Duratec 2.5 lean at idle (decel)

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 system.

Codes are P0171 and P0174.

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???

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 decelerate. Steve

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 sense. 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 problem.

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.

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 lean AGAIN.

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-------------.

Shhhhhhhh! Murphy might be listening ;)

Good luck.

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% RPM: 732 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!!

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.

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!!.