HELP....vibration at high RPM, and it's not the motor mounts either..

[low miler]

I thought I would have fixed the problem when I had brand new motor mounts installed, but that's not the case here......

5 speed GT vibrates and grumbles under heavy acceleration, but not until it hits almost redline. Runs great up to that point and then grrrrrr, rumble!!!

Naturally, I just assumed it was the motor mounts and had them replaced accordingly. My tech showed me the old mounts afterwards and they were definitely toast. The car revs perfectly now and is as smooth as a new car until the same exact point it shuttered at before. In the last 1000 RPM's, it does it's thing. I didn't get the car back until last Friday and now unfortunately my tech is on vacation until a week from tomorrow. So I haven't discussed this with him yet. So I won't have to speculate until then, I was hoping someone on here may have encountered the same situation as this before. This is a first for me, as it's usually always the mounts.

Anybody, anyone, Bueler?

Thanks, Todd

[Danglerb]

Just revving the motor with no load does it do the same thing?

When driving and it starts doing it, what happens when you let off the accelerator, does it immediately stop the grumble or does the rpm need to drop (leaving it in gear so the momentum of the car keeps rpm up a few seconds)?

If its not mechanical, some out of balance thing, spark or fuel is about all thats left.

[low miler]

Originally Posted by Danglerb

Just revving the motor with no load does it do the same thing?

When driving and it starts doing it, what happens when you let off the accelerator, does it immediately stop the grumble or does the rpm need to drop (leaving it in gear so the momentum of the car keeps rpm up a few seconds)?

If its not mechanical, some out of balance thing, spark or fuel is about all thats left.

Just revving the car produces nothing, even at the same high RPM's. The car has to be in motion for it to do it.

As soon as I let off the gas, it stops completely.

The car does have a slightly rough idle and is definitely missing a bit on the way up. As of right now, it's not running nearly as fast as your normal GT. Could the timing being off a little bit produce such a shutter as this at higher RPM's? I was going to have him check the timing when he got back anyway, change the plugs and if necessary, the plug wires. Could that be a possibility?

It does feel like something is totally out of alignment, like the driveshaft or something like that. I'm certainly no mechanic, nor do I pretend to be, so I'm simply throwing out things that it feels like it could be if you didn't know any better.

Todd

[Danglerb]

Originally Posted by low miler

Just revving the car produces nothing, even at the same high RPM's. The car has to be in motion for it to do it.

As soon as I let off the gas, it stops completely.

Todd

That says fuel or spark to me, might as well wait for your mechanic to return.

Chances are good dumping the brain error codes will tell you whats wrong.

[rhjames]

does this occur only in high gear, or when you reach max rpm in each gear?


some vibrations with tires/wheels don't show up until high speed (revolutions). again, is this at one particular speed, or at a particular rpm in each gear?

a bent wheel will show up immediately (vibration), and continue to high speed, so this should not be the cause.

Torque tube vibrations (bad bearings, bent shaft, and the like) will show up immediately as well, so probably not the cause. Same thing for axle shafts.

CV joints will make noise when going bad, lots more noise at higher speed. could this be what you're dealing with? (it's usually not just a high speed problem)

clutch/flywheel/crank balance, would also show up immediately, but would be more noticeable when the engine is under load.

engine misfire is another item, but ask yourself why it only does it at high speed (rpm's)? If this is the cause, it is probably not spark related, but fuel related. The fuel pump(s) may not be putting enough fuel volume to the engine for it to keep up with the rpm/load requirement. Another fuel issue could be filter, tank, pick up or line restrictions. G-forces can also come into play with this.

I have run many drag race cars that exhibited no vibrations or engine stumbles while revving/sitting still, but when you put a load on it (running it down the track) the fuel starvation becomes apparent.


these are just suggestions and are in no way a complete list of things to go through, but you can start with them.


good luck---

--Russ

[Danglerb]

If the car was sitting in my driveway I would .....

Jump up and down and dance around a few minutes.

Make sure the battery is good and fully charged.
Clean all the ground points.
Swap the MAF with a known good unit.
Replace the flywheel sensor.
Replace the temp II sensor.
Replace the "hall" sensor?
Run a can of Lubro Moly Ventil Sauber through the gas.

Send it to Greg if that didn't fix it.

[low miler]

Originally Posted by rhjames

does this occur only in high gear, or when you reach max rpm in each gear?

GOOD QUESTION. I'LL HAVE TO TAKE IT OUT REAL QUICK AND SEE IF IT DOES IT AT HIGH RPM IN ALL GEARS OR JUST 5TH.

some vibrations with tires/wheels don't show up until high speed (revolutions). again, is this at one particular speed, or at a particular rpm in each gear?

DEFINITELY NOT THE TIRES OR A BALANCING ISSUE

a bent wheel will show up immediately (vibration), and continue to high speed, so this should not be the cause.

NOT A BENT WHEEL

Torque tube vibrations (bad bearings, bent shaft, and the like) will show up immediately as well, so probably not the cause. Same thing for axle shafts.

