Hi all, this is my first "help with tech issue" post - I think I could use some direction here.
The car in question is a 1998 XJ8 base model with 127k on the clock.
I bought it with a bad trans - blown up input clutch drum, and did a full rebuild on the transmission right off the bat.
Once I could drive the car I could tell I had some tune up and drive-ability issues as well.
I replaced the spark plugs and filters (including fuel), oil change etc. and it didn't improve much.
After some searching and study I bit the bullet and ordered up a complete timing chain set and tools from Black Dog Manufacturing, and did the full timing chain/guide replacement deal.
So far so good, the car ran better but still a little rough. I had checked on replacement engines and subsequently decided I would see how far the current one would go.
About 2-3k as it turns out. It blew a head gasket, spit the upper coolant hose and overheated in fairly short order.
I found a replacement AJ-26 engine for what I consider an affordable price and just finished the installation last week, switching the new timing chains and tensioners to the new (used) engine and degreeing the cams for side to side balance. It needed the tensioners, having never been apart.
It's much improved now, and my apologies for the long history run-on, but anyone who offers help may want to know some of that.
I now have an idle issue to sort out.
Here's the scenario - Driving in traffic, fully up to temp and coming to a stop...the tach will drop to about 11 or 12 hundered RPM's - my foot off the gas and on the brake - and hang there. I'm having to use more brake than I feel I should, and I can feel the trans feeding lower gears in as I come up to a stop. Once I'm fully stopped, the idle will come down to 600 or so just like it should, (that might take 1/4 second, and then it's fine).
It's not terrible, nor would I consider it unsafe. But it is harder than need-be on the drivetrain - brake and transmission longevity are the main concerns for me on here.
I just got off the phone with a throttle body rebuilder who tells me I may have a bad speed sensor in the trans?? He could have just tried to sell me throttle body, but he didn't.
How would I go about checking further? I'm a little limited on scan tools, but I can get generic OBDll with my old Snap On MT2500 brick, and pin-out resistance values are no problem if I know what I'm looking for.
Any help would be much appreciated...'cause right now I'm
Thanx in advance, (anyone who reads this devserves that much).

 

I have a week on the new engine now - and noticed something new in relation to the high idle issue.
If I shift into neutral coming up to a stop the revs come down to idle immediately, as I would expect.
If I leave it in drive it still hangs at slightly over 1000 rpm's and bangs each downshift until I'm completely stopped.
I'm suspecting an output speed sensor in the trans more now than I did before.
It may be a bit before I have time to drop the pan and check.
I wonder if there's any common failure history on those?

 

Just a thought, this sounds exactly like some of our issues with the newly cleaned Throttle body. I'm assuming this one was never cleaned though? Is your car jerking into 1st gear coming to a complete stop, not wanting to downshift until you're just about completely stopped? Try cleaning Throttle body and reset the computer. hah i'm no mechanic, but worth a shot while you're waiting.

 

Yes it is doing the jerk on downshift thing, just like you say there NEWjagz. I looked the throttle body over pretty well when doing the engine swap - it doesn't look terrible to me, but I could clean the throttle plate edge a bit I suppose. