New MPS owner: Mazda 6 MPS 2005 Strange Knocking Noise

[rayd]

Hey Guys,

i bought a mazda 6 MPS 2005 model last week, and have noticed a strange knocking senstation that occurs in the car.

it happens about 1/3 times as the car is decelerating no matter if in gear or in neutral. usually happens at about 10-20km/h at 2-2.5krpm.

Feels as if the car has ran over a pot hole but it hasnt? sort of like a jerk/knocking sensation. from inside it sounds like its comming from the rear diff, or around that area.

the car is stock except that it is lowered on springs.
i should also mention that it does require a service...

what you think?

[shakespeare]

Welcome mate and congrats on your purchase. First thing to check is tyre pressures, second is are all 4 tyres the same! Reason being is that with the AWD a difference in the diameters of the tyres particularly rears left/right will cause some drivetrain knocks and noises on deceleration, also front to rear size differences will also produce knocks and vibrations.

Try that first then have a look through the Tech Talk area which contains a number of diff related threads which may help you. A number of members have lowered MPS 6's without problems so it may not be that.

[rayd]

Thanks mate ! got some big plans for the car, so you'll be hearing alot form me lol

Now that you mention it, the two front tyres are different to the two rears... but if it was that, wouldnt it cause a reoccuring knock if that were the case? not just a single severe knock?

[Velocity Red]

Welcome mate and congrats on your purchase. First thing to check is tyre pressures, second is are all 4 tyres the same! Reason being is that with the AWD a difference in the diameters of the tyres particularly rears left/right will cause some drivetrain knocks and noises on deceleration, also front to rear size differences will also produce knocks and vibrations.

Try that first then have a look through the Tech Talk area which contains a number of diff related threads which may help you. A number of members have lowered MPS 6's without problems so it may not be that.

rayd, hello and welcome to the forum. Shakespeare got it right in one I reckon. I had a similar prob with my '05 MPS6, vibration on deceleration just as you describe. My local mazda dealer (REGAL MAZDA Newcastle.... I cant say enough good things about them) went to EXTREME lengths to chase down the problem. It turned out to be Yokohamas on the front and Toyos on the back that were causing the problem.

Although both pairs of tyres were 225/40R18's and ostensibly the same size they must in fact have different rolling diameters or different coefficients of friction. The rear diff (because the car is not constant 4WD) was basically going "I'm slipping, I'm gripping, I'm slipping... " and causing the vibration you mention.

If you can, try to borrow a set of identical wheels and tyres that fit your car and take it for a drive. I think you will find your problem will be solved.

Good luck with it,

Leon

[foxy]

Swopping front to backs would be worth a try, as it costs nothing.

[Velocity Red]

Swopping front to backs would be worth a try, as it costs nothing.

Nope, wont work.

Leon

[rayd]

Ok, ill look into that, but the only thing i want to point out is that it isnt a vibration. it is just one hard knock as if i hit a pot hole. not a constant knocking... thats the only thing that is leading me to think that it isn't the tires...

[kmh001]

If there is a problem with tyre height, it will be because the car IS constant awd. That is to say, when ever the car is in motion, all four wheel are constantly driven (only the torque distribution varies). If there is a tyre height variance, it will cause the drive line to wind up, because the diff coupling doesn't slip much and doesn't disengage until the car almost stops (around 5kmh). Just to be clear, the 6MPS is not "AWD on demand".

There are two things you can do to check if it's a tyre height issue.

1. Apply the hand brake to the first notch and confirm the light shows on the dash. This will turn off the rear diff. Drive around and try to replicate the knocking sound.
2. If this experiment stops the knocking, measure the tyre height on a flat level surface, from the ground to the top of the rim. Any more than about 4mm variance will cause problems.

If there is a significant variance, you can play with tyre pressures to try and balance the tyre height. Just be careful because a significant variance in tyre pressures obviously has its own issues and this just for diagnostic purposes.

Good luck. I hope it is just a tyre issue because it's a whole lot easier to fix than some of the other drive line problems.

[rayd]

If there is a problem with tyre height, it will be because the car IS constant awd. That is to say, when ever the car is in motion, all four wheel are constantly driven (only the torque distribution varies). If there is a tyre height variance, it will cause the drive line to wind up, because the diff coupling doesn't slip much and doesn't disengage until the car almost stops (around 5kmh). Just to be clear, the 6MPS is not "AWD on demand".

There are two things you can do to check if it's a tyre height issue.

1. Apply the hand brake to the first notch and confirm the light shows on the dash. This will turn off the rear diff. Drive around and try to replicate the knocking sound.
2. If this experiment stops the knocking, measure the tyre height on a flat level surface, from the ground to the top of the rim. Any more than about 4mm variance will cause problems.

If there is a significant variance, you can play with tyre pressures to try and balance the tyre height. Just be careful because a significant variance in tyre pressures obviously has its own issues and this just for diagnostic purposes.

Good luck. I hope it is just a tyre issue because it's a whole lot easier to fix than some of the other drive line problems.

Wow, thanks for that, ill be sure to try out the hand brake trick tonight when i get home. i really hope that this is the issue aswell!

[Velocity Red]

If there is a problem with tyre height, it will be because the car IS constant awd. That is to say, when ever the car is in motion, all four wheel are constantly driven (only the torque distribution varies).

You are absolutely correct, that is a better way of phrasing it. My "car is not constant 4WD" statement was an attempt to get across the idea of the variation of drive to the rear wheels.

"AWD is in constant operation except at very low speed and when the hand brake is applied" as is stated in this YouTube vid. YouTube - Mazda MPS 6 AWD operation

Thanks for putting the record straight.

Leon