I ACTUALLY FEAR THAT IT'S ONE OF THESE. IT HAS ALWAYS FELT TO ME LIKE SOMETHING WAS BENT OR OUT OF BALANCE IN THE DRIVESHAFT. I KNOW ABSOLUTELY ZILCH ABOUT MECHANICALS, BUT I DO KNOW IMPROPERLY BALANCED TIRES VS. A BENT DRIVESHAFT OR SOMETHING OF THE LIKE....

CV joints will make noise when going bad, lots more noise at higher speed. could this be what you're dealing with? (it's usually not just a high speed problem)

I DO KNOW BAD CV JOINTS, AND THIS IS DEFINITELY NOT A BAD CV. NO VIBRATION IN THE WHEELS WHILE DRIVING ETC. IN FACT, IT DRIVES LIKE A BRAND NEW CAR UNTIL YOU DUMP IT AND IT REACHES 1000 RPM'S SHORT OF REDLINE...

clutch/flywheel/crank balance, would also show up immediately, but would be more noticeable when the engine is under load.

THEREAGAIN, THIS WOULD SOUND MORE LIKE WHAT WE'RE TALKIN' ABOUT HERE. HOWEVER, I DID JUST HAVE MOST OF THE INTERNAL CLUTCH COMPONENTS REPLACED, MINUS THE DISC AND PRESSURE PLATE, AND IT WAS ALREADY BAD BEFORE THEY REPLACED THE CLUTCH COMPONENTS AND STILL IS AFTERWARDS....

engine misfire is another item, but ask yourself why it only does it at high speed (rpm's)? If this is the cause, it is probably not spark related, but fuel related. The fuel pump(s) may not be putting enough fuel volume to the engine for it to keep up with the rpm/load requirement. Another fuel issue could be filter, tank, pick up or line restrictions. G-forces can also come into play with this.

NOW WAIT A MINUTE. IT RUNS A LITTLE ROUGH AT IDLE, HAS 3/4TH'S THE POWER OF YOUR AVERAGE GT DURING THE ENTIRE WAY UP TO REDLINE. IT ONLY RUMBLES REALLY BAD AT HIGH RPM'S, BUT STILL RUNS LIKE CRAP BEFORE HITTING THERE....

I have run many drag race cars that exhibited no vibrations or engine stumbles while revving/sitting still, but when you put a load on it (running it down the track) the fuel starvation becomes apparent.

IT COULD BE A FUEL PROBLEM, OR LACK THEREOF, I GUESS. IF THE MOST FUEL IS BEING DUMPED WHILE UNDER THE MOST SERIOUS LOAD, THEN THIS WOULD MAKE SENSE.

HOW ABOUT BAD PLUGS AND/OR PLUG WIRES? WOULD THEY CAUSE A SLIGHTLY ROUGH RUNNING AT LOW RPM'S AND THEN BE CAPABLE OF CAUSING THAT SERIOUS RUMBLE AT HIGH RPM'S?

these are just suggestions and are in no way a complete list of things to go through, but you can start with them.

I CERTAINLY APPRECIATE ALL OF YOUR SUGGESTIONS. BOTH YOU AND DANGLERB'S. I'LL BE PASSING ALL OF THEM ALONG TO MY TECH UPON HIS RETURN NEXT MONDAY. AND I'LL CERTAINLY LET EVERYONE KNOW WHAT THE ACTUAL PROBLEM WAS ONCE IT'S FINALLY BEEN SOLVED. MAYBE THAT WILL SAVE SOMEONE ELSE THE TRAIL AND ERROR LATER ON...


good luck---

--Russ

THANKS RUSS!

[rhjames]

thanks for the reply Todd.....

As I have said before, I have little experience with the later 928's, but was just trying to cover some basics (auto mechanics 101) in this discussion.

just to be clear, when I mention fuel pump or fuel system related problems, I am looking at the high speed end of things....as if it is a fuel starvation issue. (when you say "dumped" I take it you mean wide open throttle?)

much as I hate to say it, you could also be looking at collapsed rubber fuel lines, too. We need to remember that this stuff is how old....? suction on weak fuel lines could also cause fuel starvation. Runs fine at idle, but open the throttle and the engine coughs and misfires from not enough fuel to the injectors....

if it's in the electronics, please let me know what you find for my own education.....

again---good luck.

--Russ

[Shane E]

I fixed a vibration in one of my 5speeds by changing out the transmission mounts, mine didn't growl, and it came on at around 4k rpm and up.

[low miler]

Originally Posted by rhjames

thanks for the reply Todd.....

