Machupo

iTrader: (6)

Hey all,

Recently had my Z's rear end redone (replaced the old Cusco RS 1.5 with a new model -- long story). At any rate, i've put about 500 miles on the car in the past three months or so and started noticing that the car would jerk left and right last week. When I press on the accelerator I pass a certain point (acceleration load) and the car's nose jerks to the right. I usually have to slightly move the steering wheel to the left to compensate (drive straight). When I let off the accelerator (after a certain point, again) the car's nose jerks to the left (I have to bring the steering wheel back to neutral to drive straight).

Now, this has been getting more pronounced over the past two days and I'm only going to drive the car one more trip (to my local tool / mechanic shop). Wanted an idea of a couple things I could look at for problems. I'm going to change the rear end fluid at the same time (putting royal purple in). I'll also re-tighten all the mounting bolts on the pumpkin to shop manual spec.

Any other ideas? Anyone else had this weird problem?

My car has spent most of the past three months on the highway (at least 75% of the highway time in excess of 200kph), so this is very disconcerting (scary / dangerous) at these speeds!!!

Any help / ideas would be greatly appreciated!

Thanks in advance,
-nick

rocks

iTrader: (1)

You screwed up something in your suspension.

daytona350z

iTrader: (19)

^like that helps any...

well, its possible when the mechanics worked on your car, they may have adjusted some alignment bolts to gain acces to the diff, which can cause your car to veer left or right.

but, i had the same type of problem you did and it was all traced back to tire pressure. since a diff works when one wheel spins at a different rate than the other, it will try "work/lock" to the slower tire's speed.

this happened to me after i had tires mounted. there was a difference in 10lbs from the left to right side tire pressure and this is what caused my car to do, exactly as you described: veered one way upon accel and a differnt way on the decel. so check that first, then go in for an alignment to check the specs to see if they would make your car pull either left or right. so make sure they are equal from side to side, then go in for an alignment.


btw, what happend to your first cusco diff?

Machupo

iTrader: (6)

lol, thanks for the response

would having the ECU see different tire pressures throw it off? The two tires are within 3psi of each other, but one of the TPS isn't "hooked-in" to the car - i.e. I need to plug the car into a consult and get it to recognize one sensor (reads accurate in one wheel and -- in another).

At any rate, had the back two wheels off a week ago to check the brakes (had been glazing them a bit on the autobahn -- long-range decels from 280k to ~120ish), so maybe that screwed up my alignment a bit?



Long story short -- was having issues with a 3.9fd install with my original cusco rs (making tons of squealing), after three re-installs, decided to scrap the fd and went back to 3.5fd -- had continued issues with the LSD, so ended up replacing it as well.


Suspension is 30k miles old (4 years)... wondering if it is time to start replacing parts (very happy with the stock stuff, but noticing that it bounces a lot on the autobahn now).

Thanks for your ideas -- i'll check them out in the AM and report back with updates.

daytona350z

iTrader: (19)

humm....how long has it been since you had it aligned? that could aso be something to check out. your suspension should be still ok, and since you've got a euro spec car, i'd assume its the 2004.5+ suspension we have over here ion the states. dunno if its worn out, possibly sind you do drive at a much faster speed over there. (i wish i could drive the autobahn!)

i dont think the reading would affect it so much, just make sure the tire pressures are equal. good luck!

Machupo

iTrader: (6)

it's a 2003 USDM car (i bought it in dec '02 when i still lived in the US -- military)

Lol, i can't remember the last time i had it aligned -- probably a good thing to go check out!!!

rocks

iTrader: (1)

This happend on my porsche 928 before. I paid a guy to do the rear shocks after i did the front, compression coil springs without an impact wrench is horrible. He left 2 washers out that keep the coilover from sliding back and forth under the car. I picked it up and was like wtf did you do to my car. He said it just needed aligned so i took it over to my alignment guy and he told me he couldnt fix it. I took it back to him and made him fix it.

Machupo

iTrader: (6)

ok, that being said, i have a stock suspension with only 30k miles on it. The suspension has never been altered. This problem has been getting progressively worse over a couple days, which leads me to believe something is loosening or unwinding slowly as opposed to breaking. At any rate, i'll take these suggestions in stride and see what i can come up with tomorrow.

thanks,
-nick

rocks

iTrader: (1)

Something is the suspension is moving improperly. Such as a bushing has been removed or something like that. Some kind of washer. Was the diff taken out of the car to do the gears and lsd?

Machupo

iTrader: (6)

I imagine so, have to call up the shop to make sure.

Regardless, I was not feeling this movement a week ago, nor for the first two months, so it's not something that would result from a missing part (I would've felt that from the get-go). Maybe something that has loosened. I'll add some suspension checks to my to-do list (looks like a couple books of shop manual are coming with me tomorrow)

thanks,
-nick

rocks

iTrader: (1)

Maybe one of the diff bolts walked its way out if i were i would get under the car and see if it looks like anything is wrong.

ouch1011

I'd start with a good inspection and replacing the gear oil with one that has a limited-slip additive in it. Royal Purple should, but read to make sure. If it doesn't include a limited slip additive, you can buy an additive separately and put it in as well.

Machupo

iTrader: (6)

turns out the tire pressure stick i was using was defective, read between 33-36 no matter what i put it on -- luckily i had my autoX big dial pressure meter -- had 36psi in the left rear and 12 psi in the right rear. Evened them out and works like a charm. Now the question is how did the pressure get that low, so i'm going to watch it for a day or so and see if it drops.

Might go ahead and change the rear end fluid while i'm at it (this is the first fluid in there since the rebuild... 500 miles is a good break in, i think )

daytona350z

iTrader: (19)

glad u figured it out yeah, change the diff oil, its probably been working a lil more because of the difference in the tires.

btw, if your changing suspension and like the oem stuff, i would suggest the 2004.5+ suspension, or the euro spec. it'll give you a much better ride with less bounce.

Machupo

iTrader: (6)

awesome -- i'll have to see if I can snag a set -- thanks again!

Sharif@Forged

iTrader: (92)

I was just about to tell you to check the tire pressure. With my stiff sidewalls, its impossible to tell if a tire is flat. I had a nail in one tire, and 5psi of tire pressure in the left rear....didnt notice a damn thing visually. But the car would pull to the side, as you described.

daytona350z

iTrader: (19)

no problem always glad to help