Strange problem on a newer car... Cannot find an explanation.

Hi all,
Please be patient and read the whole posting !

I have a 2000 Nissan Sentra with a 1,8l engine, AT. I bought it a year ago with 54.000 miles. Now I have 66.000.
After a couple of months of driving it (that's when I first noticed the problem), when I was slowly decelerating (very slowly releasing the gas pedal) from like 50-45 to 40 mph the car started to shake pretty bad the same way an MT car would if you would use the 5th gear at 20-25MPH. And I can "obtain" the shaking only on a perfect smooth road and more often when the engine/transmission has been running for less than 5 mins. But sometimes is very hard to get the shaking...
And let me remind you all the important aditional info:

I changed the trans fluid/filter and the problem didn't go way. The old filter looked like new, so I gues the transmission should be ok.
My car, between 40 and 50mph is in the last gear(4th) and fully engaged because I look at the rpm and it's somewhere around 1500-1700rpm range. If I go under 40 then the transmission wouldn't be engaged with the engine and the rpm falls at 1000-1100rpm.
My problem appears ONLY when using SOME of the engine's breaking power(slowly decelerating).
If I release the gas pedal completly, the problem would not appear.
If I use cruise control between 40-50mph the shaking doesn't appear.
The car runs really smooth at ALL the speed ranges, even at over 85mph, no noises or anything.
I feel NO vibrations at the steering wheel, ever.
When the shaking appears, the whole car feels is shaking, not just the steering wheel for example.
When I turn while driving there are no noises or anything unusual, at any speeds, even when taking really tite corners. (no CV-SHAFT, BEARINGS noise).
While the car is shaking, if I release the gas pedal complettly or if I accelerate, the shaking goes away.
I had the wheel balancing done.
I've done the oil changes by the book.
And the last thing, witch I think is really important is that when I first looked under the car (2-3 weeks after I bought it), I've noticed some grease at the caliper area. The clamp that was holding the outer cv-shaft boot (the bigger clamp) was loose, and I could rotate the clamp without too much effort. That's where all that grease came from ! But the boot was perfect, no cracks. I now I should have done this earlier, I refilled the boot with grease and changed the clamp on the boot only after 10 months after I saw the grease leak... When I took the boot off the joint it still had some grease inside, not much, and the joint looked really clean, no contamination or anything. But i'm reminding you, when I turn, there is no noise coming from the cv-shaft, like we would normally hear when a cv-shaft is bad.

What could be the problem ? The transmission, the cv-shaft ? :banghead:

I bought an extended 3 years/30.000 miles warranty from the dealer when I got the car and the problem is that it's very hard to find the perfect conditions of road/traffic to simulate the problem and demonstrate it to their technician... I know the warranty doesn't cover cv-boots, so I would want to know if the cv-shaft is the problem then I would buy a new one ($100) and change it myself and save a trip (long one) to the dealer, but if something else is like the transmission, then I would want to use that warranty.
Myself I've done all the work on my cars, with success (including a head gasket change on a 6 cyl in line, Maxima engine) but this is really puzzling me...

Please help me !
Thank you

I know you said there is no noise from the CV joints, but a loose boot for 10 months is a huge deal. I wouldn't be surprised if that was the problem.

Check all the motor/tranny mounts. Its common for the engine/tranny to make terrible vibrations under certain conditions if they're deteriorated.

Check all the control arm/strut mount points. A loose bolt from the strut to the spindle or a worn bushing on the control arm might make a vibration. Sometimes the geometry is such that it doesn't cause vibes in the steering.

Last and probably least, pull the boot off the inner CV joints and give the shaft a good try in all directions. Sometimes a previous bad CV joint can cause output shaft bearing failure in the tranny.

Those are my guesses.

""know you said there is no noise from the CV joints, but a loose boot for 10 months is a huge deal. I wouldn't be surprised if that was the problem""

Good one, I also think it is a cv joint. The conditions which you describe is called "float", and a CV joint with play in it will rattle around, and since it is connected to the trans, it'll make a lot of vibration. Bad trans & engine mounts would exaserbate the problem.

Thanks guys, I will look into these things, check them all and I gues if everything it's going to be ok, then maybe the cv-shaft it's bad in some way...
I looked today at the engine mounts : front,left and right and they all look ok. I still have to check the trans mount.
Have u seen cars with only 66k or less miles with bad engine/trans mounts ?
Is there any way I can check the mounts, other than just looking at them ?

But I need some more time (a couple of days) to check the rest... I'll get back to you when I know more.

I have one more question: If there is less than 25% of the grease that's supposed to be in the cvboot, does this mean for sure that the cv joint will go bad within a couple of weeks or months, even if there is no contamination inside ?
In my experience, so far, I dealt only with cv-joint failure caused by boot cracks and contamination of the joint... I've never seen a situation like the one described for my Sentra.
The documentation I found so far only tells about the cv-joint failure caused by boot failure...
Maybe you guys have some links available.
Thanks again.

jack the wheel up on which ever side the boot had the loose clamp, & check for play in your cv joint, but really, every bad cv joint i've ever heard if it's got problems, it will make noise under turning one or the other directions. i'd of thought it was a engine/trans mount too except you don't seem to have enough milage to have that be the problem. perhaps your torque converter is locking up when it's cold (since you said that's the only time it does it - when it's cold) & lockup torque converters aren't generally supposed to lockup until the car goes into closed loop mode - generally.