As I have said before, I have little experience with the later 928's, but was just trying to cover some basics (auto mechanics 101) in this discussion.

just to be clear, when I mention fuel pump or fuel system related problems, I am looking at the high speed end of things....as if it is a fuel starvation issue. (when you say "dumped" I take it you mean wide open throttle?)

much as I hate to say it, you could also be looking at collapsed rubber fuel lines, too. We need to remember that this stuff is how old....? suction on weak fuel lines could also cause fuel starvation. Runs fine at idle, but open the throttle and the engine coughs and misfires from not enough fuel to the injectors....

if it's in the electronics, please let me know what you find for my own education.....

again---good luck.

--Russ

Thanks Russ, will do!

Todd

[low miler]

Originally Posted by Shane E

I fixed a vibration in one of my 5speeds by changing out the transmission mounts, mine didn't growl, and it came on at around 4k rpm and up.

Interesting! So exactly what were the symptoms anyway? Mine doesn't do anything at all until it nears the top, but it is slower than it should be. But then near redline, grrrrrrrr, ruuuuuumble! Tranny mounts, huh? Interesting indeed.....

But the fact that mine does idle a little rough and has half the accleration it should on up to the grrrrrrr, may either be something else, or two seperate problems entirely.

Good times!

Todd

P.S. From now on, I'm definitely going with the cars that gave me my infamous username......Low Miler!

[911blex]

Although not on a 928, I had identical symptoms after a repair shop replaced the clutch on a 1967 Ferrari 330 for me. Problem was solved by balancing the driveshaft and taking care of an alignment problem somewhere in the driveline. Unfortunately, by the time the problem was solved, the vibrations turned my once quiet transmission in a very noisy one (i.e. vibrations rattled it apart), resulting in a $7K rebuild.

IOW, I would try keep driving to a minimum until you get a driveline vibration corrected.

[Louie928]

I had a similar problem in a GT. It was the front driveshaft bearing had worked it's way back a few inches. IIRC, the front bearing should be 9" back from the torque tube flange.

[low miler]

Originally Posted by Louie928

I had a similar problem in a GT. It was the front driveshaft bearing had worked it's way back a few inches. IIRC, the front bearing should be 9" back from the torque tube flange.

Hi Louie,

Did yours show no signs until around 5-6000 RPM's? I personally feel that the fact mine is a little sluggish at idle etc. is unrelated to it's vibration near redline. Let's just say hypothetically that is the problem with mine, is this going to be an expensive repair I'm looking at here?

Todd

[Louie928]

Originally Posted by low miler

Hi Louie,

Did yours show no signs until around 5-6000 RPM's? I personally feel that the fact mine is a little sluggish at idle etc. is unrelated to it's vibration near redline. Let's just say hypothetically that is the problem with mine, is this going to be an expensive repair I'm looking at here?

Todd

Yes. High RPM and high road speed only. Worse when 6000 rpm and shifting to the next higher gear.

[low miler]

Originally Posted by Louie928

Yes. High RPM and high road speed only. Worse when 6000 rpm and shifting to the next higher gear.

Thanks Louis. Sounds damn near identical to mine. BTW, is this gonna cost me alot of dough to fix? The answer I'm hoping for is, "Absolutely not!". "Maybe $30 tops".

Todd

[low miler]

Originally Posted by John Taylor

The parts will cost next to nothing. If one bearing has migrated, you might as well replace all 3 of them. About $17 each if I recall.

Since Scotty's ability to transport items back and forth between the transporter room is movie fiction only, you will actually have to pull the tranny back, drop the torque tube, press bearings out, press new ones in, etc.

So what, we're only talkin' about maybe $100 total here? Sweeeet!

Not!

Todd

[Louie928]

Originally Posted by low miler

Thanks Louis. Sounds damn near identical to mine. BTW, is this gonna cost me alot of dough to fix? The answer I'm hoping for is, "Absolutely not!". "Maybe $30 tops".

Todd

How many $$ to fix depends on whether that is the problem, and how you go about fixing it. First, measure how far back the front bearing is from the end of the torque tube. You might be able to get a small wire in through the hole where you check the clamp bolt to see the distance back to the bearing. Otherwise, take off the clutch housing and measure back. If that is the problem, then you have to take out the torque tube to fix it. The easy way is to get a replacement from 928 Int'l. If you want to replace the bearings yourself, there are procedures written on how to do it. One procedure is written here, http://home.comcast.net/~gq-beej/928/ttrebuild.htm. You can do a web search on "928 torque tube rebuild" to find other procedures. I've never done it, but seems like a fairly common DIY task.

[rhjames]

Hey Todd,

If we're now talking about torque tube bearings being the cause of the vibration...and then also having sluggish engine performance issues, you know where this is going.

my best guess was both issues combined, but not necessarily the case it seems.....

sorry I couldn't be better at my diagnosis-

--Russ

[ROG100]

Todd,
How many miles on the car?
You say the car has a bad idle and gives you issues at the top end.
What is it like overall for a GT - good, bad or middle of the road.
I am sure Louis is right but I would want to at least swap out the MAF if it is the original and the miles are high.
Swap it with one out of your other car or cars - worth a try. Will not cost you anything apart from 30 mins with a srewdriver and 10mm wrench.
Roger