Car History: Second owner car in perfect condition (as reported by 2 Jag mechanics) been in storage for 6-7 yrs.

Recently removed large rats nest (and accompanying urine and fecal material) on top of distributor and block, changed oil and radiator fluid, performed Kirby rotor mod (silicone under rotor) and replaced plug wires. Also rebuilt front calipers and replaced front and rear pads.

During test drive/brake burnishing the car makes thumping sounds I have NEVER heard before during acceleration. The sound is like someone thumping on the undercarriage. Could be described as "fart-like." It is VERY disconcerting. PLEASE HELP!!!

 

Exhaust pipe thumping the underbody?

Transmission mount broken/weak?

Cheers
DD

 

Everything looks pretty tight and intact under there...

 

Ours maked a sound similar to what you are describing and the local mechanic diagnosed it as a bad differential.

 

[QUOTE=ECBJAG;216971]Ours maked...... Ours made.... sorry

 

Considering it was the "local mechanic" makes me feel a whole lot better. "Bad differential" does not sound like a good thing. I bought this car in 1997 and had it completely examined/scrutinized by the most renowned XJS guy in So Cal. She had 60M at the time and I've put about 900 mi on her since then.

Maybe I should re-describe the sound - it sounds like low/mid frequency "popping," almost combustion-like. When you rev her at idle, however, it is nonexistent which typically indicates that it's non-engine/combustion based.

 

This is way, way out there, but have you ruled out detonation?

 

More info- Prior to this strange problem, she was idling at around 900rpm (after having turned the aux air screw about 85% inward). I have not touched the screw, but now she idles at about 600-650.

ALSO- I cut off the 1/2" stub underneath the rotor (as recommended in Kirby) and the stub flew-off somewhere in the engine compartment.

ALSO poured in a bottle of STP injector cleaner for the first time with a half tank of premium gas.

 

Wow, pretty weird.

Maybe you have a misfire. With V12 you can have a misfire and not feel it.

I had a Chrysler minivan once (no laughing, please) with a misfire problem that made a whomp-whomp noise in the exhaust under the car. Never understood why.

If it was me I'd backtrack over all the work just done. Maybe a couple plug wires mixed up or something.

Sorry I don't have any solid answers

Cheers
DD

 

Thanx very much, Doug!!! Also thanx to everybody who chimed in. I would have never re-examined my work as I am extremely detailed when working on Kitty. Per your suggestion, I discovered 3A and 3B were crossed...

Going for the REAL test drive tonight!
Cheers!

 

Ahhh... She's back to her old fast and smooth self again ! Phew!