I looked today at the engine mounts : front,left and right and they all look ok. I still have to check the trans mount.
Have u seen cars with only 66k or less miles with bad engine/trans mounts ?
Is there any way I can check the mounts, other than just looking at them ?

I've seen them with 7400 miles with bad mounts and that's not a typo. Its not so much a function of mileage, its a function of time, deterioration from gas and oil, and oxydation and UV. They don't get much UV, but plenty of reflected light to shorten their lives. You can check them by digging at them with a screwdriver. If you can get a chunk off or if you see a crack or two, they're on their way out.

I have one more question: If there is less than 25% of the grease that's supposed to be in the cvboot, does this mean for sure that the cv joint will go bad within a couple of weeks or months, even if there is no contamination inside ?

A shortage of grease might cause short grease life or higher temps which would shorten the life of the joint, so the short answer is yes. The capacity of grease in the joint is there so that the grease outlives the joint. Its much like engine oil. An engine could run fine with only 2 quarts, but that oil will lose its viscosity and pH very quickly requiring oil changes more frequently. The extra grease in the boot is there to do many things like displace water, but primarily so the joint can cycle new grease through the joint as it encounters it.

The problem in your case is that loose boot. If it was loose enough to let grease out, it was loose enough to let grit and water in. May not seem like much, but it can be a killer. The boot flexes as it turns. The place where the clamp is (if its loose) will move back and forth. As it goes out, it picks up grit deposited on the bearing flange and scrubs it back in. If you take that boot off, my guess is you'll find that flange is polished shiny from the grit and motion of the boot. That grit gets picked up by the joint too.

Hi everybody,

AGAIN: The problem I have it's not a vibration (like the one when wheels are unbalanced or you have a bad tire or a tie rod or something is bad).
It's like somebody would "grab" the whole car and start to shake it FRONT AND BACK ONLY. It's not just a steering wheel vibration it's the whole car that shakes...
I have noticed that is more likely to start shaking when I drive on a slightly uphill road. (maybe 2-3% or less, zero % being flat road).
And like I said it doesn't always do it. Sometimes I can barely notice it.
I had a small rack&pinion leak (at the boots), so I took it to the Nissan dealer and they replaced it for me and now it's ok (under warranty). They also checked the cv-shaft and they said it's ok, it would be good to change the cv boot, they said (to me the boot is perfect...).
I just put 2 new Michelins, the other two tires are perfect !
The car runs perfect (except when the shaking appears)
I checked all the mounts and they are all perfect !
I drove the car in circles left, then right, many times and no sound comes from the cv-shafts. :banghead:
I went under the car and tried to spin the shaft with the wheel blocked to see if it has any "free play" but it's perfect. No movement at all.
A mechanic said it might be the transmision (this is what I'm most worried about).
Another one said it might be one os the sensors (I'd say less likely) MAP sensor, TPS sensor.
Another one said it might be the EGR valve.
To me it doesn't feel like it's an engine problem (Engine runs great, maintenance done by the book).
The problem doesn't appear only when the engine - transmision are cold, but it's more likely.
And I can sometimes feel it even at 50mph (but much less than at 40-42MPH, when a couple of times the shaking got really bad, that even the exhaust pipes started to make noise like somebody kicked them, that's how bad the shaking was!)
Is there any way the cv-shaft is bad, even if there is no noise coming from it ?
Should I replace (spend about $350) to replace the cv-shaft that had the bad clamp ?
Is there any way I could test the transmision to see if anything is wrong with it ?
What else can it be ?

Please, any ideas are welcomed ! Thank you !
What's wrong with my car ?!?

P.S.: Please read all my 3 postings to get all the info !

Nah, I'd say your CVs wouldn't cause that type of problem if they're as tight as you said they are. You're doing a great job of describing the symptoms, but this is one of those where seat time is probably the best diagnosis. What I might suggest is just taking the car to all the transmission/chassis repair shops you can and get a free estimate. You'll probably get 30 techs that say its a CV joint or loose mount. You'll probably get another 30 who say its the MAF sensor. Most of them will suggest the most likely thing that pops in their head, and a few will suggest the repair that makes them the most money. But you might get that one tech who says something like, "oh, yeah, its the transmission's 2-3 apply clutch pressure solenoid. Happens all the time in Sentras that were assembled in Haiko with the 1.8 engine" Then (although you're never sure of what it will be) that is at least an inspiring answer that isn't a carbon-copy response.

I wish I were closer to San Jose or I'd come drive it. Keep taking techs for a ride and the problem will repeat itself eventually.