Mombu 2009-03-14 07:48:03
I’m back trying to solve the lean bank 1 and lean bank 2 problem on my
wife’s 1996 Mystique. 2.5 litre V6 Automatic hiLine
Intake manifold gaskets were leaking – have been replaced. Intakes
were coked up – have been cleaned. Injectors have been cleaned. Bank
two O2 sensor #1 has been changed. (was getting an intermittent O2
sensor action code)
I had installed a restrictor in the Idle Air control line to (very
effectively) stop the “moosing” before I did the manifolds. I have
since removed it – with no improvement. (no moosing – yet since
manifold repairs completed.). Car has about 130,000Km on it.
The data capture shows the engine at 750 RPM, engine temperature
between 75 and 95 C, and vehicle speed zero when the light is
triggered. Not sure if it is triggering as it comes to a stop, or when
attempting to pull away (crack throttle open)
There are NO driveability problems at all – never a stutter, miss, or
hesitation. Fuel mileage appears to be normal, the engine temperature
guage is always very constant. The CEL is consistent – usually comes
on the second or third time the car is driven.
Occaisionally it will only report bank 2 lean. This car has the
“drive by wire” traction control system, so is a bit more complex than
most – and my OBD2 tester does not address the extended codes.
Anyone have experience with this problem – and a solution??
Mombu 2009-03-14 07:48:11
I forgot – the PCV system has also been checked and the valve
replaced. The MAF has been cleaned, but not replaced.There are no
exhaust leaks, and no leaks can be found anywhere in the intake
Codes are P0171 and P0174.
Mombu 2009-03-14 08:54:32
I’ve done some more troubleshooting.
With my OBD2 scanner connected and the key on, engine off, I am
getting a reading of 1.7 on the air flow.
It should read very close to zero, I would imagine. On my brother’s 96
Taurus it reads 0.03.
Suspect I have a bad MAF – and yes, it HAS been cleaned. At least
twice in the last 2 months.
What all vehicles does this particular MAF fit???
Mmmgall 2009-03-14 10:42:07
Not familiar with your engine but is there an Idle Manifold Runner Control?
Has to do with restricting airflow as you come off the gas and
Mombu 2009-03-14 11:10:28
Yes, it has a runner control, but it does not restrict airflow – it
makes the 24 valve engine into a 12 valve a low speeds for better
intake velocity – and better low-end torque. It has been cleaned,
checked, and adjusted.
Restricting the airflow would also tend to make it run RICH, you would
think – but then again nothing about this really seams to make any
However – I am not the first owner of this car, and I am suspicious
someone fiddled with the throttle valve closing adjustment. I have
turned it open 1/2 turn and “relearned” the computer so it idles
properly again – and it has, so far, gotten through FOUR drive cycles
without throwing a code.
I tried 4 different MAF sensors and they all read the same 1.7 gr/sec
with KOEO – including one that did not register a change in airflow
even if I blew on it – so I’ve come to the conclusion THAT’s not the
Mombu 2009-03-14 12:18:49
Still no check engine light after 6 full driving cycles – but I think
the plastic water pump impeller just let go. Blew a bit of antifreeze
out of the bottle, and running a touch on the hot side – thankfully it
happened this afternoon less than a mile from home, and not tonight on
the highway 60 miles from home at midnight!!! Had to take the van to
the airport to pick up #1 daughter.
Mombu 2009-03-14 12:43:56
Was the water pump – expensive little sinner too!! And the CEL came on
as it was driven into the shop to replace it — both banks showing
Mombu 2009-03-14 21:16:15
Well – the ongoing saga of the P0174 and P0171 codes on the ’96
Mystique 2.5 Duratec continues.
I have done every test imaginable on this car – and EVERYTHING passes
– but I still get the P0174 code.
I have replaced upper and lower intake manifold gaskets (which WERE
leaking) , and checked the runner control shaft bushings for leak. I
have checked fuel pressure, cleaned injectors, replaced PCV, and
cleaned the MAF sensor 4 times. I also changed the Bank #2 O2 sensor
#1 when it threw a code and occaisionaly did not “clock”
After running one or two fuel monitor cycles (0-65 and back to 40mph
without touching the brakes) there was always a code P0174, and
sometimesa a pending P0171 – whether the CEL was on or not. The long
term fuel trims were quite different bank to bank.
I tried re-adjusting the throttle valve stop on the advice that IF it
had been tampered with, it COULD cause the problem.
Finally, I got cheezed off enough to get a replacement MAF sensor. The
readings were virtually identical between the two sensors both at idle
and at 2000 RPM – but today I changed it anyway – SOMETHING has to be
wrong SOMEWHERE – right?
Well, after I changed the MAF sensor, and reset the CEL, there was
STILL a P0174 code. I disconnected the battery (which I have done
several times in the past) for 3 minutes, and reconnected it. When I
restarted the engine, and checked codes, THE P0174 CODE WAS GONE!!!!!
Funny – never had that happen before.
Ran it through 3 fuel monitor cycles – still no code – and the fuel
trims are VERY close to the same bank to bank.
Do I DARE to think, perhaps, the problem is now solved??????
Stay tuned – we’ll know in a day or so————-.
Tom adkins 2009-03-14 21:16:43
Shhhhhhhh! Murphy might be listening 😉
Mombu 2009-03-14 21:51:03
Murphy is alive and well.
I had to go pick up my daughter from work – checked for codes before
starting – nothing. Started car, backed out of driveway, 1 stop sign
and 2 stop lights in just about 1 mile of driving – checked for codes
when I stopped in the lot –
pending code P0174
Data capture says:
Fuel system: closed loop
Load Value: 19.2%
Coolant Temp 82C
Short term fuel trim 1: 12.5%
Long term fuel trim 1: 25%
Short term fuel trim 2: 14.8%
Long term fuel trim 2: 25%
Vehicle Speed: 0
EVERYTHING CHECKS OUT OK!!!!
What can be the problem??????
Whatever it is, it ALWAYS hits bank2 first – but bank 1 follows. It
ALWAYS occurs at idle, with the engine warm but not hot, running in
closed loop- with the vehicle at a stop. The O2 sensors check out just
fine. The MAF sensor has been cleaned AND replaced. The intake gaskets
have been replaced, along with the PCV, and there are no obvious
vacuum leaks. The dipstick is sealed.
The only thing that seems off is the long term fuel trim hitting 25%.
On BOTH banks at the same time.
Guess I’ll have to check fuel pressure again – but you would think a
stuffed filter or fuelpump would give trouble at speed, not at idle.
Well, enough for tonight – I’m going to BED!!
Chevy 2009-03-14 22:29:09
Just a Guess , but did you check fuel pump presure and Volume ? change out
fuel filter ?
Have seen where presure is good but voluem is bad ,
Mombu 2009-03-14 23:05:26
I’ve thought of that – will check farther, BUT volume will be an issue
at speed, not at idle – at least one would think. The pressure was
checked at idle – will check it at speed – if the pressure does not
drop, volume has to be OK.
Mombu 2009-03-14 23:35:51
Well, checked the fuelessures – OK – and guess what? The pending code
P0174 was gone this morning, and has not come back. First time that’s
happened – so mabee we are getting somewhere. Looks like mabee the MAF
sensor was the culprit. The one that is on now SEEMS to have made a
big difference – but we’ve been there before!!.