I tried searching for this but all the key words I could think of return a ton of hits and false positives.

The car is a 1984 Vanden Plas, 4.2L XK engine, American edition (California actually). It passes California smog, FWiW.

If I start the car cold, it idles well.

If I drive the car and get it nice and hot, stop somewhere and shut it off for only a few minutes and get back in--it idles well.

If the car is hot and I stop somewhere and I'm gone for say, 20 minutes -- enough for the car to cool but not back down to cold -- it idles very low (tach says maybe 600 RPM) and rough. Continues on until I accelerate and the car gets back up to operating temp. I.e. if I drive a few minutes and it's hot again, it'll idle fine at a stop light.

Ideas on why the car is fitful in this middle-ground zone of not being either fully hot or cold?

It's been doing this for quite a long time--not recent, doesn't seem to be weather or season related.

Thanks in advance
-J

 

"Poor running after warm soak"

One possibility is that you have some drippy injectors. Over fueling the cylinders

A cold engine likes the extra fuel. And, the drips are not bad enough to cause a problem after sitting for just 5-10 minutes. But.....20 minutes of dripping might be more fuel than a warm engine can tolerate.

You could lift the fuel rail off the engine, injectors and all, and simply visually inspect for drips.

Cheers
DD

 

your 1984 could also benefit from a new Oxygen sensor with a separate Ground Strap. It stabilizes idling. When was the last time it was replaced?

Go to the O2 sensor located in the driver's side of the engine, in the exhaust manifold "Y".

The 1984 XJ-6 has a 1-wire 02 sensor, color of wire is black. O2 sensor looks like a spark plug fitted to the exhaust manifold.

If you insert a Ground Strap between the exhaust manifold surface and the sensor, then connect the other side of the ground strap to the body or chassis, it tends to correct this idling issue. After all, the factory did it starting in 1986.

you can see this fix in my website:
​​​​https://jagupgrades.webstarts.com

I did it to my 1984 years ago and the idling has remained stable even with an old Oxygen Sensor.

 

Doug: thank you. I'll check that out.

 

Jose: thanks, I'll try that. The car went through hell twice getting it through California smog. I assume the O2 sensor got replaced on one or the other rounds, but don't recall when. I'll look into adding the ground strap over the weekend.

 

an O2 ground strap might not resolve an issue caused by another component but it will help.
I know about California, lived there 3 years. Personally I am convinced the smog testing is a scam in any state. The 1980's Jaguars came very healthy from the factory. Next time you need to pass a smog test, drive the car for 1 hour or more before stopping at the smog station.

 

CA SMOG I A REAL PITA!!!!!!
A local car guy asks us to get our reps to support updating the cut off to newer than 83. i would like that. but, fat chance!!!!

When my car was a DOHC it had a sensor. Loose in the pipe. I doubt if t had any effect. It did pass SMOG, and ran well...

MY Lump idles cold at 1000 and warm at 500. No stalls, anytime.. Or any hot start issue.

The EFI on these cars is a foot off throttle start scheme. However, if the injectors dribbled and the engine is in a flood condition, open the throttle and crank. Added air may allow a start... And confirm the flood hypothesis...

 

"The EFI on these cars is a foot off throttle start scheme. However, if the injectors dribbled and the engine is in a flood condition, open the throttle and crank. Added air may allow a start... And confirm the flood hypothesis..."

Easy to Check Carl's suggestion, if not too much fuel, may be too little fuel see below thread... Would be interesting to see your CO value at idle on smog test.

https://www.jaguarforums.com/forum/x...%25-co-215769/

Rgds
David