Hey all!

I've been thinking about my jags idle and I cant seem to wrap my head around why the idle is great in park and neutral (maybe a tad high, like 900) but lower and lumpy in drive (600 - 700 roughly).

I mean the obvious reason would be that there is a load on the engine but some how I think that the idle should be in the same ball park in D as in N or P.

Now the stupid bit: I'm a young fella so I'm used to these fan-dangled new auto car in which you leave the car in drive with your foot on the brake at red lights. Should I be chucking it in N when stopped?

Follow up question: By adjusting the idle to bring up the RPM in D would that not also increase the park idle?

I think that someone has adjusted the idle to bring up the RPM in D to stop the car from stalling, so in my mind that points at a transmission issue? Amongst all of the other lumpy idle causes that I have read about.

Unrelated Question: I found a vacuum hose on the exhaust side of the engine bay that is not connected to anything, it just hangs down from the firewall, seems strange to me. Is this normal?

 

No question is stupid Harry, not everyone is an expert, I most certainly am not! Just a few basic observations though, until others are able to comment:

Most older gen automatics do idle significantly slower in D & R since they are under load. If you increase the revs when under load, it will correspondingly increase the revs in P & N. The idle speeds you've quoted sound about right.

Shouldn't be any need to select N at the lights, other than to improve the idle "comfort", or unless you are stuck there for an extended time.

The 4.2 XK engine does not idle very smoothly at the best of times, well known for it. A rough idle is more to do with engine tuning than a transmission issue.

 

Awesome thanks for the reply! You gave me the exact answer I was expecting haha, there was just this niggling wonder about maybe the auto trans doesn't like sitting stopped in D, which is why I had to ask.
It did used to idle quite a bit rougher but after I adjusted the valve clearance back into spec (0.13) it instantly got smoother, but not yet smooth.

Since then I have changed the pintle caps and all the O rings on the injectors which I'm hoping will also smooth it out a bit more. I have read that even a vacuum leak from the injectors can be a part of a rough idle issue. (Idle Speed Adjustment)

Plus all the fuel lines, coolant hoses and radiator which is why it hasn't been out recently, can't wait to get back on the road!

 

Hi Harry,

Vacuum leaks in my experience made the engine revs hunt, not idle roughly.

As Jagent says, old school auto boxes do drop revs when you select a gear.

I was always taught to not put an auto into neutral at the lights, something about they can get grouchy when you drop them back into drive, but that could be an old wives tale??

XK engines don't idle really smoothly so when someone makes a comment to me regarding this observation (note I did not call this an issue), I just remind them that it is a race bred engine and doesn't like idling around doing nothing!

Cheers,

Nigel

 

hi,
if you add your location to your Profile, and a picture of your car in your post, some of us can figure the model you have and the "market" equipment the car might or might not have; (Different "markets" (countries) have different engine equipment for emissions and whether it is a Daimler or a Jaguar version, etc.).

also: is it a 1980 XJ-40 body style or is it a 1980 Series 3 XJ-6 body style? (I imagine it is a Series 3 XJ-6 based on the 4.2 engine).

also post a picture of the hose that is hanging from the firewall, it must be a vacuum hose, not normal, it must connect to something. On the exhaust side of LHD 4.2 engined cars you have a vacuum tank vacuum connection, a Speed (Cruise) control unit vacuum connection, and other vacuum inter-connections.

 

Jose, it would have to be an early Series 3. XJ40 introduced in '88...

 

Hi Jose, thanks for the reply. Ill update my profile and add a picture when I get home.

The car is a RHD 1980 Daimler Sovereign 4.2, no aircon, no speed control. Hmm Ill have to find out where this vacuum hose is meant to live haha I'm sure that its not meant to be where it is now haha.

Being RHD the Vacuum tank is on the intake side, with the brake booster. The hose seems to be hanging off the vacuum operated heater valve (from memory, Im not at home currently) I wonder if there is any vacuum connections on the battery mount box thing, it has a fan but I can't remember if its powered or vacuum. Will check after work.

Thanks for all you're help people! This forum is so much friendly then the other jag forums I've read, so glad to get involved, and sometimes I can even answer a question or two

 

UPDATE: Ok so I found that vacuum connection, it had come off of a couple of nipples hidden down on the LH side of the engine bay, looks like it connects to the heater controls to supply vacuum to them. Thanks for the help again, I'll live with my slightly rough idle. Maybe I'll look into changing the vacuum hoses at some point.