Dwayne

I've had a significant vibration around 1200 RPM for some time. I seem to remember this vibration was present before I took the motor out and disassebled/reassembled it this last summer. It does it while parked, in neutral, and while driving. Is there a simple test or inspection for tracking it down?? I'm suspecting either the motor mounts (which I did not replace when I had the engine out), the torque tube or the TC bearings??
I researched some of the posts here and I have not noticed any noises (which may indicate TT or TC bearing problems). The vibration does smooth out after about 1400 RPM but doesn't go away entirely. The engine reves without hesitation and is responsive (more so than before the rebuild).
While reading posts here, I ran across some good information on checking the flex plate, releasing flex plate tension and checking for crank shaft end play. I plan to do these this weekend. THANKS for the help!

David L. Lutz

Dwayne,
On my '87 I had a vibration at around the same RPM and it was the motor mounts.

In my experience the Torque Tube is more of a varroom or grind at more that just one RPM, if not all the time.

Did you inspect the motor mounts when the engine was out. Mine were completely Flatened.

David

Mrmerlin

Put this in memory if you ever pull your 928 engine out, always replace the motormounts, There are a few good options here, the best seems to use the solid MM that are from Rock Auto,

When you do this please note that the stud that comes out of the bottom of the crossmember will probably hit the top of the steering rack and thus the stud should be shaved close to flush after fitting the securing nut. as a side note the TT will now fit into the hump better and the sealing foam will add some quiet to the ride, also check the transaxle mounts they could be sagging, and check the heat shields, and the exhaust system for hitting on other parts. the oil pan should sit flush or just above flush with bottom of the crossmember, when the mounts are good

Bill Ball

TT vibrations are usually worse at higher RPM. Idle area vibration points to the MM.

The simplest check for MM is to get under the car and look at them and at the oil pan and crossmember. The MM have top and bottom plates each have a "finger" that interlace. You'll see the "fingers" on the rear on the passenger side, and on the front of the MM on the driver side. When the mounts collapse the spread between the tips of the fingers increases. You can use your fingers as a gage. If you can insert more than one finger between the tips of the plate fingers, the mounts are collapsed. Also, if the oil pan is resting on the crossmember, that's confirmation. Even if it's close (less than 1/4"), it can make contact under load.

Sab

If you have some pressure at the flex plate after reinstalling the engine and bolting it all back together, vibrations can be amplified by it. I would defenitely check that first.

Dwayne

Good advice on simply replacing the MM when the engine is out - should have done it. When I had the mounts out this summer, I noticed the rubber was cracked but otherwise looked the same size and shape as a new pair I viewed on-line. Oh well!
I'll do an inspection on the clearance of the oil pan and and cross member and look for 1/4" or more. I'll also check the finger measurement at the MM plates. I'll try to get to this ASAP before the weekend. This weekend I want to check the flex plate tension and crank shaft end play.
THANKS for the tips to confirm MM failure. I'll let you know what I find.

Dwayne

OK...I took some measurements and some pictures of the clearance between the oil pan and cross member as well as between the MM plates.



The angle of the camera is deceiving. The actual measurement is about 1/3 of an inch.



I then looked at and measured from the bottom of the lower MM plate to the bottom of the "J" or hook that comes down from the top plate. And the measurement is about 1.375" or about 2 finger widths. Unfortunately, I realized I measured the wrong thing after I re-read Bill Ball's post. I need to measure from the lower tip of the "J" from the top plate and the downward protruding tip of the bottom plate. It will obviously be less than 2 fingers since 2 fingers fit snug with my incorrect measurement. Maybe a finger and a half.



I will measure again just to confirm. But given the oil pan clearance, it appears the MM may be OK?? What do you think? Should I look elsewhere?? I'm going to check the flexplate tension this weekend. I've also been testing the throttle response and I'm not sure yet if there's something here. But it's almost like a non-linear acceleration. I'd have to make measurements to be precise but it's like this: I move the throttle about 1/2" from idle and it gets me to about 1200-1300 RPM (the vibration region), I move the throttle another 1/2" and I'm at more than 2500 RPM - it really seems to take off with the same amount of throttle.
When I had the engine apart, I tested the Air Flow Meter (I don't have a MAF) and I remember at lease one of the terminals did not match spec from the manual when tested with the Ohm Meter. Should I take another look at the Air Flow Meter?? Could it be sending bad data to the computer when smaller volumes of air are flowing and causing a rough spot at the 1200 RPM range?? Just thinking out loud....
THANKS for any recommendations!

Mrmerlin

you dont need to measure anything just look at the bottom of the pan and see if its flush or in line with bottom of the crossmember, if it is just above or flush the MM are still good if below the crossmember then it indicates the mounts have sagged, the last car i did I could fit almost 3 fingers between the pan and the top of the crossmember

88porsche928

my car made similar noises while at idle and near the name rpm.

88porsche928

Hey dwayne can you pound on the intake manifold and hear the noise? It sounds funny but I did it to my car and heard the same noise from when it was running at idle and at the rpm range it made.

Dwayne

I just took a look - sorry, I didn't understand what you were saying in the first post Now I see. The bottom of the oil pan IS below the bottom of the crossmember/steering rack cover plate. And there's no way I can get 3 fingers between the pan and the top of the crossmember. So to make sure I understand, when I put the new MM in, I should see about 3 fingers gap between the oil pan and top of the crossmember?? If that's so, then the fingers on the MM bottom plate and top plate (as Bill Ball mentions) should actually be interlocking (i.e., touching) with new MM??
Do you or anyone else have a picture of a new MM installed - just to get some perspective?? THANKS!

Dwayne

It's not really a noise, just a vibration at about 1200 RPM.

Bill Ball

Stan: I've never seen 3 fingers distance between the oil pan and crossmember, new mounts or not. I must be misundertanding you and my fingers are on the small side.

Dwayne: With new mounts you should not be able to get a finger between the interlocking tips of the j-hooks. Anything more than 1 finger means the mounts are on their way out and two fingers means they are dead.

Dwayne

Thanks for clearing that up, Bill. I'd say my mounts are on their way out - there's a little more than a finger width difference but not 2 fingers width - maybe a finger and a half. Although they are not dead, I'll plan on replacing them soon. I need to go shopping for motor mounts...

Barry Johnson

Who was the guy that sells the correct solid Anchor mounts with a shim as a kit on here? Roger was his name? I need a set myself...