So my engine has been running a bit rough for some time, not terrible, not stalling and not lacking power, but I can feel that it has a little more "shakiness" than it used to. I was thinking plugs / wires / cap & rotor, since the plugs and wires looked pretty old, - completed that a couple weeks ago. That seemed to cause little or no improvement, which surprised me since it appeared that mice had chewed the insulation off several spots on the wires down in the vee. Anyhow, the gaps on all the old plugs were way oversize - .050 to .060 - but they all looked more or less ok otherwise - not real dark, wet or fouled.

Now I'm wondering what direction to look in next. Its a Marelli car so I don't think the timing is adjustable in the normal way. Idle speed is about 600, there aren't any faults that are obvious, to me anyhow.

The thing that comes to my mind is the fuel injectors / wiring, and I am planning to send them out for cleaning this winter as they should have those original hoses replaced. Also those bushings in the throttle linkage are gone, I'm going to replace them soon, but don't really think that's the cause as they weren't there when it was still smooth either.

I'm hoping the wisdom of the group can give me some advise in how to diagnose this - I certainly don't want to just start throwing parts at it without a real idea of what's going on, and would like to start with the most likely culprits.

Thanks, Eric

 

I probably should have mentioned, it doesn't improve much if at all as the engine warms up. It also doesn't seem as rough at higher rpms, though it may just not be as noticeable. This didn't start as a result of some sort of "event" like overheating or anything as far as I know, it just sort of appeared independantly.

 

It's often hard to trace the cause of subtle faults. Your idea of having the injections professionally cleaned is as good a place to start as any, IMHO.

I had injectors cleaned on many of my cars. I've always felt an improvement.

Cheers
DD

 

Hey Eric -

I think some folks evidently forget to change the fuel filter too! (As well as air filters.)

Also: I know any discussion of "additives" always generates "interesting" discussions, but I really like the Lucas fuel injector/upper throttle body fluid. Always cheapest at the farm-type stores (e.g., Rural King) in the gallon size container. I add it during every tank fill.

Take care,

Jerry

 

Hi Doug and Jerry, thanks for your suggestions!

Doug, yes, I think it's about time, 110k miles and 24 years, even if that's not my problem I think it's due for the hoses at least.

Jerry, I'm hoping to go through the fuel system all the way through this winter, don't know if it's ever been touched. More just to renew the hoses etc and clean up, also not surprisingly there's some gas odor in the trunk / "boot". To me it doesn't seem like there is any symptoms of fuel starvation at highway speed so would the filter cause a problem at low rpms? I think it would be fine to replace it either way.

One other thing I didn't think to mention, I've been able to hear the fuel pump running for some time, can't remember now if I could always hear it or not. It doesn't sound exceptionally rough or anything but maybe a little "uneven", does this seem normal?

Thanks, Eric

 

Hey Eric - my fuel filter was in horrible shape - I bet no PO ever changed it before I got my Jag a few months ago! It did run noticeably better after the change!

As for the smell issue: see this link...

https://www.jaguarforums.com/forum/x...smell-xjs-111/

Take care,

Jerry

 

On the noise issue - perhaps this is on point?

https://www.jaguarforums.com/forum/x...el-pump-55988/

 

Hi Jerry, Thanks for the link, I have also done a lot of reading on that subject - the gas smell I have is definitely noticeable, but far from overpowering - hoping it's not a huge deal to remedy, or at least minimize. I'm guessing my filter is also way overdue to be changed, I'm hoping to go through the fuel system entirely from back to front this winter. My main impediments are a very busy family and work life, and a serious tendency to procrastinate. I am getting better at it though!

Thanks, Eric

 

Hey Eric - and I've learned my CAT is one jealous lover who demands attention quite a bit! Hang in there and good luck. Try the filter - it won't take you long. Jerry

 

First you need to establish if the engine is running ALL cylinders, and if not which ones are not firing.

Start the car and put your hand near each exhaust outlet, (Be careful here so as NOT to BURN your hand) you should feel an almost constant stream of exhaust, with no pulsing.

Pulsing is an indication of cylinders not firing. If this is the case let us know which side is not working correctly and we’ll go further into how to locate the dead cylinder.

 

Hi Warren, I checked the exhaust, neither side is completely smooth, like maybe the flow of air from a fan, but the right side ("A" side?) is definitely less smooth, at idle anyhow. Regarding finding the dead cylinder, did you mean totally dead, or just not firing correctly? I'm asking since when I changed the plugs none were really badly fouled, it seems like if it was really bad the plugs would show it. As I recall two or three were a bit darker, but to my untrained eye didn't look that bad. Gaps were greatly oversized on all.

I did save and mark the plugs so I'll go through them again more carefully and see which looked different.

Thanks, Eric

 

I have five generic suggestions. Never messed with a V12. Narrowly missed an opportunity a couple of decades ago???


1. Compression test. Said to be anything but easy on this engine. Essential nonetheless. All the tuning in the word will not fix a mechanical defect.


2. A vacum guage on each bank. Might isolate the missing cylinder's bank. Smooth on one bank, erratic on the other??? Erratic on both....


