Okay, I'm stumped

Sorry for the long post but this has me really stymied.

Usually I am pretty good at diagnoses but this one has me running in circles. Bought the Caddy 4 years ago and used it for the first couple of years as a tug boat for the camper. First trip to Myrtle Beach and back, noticed a slight growling at highway speeds but put it down to odd wind noise (bunch of plastic missing from front air dam and suspension pump cover). Did notice that it seemed to be RPM related??? Less in forth, more in third etc.

The following summer took the rig out east and on the return trip blew out a rear wheel bearing. Ah ha!! That must be the problem. NOT. After going through three sets of used axles (why do people store this stuff outside?) finally found a set with usable surfaces for the inner race. Decided to do the job at the same time as changing the gears (from 2:92 to 3:42) and had a drivetrain guy do it for me. After 40 some odd years I still can't set up a set of gears right. Figured I had the growling thing licked....NOT. If anything it was worse. Note the noise sounds a lot like a set of bearings going south but it still is RPM related???

Before going down to the Finger Lakes last month, I had my tranny guy take it out and he figured it was a worn U joint. Had a pressure gauge on the tranny and shift points and pressures are right where they are supposed to be and the fluid is bright red and clean. He assured me there is no issues. Took the driveshaft out again and sure enough, while it was still tight, the rear U joint had a hard spot in the travel. Ah ha!!! That must be it. NOT. Now the noise is getting louder but still not bad enough to be a concern so hooked up the RV and away we went. Now at 50-60 MPH in 3rd gear it not only is really noisy (with 5,000 lbs. of trailer behind)but it now is causing a body vibration when under moderate to high load. Arghh!

Found a good shop in Romulus, and a real old school gear head to check the U joint to be sure it was done right. No issues found so I had him replace the rear shocks that I didn't have time to do before I left. While he was doing that I noticed that the passenger side motor mount was totally collapsed and the clam shell was resting on the frame. Another ah ha moment. Replaced the mount then and there and checked the other mounts. They were fine. What we did find was that those new wheel bearings we installed? Now have too much play in them. So much for factory axles and American made bearings. Needless to say, none of this corrected the problem.

So after getting the RV winched out of a mud bog (did I mention it rained all week?) we set off home last Monday. Now that the noise is truly obvious, I decided to do some diagnoses on my way home (what else do you do on 81 in the rain?) and narrowed down the symptoms:

1. It is quiet up to about 1,800 RPMs and the real noise and vibration happens, starting at 2,300 RPMs, under moderate to heavy load such as climbing a hill. Lessens some what on downhill slopes or constant cruise but still evident at anything over 2,300.

2. Is more pronounced when torque convertor is locked up.

3. Discovered that it is not speed related. After having dinner in Watertown, tried putting it in forth and it almost totally disappeared. Still there but not nearly as bad and no vibration. Took the rig up to 70 MPH and as soon as it hit 2,300 RPMs, it all came back so it is engine RPM related for sure. Wheel bearings are another issue but it seems they are not contributing to the problem as is the rest of the drive train downstream of the tranny.

4. Had removed the center covers on the rear wheels so I could check for overheating. At no time was there any indication that the bearings were going bad. Just lots of play. They stayed cool all the way home.

5. The vibration is felt through the floorboards and not the steering wheel so we should be able to rule out front end. All the tires were balanced before I left and plus this is the third set of tires I have had on this car and the problem existed with all of them.

So after all that I am now really open to any and all suggestions. Still want to use it to tow the RV down to Williamsburg in September but until I get to the bottom of this, no camping for this family unless I can find a friend with a truck or an Avalanche with a cheap price tag. Tired of spending the vacation fixing the car. Your help with this is really appreciated.

Going to start another topic about axles. Must be a fix out there.

Bill

Will it do it without the load of the trailer on it?

Are the tranny mount and rear suspension bushings still good?

Do you have airbags in the back? Or some sort of helper spring? I wonder what your driveshaft angles are like when it's loaded up?

I was thinking transmission output shaft bushing or yoke, but that's going to vary with road speed, not engine RPM.

It could be a flex plate or torque converter problem, but first I would check all of the engine-mounted accessories.

Water pump, power steering pump, balancer, A/C compressor, mechanical fan and/or fan clutch can all contribute. It could also be the AIR pump, if it has one.

It could be a trans input shaft or pump issue, too.

I had a similar experience with my Bonneville shortly after I got it. In that case, it was the water pump warning me that it was about to disintegrate...THAT was fun.

toomanytoyz wrote:

Will it do it without the load of the trailer on it?

Are the tranny mount and rear suspension bushings still good?

Do you have airbags in the back? Or some sort of helper spring? I wonder what your driveshaft angles are like when it's loaded up?

Bill:

I can duplicate the situation unloaded if I really lay into it.  On the highway at the above mentioned RPMs, it comes back but not nearly as bad as when the trailer is out back.

