Quick diagnostic question...

The engine, at idle, sometimes seems to shake; a lumpy idle. The RPM is consistent and according to my Ultragauge there are no significant vacuum leaks. Spark plugs are new. Fuel pressure is good, too.

It really feels like a vacuum leak, but the fuel trims are pretty good.
Any ideas?

 

"pretty good" ??? How about your spark plugs? Right number, right gap?

 

Bank one: +1 to +4 while driving/ +3 to +6 at idle. (LTFT)

Bank two: +2 to +6 while driving/ +6 to +9 at idle. (LTFT)

I have slowly improved my fuel trims over the past couple of months. Even when they were much worse than above, the engine idled smooth.

The plugs are NGK IFR5N10 - gap about .039 (thousanths?) - inches. The plugs were replaced 4,000 miles ago so when I say "new", they are "pretty new".

 

Check the engine mounts. In my case both were damaged and leaked. Replacing them solved the problem of shaking engine at idle.

 

Good advice on the engine mounts, I solved a similar problem on my BMW by replacing the engine mounts.

But, I inspected them and visually they look clean and shiny with no indication of leaking. I snaked my hand up to feel the rubber portion and it feels smooth and intact. Could they be bad and still look good?

 

If the rubber looks and feels solid and there are no leaks the mounts are most likely not the problem.

 

I am near certain it is your engine mount. I had the same issue.

 

Ok, so directly contradicting advice on the engine mounts, haha.

I am going to crawl under the car again today with my trusty inspection camera and try to get a better assessment of the engine mounts. Yes, they do "appear" to be in excellent condition, but all the symptoms point directly to them as the problem. I am even feeling front end vibration under high speed acceleration; would this not also point to the mounts?

Thanks

 

With your foot on the brake and in gear hit the gas a couple of times. If the mounts are bad you will see a lot of movement .

 

I just went out and did that, RJ.

Engine doesn't really move. Maybe a mm of play, but even when the front end was lurching from torque, engine was steady.

 

Try it in reverse and repeat RJs procedure. Could be only one mount. Long shot but possible. You said it was lumpy at idle. What is your RPM.

 

I will try that later, thanks.

Idle is steady at a little over 600 RPM, UltraGauge confirms it is steady.

Is it possible for a coil to be somewhere between good and bad? I thought they either worked or failed.

Maybe it's just bad gas. I will try a different gas station.

 

I have another thought but I hope it isn't your problem. I believe you did a timing chain and tensioner upgrade. Did you torque the cam sprocket bolts to their proper torque and not just guessed. I still learning these cars myself but if the sprocket slipped slightly it would throw the engine out of time and cause a rough idle. I might be over thinking but I trying to figure out what would cause a rough idle without giving you any codes.

 

I did the timing chain upgrade on the 2001 XJ8 that I sold.

This XJR is a 2003 model and when I took the cam covers off to install new gaskets a couple of months ago I was treated with the sight of big bulky metal timing gear and upgraded chains, from the factory. Everything looked pristine and the chains were tight. I don't anticipate having to mess with the timing on this car....ever! haha.

I did have to re-torque the driver side valve cover a couple of weeks ago, I suspected a leak because I saw a little smoke. Even though my fuel trims have not changed since I did that, is it possible that a small leak has developed in a particular spot that would create this issue? A leak is not big enough to significantly effect fuel trims but that could still cause a stumble?

Talk about reaching... haha.

Edit to earlier info I gave: The RPM is steady just a little over 650, not 600.

 

Harvest14,

Thanks for this post. EXACT same problem here, and have checked the same things you have including replacement of snorkel and full load breather. MAF also recently replaced before this shaking problem started.

I also keep coming back to engine mounts, but have not had the time to replace them yet.

I will watch this thread with interest.

 

H: Go back to Ross' cut at your problem. Double check that your plugs are all torqued correctly. Are you getting any misfire codes? a 1 mm engine rock when engaged, braked and raising the RPMs does not point to engine mounts.

Sometimes, when you have done lots of work, it pays to go back over everything you have done to be sure no mistakes or bad seals or alignments were done inadvertently. I am betting on a gasket leak from your gasket repair.

SMT: how close does his symptoms match yours? Does your engine rock when you hit the gas?.

 

The MY 99 JR I have now drive has never actually had an idle as smooth as my other Jaguars. Careful scope analysis does not point to a particular cylinder as the offender. I think injector cleaning and flow balancing will be my next try, if I decide to try some more!

 

I agree, Jim, that it seems most likely the cam cover seal is causing this issue. I want to rule it out because of the unchanged fuel trims, but it is just too coincidental. I am getting no codes of any kind, save for the pending P1111 of course.

I ran a can of BG 44k through it about 5 tanks of gas ago. My fuel mileage increased almost exactly 1 mpg but I could detect no other differences. I drive the exact same route to work with only a couple of stop lights for the whole 30 miles. My MPG has been steady for many weeks so when it suddenly showed an improvement I actually believe something happened. Perhaps the BG 44k changed the dynamics of the fuel injectors somehow while cleaning them? I don't know. I am now averaging only about 1/2 MPG improvement as compared to before I ran the BG 44k. Maybe it partially cleaned the injectors causing them to flow unevenly. Maybe I should run another can to clean them better.

I know my wife's 2004 XJ8 (x350) is only a year newer than mine and has over 30,000 more miles than my car, yet it has always idled sewing-machine smooth. I think maybe it just has a better engine management computer than the x308.

 

Jim, the problem sounds identical to Harvest's. I did do the engine "rock" test, and it didn't seem to move much. No codes on my end. I have also suspected injectors, but have not had the time to dig into them. Will probably do plugs anyway first since they are past 60K.

 

My guess is that something electrical connection or in the intake or vacuum system is not hooked up properly or not at all, it is easy to miss something when you put it all back together.

It really sounds like an air leak in the intake to me.
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