It's a 1994 XJS V12 with 40,000 miles and has had all regular maintenance.

Tonight, while in normal city driving, I experienced a loud popping sound and what seemed like a loss of power. The popping only occurred under acceleration so the loss of power may have been due to my quick removal of my foot from the pedal! I made it home without issue and while in the garage, I recreated the popping by manually opening the throttle. It seems to be on the left bank. Again, it only occurs under acceleration. Nothing at idle or light driving.

I read one thread from awhile back with a similar description. The thread title asked what a dropped seat sounded like. There is not a regular or ongoing noise....so far anyway.

Any thoughts??

Kelly
1994 XJS V12

 

This sounds to me more like an ignition issue. Spark is going to a cylinder before the valve shuts. I don't know if this is a Marelli system or not, if it is that would be my first guess. Others will chime in I'm sure.

 

It's the 6.0L V12 which as I believe, is the Marelli system. The cap, rotor, wires and plugs were all changed about 1,000 miles ago. The cap and rotor are Beck/Arnley parts as are the plug wires. Plugs are Bosch.

I did have a British shop owner tell me I have a Denso system???

 

It shouldn't have gone bad in such a short time, but it is still a good possibility. You could have gotten a defective cap or rotor. It's a bit of a pain, but it's worth looking into.

 

I just went out and quickly, visually inspected everything. I did not pull the cap yet but will tomorrow I guess. There are no visible burn spots on the cap, no leaks (beyond normal). I started it and let it idle to temp. No issues. Revved it cautiously several times. No issues.

I do recall when I first left this evening and prior to the popping sounds, I experienced an usual smell, similar but not exactly like a fireplace burning. However, since the top was down I figured it was from a neighboring house. But, it did continue up to the popping. What does a buned cat smell like? And, would this popping not occur until the failing cat was fully heated? Again, only 40,000 miles on this XJS and it's been very well maintained. It looks nearly new all around.

 

Unusual smell

 

I think you need to find a new British shop owner. ;-)

You have a Lucas/Marelli engine management system, the Denso started with the '95 XJ12.

With your description, I would first suspect the probably aftermarket [meaning non-Marelli manufactured] distributor cap and rotor. These parts are notorious for very early failure. The Bosch plugs are suspect as well. Everyone touts the Bosch Platinum as the be all, end all of spark plugs. Then they don't put the correct ones in. Many times they put a plug in that is meant for a wider gap than the .025" intended for our system. That drives up the secondary firing voltage and usually manifests by burning through the rotor inside the distributor. The non-original rotor especially is already a weak link as compared to the OEM parts. That could be the cause of the popping back in the intake during acceleration.

This isn't the only possibility, of course; but it is the first place I would start looking, given the information you supplied.

Good luck!

 

Thanks for the replies everyone. There certainly seems to be a common thought here! I will pull the cap tomorrow. But assuming both the cap and rotor check out okay, what other areas might I tackle while in there? Could a coil be failing? And if so, might it only begin to fail when hot?

Also, what thoughts, if any, might pretain to a failed cat? If this is an option, would it also potentially become worse as it heats and then causing back pressure resulting in hard popping?

I will also perform the rotor guide post trimming proceedure and silicone per Kirby's book.

 

Popping in the intake is usually a lean mixture.

 

Thanks Greg. However, I don't know for sure if it's in the intake or exhaust. My initial thought was exhaust when I was driving and experienced it. From inside the vehicle, it "felt" almost like a random series of backfires, but the sound was coming from the engine compartment driver's side. Would that be considered the A or B side.

What could cause such a lean mixture? I know next to nothing about injected engines.

 

There were no denso XJS's, only the 95/96 XJ12 sedans.
If you inspect the ignition system, remove the rotor, and look at the underside for burned/melted plastic. Also I have found that when one of the coils first starts to fail, it is an intermittant fault for a while...

Dave
Atlanta
1994 XJS 6.0L Coupe

 

Geez, I wondered what made you make that comment about Denso XJS........

Then I rechecked my post. Stupid me..... I have no idea why I stuck /XJS on that Denso statement. Thanks for the correction. Yes, the last of the XJS V12's was the '95 model, but it continued the Lucas/Marelli engine management. The sedan got the Denso system for '95-'96.

I've corrected my post, thanks for keeping me honest!

 

Well, I inspected the cap and rotor and found zero issues. However, I did reinstall my used Marelli rotor just to say I did something.

HOT SMELL
After this mornings exercise with the cap/rotor, I warmed her up. No issues in the garage except I began to pick up on that unusual smell again. It's a "hot" smell. Faint, but there and new.

POWER LOSS
In my original post I mentioned I may have experienced a loss of power but wasn't sure due to the popping sounds and my reluctance to press the pedal. I did a VERY short drive this morning and there is a significant power loss. I can hear the throttle opening, but "no go". It's clear the engine is not happy with a load on it right now.

Any thoughts?

I guess it's off to the shop tomorrow.

 

You have definitely lost half of the engine. You are slowly but surely cooking one of your cats. Please do not drive the car. Get it towed to A workshop. Despite being completely in the know about Marelli failures I kept driving mine for about 1 mile and almost lost the car to fire.

MrTalkRadio did exactly the same thing a few weeks back and almost lost his hands trying to put the resultant fire out.

You have been warned.

 

Ktrumper, please heed Sarc's warning. Do not run the engine further. Catalyst meltdown and possible fire is eminent. Also, be sure to utilize a shop and tech EXPERIENCED with Marelli V12's. Someone not experienced with Marelli can cause COSTLY additional damage in attempting to diagnose.

To continue Sarc's theme........ second warning!

Good luck!

 

Or, you can "gut" the cats :-) and never have that issue again :-), not that i would even think of doing such a thing myself.

 

You guys are great! Thank you.

When I confirmed there were no issues with the cap/rotor and adding the "hot" smelly smell I was having the same conclusion - a cat meltdown. Another noticeable point as of this afternoon in the sunlight which I believe contributes to the cat diagnosis is I could see what appeared to be tiny gold specks on the chrome of my rear bumper. I think one could probably call these "Cat Guts"!

I thought these things were supposed to last "forever". I was just at the SEMA show and spoke with a few cat manufacturers who basicaly said just that, or at least twice the miles I have anyway. Of course I have no true idea of the actual maintenance by the original owner, or fuels used, etc.

I wanted to change out the exhaust system anyway but in my own time. If this ends up being the case, and if you were to design a replacement system which possibly moves a single CAT further back on each side, what combination of resonators/mufflers would be preferred? I would like a nice healthy tone but nothing close to an open header sound.

 

And of course, I do have to be concerned with emmision testing here in the Phoenix market. So another question, would a single cat on each side do the job?

 

Obviously you get what you pay for, but generally the newer cats are much more efficient than the older ones. You can run a single cat on each side, but you need to make sure you don't mount it too far from the engine because it uses the heat from the engine exhaust to bring them up to temp. The further away from the engine, the less efficient they will be.

 

My undestanding on the V-12 cats melting and causing a fire was that one bank of cylinders was not firing and letting unburned fuel into the exhaust system. If true, you need to find out why the plugs aren't firing to burn all the gasoline being injected.
RagJag