Engine Surging and Running Rich

[Corbin]

Hi all,

I was hoping someone might be able to give a little insight into a surging and hesitation issue i've been having. I bought the car (2004 Liberty GT) a few months ago and have been noticing poor fuel economy (town driving yields 375km to a tank). Since buying the car I have noticed a hesitation in the lower rpm which can sometimes develop into a violent bucking on very light throttle applications. The idle also seems to be "pulsing".
The car is mostly stock other than a Pulse Racing ecuTek reflash (done by a previous owner in 2009) and what appears to be a hacked downpipe.
The car was serviced at MRT just before I purchased it which, as per the invoice, included both MAF and front O2 sensor replacements. I have recently changed spark plugs which had a lot of carbon deposits.
Most recently I have taken the car to my local Subaru (useless) and a specialist tuner. After datalogging the specialist discovered the that the jerkiness in acceleration and erratic idle is due to excessive O2 correction. The fuel trims are way out (-25% Short Term and -15% Long Term) and the AFR is excessively rich. He also advised that fuel pressure is diminishing at high RPM/Load due to a wearing fuel pump and that the rear O2 has visible external damage (could this affect fueling?).

My question is whether this problem would be consistent with a faulty O2 sensor (despite it being replaced) or possibly a bad base tune. If anyone could chime in with possible solutions it would be greatly appreciated.

Thanks in advance,
Corbin

[dr20t]

Check entire inlet system for boost/ vacuum leaks. This is most likely your cause if indeed mrt replaced maf and 02.

If no leaks at all, then get someone to verify that maf and 02 were indeed changed (pretty easy to tell)

Would say if no inlet leaks then its a maf or 02 sensor

Mick

[Corbin]

Thanks for the reply mate,

From what I understand, a boost leak would cause a car to run rich. However shouldn't this only affect the engine when it is on boost? The car also holds a steady 15psi on WOT.
I have yet to check the O2, however the MAF has definitely been replaced and is apparently reading the correct voltages.

Also, should I be worried about the fuel trims and the engine running so rich? Don't want to be doing any more damage.

Thanks again,
Corbin

[jslayz]

Sounds pretty bad

Have you looked for air leaks?
Bottom edge of the hose going onto turbo inlet is often an issue, can look fine from the top.....
other smaller hoses on TMIC side of intake manifold can also pop off....

Spray some aerostart around suspected leak areas with engine idling and wait 30 seconds, if the idle increases then check further where you just sprayed the aerostart to track the leak.

If possible ask for a forum member to do a learning view and post it up?
Also check for any codes being logged. 02 Sensor out of voltage etc.

If still no luck, I would go and try to find out if plugs have been done recently?

Then investigate tune.... but i'm thinking air leak.

Anyone got any other suggestions..???

[fibble]

Agreed 100% J

Post MAF leak but pre-turbo/intercooler (if you are holding boost) - this is where to start....I would look for holes/leaks in the bottom of hoses.

I am assuming that you know you are holding boost because you logged it?????


Edit: actually, just check something as well....there is a little hose going from the intake manifold (on right side as you face car) which goes to fuel pressure regulator circuit (which is mounted n body in front of strut top) - is this still connected? Long shot and might contradict what you are telling us about your log......symptoms may be similar though......

[dr20t]

Any intake leak can cause rich conditions. Particularly given the factory twin scroll turbo spools so early (full boost 2400rpm), which means as soon as on boost, you're leaking metered air, so ecu is expecting a certain airflow, but instead receiving less (due to the leak), so your fueling is too high for the actual air being ingested by the engine.

Could be a bad tune but would have to be atrocious to be that bad (not out of the question given the history)

Silly one - but check the maf sensor is on the correct way.

Mick

[Corbin]

I just finished up searching for vacuum leaks for the third time. This time I removed the turbo intake tube but it was in good condition. All vacuum hoses are in relatively good condition and none where disconnected (although I remember now that when I got the car the hose that runs from the nipple on the recirc valve to the back of inlet manifold was disconnected).

No codes have been thrown since I have owned the car and I believe the tuner who looked at the car for me said that he investigated the O2 but hasn't ruled it out (time restraints and found it unlikely to be faulty given it's recently been replaced). I replaced the plugs only last week.

The problem has got me really confused as it is present when the engine is both cold and warm (although I used to only notice it on cold startups) and is only really present on light throttle/low rpm. In neutral, pressing the throttle a tiny bit to maintain around 1500rpm will cause the rpms to rise and fall repeatedly as though it is hunting.

MAF is on correctly, I wish it were that simple .

EDIT: Just had a thought. The front O2 sensor is behind the driver front wheel on the manifold right? And the sensor for exhaust gas temperature is on the downpipe? It could be possible they replaced the temperature sensor and not the O2. Will investigate tomorrow.

[fibble]

The front 02 sensor is on the dump pipe.

Rear 02 sensor is after CAT.

[norbs]

Mate, whilst the car is running give the MAF cable a jiggle and see if it changes the fuel correction. Look at MAF volts. At idle should be 1.2 to 1.28. My fuel trims would sporadically go south (-ive) and a faulty MAF plug was the culprit. I had high MAF volts at idle.

Otherwise either an air leak or possibly bad 02.

[Corbin]

UPDATE: Alright, after countless hours I have a little breakthrough. Removed intercooler-throttle hose to check for leaks but there were none. While I was there I decided to inspect the throttle itself. While getting someone to smoothly open the throttle part way (about the distance I have always experienced hesitation/bucking) I noticed that the throttle movement is not smooth at all. The throttle gradually opens before fluttering a little and continuing. At about this position in the throttle there is a "clicking" sound (not sure if this is normal). The throttle movements seem quite random, sometimes opening/closing smoothly from the closed position, others it seems to be jumpy despite smooth pedal control.

Also, there is a rather distinct ringing noise coming from the throttle body assembly. I confirmed this by unplugging the connector on the throttle itself (the ringing stopped). Anyone else have this?

This seems to explain a lot of my problems. Any idea what could be causing this?

I also got a brief video that sort of shows the problem. Could possibly upload it.

Thanks in advance,
Corbin

[norbs]

Interesting. But do you think that would explain the -ive fuel trims?

[Corbin]

That's where my brain gets confused.

[norbs]

Check out this thread on legacygt.com showing common 'potential' air leak areas:-

http://legacygt.com/forums/showthread.p ... 35626.html

[dr20t]

The humming is normal.

Depending on how much the throttle plate is oscillating, this is also normal for a dbw to get the exact throttle position requested by ecu. However large variations (more than 10 degrees throttle angle change) would suggest either faulty tps / throttle body module or accelerator pedal module. Check the wiring on the back of your actual accelerator plug

[Corbin]

The humming is pretty loud. I feel like it has gotten louder since I bought the car as I can now hear it inside the car and sometimes when the car is idling. Is that still normal?

I have finally hooked up my computer with a live data program (ecuExplorer as I couldn't get romraider to work). I discovered that the throttle flutters at light throttle, but also slightly in sync with the stumble in my idle. I'm sure that it's not supposed to do that.

But, the now for bigger breakthrough! I believe the MAP sensor is faulty. ECM data is showing MAP values as being around -4.9psi at idle while my installed boost gauge is reading (albeit slightly inaccurate) around -10psi. This would explain the terribly rich fuel trims if I am not mistaken. Might drop in to Subaru and the tuner again tomorrow.

Thanks for all the help!
Corbin