My 1994 4.0 xjs has a unique rough idle. It runs smooth, the stumbles a bit runs smooth stumbles again etc. I've replaced the valve cover gasket and changed all the plugs, new alternator and coil. Any thoughts?
It's the last annoying thing left. Did interior, awesome paint job, tires, tires, brakes, electrical gremlins all gone this is all I have left. I'd love to upload some pictures but apparently I'm not terribly successful. I guess I need to find someone under the age of 14 help me lol.

 

Does it do it from a cold start or after it has warmed up?

If it is after it has warmed up a bit, you might have a lazy O2 sensor which could definitely cause that idle cycling. If you back probe the o2 sensor at the connector with a scope or DVM, you should see the voltage rapidly cycling up and down between about 200 and 800 mV. If the cycling is slow you will need to replace the sensor.

Other things that can affect the idle quality are a dirty throttle body, poor connections at the MAFS, dodgy or dirty idle control valve among many others.

 

I think you nailed it. Runs smooth cold gets rougher as the car warms up. Is there a "right" o2 sensor to buy?

 

I use the Bosch 13313 O2 sensor, it's available at most big chain auto parts store.

 

If this fixes it you get a steak dinner when you come to ny!!

 

Here is a little check list that might help...

jaghelp.com: That pesky rough idle

 

Very kind but let's not count our chickens just yet

As mentioned the O2 sensor is just one of many possible answers and again I would encourage you to test the sensor first before throwing parts at it. If you have a voltmeter, it's very simple and quick to look at the voltage changing.

The old tailor's adage of measure twice cut once is sound advice when working on jag's too

 

Although I wouldn't call it rough, would the O2 sensor also make the idle revs cycle up and down when it warm?

 

Depends what you mean by up and down? About a 50rpm variance at idle is normal and it will be cyclic. The O2 sensor would be partly responsible for that.

 

I have a '93 XJS 4 L that has begun the bad habit of stalling when coming to a stop. The engine runs smooth as silk but sometimes stalls at the most inappropriat times. It seemed to get worse after I pushed the sport button. When the car slows down the engine slows with it like I have forgotten to declutch and when the car stops the engine stops. The car has an automatic xmission. Any ideas. Thanks. Jim from Iowa. stix66rb@msn.com

 

1. Dirty throttle body
2. Dirty EGR Valve

 

Could be a number of things. Does the transmission change down when you decelerate or stay in the same gear. If it doesn't change down you might be having an issue with the TPS. On 93 and 94 MY cars a twin track potentiometer was used in the TPS. One track communicates with the ECM and the other with the transmission. If the wiper on the transmission track is worn that might lead to some issues with stalling. You can measure the outputs of the wipers with a voltmeter, they should be identical to each other and vary from about 0.6v at closed throttle and 4.8v at WOT. It's an easy and quick check.

 

Thank you AllanG and Vee, I would love to carry out the suggestions given but being a retired oral surgeon the TPS and ECM , throttle body and EGR valve is all Greek to me. Hopefully a Jaguar dealer will know how to do these things. I really appreciate your time and expertise in answering my question as I really love the car. It has only 35K miles on the clock and looks new. Thanks again,
Jim
'93 XJS two place convertible (stix66rb@msn.com)

 

Whoa! Stop! Hold on!!!

Check out the attached picture!

The EGR Valve is #22 and is easy to get to and replace.

The throttle body is shown at #23. It's easier than it looks to get at and clean. The TPS (throttle potentiometer solenoid) is just under the throttle body. You could ruin it if you try to clean the throttle body in place. Oh crap, it even tells you that on the bottom of the pic.

The ECU, or ECM is the Engine Control Unit or Module. It's a silver box located behind the footwell carpeting in the passenger side. It's straightforward to get to, just some bending needed with screwdrivers and sockets.

If you need to look up what these parts look like, I find that an eBay search sometimes helps more than a Google search.

That being said, if and when you find out what is causing your problem, please let us know!

The EGR Valve will eventually kick out a code, so it may not be that. A dirty throttle body is common.

 

Thank you Vee for your tutorial and encouragement. When the weather finally gets better I'll try these thinks out. At the moment the wind chill is -54 and the car is storerd for the winter. I have heard that a fouled catalytic onverter might cause this kind of problem. Any experience with this?
Thanks again, Jim from Iowa. 4L Convert, '93, 35K on the clock. (stix66rb@msn.com)