The crazy thing is it seems to run nice and smooth at idle and at higher RPMs...until it heats up. A really "hot smell" becomes noticeable and when a load is placed on the engine a "power loss" is experienced. If a cat is melting (and expanding) as it heats, then it would make sense there is backpressure building. I also have "gold dust" on the chrome of the rear bumper. It's probably a combination of everything possible!

But, and to your point, if it is a melting cat something caused the melt down. I inspected the marvelous Marelli distributor cap and rotor: no issues. So if it's not an ignition issue, it could be a fuel issue.

NEW THOUGHT
Another thought which came to mind earlier today was a raod trip it was on last week. Driving back from Las Vegas I had an opportunity to pass a string of traffic and really opened the throttle for the first time. The RPMs reached around 5500 on two occasions. These were not sustained runs, maybe 3-4 seconds at most at that level. But, my thought is if during those high RPM runs, the exhaust heat became high enough to damage a cat. Then, over the course of a few days of light driving, the effect is now being felt.

Any thoughts????

 

It's a coil.
The classic failure mode is it works fine cold. Then it starts to break up hot, and under load. Soon it will fail completely. Even though this isn't the classic "Marrelli Failure", the end result will be the same, no spark on 1 bank, raw fuel in exhaust system, cat meltdown and fire...

Dave
Atlanta
1994 XJS 6.0L Coupe

 

+1

Dave has you covered, right on the money.

Think of your engine as basically a marriage of two 6 cylinder engines, each with it's own ignition system. When you loose one system, it will still be smooth as glass; but it will feel like it's dragging a house. All the while fuel injection continues and pours raw fuel into a hot converter. Along with that raw fuel, it also pumps in all the oxygen that WOULD HAVE BEEN CONSUMED if it were running normally. Fuel, heat, oxygen......equals fire. The catalyst becomes a sustained reactor and I have seen them become white hot. Even seen them melt the insulation and carpeting in the passenger compartment. This is why its important not to run the car, at all basically, until it is diagnosed and corrected.

Good luck!

 

Interesting. I completely understand. But when last ran, I had exhaust , fairly even I believe from both banks. Of course I cant check this when driving! Could the coil fail under a load but not at idle or while revving in the garage? It's a relatively cheap fix, so I'll just replace them both and see. Thanks again.

 

COILS
Is there a preferred brand or better yet, brands to shy away from? I did a search and came up the following:
-URO
-BeckArnley
-Standard Motor Products (European Division: Intermotor)
-Airtex Wells (Aluminum Heat Sinks)

Thoughts?

 

SPARK PLUG CHECK
I pulled a few plugs from each bank tonight. They are so close it's hard to remember which came from where. A little lighter than I think is preferred, but they do have less than a 1,000 miles on them.

So the plugs are evenly colored, but very light to almost white. If I recall correctly, that would imply a lean mixture. I also have the original plugs with 39,000 miles (or so). They have a very similar look.

Could this play into my issue of popping and power loss? Could it destroy a cat over time?

 

Coils are under the most strain when they are at higher RPMs. At idle they may appear to work fine but once they are under a load things start getting wonky.

You will have exhaust on both sides regardless of the ignition. It may be cold air mixed with raw fuel, but it will continue to come out unless it becomes obstructed. Besides, as I said, you may have near perfect spark at idle so it will appear normal AND burn off any excess fuel built up on the plugs.

 

Well, two new coils later and the same result as before. Idles good. Even, warm exhaust on both banks. Clean, evenly colored plugs. Within last 1,000 miles: Plugs, Wires, Cap, Rotor. I reinstalled the used Marelli rotor.

The original and new coils measured resistance as follows:

Bank Primary Secondary Temp F
A 0.8 4.93 70
A 0.8 5.60 200
B 0.8 4.72 70
B 0.8 5.80 200

NEW A 0.9 7.10 70
NEW B 0.9 7.20 70

After returning from my 1000 foot test drive (at running temp), I let it idle in the garage (with FE in hand). With the hood raised I could hear what sounded like the AC compressor cycling; but it was off. Further investigation determined the Air Pump is cycling and is noisy as well when engaged (I didn't know Air Pumps cycled). With a stethoscope on the Air Pump, it sounds like dry, very rough bearings when it engages and only when it engages. Could this contribute to engine power loss? It cycles every 10 seconds or so but only seems to be engaged for 2-3 seconds.

 

Now, with coils eliminated, it sounds like you may be treading on an area that may be harder to diagnose.

