[nzjdm]

Hi,

I have a Japanese imported 2001 legacy outback that i've owned for 3 years.

Over the last few months I have been trying to diagnose a problem that gives a symptom like the rule being restricted at part throttle cruse, more noticeable up hill with a steady foot. Engine speed 2500 rpm approx 60 miles per hour.


This makes the engine feel like its violently knocking on the engine mounts, the issue is more noticeable at a steady speed, but does have this this symptom during slower road speeds intermittently. Along with this when you floor it, the car seems to bog down, at random like it is in a too high gear, but moving the auto stick to a lower position doesn't effect the rpms (so its probably in the right gear).


I have done some troubleshooting via threads on this very useful forum, (a lot of good knowledge here).

Had a CEL for TPS so that was replaced.
Now occasionally CEL for P0171 bank 1 system lean when taken on long trips.

18 Mpg Average on combined city / highway.

The car:
2001 Subaru Legacy 3.0 H6 VDC Automatic
Japanese Domestic Market - it has no MAF sensor.
18-20 Mpg Average on combined city / highway.
128K miles

Trouble shooting so far:
Replaced the TPS with a new unit as has a CEL for it
Replaced the EGR valve
Replaced the IIACV
Spark plugs replaced with correct NGK ones
Air filter changed (paper type)
New battery 530 CCA
Vacuum lines have been checked (ends clamped down to reduce possibility of leaks)
O2 sensors positions swapped over to see if fault was with sensor or engine bank, but fault P0171 remained.


This problem comes and goes, disconnecting the battery helps it to drive more normally for 80 miles or so. Occasionally the VDC OFF light will appear for a few seconds while it misbehaves.

Starts usually first time, except for one instance 3 months ago where for some reason it just would not fire and then did.


Has anyone out here come across this before, or any idea where to look next. ?

Thanks.

 

[nzjdm]

Update:

Had the CEL on again with P0051 Heated o2 sensor control circuit low bank 2 sensor 1

and

P0031 Heated o2 sensor control circuit low bank 1 sensor 1.

I forgot to mention, I am in New Zealand.

 

[idosubaru]

When the symptoms happen - does the CEL come on?

Did the symptoms change at all after replacing the TPS?

Have the spark plugs ever been replaced?
Have you checked for oil in the spark plug tubes?

Doubt these are it but has the air filter, PCV valve, fuel filters ever been replaced?

CTS and Crank sensor are easy to replace I believe. CTS are cheap and crank sensor can be had used probably to test.

Have you tried different gas? Do you always buy the same gas from some place?

 

[nzjdm]

Thanks for the reply...

It takes a while for the CEL to come on - it doesn't light as soon as the fault happen. No fixed pattern to it.

Changing the TPS seemed to have no effect (other than maybe slightly sharper throttle response)

I changed the spark plugs 8K miles ago. Correct NGK ILFR6B Iridium plugs. (feels a little more powerful when it does run properly but again no improvement)

Air filter changes 1k miles ago. (no improvement on problem)


PCV valve and fuel filter no

CTS and crank sensore I haven't looked at yet - (Diagnostic code scanner dose shoe believable temperature)

Gas - I have tried different places, different grades all on a full tank. Made no noticeable difference to the way the engine runs.

 

[cardoc]

Check the battery ground cable and all the ground lugs, check your alternator output. The sporadic nature could be poor grounds, especially with 2 heater circuit codes coming up.

There are specifically 4 on the transmission torque arm bracket at the firewall and another that attaches to the bottom arm bracket. There is also one that runs from the block to the firewall on bank 2 side of the car (passenger for the JDM).

And yeah, replace the fuel filter if its been more than 15k miles. Too lean is lack of fuel. And the way you describe it running in the first post is indicative of a clogging fuel filter. If it doesn't improve with the filter, you'll need to check fuel pressure and flow.

 

[nzjdm]

Thanks cardoc, i'll check the grounds and voltage over the weekend.

What makes this even more strange is that I did not clear the error codes, they are still in the computer. The CEL is not on, but since the o2 sensor codes were set, the car has run absolutely perfectly over the last 180 miles. This makes me think that it could well be an electrical issue of some sort.

In regards to the fuel filter, I have heard that it is in tank and part of the pump. Again being JDM i don't know if its any different, are you able to clarify this?

Thanks.

 

[cardoc]

I have limited JDM info, but if yours has a filter it will be on the passenger strut tower between the fuse box and the firewall.

 

[nzjdm]

Cardoc,

There is no fuel filter on the strut tower (space for one) but lines come up and go straight to the engine. So I guess this car has the in-tank fuel filter.

Voltage check:
Battery after sitting for 18 hours. 12.38v
Fast idle: 14.40v
Slow idle: 14.36v
Rev up and let it fall to idle voltage stable @ 14.36.

Re did test again 30 min later, battery voltage 12.8v but under all other conditions was 0.7v lower than the first test. Don't know if that means anything?

 

[cardoc]

Amperage availability from the battery is key to proper function of all the electrical components on the car. Voltage is different from amperage. The voltage you posted is high. The alternator may be overcharging due to a weak battery or a bad engine ground.

I would replace the battery, check all the grounds and recheck the alternator output. It will most likely drop and you will see a difference in the way the engine reacts.

Fuel: Pressure and flow need to be checked. Is the fuel available to you blended with ethanol? Does the engine change with the ambient temperatures?

Oh, and the ECM stores inactive codes for an average of 40 drive cycles, so don't fret the history codes. Pending gives you an indication that the ECM is watching something and trying to determine where the failure is and the hard code tells you what system the failure may be in.

 

[nzjdm]

Battery was replaced about 10 months ago with an Exide 530 CCA, so I would hope that hasn't failed, when the old one was dying i did experience very bad idling so knew it was time to go.

I hear what your saying about the current of the alternator.

Gas here has no ethanol (other then one brand that uses it to enhance performance) and I am filling the tank twice a week so its always fresh.

I will go through and clean the ground connections, and check their wires hopefully ruling this one out.

 

[rickjames8]

Did you ever find a solution to this?

I have a very similar symptom. I have a rythmic surging around 2500 rpms when I'm at about 50mph. Tried changing trans fluid as it feels similar to the car switching rapidly between gears (it needed a change anyway). I also checked for vacuum leaks and checked the plugs. Car starts fine, idles fine, and generally drives 100%. I have no CEL on. My battery/ground connections look good. The fact it only happens at a very precise point on the rpm/speed range makes me question the drive-by-wire. What are your thoughts?