I recently purchased an ‘89 XJS convertible that had sat idle for 15 years or so. The car seemed very underpowered and came with no service history.

So I decided to replace the spark plugs, fuel lines, fuel injector harness and have the injectors refurbished. In the process I also removed some nesting material, a lot of mouse turds and a fair amount of oil residue and melted wire insulation.

The car seems to have much more get up and go now, which is great.

But now there is a very noticeable vibration / shudder at low RPMs (below 2,000) regardless of whether the car is in drive or park. So it’s defintiely not the tires, wheels, suspension, transmission, etc.

Any suggestions as to what the cause might be or what steps I should take to better diagnose the problem?


The engine bay after all the work

 

Before going down any rabbit holes, just to clarify, you're not talking about a rough running engine,correct?

.You're talking about vibration in the sense of something being out of balance? Or perhaps vibration in the sense of two things vibrating against each other?

Cheers
DD

Aside:
-the cover over your cruise control bellows is installed upside down
-even if the engine is smooth running remove the distributor cap and check the rotor. This is important !!

 

Thanks for the heads up on the cruise bellows!

My first thought was that it was idling too low and on the verge of stalling. It idles at about 600-700 RPM. But the vibration is still there when accelerating from a stop until the engine goes above 2,000 RPM or so. When sitting at a stop, the vibration is quite noticeable. But it doesn’t sound or feel like anything is grinding together. More like the whole engine is wobbling slightly, if that makes any sense.

When I replaced the spark plugs, I removed the distributor cap and the rotor looked to be in good shape, but I didn’t know how to test it further, so I simply replaced th gasket and reattached the cap.

 

Hi Hedman

Maybe one of your Spark Plug leads is shorting out?

If you can start her up somewhere dark, then you might be able to see it, which you wouldn't be able to notice during the day

 

Could be a shot engine mount. Pop the hood with one foot on the brake and in Drive rev it if your whole motor lifts off then there you go.

Perhaps a video would help if you can upload so we see what your seeing...normally you can balance a coin in the engine with these cars.

 

Basics kicks in here for me.

Its a Marelli Engine, and they havde their own unique issues.

BUT

A V12 running rough and shaking, is cylinders not firing.

You have done heaps of GOOD things. HT l;eads is my guess.

Have the engine running, unplug ONE injector at a time, and note any difference. Simply "unplug, note any change, replug" is what I mean.

Once the culprits are found, fixing is simplified, BUT, I still reckon your HT leads are way past use by date.

OR

The leads are in the wrong sequence, rare, but we humans do get it screwed up at times, and blame the car, HA.

 

Agree with OB and Grant, an injector not firing, or an HT lead/spark problem.

 

Not to get off topic but how long should the leads last? I'm asking the Master as I've heard differing answers.

 

HAHAHAHA>

Down here that is like asking "how long is a piece of string".

There is NO rule as such, however, whenever I get, or appears in the driveway, a new to someone V12, HT leads are waaaaay at the top of the post purchase to do list.

Normal, daily type driver, good on ya Grant, about 5 to 7 years in OZ.

BUT

That depends on what you fit, and sadly most owners are after the cheapest. If that is the road you travel, 5 years maybe.

I ONLY use Magnecor, and they COST. The Red HE has been there since 1994, still good as. The 1964 S Type 3.4ltr, "guess" 1990ish, when fitted.

 

I've tried to acquire those, they seem to be NLA or backordered into oblivion. My cables are on the 70,000km mark. (Daily driver all weather)

if it gets any worse I'll be doing a very costly permenant aftermarket ECU with coil-on-plug setup and wash my hands of distributor nonsense! After all...you did say do it all do it once and do it right.

 

Should be fine, I average 35K kms per year, and HARD kms.

Just looked at that Magnecor site, oh dear, they have stuffed that up seriously. It is a UK mob, and I would have thought better of them. They list 14 leads for a 1987 V12, OOPS.

The Aussie mob appear in limbo at the moment??

NGK Leads, Nope.
TopGun leads, confusing also, with the "14" sneaking in also.

Memory, 1001 is teh Magnecor lead kit I used on all mine. Just make sure you get 90deg fittings for #1 both side spark plugs.

SNG USA would have OEM sets I am sure.

 

Grant, thanks for your comments. My plan was to replace the HT leads at the same time I did all the other work. I’ve ordered a set of leads from a supplier in the UK, but they’ve been hung up in customs limbo due to the Coronavirus. I got impatient and decided to re-use the old leads so I could get it back on the road.

Now regretting that decision...

 

These are what I use, but as Grant says, Magnacor are terrific too.
https://www.formulapower.com/product...-ht-lead-sets/

 

Not me! In all the decades I've been playing with cars I've never mixed up the plugs wires. Never.

Well, OK, maybe once.

OK, OK. You got me. I'll admit it. It was more than once. But definitely fewer than a dozen times. I'm sure of that



Cheers
DD

 

I'm a bit contrarian on the subject of V12 spark plug wires.

I've used both OEM Rists brand and, more recently, Bosch brand....which came from Ebay. Operationally they've worked perfectly well and they're much less expensive than the Magnacores and other. OTOH, I've not owned any single V12 Jag for 10-15-20 years so I can't comment on long term durability from personal experience. Nor do I have to. Grant's experience trumps mine, for sure!

IMO, the durability of most anything in a V12 engine bay comes down to heat. How long is something exposed to heat, and how frequently is it exposed. A V12 driven just a few hours a month versus one driven almost constantly....and for longer duration runs. A five hour, highway speed road trip is gonna result in a lot more 'baking' than some occasional puttering about for an hour or so at a time, two or three times a month.

Any plug wires that are more resistant to heat will last longer. I think we can all agree on that. But the higher cost products might not really pay dividends for every owner.

This isn't me disparaging anyone's advice nor any particular product. It's really just a reflection of how my own thinking has changed over the years. Once upon a time I wanted to lavish my Jaguars with only the 'best' stuff. I insisted on certain brands. Money was (almost) no object. Eventually it came to me that, in almost all cases, the fussing, lavishing, and extra money spent made ME feel really good but the car itself was totally indifferent to my efforts.

As a result I've become outright nonchalant about certain things, which is a welcome relief. But, in a bit of shooting-myself-in-the-foot, I've also created more work for myself in some cases. Rather than simply buying the best or most expensive stuff and "being done with it" I now spend more time analyzing my repair/maintenance strategies and purchases.

Sunday morning "ramble switch" now turned off

Cheers to all

DD

 

Thanks to everyone for their comments — especially Doug’s observation about the cruise bellows being installed upside down. After a long weekend, I didn’t have the energy to tackle much, but figured I could at least put the cruise bellows right side up. In the process of doing this, I noticed that the injector lead on cylinder 2A was disconnected! I must have accidentally pulled it out when zip tieing the injector harness to the fuel rail. Plugged it in and the engine purred like a kitten, with no vibration.

 

HA, BEERS ON YOU, WELL FOUND.

SIMPLE cars these V12's, say that many times on here.