I have a feeling everyone's gonna tell me it's my torque converter, and I'm hoping someone can help me diagnose.

i have added a bottle of Lubeguard with no help.

i still have weird swings on my LTFT, right now it's at 6.3- both banks.

Is there anything else it could be, other than bad wheel alignment/balance?

by the way, it's a ZF 4HP24 with about 105,000 miles

 

Where do you feel it? Wheel balance can often be felt in the steering wheel if you hold it very lightly between 2 fingers, driveline imbalance often seems to be felt in the seat, in your lower back.

Is the vibration road speed or engine speed dependant? If you're at 50 mph in D and shift to N does it stay the same or change? What about shifting to 3?

 

When you get to that speed does it continue to get worse with speed? Does it drop as soon as speed is reduce below a point? Do you feel it in the gas pedal or the steering wheel?

Gas pedal is usually driveline, like the UJs and wheel is suspension/tires

 

I did get an EGR related code a week ago. I tightened down the bolts, even though they were already pretty tight and it was not as apparent this morning.

The whole car vibrates, to the point that a passenger can feel it...almost like its deciding between gears, thats why I'm suspecting the torque converter. I did try to press the brake while accelerating at the same time and the vibration did not go away. I thought that actuating the brake sometimes (temporarily) stops the vibration.

 

From your latest description, this sounds either like the gearbox mount going home. Take a good look at that. Or, asyour car is a 6 cylinder car, there is a bearing halfway down the propshaft that also fails. Look at this also. No 2 below

 

Vibration at speed that occurs at a specific speed is usually tire related. Flat spot, broken belt or out of balance.

 

I did change the gearbox mount about a year ago. Spring, bushings, everything that can go bad.

How could I check that propshaft bearing? Is there some way to diagnose that?

I am beginning to suspect that it really is a tire and suspension thing. I think I'm going to take that plunge first.

Will report back.

 

The 96 XJS apparently does not have a prop shaft bearing.

 

Problem solved!

I did two things that could have solved my problem:
1. I removed the EGR, cleaned it. It wasn't too dirty, but I brushed off the surface and sprayed the inside with carb cleaner. I also cleaned the holes coming in/out of the EGR cleaner at the engine. The hole going into the intake manifold seemed pretty dirty. Cleaned it up with a wire pipe cleaner from harbor freight, and everything was a-ok. I wish I had seen how bad the blockage was before wire brushing that thing out.

2. I replaced the EGR gasket (it looked pretty mangled) and smeared Hylomar AF on both sides of the new gasket, as well as the face of the EGR and the face of the part the gasket gets put on. Reinstalled and torqued, then loosened and re-torqued down to 19lbs-ft. (Book says 17-20)

All is good. The shudder is gone. My mileage is back where it should be. I will expect to see my LTFT drop back to a consistent value now.

I'll post back if something new pops up.

 

Hylomar was a bad idea. It cooked right off the engine to gasket side. Oddly enough, the asked to EGR was still thoroughly adhered together with the Hylomar.

I did exchange my 3 year old EGR that I bought from AdvanceAuto for a new one, under lifetime warranty, no hassle since I was able to pull the receipt off my purchase history on their website!

I also bought a tube of Ultra Copper RTV and smeared it on both sides of the new EGR gasket, tightened the bolts finger tight for an hour, then came back after an hour and torqued it down to 19 lbs-ft and drove home!

Mileage is back up to an average of 24mpg for mostly freeway driving! This issue, I hope, has been resolved permanently.