Hello all,

I bought a 2004 RX8 with 69k miles yesterday.









Everything seems ok with the car but one thing. When I am idle and the ac is on, whenever the compressor kicks on there is a major either vibration or rough idle happening. If i give it a bit of gas it goes away. The car does fine without the AC on, either that or the vibration is so small I am not noticing it unless the AC is on. Any ideas?

 

Could be motor mounts going out.

 

Seems like a common thing from what I am reading. It only does it when the compressor kicks on.

 

Welcome what part of florida you in?

 

Just outside of Tampa. Maybe 20 mins north.

 

They could be GOING out.

 

Ah missed the "going" in that. Sorry skimmed over the response. Makes sense tho I have an LS1 that acted very similar when motor mounts were going out. Thanks!

 

my car does the same thing and far as I can tell and Mazda Techs, the mounts are fine.

 

Just called mazda and talked to the service dept. Very friendly by the way. He gave me some tips on the car and he did say based off of the description I should have the mounts looked at. He did say that without oil change records it will be hard to get them to move on it but he said as long as I show something it will help.

 

What kind of car is an LS1? Is that a Lincoln or a Lexus or something?

Welcome to the board!

 

ls1 is a chevy v8

 

Welcome.

Given the age (I have an 04 as well), check out the $100 thread below, as well as the other one, then think plugs and coils if it still shakes. Honestly, the clean up stuff I did in $100 took care of most of it. But, you are at the end of life for the plugs and coils if they have not been changed.

Car is beautiful by the way.

 

+1 on 04green.

The other thing to understand is when you have the AC on, you are running an entirly different motor, so the car is going to be doing a few things different as apposed to having it off. On low rmps (like 1-2) your car is getting overloaded with the second motor turning on, so its struggling as it draws power from the current motor. while driving, it draws the same power - but you dont notice it as the high rpms are masking it.

I suppose what im trying to say is its normal. Something that most cars do when you haev the AC on. unless you have extreme loss of power/really notable rattles or bad shuddering under first power there is nothing to worry about.

Nice car btw.

 

+1

Mine does it too. It's my motor mounts.

 

Just check the right side motor mount.
If it's anyone it will be that and look for any cracks.

 

98 Trans Am

 

Awesome thanks guys. I am going to have the mounts checked out and I want to say new plugs and coil packs were put on the car before I purchased it. I am going to double check on that. This car is radically different from anything I have driven or owned. I have only owned big 8's for about a decade now so this a lot different mechanically.

 

and it is different again above 5K....

 

Ha yeah above 5K is interesting. Amazing how a motor like this can put out 238 hp. Not having a massive amount of torque is something I am still adjusting to. But I will say the thing will kick out and slide like it is not even trying. Just in a rainy parking lot not even trying lead to the most effortless drift I have ever done.

 

You get the torque from the gears. Other cars are falling off at the higher RPM. You get to go to 9K, ie, stay in a lower gear. Think of it as a 50% boost in torque, because you can apply your 168 at half again as many RPM.

 

What the hell are you smoking.

 

beer,

Short form, you got a something like a 4.11 rear end, right, that gives you a lot of torque multiplication (just over 4x). Assume about the same shift points (speed wise), assume the other guys are running a 3:1 or so rear end. That means you gain about 33% more torque at the wheels from the rear end than he does. Say he has 250 ft lb, runs it through a 3:1 rear end, gets 750 at the wheel. You have 170, run it through a 4:1, you got 680. Yeah, down 60, but that is less than 10%. Now where near as much as it seems when you compare 170 to 250.

And, he has a sucky car with old fashioned pistons.

You have to pick a car, and know the gear ratios in the rear and transmission to get more detail, but you will be surprised at how much difference that extra 3,000 rpm might make with the rear end ratios we have. Now, they are hell on gas mileage, but that is 14 other threads.

And, I am smoking more bear at this time as well.

 

 

@ moochie,

Different version..

Say you are pulling 3rd gear, coming out of a turn, at 8,000 RPM, you have a torque multiplication based on 3rd gear and the rear end that gives you a specific torque value at the rear wheels. Your buddy who cannot make power that high, say only about 5,500, has to shift, and likely had a shorter rear end. He is in 4th gear, and has an overall lower torque multiplication. While you got less torque, you multiply it by a higher number by the time it gets to the rear wheels. You may not have more, but you are not as far behind as you might believe.

I think this is why a Boxster S, with an extra hundred or so foot pounds of torque, cannot drive away from me coming out of the above corner.

It used to be that you compared hp and torque, but then again, everyone had about the same gear ratios and RPM limits. Once again, rotary is different.

 

^ Whatever you say bro