94' XJS 4.0 coupe sputtering at idle and driving

Hello. I just purchased a 94’ xjs 4.0 coupe with 110k. I have changed the spark plugs, distributor cap, oil, oil filter, fuel filter and air filter. I have also cleaned the throttle body with specified cleaner. it is running better than when I first got it, but it is still putzing/sputtering in idle and while driving. there are no lights on the dash except for the oil pressure light. the oil pressure gauge is in normal range. we think the light may just be bad (the car came with a brand new oil pressure switch in the glove box) because we keep checking the oil level and its right at the right line/level.

posting to see if anyone could advise further on what could be causing the sputtering

when I got the car it did have a full tank of gas and the previous owner said it had been sitting for a while. could the gas be old and causing the sputtering, also there is a small gassy smell coming out of the exhaust

thank you for any suggestions or feedback

Search this forum. There is a lot on this subject.

ok will do! I appreciate it!

particularly on how the OEM oil gauge pressure senders are notoriously faulty/flawed … :wink: Welcome to the forum, btw, and your new cat … same MY as my Superblue and about the same mileage … :smiley_cat:

Oxygen sensors. I’ll bet you what is on the car came from the factory. Its time.

Perhaps the Coolant Temp Sensor is bad as well. That’s a $20 part and 5 minutes of labor.

Would a bad O2 sensor really cause our cats to do that, VK? :confused: I think I’ve mentioned on here how I’ve had the JDS code # for “bad heated O2 sensor” showing on my dash for a LONG time now, yet I haven’t noticed any change in idling, performance, MPG, etc. Everything is “copasetic” … :crazy_face: By now I’ve just attributed it to a glitch in Superblue’s JDS (i.e. erroneous code) … :thinking:

So, I’m thinking now I must be right on that assumption, b/c if the O2 sensor were really malfunctioning Superblue would have running issues, like mentioned … ?

Absolutely. An oxygen sensor that is dying might not be bad enough (yet) to trigger a code. My car hates doling out codes, so I can’t rely on a code to indicate a failure, I can only appreciate a code to help me identify a failure. Basically “code = problem”; “no code = possible problem”. The lack of a code certainly shouldn’t ever be the reason to cross off a potential suspect.

A dying oxygen sensor will cause bad fuel adjustments to be made by the engine. It will do exactly what is being reported, before it gets worse, maybe bad enough to award you with that code. Now I will say, if the car is sputtering cold, then I would probably look elsewhere as the oxygen sensors shouldn’t be controlling the fuelling yet. Once warm, when the car is in open loop, would the oxygen sensors sputter the car.

Coolant temp sensor could cause this problem hot or cold.

Paul, your problem seems to be something else. I’m assuming you’ve changed your oxygen sensors without any change to the JDS code?

No, haven’t seen a reason to, since again I’m assuming there is really nothing wrong with them, for the reasons mentioned. I think there was some talk some time back that our facelifts have some kind of an issue, after time, with the grounds to the sensors being “iffy” … and that sometimes a double-wiring is needed running from the sensors to the grounds for a proper connection, or some such. Supposedly Jag even issued a TSB on this matter. ? Do you remember any of that? :confused:

But, you’re right in that if I change the heated sensors (and make sure the ground connections are adequate) and yet the code persists, then the JDS is definitely to blame. :thinking:

How long do oxygen sensors usually last?

And lastly, how long do oxygen sensors typically last? Older vehicles have O2 sensors that will typically last 30,000 to 50,000 miles, or 3 to 5 years. Newer vehicles employ sensors with an additional heated element and these new sensors are more likely to last to 100,000 or 7-10 years.Oct 2, 2019


Holy cow, so the car is telling you there’s a problem with the oxygen sensor, but you won’t change it because everything seems fine? Yikes.

You might be driving in closed loop and everything seems fine because the car is ignoring the oxygen sensor data and running on a preprogrammed fueling map. You’d be overfuelling the car, and you probably don’t realize it because you’ve had this going on for a LONG time. If you’re running in closed loop, you wouldn’t see a change in idling or notice a performance issue. MPG will improve, but hey, if you’re happy, then keep on trucking.

When I screwed up the ECU in one of the XJ40s I used on a charity run I replaced it with a non o2 ECU, I could tell every time the car tried to go from closed loop to open loop there was a hesitation where it looked for the signal, didn’t see one and went back to closed loop, and yes it drank fuel☹️

And, oddly enough, for some reason her MPG went up this past week to the highest ever, over 17 MPG (I drive only “city”, no “highway”, btw) … Usually I’m lucky to get in the 14 MPG range … Strange, and I’m wondering if there was just a glitch with the trip computer this time around (although the manual odometer is spot on with the “miles” shown on the computer) :confused: And, I ain’t complainin’, given that gas is now up to $4.60+ per gallon (reg.) here … Like they say, “if it ain’t broke, don’t fix it” … :cowboy_hat_face:

and, btw, VK … whenever I take a plug out to inspect, it looks the way it should … no signs of “rich running” … :thinking:

bottom line (and I’ve asked this question before w/o answer): is there a simple and easy way to test a (heated) O2 sensor to tell if it is bad, w/o removing it? :confused:

Have you tried a ‘hard reset’ ?
It might be that its an old code that needs to be cleared.

Looks the way it should? If thats the way you diagnose a failed part, good luck.

Most every sensor will look “the way it should” after failure. TPS, MAF, CTS, Thermostats will look normal, IACV, EGR, all of them will visually appear great. Sometimes like new.

I’m not sure if you can find out if you’re in open loop when driving, but that would be one way to know if the car was using sensors. The other way is if you get a code. The car is literally telling you something is wrong.

I’m not sure I’ve ever read a post where the car is giving you a code for a specific part gone bad and it was wrong.

Paul is talking about his spark plugs not the o2 sensor.

Ahh yes, my mistake. I see that now.

Still…you have an on board computer for a reason, I don’t understand why you would want to ignore it.

Agreed, easy enough to back probe the o2 feed to see if its operating properly.

I didn’t know we could do that … other that disconnect the battery and touch the ends together for a few seconds, you mean? :confused: Seems though she has always had a “faulty fault” on her readout almost from the day I got Superblue. First, it was the one for the fuel pump circuit (“resistance too high”, or some such). After a year or so, that one disappeared, to be replaced by some other supposed fault code (IIRC, it was for the TPS) … then a year or two later, that one disappeared to be replaced by the heated O2 sensor one, to date. :crazy_face: I put Superblue’s smooth running condition up against that of any other stock facelift 4.0, despite supposedly her having a bad fuel pump, malfunctioning/incorrectly adjusted TPS AND a screwy heated O2 sensor. :person_juggling: :laughing:

And yes, disconnecting the battery = the code goes away. Over time, though, it eventually returns. :angry: I am afraid though to try the “touch the cable ends together” thing b/c I have had some owners opine to me that doing so could cause some kind of damage to the car’s electrics/electronics. :grimacing:

I’m talking about the spark plugs, VK … Haven’t you ever seen one of those charts which shows with photographs what plugs should look like at replacement time? One of them is always, for purposes of demonstration, of a plug from a car which was burning too rich of a fuel mixture (i.e. covered with black carbon deposits, etc.). Superblue’s plugs do not look like that at all, or FTM, any of the other types of “bad” plugs in the pics … :bulb:

And can that be done with like a multimeter (i.e. resistance check) ? :confused: