1999 Mystique, 113,000 miles, 2.5L 6 cyl. AXOD trans. For informational purposes: Within last 6 months I have replaced alternator (and serpentine) and right rear brake line. Unknown service history prior to about 80,000 miles.
For 10 days or so, have LOUD humming noise (sounds like pump or vacum noise) from underneath under certain circumstances. Does not hum at start up or idle so I'm not able to exit vehicle and determine exact location.
When trans is engaged, humming begins under very light acceleration and when maintaining speed (Light pressure on gas pedal) whether that speed in 25 MPH or 70 MPH. Does not occur with more firm acceleration. At slower speeds, if you pop the trans into Neutral, humming stops. Any increase or decrease in pedal pressure, hummming stops but returns once you begin to 'maintain' speed again (light pedal pressure).
Humming does not vary with RPM's and remains constant in both pitch and volume as long as it is humming. When it hums, it hums. I've done visual for vacum leaks and seen nothing obvious. My guesses are fuel pump or Tourque convertor.
Reasons why NOT fuel pump:
There is no problem with fuel delivery (yet).
Smooth, powerful acceleration when asked for.
No hesitations, stalling or sputtering.
No detectable decrease in fuel mileage.
Engine performance remains good whether humming or not.
Reasons why NOT Torque Convertor:
Trans engages smoothly and crisply.
Upshifts and and downshifts without trouble. No slipping, clutch packs seemingly OK.
I have not serviced the trans since I purchased with 80,000 miles but trans fluid is pink, does NOT smell burnt, very little clutch material on dipstick.
I know there is at least 33,000 miles on the fluid that I'm looking at.
No vibrations detected in gas pedal or in vehicle overall.
It's not a wheel bearing as it is not a constant humming sound and humming is not related to RPM's or actual car speed. Hums under outlined circumstances whether you go straight line or around corners.
My secondary guess is the alternator (I bought the store brand) or serpentine.
Reasons why NOT alt or serpentine:
You would think that any increased load on engine, such as turning on AC should start, stop, increase or in some way affect the humming. Turning on/off AC makes no difference.
I'm not able to remove serpentine and start engine as my problem doesn't occur at start up or idle.
No, not the power steering pump. Typically the power steering pump gives you a louder growling noise or quiets some as you turn and straighten your steering wheel. None of that happening here.
I'm really confused with this one. I'm no mechanic but I can usually work my way through an engine compartment and at least figure out if it's something I can handle or not. I just can't figure out what there is to handle (or not) in this case.
I have managed to exit the car while the humming sound was occuring and it seems to be coming from the top/rear of the engine right near the firewall, right around the coil pack. There are a ton of vacum hoses back there and I'm starting to think it may be related to a vacum leak or collapsed vac hose. Strange that the engine performance isn't affected though if that's it.
Ever known a vac leak or even an ERG or PCV valve to cause a REALLY loud humming noise?
Could be , as you suggest vac leak or PCV , worth checking . Not the first time I've been surprised at what caused noise in engines . Came across this , re. moosing , might help .Try hose first , rather than IAC .
Posted 09 January 2011 - 05:59 PM
OK! i have a 1998 mercury mystique LS V6 2.5 litre, i was getting a loud humming from my air filter but only in the winter does it do this, this year i decided to try and figure it out. everyone says its called moosing..... the IAC so i took that off, i cleaned it got my own gasket sheet and made a new one very well at that i might add. and now my car idles rough as to before it did not, not intill i cleaned it. any help would be greatful.
Points to note on V6 IAC valves..
*If you change the valve use only genuine Ford part. Other pattern copy parts do do have the all the Ford revision built into them,genuine Ford part does.
*Remove the stock air feed tube/pipes that go into the IAC valve from the air cleaner tube. Replace with a hose of the correct bore size approx 2'> 2' 6" long..loop it to take up the extra length. This longer feed tube will change the harmonic of the air feed to the ISC valve and will stop the 'moosing' sound you have...
We have all assumed the car is a V6 re the noise...If it is,look at where the IAC bolts to the intake manifold,just between it and the EGR valve is the hose/connection point where the longer hose shouild be fitted.This hose replacement is not a major task. Been doing the hose fix for years ...
I thought the trouble was back near the firewall and it turns out, it was. As I was also hearing it from the throttle body and because of it's location, your suggestion of the IAC valve made sense so I looked into that.
Bottom line..........40 bucks and 20 minutes in the driveway later, I'm on down the road in a quiet Mercury Mystique. Didn't even bust open a knuckle on this one.
I've had 4 of these cars, and only once did the moose ever come to visit. Scared the hell outa me !! Embarrassing at the stop light too. Fixed it with a copper plumbing cap with a drilled hole in it to regulate the air flow into the IAC. This solution also solves the 1-2 bang shift and RPM hang after acceleration. BONUS
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