3. Vacum leak. Examine. And, remove and block at the source of each hose. Listen tot he engine! Any change?


4. New idea: Run the engine. Hold a clean sheet of paper behind each exhaust pipe. A bit of black soot tolerable. Wet fuel not so!!


5. Carefully examine the distributor cap and rotor. Particularly he center button for
free to contact or burnt or broken.


Yup, any engine with a weak or dead cylinder will "clean up" on revving??? the miss is merely masked...


Carl

 

Carl, all good suggestions. In hindsight maybe I should have done a comp test while I had all the stuff out of the way to change the plugs... but I didn't, wanted to get back on the road before winter, it comes early and lasts a long time here.

I did check the cap and rotor thoroughly, they both looked normal, better than I would have expected for their age, I also cleaned all the contact points up, also the cap itself.

Regarding the exhaust, I have checked it out, not with paper though but not a lot of soot or smoke, also no excessive fuel smell.

As for vacuum leaks, it would be no surprise to me if there are some, more surprise if there aren't really, the hard part for me is going to be finding them. I have found and corrected several in the past that were bad enough that I could hear them "whistling", harder for me to find if they are not that severe.

Thanks, Eric

 

Before you go and do a compression test I would the look at the easy stuff first, you may have a dodgy plug. If everything tests ok then do the compression test.

Lets diagnose the A bank. Pull ALL of the injector connectors off the B bank, the car is running on the A bank only. Then pull the connectors off the Injectors on the A bank one at a time, You should hear the engine change note if it does then that cylinder is firing so put the connector back on and move to the next one. If you pull one off and the engine note does not change then leave that one off as this cylinder is not firing.

When you have all 6 on the A bank, turn the engine off, pull the plug/s of the cylinders that are not working (check the gaps again) and connect them to the leads, leave the injector connectors disconnected. Start the engine and check the spark on the plug, alternatively you could swap the plug to a known good cylinder to check it.

If you are still unsure put everything back, start the car and take a video with your hand near the A bank exhaust and post it.

Let us know how you go…….

 

One thing I forgot and do this first is an Italian tuneup. I used my car daily and only drove 10km a day to and from work, having a bad RH O2 sensor mine runs very rich when cold.

After a good 110km/h run down the motor way she idles much better.

SO take yours out and rev the rocks off it. 100km/h in 2nd gear for a few km's and mix it up change to 3rd for a while then back to 2nd and see how it runs.

 

Hi Warren, first off thanks for all the advise, it all seems good to me and I'll try it as I can, unfortunately its back to work for me tomorrow so time will be somewhat limited but I will get to it.

I'm pretty confident that the plugs, or really the ignition from the distributor on down are not the problem, as I did just check and clean up the cap and rotor, and replaced the wires and plugs, with no real change in how it runs. Also to me seems unlikely that the coils etc would just affect one or two cylinders. It seems like a long shot that the new parts would be bad in the same way as the old, not impossible but unlikely.

Regarding the "Italian Tuneup", my general driving pattern seems pretty similar to yours, fairly short runs plus being stored for our (long) winters so it would be no surprise if there was some build up of crud. One question, are you sure 110km is fast enough? ;-) Most of my driving lately has been taking my daughter to school, and going to work, one is during "rush hour" so can't go fast, the other is only about 4.5mile/8?km on city streets. One other thing to mention, when I was changing the plugs I did try to look down into the plug holes, hard to see much of course but the little I could see of the piston tops looked pretty crusty, not sure if that's normal or not, or how accurate my vision was for that matter.

One question regarding running the engine with the injector plugs pulled, I assume you wouldn't advise me to do it if it were wrong, but could that cause any damage to the engine electronics? I seem to remember being told not run the engine with something disconnected unless that something was grounded or it could wreck ... something? Don't remember the details now as it was just something I was told, not actually did.

Thanks, Eric

 

I have a 91 xjs v12 with the exact same issue as yours. It also has a newer dist cap, rotor, plugs, wires, and a new coil. Air filters are new. I have to check the fuel filter. The PO redid the fuel hoses but did a poor job. I do not think he replaced the injector seals and I have a slight leak on one injector. Not sure if this would cause the rough idle.

Rough idle is hard to diagnose because a lot of things can cause it. I have random idle changes every once in a while which is seemingly an IACV. Also the tps sensor could cause a rough idle, along with other sensors.

I will be following your thread to see what it is. At first I thought I was being **** about it. Though, it is definitely a little shake when waiting at lights. 70% of the time.

I am a city driver so an Italian tune up is needed. I will do that soon.

 

Hoodun
If that is a leak of fuel to the outside of the engine, then I very strongly suggest you fix it before driving any further.
Greg

 

Vorsicht:


The Italian tune up must be done with a tad of care. Do some warm up laps at moderate speeds. then a bt faster and then more.


That coke on the pistons from short runs can ignite. Premature combustion. Might get away with it or might hole a piston....


I have to it for as pre SMOG test run for my car. Today, mebbe, mebbe not. Hate the process!!!


Carl

 

I tightened the clamp which seems to have fixed it. I still have to redo all of the hoses. He used carburetor fuel lines and did not attach them properly.