Bill Ball

Bad motor mounts give low speed vibration, very noticeable at idle and just above. Torque tube vibration is throughout the RPM range although worse as RPMs increase [Note: In a stick shift, you can isolate this in the driveway by putting in the clutch and reving the motor with the TT motionless; not so in an automatic as the TT always spins with crankshaft.] High speed only vibration is most likely tire balance. I've had that problem twice, with vibration just above 85-90 after new tires and a supposedly good balance. Had it checked at another shop and found one rear wheel an ounce off.

Charley B

I'd be real curious about how many miles elapsed while the odo gear was bad. In reading through the receipts, when did the mileage stop changing between receipts. Are the Potenza's still on the car?

Landseer

If they are still on the car, they might have taken a lopsided set, flat spots, from sitting, plus be hard. New set might solve your ride issues.

Looks like some solid maintenance. Good sign.

Ditto on odo. On one hand, its hard to believe some of these cars were used so little over 20 some odd years. On the other hand, seems like they tended to be "3rd cars" for many owners. More than a few of us have cars with documented lower-than-expected miles (for the model year).

cold_beer839

These cars routinely go through lean years mileage wise. Some owners come up on needed repairs and simply park the cars long term. That's why a lot of these cars are sold with little mileage over the last few years of PO's and a long list of needed repairs.

My '87 S4 had cobwebs under it when it was delivered and it needed pretty much everything done (according to the PO it needed nothing, i.e. he broke it off in my ***).

JoeTaylor

Thanks Guys,

-> Bill - You might be right about the tires, from the receipts/milage the tires are amost new, but 6 years old so there could be a balance issues and/or flat spotting. Visually, the tires look brand new with lots of tread. Mel (local contact for 928 OC here in Jax) called me yesterday and suggested a local tire shop that the owner knows how to balance tires on a 928, so I am going to check that out this week. If its the balance or the tires, I can take care of that pretty quick.

-> Charley - The Potenza's are still on the car. Odometer: I wondered that too, but the Ga State inspections (per Carfax and Autocheck) recorded the milage from year to year and the owner put only about 400-500 miles per year on the car - very consistantly for the last 5 years he owned it. The shop that worked on the car during that time for the previous owner is 300 feet from his doctor's office, so he would just drop the car there and let them work on it and pick it up when finished. I dont really think there is any monkey business with the odometer.

-> Chris - I agree, the tires might be lopsided or flatted from lack of use, I will get that checked, plus the tire balance checked out, ASAP. I think from looking at the records and the amount of money spent, the previous owner took good care of the car. That was also the verdict from the PPI at the local Porsche dealer as well. FYI, the 928 was the previous owner's "fourth car", he also has a Lexus, BMW, Audi for him and his wife - not including his kid's cars.

-> David - The repairs I did before driving from ATL to Jax were my choice, both the Porsche dealer and local shop said the car was fit enough to drive back to Jax, as-is. I really wanted the timing belt changed due to age and the CV boot fixed before I left ATL.

Thanks for the kind words and advise, keep 'em coming. The more I know, the more I learn.

VTY,
Joe Taylor

RSKY BIZ

Hi Joe-

Looks like you 'done good!'
Happy you got it all sorted out in ATL and are safely home with your new ride!
I thoroughly enjoyed my 'bonding' delivery drives home with each car.
Something about that 'first date!'

Glad to help.
Miles of smiles,
Doc

Bill Ball

I don't think there is anything special about balancing tires on a 928 other than the wheels are "staggered" starting with MY87 so the wheel width entries into the spin balancer computer need to be correct. But even with that, there are other competence issues when it comes to balancing tires with the same balancing machine from shop to shop. No need for road force balancing or anything more than a good computerized balance by a competent tech who is not a hungover teenager who hasn't recovered from the night before. One supposedly reputable local "race" shop tried to convince a 928 buddy whose rear wheels vibrated at high speed after a tire install and were found on re-check at another shop to be 1 ounce out of balance that 1 ounce was "within spec". Sorry, not so. The spec is more like 10 grams (1/3rd ounce). That shop is now off our list.

White Lightnin'

Joe- I had to return some Potenza S-03s due to vibration and it turned out to be a belt separation issue. I ended going to the Potenza RE-050A series. I had the Pole Position compound tires on the rear for increased traction and they wore out pretty quick

Check your tires carefully -no need to ruin the car or the experience of ownership with faulty tires...

bigmac

Joe, "old" tires may not be up to high speed usage. I think there may be some threads here about it

JoeTaylor

Hi Everyone,

You may be right Bigmac.

I ordered a new oil pressure sender and replacement hood struts online.

Hehe, the guy at the shop in ATL said the sender was $180, I bought it online for ~$45 with shipping. Struts were not bad either, bought those for ~$43 with shipping.

I just need to figure out how to "slither" up under the 928 to replace the oil pressure sender, haha.

VTY,
Joe Taylor

JoeTaylor

Hi Everyone,

Drove my sister and brother-inlaw up to Fernandina Beach for some seafood this evening. We took the 928 and zoomed up A1A from Jax. Broke 110+ mph with the car - while there was some vibration around 90-95 mph, the vibration went away above 100.

I think you guys were right and it might be the tires or tire balance. I am going next week to check the tires out and possibly buy a new set. I am thinking of buying some V-rated Michelins. I have run Michelins on all my cars with great results, but I am open to your comments and suggestions about tires for the 928.

Thanks again for all your help,
Joe Taylor

Charley B

First rule of thumb, Joe. Do not exceed 100mph on tires that have been sitting around for six years.

blown 87

QFT

Mike Frye

Joe,

Have you checked the tie rods at all? I've had situations where the tie rod was badly worn and it only showed up at certain speeds (and went away after you got above it). Also jack up the car and test the bearings (look up the test but basically if you grab the 12 and 6 and have play it's one thing and if you have any play at 9 and 3 it's another, I forget which means what).

If you have any kind of play in the tie rods or bearings it might be a situation that can only be felt at certain 'harmonic' RPMs.

Oh, and congrats on the new car and welcome!

jsherid1

Check the thrust bearing first, then work your way back to less potentially catastrophic causes....