Vibrating/Rough Idle Issues
 

Lately the Jag has been manifesting some odd characteristics. First, there is a vibration at idle when stopped at a light. When placed in Park or Neutral, it lessens slightly, but its still there. Vibration worsens slightly when AC is on. The vibration isn't enough to make the car shake, but it definitely wasn't there before. This car used to be silky smooth. You couldn't even tell if the engine was on before!

Second, RPM drops down to 500 randomly at idle, then back up to 600 right away. It does this maybe once or twice every other day. Also, when turning the wheel in a parking lot, the rpm drops very noticeably, and it does this every time. Sometimes you can feel the car bog down at a stop when you lower or raise the windows even. I haven't noticed any light dimming issues yet.

The belt started giving off a whistling or chirping sound at startup recently, so I changed it out, and the noise is gone, but now I'm wondering if I should have replaced the idler and tensioner pulleys as well. I didn't even bother to check if they were any good still. Stupid, I know.

I almost forgot...there's also a loud whining noise when accelerating. It almost sounds like a supercharger. Does this point to the pulleys?

I want this car to be the smooth V8 it once was. Its nearly at 140k miles, so maybe this is just how the car ages? Every Series III XJ6 I owned had rough idle, but the XJ40 was smooth as silk even at 200k.

Any thoughts, tips, suggestions would be greatly appreciated. Thanks!

Hi XJ8JR,

I was recently reading about the active engine mounts in the X350, and your vibration symptoms sound as though they could be related to an engine mount that is not operating properly. The mounts are activated only at idle to reduce engine vibrations perceived by the driver and passengers. I'm attaching the relevant page from the Powertrain section of the Workshop Manual.

Hopefully others will have some ideas about your other symptoms.

Cheers,

Don

Rpm drops+vibrations+supercharger alike whine=time to check your alternator. These are a very common sympthoms of a failing alternator. Start an engine,turn or rear glass heater,drivers seat heater,headlights and heater blower. Check voltage and if it is below 13 volts then it's time to remove your alternator.

P.S. idiot light may not be lit even if an alternator is completely shot.

I'll look into that, thanks. Interestingly, my mechanic replaced replaced one of the mounts about 2 months ago. The idle vibration and whine was around before that, however, the dropping rpm issue started afterwards.

I was thinking it sounded like an alternator issue. I'll check that out asap. Is replacing the alternator a DIY job or will my mechanic have to take care of it?

I went ahead and ordered a new idler pulley and tensioner pulley assembly. I figured it couldn't hurt to give those a try. I'm pretty sure they've never been changed before.

It's DIY job. There's no need even to lift a car.

I took a quick look at the manual. Do you really need to lift the engine?

Nope. Detach a wires, unbolt an alternator, remove fans and shroud and an alternator will come out between an engine and radiators.

Wow. That's a lot easier than the manual describes it.

Can failing alternator pulleys cause these symptoms? Can an alternator pulley fail by itself if the alternator is still good?

No/No.

That rules that out!

Ok, so I'll test it tonight and report back tomorrow. Thanks very much!

UPDATE: So I tested the battery and alternator last night. The battery tested around 12.2 with engine off, around 13 with engine on, and between 13.5-14 with all accessories on and engine revving. So it would seem the alternator is not the issue.

That's a lot of miles. I would change the coils and plugs at this late date if they have not been changed.

They are a wear item and you got your use out of them!
.
.
.

I changed the plugs a year ago, and I was under the impression that coils needn't be changed unless they specifically fail.

I received the new idler and tensioner pulleys, so I'll be installing them tonight. We'll see if that handles the issue.

However, per Don B's suggestion, I have been doing more research on the active engine mounts. I read somewhere that you can test them by electrically disconnecting them one at a time. Any idea where I can these connections? The manual is very vague and I can see no mention of any electrical connectors.

I haven't crawled under our car to look for the connectors, but page 899 of the Powertrain section of the Workshop Manual shows the pin-out for the connector, so it must exist if your mounts are active. Page 880 also contains a description of the operation of both the passive and active parts of the mount.

I googled images "Jaguar X350 Engine Mount" or "...Mounting" and though a lot of images come up, none seems to show an electrical connector... ???

Curiously, the EPC shows that up to VIN 45653, an "Engine Mounting," part number C2C22659, costs $294.84 each.

From VIN G45654 the part number is C2C31215 is $184.80, and from VIN G49701 part number C2C31128 is also $184.80.

I wonder if this indicates that beginning with VIN G45654 the active engine mounts were discontinued? What else could account for the price of the mounts dropping about $110 each?

I just scanned the X350 TSBs but don't see one related to the active engine mounts. I wondered if perhaps Jaguar service departments might have replaced failed active mounts with passive mounts, but I think there would be a TSB if that were the case.

I'll be anxious to know if you find the connectors, and If I get a chance I'll crawl under our '04 XJR to have a look.

Cheers,

Don

how old is your battery? If it's a Jaguar battery the week/year code is stamped on the negative post of the battery. Anything over 5 years old - have the battery load tested. Anything over 7 years old / regardless of load testing - replace it. 12.2V off is low (should be in the 12.5+V range - engine running should be 13.5 to 14+ volts - good voltage w/ accessories running. Just my $0.02

The new battery is just a year old. The Jaguar battery was original, nearly 9 years old!

My numbers may be off a tad, I'll probably give it another test.

I didn't get around to replacing the tensioner last night, however I took a peek at it while running and noticed that it seems to shake a little bit.

I had the transmission fluid completely flushed and replaced this morning, and the car feels completely alien to me now! But in a good way. Acceleration and shifting is so much smoother. Its amazing how you can forget over the years how a car felt when new.

Well, I replaced the idler pulley and tensioner assembly last night. The old ones were pretty crummy. Unfortunately, it did not resolve the idle vibrations. However, it does seem to have lessened the rpm drops a bit, but not completely. I guess my next venture will be to check for any vacuum leaks.

Don B, for some reason I didn't see your latest response. Sorry for ignoring you. Thanks for the interesting info. I'll definitely update if I get a chance to crawl under and take a look.

I'm also in the middle of tracking down an annoying clunking noise from the front suspension, so I'm sorta going back and forth between that issue and this one.

I noticed a pattern yesterday regarding the random rpm drops. It seems to only happen after I've been running at highway speeds for a while. I was doing around 70 for about 15 minutes, and at the first stoplight after I exit is when the rpm fluctuates and the vibration is harder. Still not hard enough to make my mirrors shake or anything, but definitely not normal. Almost as if there's a random pulse emanating from the engine. Or likes its shivering ever so slightly.