Revisited, again. I have just reviewed prior posts regarding this subject, and I come to the conclusion that my SType 3.0 with 46K miles has either one or more bad coils or a vacuum leak. OK, so be it.
Before I take it to the shop (not a dealer!!) to replace all 6 coils/plugs/O-rings and spend probably $1000 or more for a fix,,,, Question?? Would a vacuum leak cause a drastic drop in fuel MPG as measured by the console readout??
Highway MPG as I remember it is about 27. The readout now for city driving is 16 MPG. Is that a clue toward coil vs. vacuum?? Thanks in advance, Mack

 

It'd be so much cheaper and more certain to get a cheap OBD tool which does live data, then look at the fuel trims. It's so easy, and no need to get oily or even lift the hood.

If it's got a problem it may be guessing the fuel so sticking lots in. Engines must not run lean, but rich isn't too serious. Tends to destroy the cat(s) instead.

 

OK. Prior to buying an OBD Scanner, can you tell me where the scanner plugs into the car?? I have looked around the steering column, the fuse boxes, the glove box>>> Cannot find the recepticle for the OBD. Help??

 

Mack look right around here:



And yes a vacuum leak would throw off your MPGs as many posts here would reveal. A VERY common vacuum leak for your MY (Model Year) is located in this link:

JagRepair.com - Jaguar Repair Information Resource <--Thank you Gus!!!

Go to any BIG box auto parts store and have the codes read for free and post back.

 

DUHHH. Got it. Thanks for the help.

 

One final (?) question to you guys. The symptoms, the car stutters at about 50MPH going up an incline, prior to downshifting. NO ENGINE LIGHT of any kind ever. The engine stutters one day, but NOT the next on the same incline.
Ordering either a Autel/Actron/Equus code scanner soon, per your advice.

The question? If one or more coils were randomly failing, would that not illuminate the "Check Engine" lamp on the dash during a failure? And if not, should I expect to see a code on the scanner that indicates a misfire anyhow?
I wanted the OBDCOM scanner, but it requires a laptop in the car to use it, not.
Thanks in advance, Mack
Oh, 2001 S-Type 3.0L @ 48K miles.

 

My car had 2 coil failures and both produced a code, I then replaced all 6 along with the plugs. I got them from NAPA auto parts. I would recommend adding a fuel additive BG44K and see how the car performs. With no codes you are chasing a ghost.

 

I would suggest if the MPG is low then if you're lucky it's running OL (open loop) which is a bit on the rich side and will slowly destroy the cats. If you're unlucky it's misfiring so badly it can't figure which codes to throw and will destroy the cats quickly.

If OL, there are few codes it can figure out to throw.

You probably have the usual air leak(s) at the usual hose(s) for your era car. Tons of detailed threads with fixes. If you want to check then get an OBD tool with live data. Cheapest is an elm327 and borrow a friend with laptop as needed. Look at fuel trims. See what they are parked at idle and 2500rpm. If you like, drive it and see what they are at 50mph.

Unlikely to be plugs at that mileage. They're not even due till 70K.

If not the air leak then coils.

Fixing hoses / coils is much cheaper than the cats...

Could be cats already

 

I got an Annova 3040. No DTC codes reported. Per your suggestion, I recorded Fuel Trim data as follows: (makes no sense to me)
800 RPM 2500RPM
LTFT B1 .8 -.8
LTFT B2 4.7 -2.3

STFT B1 -1.0 -3.1
STFT B2 -1.0 -1.8
STFT B1S1 -2.0 .8
STFT B1S2 99.2 99.2
STFT B2S1 -1.0 3.0
STFT B2S2 99.2 99.2

Any ideas??

 

If those are CL (closed loop), they're OK. You don't appear to have an air leak. (Pity: well known how to fix those!)

Are all the monitors Complete (aka set)?

edit: it's important that you checked with the engine properly warm - have another look if not - and that it was indeed CL

 

Mack... Fuel my curiosity... Get it out on an open highway and floor it. I mean mash the pedal to the floor (warmed up of course). See if you trip the CEL light then. If you do, read the codes...

ALso, have the O2 sensors ever been replaced? They are over ten years old you know. Old dirty O2 sensors will also cause poor MPG and poor engine performance without tripping any codes. See, they aren't "bad" just giving poor readings.

 

Good ideas. The first may help to detect failing coils, battery or blocked cats. So-called "lazy" O2s are indeed a real possibility. If bad enough you'll get a code but before that you can just get poor MPG.

 

Stutter problem seems to be resolved. Replaced all coils/plugs and all vacuum lines under the intake. Running smooth.
During all this, the CEL never came on, and the code reader never showed a code. I had the mechanic "break" something on the engine to see if the car is capable of throwing a code at all or maybe the CEL is not functioning?? Disconnected the MAF and pulled off a coil wire. Started throwing codes. That's good. But, but,,, the CEL never came on. Bulb check passed.
Something to worry about or not??

 

Sounds like one of the coils had an intermitent misfire - for some reason the CEL didn't flag it. I had the same issue - shaking bet 50 - 60 MPH - No CEL. I wound up purchasing a COP (Coil On Plug) tester and sure enough - cyl #4 coil was misfiring randomly at idle and I mean very slight. Sounds like you changed the coils so you should be good to go.

 

To throw a code the problem needs to match what's in the codes PDF (and in many cases dictated by your laws). Some misfires occur in normal running (like you go over a road bump etc) and the decision by CARB/EPA was to try to avoid endless false codes so I guess this car was just not quite able to flag it up. There are misfire counters but a cheap reader won't show them

 

Mack;
I have posted this before but it helps to understand why you got the results you did.

All engines have misfires all the time. Because combustion is not an all or nothing process it is a statistical process and there is NEVER perfect combustion. So the electronic control systems have to have a threshold of some kind or the dang check engine light would be on all the time!!

The threshold is related to emission standards and basically if the engine with this level of misfires is now out of compliance with emission standards the check engine light will come on. Note; In the not too distant future the cars will automatically report these emission violations to the US government so the owner can be ticketed.

So misfires are occurring every time you drive the car. Most of the time if everything is operating correctly they will be minor and not be detectable by you. Even though the electronics are seeing them. No codes stored. As the misfires get worse you will notice symptoms such as rough running or transmission shifting issues. You may or may not have a check engine light at this point.

Next level up is the misfires are bad enough to trip the check engine light but not all the time. So you might see the dashboard light on or not BUT you will have a stored code telling you where the system detected the misfire.

Next level up is a constant check engine light and by this time the car is obviously showing misfire symptoms when driving. Code or codes are stored and retrieved.

Finally if you are driving and have a flashing check engine light. The system is telling you that you might be damaging the catalytic converters if you keep driving around with all that unburned fuel getting burned up inside the cat instead of inside the combustion chamber!! Please DON'T drive with a flashing check engine light!!!

As you can see there is a wide variety of misfires and what they trigger.
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None of the vac hoses appeared to be bad. Replaced them anyway. The problem was coil(s) after all. The car is now running very well and MPG is already showing @ 4mpg improvement in the first 60 miles. It was an expensive process, but the results were good.
Thanks to all for the guidance. I don't do the work any longer, but the Jag Forum helped me a lot to understand the causes. Enabled me to speak intelligently to the technician before the actual work. Thanks again,, Mack

 

Thanks for the update.
It was probably a good decision to change the vac hoses when you were in there.