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Brad B
Brad B, Ford Technician
Category: Ford
Satisfied Customers: 1091
Experience:  ASE Master Certified & ASE G1. Ford Senior Master
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2008 Mercury Mountaineer Premier(v8) Only experienced during

Customer Question

2008 Mercury Mountaineer Premier(v8)
Only experienced during this summer, when A/C is on, sometimes on acceleration the engine sounds like a turbo diesel taking off. It will stick around for a while and can go away on its own, but usually doesnt start till after the AC has been on for a while. Turning the ac and rear fan off stops the noise and assumes normal HP.
Submitted: 1 year ago.
Category: Ford
Expert:  Brad B replied 1 year ago.

Hello;

Thanks for choosing Just Answer! The noise and loss of power you are experiencing sounds to possibly be a bad A/C compressor clutch. If you turn the A/C on and hear the loud noise and can confirm the noise is coming from the compressor itself (mounted on the front of the engine and ran off the serpentine belt). If the compressor is locked up this will cause a loss of power as the compressor is still engaged when the vehicles computer has cut down the fuel trims when the compressor has been commanded off. Is the noise only present when taking off or at an idle when the air conditioner has been turned on?

Customer: replied 1 year ago.
I only notice the noise on acceleration, on idle sounds normal. It takes a while for the sound to start. Have noticed it on hotter/more humid days. Won't be able to test the compressor for a few days...waiting for a strut mount so the truck is on blocks.
Expert:  Brad B replied 1 year ago.

Once the noise starts, does the vehicle have to be accelerating to get the noise to continue or will it happen at idle after that point? And does the noise go away if your foot is taken off the accelerator as you are accelerating? Just trying to see if this is a 'pinging' noise issue or something with the air compressor. Also how many miles are on the vehicle and do you use the same fuel station, has the vehicle been sitting for some time and have you had any maintenance done such as spark plugs or have a check engine light?

Customer: replied 1 year ago.
114,000 miles. Dealer switched plugs at 105k. Not the same gas station. No check engine. 1 owner before me and was very clean/well maintained.Only hear noise when gas pedal is used to accelerate. When coasting I do hear a louder than normal idle but is worse when accelerating. I have also heard possible Trans temp, but I ruled that out when noise goes away with turning a/c off.
Expert:  Brad B replied 1 year ago.

It sounds like you have a pinging concern sense the noise goes away when you take your foot off of the accelerator. And it also can be confirmed that the noise is not from a transmission issue as the A/C compressor makes the noise more present. The issue can be from a restricted fuel filter, but this should cause a power loss at all times. The spark plugs have been replaced so the chances of carbon buildup is small. I would suspect at this point that the Mass Airflow sensor is dirty or the fuel quality is low and is causing a lean condition when accelerating. If the fuel tank is near empty you may try adding a full tank of qualtity fuel to see if the issue gets better, the mass airflow sensor should also be checked by looking for contamination on the small wire of the sensor and if anything is present (even a small piece of hair), the sensor should be cleaned which you can buy the special cleaner at most auto parts stores.

Customer: replied 1 year ago.
I recently cleaned the flow sensor and throttle body. Also just replaced the fuel filter 2,000 miles ago.
Expert:  Brad B replied 1 year ago.

Okay, can you recall if this happened after refueling or have you been through several tanks of fuel after the issue has happened? You may also try to reset the PCM to see if the fuel parameters can be relearned to make the pinging go away, you can do this by disconnecting the battery and waiting 15 minutes for the computers to discharge all power then reconnect the battery and drive to see if the pinging is still present. You don't notice any type of bucking of the vehicle when the noise is present do you?

Customer: replied 1 year ago.
Just noticed it within the last 30 days. But it hasn't been present every day. Only on days where I believe the air is working harder.No bucking...and I will disconnect and see if that helps. Like I said it might be till the end of the week waiting for suspension parts and my schedule is busy till end of the week.
Expert:  Brad B replied 1 year ago.

Okay, let me know what you come up with and we will go from there. If the Mass Air Flow sensor was contaminated and the computer has not relearned the ping could still be present, adding a fuel cleaner like Sea Foam may also be good to try at the same time.

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