for highway... lol.

As far as the codes... is there any good chance that maybe the car is trying to spit out a code and it's not able to? I know the light works, but could there be something else wrong that wouldn't show the code even though it wants to?

Where would I hook up a vac meter to get readings? There is a very good chance that the valves could be off. For some reason, I have a feeling that the previous owner did nothing with valve adjustment.

Andre

 

We are in the same boat as your car with our 97.
My wife is a Home visit nurse and when we got the OB it has just been overhauled by a master subie tech due to head gaskets going. It was getting 15mpg around town.
I did the filters plugs and wires and it is up to about 19 around town. On a few long trips (like 4-6 hours) it is all over the place. it did 25mpg on one leg of the trip, 20 the next, and on the way home with the roof bag it dropped to 16! (and I can understand the drop)
Kind of sad, as my 05 FXT gets 19 in town with me beating on it a bit, and does 26 on the same long trips!

I hear you. It is frustrating in a way but I'm sure it will be figured out eventually.

Has anyone ever gotten a valve adjustment done? I know that on our Honda S2000s, after you get a valve adjustment, it's like the difference from night to day. Smooth power, more power, and it runs so nicely.

Andre

 

You can hook up the vac gauge at almost any vac line. You may get reduced vac or no vac, so just try another one, as there are lots to choose from.

It sounds like you may have a lazy o2 sensor, bad engine temp sensor. The vac will tell us if you need a valve adjustment.

Have the codes rad to see if anything gets spit out.


nipper

 

to answer the question "is there any reason it would be trying to spit out a code and not doing it"

i can't think of any.

when i comes to things like this where there isn't any obvious cause for poor mileage, finding out the "overall health" of the motor with a vac gauge should be the first step.

you can have low compression but not low enough to cause misfires, you can have a small vaccum leak that is causing the car to compensate with added fuel without tripping any sensors.

my car was peppier and quieter after i did the valves, some were out past the limit but not too far. i've got 125k or so on mine and still get 20 ish around town and last trip up north got 27 on the highway.