Checked the tranny mount and it seems fine.  All the suspension bushings are intact but I haven't put a bar on them to check.  With over 1/4 million miles, I would suspect they are getting a little mushy.

Use airshocks, but between the trunnion arm hitch and the airshocks, the ride height is within 1/2 inch of where it is supposed to be.  Again if it was driveshaft angles it would be road speed related, not engine RPMs.

JoeT wrote:

I was thinking transmission output shaft bushing or yoke, but that's going to vary with road speed, not engine RPM.

It could be a flex plate or torque converter problem, but first I would check all of the engine-mounted accessories.

Water pump, power steering pump, balancer, A/C compressor, mechanical fan and/or fan clutch can all contribute. It could also be the AIR pump, if it has one.

It could be a trans input shaft or pump issue, too.

I had a similar experience with my Bonneville shortly after I got it. In that case, it was the water pump warning me that it was about to disintegrate...THAT was fun.

Joe:

Good idea about the flexplate.  Makes sense. From day one, I suspected the torque convertor but there is absolutely no evidence of any internal issues.  When the problem first appeared, I could sometimes get it to go away by cycling the lockup in and out a few times. That doesn't work now. Pan is clean, fluid is bright red and clean, pressures are where they are supposed to be and it shifts well. Never thought of the flexplate.

If I had a mechanical fan, that would have been my first stop.  Have had a few go out of balance on me causing some weird problems.  Air pump is on but it is electrical and only comes on when I first start it.  Had the front off the engine last year to replace the Opti and checked the WP then.  Seems to be in good condition (Delco replacement).  Also no temperature issues, even when pulling hard.

If it was trans input shaft and or the pump, would it not show up as bits in the pan or wonky line pressure issues?

Thanks for your help guys. Will check the flexplate and the rear end bushings next week when my friend with the hoist comes back from vacation.  May replace the left motor mount and tranny mount just on spec.

Bill

When I mentioned the AIR pump, I wasn't referring to the compressor for the air shocks. I meant the air injection reactor ("smog") pump. But I don't know if LT1s even have that pump.

I'd also check the belt tensioner and the alternator.

JoeT wrote:

When I mentioned the AIR pump, I wasn't referring to the compressor for the air shocks. I meant the air injection reactor ("smog") pump. But I don't know if LT1s even have that pump.

You need to research LT1's joe,.
the air pump is electric and only cycles for a min or so just on cold start,.
there is a tsb and a carb exemption for them now,. they can legally be removed,.

I,d check the ignition/fuel,. sounds like you have one/two laying down on a hard pull,.
the converter locks up in third and amplifies the problem,.

Just my best internet guess,. unless you back it up to the keyboard and then i'll have a listen.

Does the noise change when the road surface changes suddenly?Is your frame absoulutely square?The fact the vibration changes when the trailer is hooked up makes me think the frame,s not square.As far as the TC being cracked, I had a 70 corvette that had a terrible vibration that became worse with every mile.As I pulled the trans away from the engine the flex plate fell off and hit my friend who was helping me.That new flex plate completely changed that Vetts driving characteristics at all speeds.Are the rear tires wearing evenly with no edge wear or feathering?Get a legal pad and start a list and then work your way down it as you eliminate any possible problems.

I'll throw in rear differential.
At least that was the noise on mine last time...

phantom 309 wrote:

 I,d check the ignition/fuel,. sounds like you have one/two laying down on a hard pull,.
the converter locks up in third and amplifies the problem,.

Just my best internet guess,. unless you back it up to the keyboard and then i'll have a listen.

Nick:

In the past 4 years I have replaced plugs, opti, coil, injectors (twice), FP regulator etc.  Have done numerous hard runs running Datamaster with no indication of misfire.  Non of this has changed the issue.

Flasheroo wrote:

Does the noise change when the road surface changes suddenly?Is your frame absoulutely square?The fact the vibration changes when the trailer is hooked up makes me think the frame,s not square.As far as the TC being cracked, I had a 70 corvette that had a terrible vibration that became worse with every mile.As I pulled the trans away from the engine the flex plate fell off and hit my friend who was helping me.That new flex plate completely changed that Vetts driving characteristics at all speeds.Are the rear tires wearing evenly with no edge wear or feathering?Get a legal pad and start a list and then work your way down it as you eliminate any possible problems.

Car took a hit in the front right at some point in its life but it does look like it was repaired correctly.  Before committing to the car as a tugboat I carefully went over the entire chassis checking for abnormalities and weak points and everytime I get it up on a hoist I do re-check it.  The torsional stresses caused by load equalizing hitches do tend to crack the chassis either at the joining rails or the wheel arches so I tend to carefully check these areas.  Actually hoping its the flexplate.  Cheap fix.

Am on my second legal pad.

s1l1sc wrote:

I'll throw in rear differential.
At least that was the noise on mine last time... .

I definitely have issues with the rear wheel bearings but as I said before, this issue is RPM related.  Not totally ruling it out but if it was the rear end, the problem should be road speed related.

Thanks guys.

Bill