First things first. The Air Injection can pulse on/off at times that's not bothersome. Continued operation above 115 deg. F coolant temp is. The Air Injection should engage on all starts, hot starts included for about 15-30 seconds. It should engage on all cold starts. It is there to light off the catalysts early and get them up to temperature. The noise you noticed probably means there is a pump in your future sometime soon.

It's also possible the culprit is a restricted catalyst itself. I've had to pull the O2 sensor out before to see if I get any power back. You can also check the catalyst temps side to side [with an infra-red thermometer] to see if you find one much warmer than the other, that can tip you off to a bad one.

Now, the fact you idle OK but loose power when under load has another possibility, much harder to pin down. The ECM fires the injectors in packs of three at a time. If you look at the wiring diagram you will see that the circuits from each injector pack to the ECM are one wire, but the wire splits and goes to TWO different pins at the ECM. That's because there are TWO sets of transistor drivers that fire [turn on] the injectors. Each pack has a TURN ON circuit and a HOLD ON circuit. The TURN ON driver fires the injector from closed, and then the HOLD ON circuit maintains the injector open for the commanded time, based on load. Obviously higher load means open longer. It's possible some HOLD ON drivers may have failed, which will mean the injectors are fired and then never get HELD ON for the remainder of the required time. That can account for the power loss under load, and the fact it seems to idle fairly decent.

Hard to follow, I know. This sort of thing is very hard to diagnose, and really requires an oscilloscope to find or prove. If you check with an injector light, it's going to flash just fine making you think the injector is firing OK. But if the HOLD ON portion isn't there, you end up with no power and a lean bank; scratching your head as to why.

There is a lot to digest; ponder that a while and maybe discuss with your tech.

See why I said find someone with Lucas/ Marelli experience??

Good luck! Keep us posted.

 

Thanks Steve. I appreciate the comments. I have been reading the Jaguar shop manual (not the most exciting reading by the way) and along with all the comments here, have learned a lot. What do you think about the Crankcase Speed Sensor and/or Flywheel Sensor? From what I've read, these too seem to be able to affect power loss under a load if they are failing. A lot to consider, too much for a backyard mechanic like me!

I think I'll take it to a "new to me" independent shop I learned of. The owner worked for Jaguar as a lead mechanic (or whatever the correct term is) for around 20 years or so and has great references locally. I just don't have the high tech tools to speed the recovery; I need to drive it and he has the tools/knowledge. I can R&R all day but as soon as an ECU shows up I am out of the ball park!

 

MELTED CATS SMELL BAD!

Well, as it turned out, my original thought was correct. I melted a cat! Here's what I believe led to the meltdown (btw, everything else has checked out fine). I mentioned this issue occurred shortly after I returned from a road trip to/from LV. During the return trip, I had the opportunity to open up the throttle for a short period a few times while passing strings of traffic. On each occurrence, I would reach a little over 5,000 RPM. The second time, I approached 5,500 RPM and experienced detonation so I backed off. Everything seemed fine. It was the next day when while reaching running temp from a cold startup did I notice the odd smell I mentioned. I shrugged it off as the top was down and I was in a neighborhood with countless smells all around. What I did not remember until I was educating myself on cat failures over the weekend and reading the likely causes was I reminded of the fuel additive (SeaFoam) I had placed in the tank when I fueled up for the drive home. BINGO! My fuel mixture was not only modified, but when I pushed her to high revs the subsequent heat increase coupled with the additive damaged a cat. Once that occurred, it was only going to be a matter of time until the damaged cat would collapse and eventually restrict the exhaust flow to the point of no return.

I replaced the primary cats with high efficiency OBDII cats by Eastern Catalytic and eliminated (gutted) the precats (the car is OBDI but the OBDII will add extra emission power to offset the precat loss). While at it, I also eliminated the muffler blocks under the passenger area. She sounds and runs real nice now!

However, it's always something! The CEL is now on so I need to check the code. I suspect an oxygen sensor or two were damaged by the additive or during the exhaust mods. Ordered a pair today!

Thanks for everyones comments. It's a learning experience for me and I do appreciate your insights.

 

@Kelly

Glad you are on the road to recovery. It does sound like an interesting failure mechanism. Any post with "SeaFoam" mentioned anywhere will generate bi-polar responses "It's awesome, I put it in everything I own" or "It's awful, I used it and [INSERT BAD THING IN HERE] happened"