twebb

This is a follow up from a post I had a couple of weeks ago. My 996 has a distinct vibration present when over 55mph. I thought this could just be the rear tires, but after replacing them it's still present. It also persists when the clutch is engaged or disengaged. The tach does not fluctuate rapidly. I'm thinking clutch? coil pack? maybe just replace clutch fluid? What do you guys think? Thanks in advance.

RPMulli

maybe look into getting your front wheels rebalanced.... if had a problem in the past where the stick-on weights will fall off and cause the wheel to vibrate (only at high speeds). i seriously doubt its the clutch, coil pack or fluid..

Pac996

Not saying the vibration isn't there but if you haven't been driving a car that stiff and down on the road it might just be the road.

But I've seen warnings about having non qualified tire change handlers slopping rims on and banging the rotors. On different cars I had a problems with water in brake fluid and warped rotors. To elimnate its water expanding applying brakes a little to give the vibration try seeing if the car still rolls easy after the symptoms show up. If you can hit the said 55 right off the bat and the symptoms don't appear then it might be water in your brake system or warped rotors that need time to heat up to cause the symptoms.

What evers good luck man. Notice I didn't mention checking the true of the rims or fine cracks since you had the wheels off recently.

Oh yah, check your CV joint boots.

Macster

Could be any number of things. Not knowing the history of the car impossible to say what the problem is.

A few things:

Air pressure. My experience is overinflated tires -- front ones especially -- cause a vibration at speed. My experience is higher than 55mph but depends I guess on how overinflated (in my car's case about 4 to 6psi from cold pressure) adn type of tires, road, etc.

Buy a good dial pressure tire gage and use it to bring tire inflation pressure to the right settting cold. I check tire pressures in the morning after the car has sat overnight and everything's cold.

Tire balance. The best tire balance uses a Hunter system with load force balancing.

Alignment.

Now a comment or two on the above. The tire balance and alignment only as good as the tech that does the work and the quality of equipment. I have found indy tires stores terrible for my Porsche tire/alignment needs (with just one exception) and I use a Porsche dealer and its service department, several in fact, depends where I am. Now I have gotten one bad alignment at one dealer but every other alignment at this dealer and other dealers (and my cars have had a few of these) has been very good. No problems.

(However, I no longer use the Porsche dealer that gave my Turbo a bad alignment.)

Also, I came upon one Porsche owner who had a vibration problem with his car and had it looked at several times and was told each time alignment ok. Finally he went to another shop and the shop found the alignment wrong and brought it into proper alignment. Symptoms gone. The 1st shop just blew this poor guy off.

More on tires: If you run non N-rated tires these may not balance properly, run true, fit properly, etc. I forget the numbers, but if the tire balance requires too much weight the tire is not acceptable. A good tire shop and tech will know this and should advise you. But ask and see what you're told.

More on wheels: If you run aftermarket wheels these may not run true or fit properly. Or be mounted properly. There's always the human factor to consider.

Also, some wheels very prone to being damaged by hitting potholes. I see cars -- one car belongs to someone who visits a neighbor -- and I swear the right front wheel is square from God only knows what the driver has hit.

Some years ago I got to talking to a guy -- we met at Audi dealer when I was looking at Audis -- with a nice Mercedes. He had aftermarket wheels (don't know the brand) and while they looked nice he told me the rough roads in the area (KC Mo area) just beat the wheels so bad he had to have the car's wheels/tires balanced at least once a year and have the alignment checked often cause it would go off.

Feel the tires. If you feel any feathered edges alignment possibly off. Uneven tire wear from side to side or across tire tread faces another sign of alignment problems.

Tire cupping can arise from a bad shock, too.

You really need to eliminate what you can -- tire pressures mainly -- and hve the other things checked and eliminated.

Sincerely,

Macster.

Meister Fahrer

Do you have spacers?

philooo

Is there a way to check for warp rotors ? are small warp causing problem or it needs to be big enough that a visual inspection could spot it ?

Pac996

Easy no frills is feel the wheel where it contacts the hub after a drive you get the vibration during. If its hot and you weren't racing around it means extra contact between rotor and pads have been going on or a bearing is shot. Normally applying the brakes with a warped rotor is going to give pulsing. You never mentioned anything like that. A bad wheel bearing can be snapped out of vibrating some times from applying the brakes. Anyway if you get the pulsing its the rotor warping from extra pressure in the brake linefrom water heating up and warping the rotors or the rotor is already permanently warped. Would be nice if you could pickwhat it is by price but you have to diagnose the problem. All simple physics. Did you check the CV boots? Change to another road you also think is smooth.

Some guys have been amazed to find out they have a cracked wheel like the last one I saw on here on the inner rim edge. Funny how we forget misadventures some times and don't investigate how screwed up we got things. I'm nobody to be talking on that subject hint hint

Macster

That's a very good question.

In trouble-shooting these types of problems anything different, or recently (or not so recently) changed always must be considered.

Sincerely,

Macster.

twebb

Thanks for all the input. I had new tires put on the rear, checked the fronts for air pressure, went through and replaced plugs and checked coils packs (eliminating possibility for slight misfire), and cleaned out my front radiators while I was into it since summer is here. Took it out for a drive late last night and there was a noticeable improvement. Time will tell if what I was feeling will return. From talking with a P-Tech friend, the next step will be looking at the clutch, clutch springs, and flywheel, if the vibration returns.

As for replacing the plugs, I have read many posts about doing this. Unless you're gumby I don't see any way to get in there without taking off the mufflers. Thanks for the posts of how to do this. On a scale of 1-10 I would rate the job as a 3-4. It's just a PIA, not too technical.

Thanks again to everyone.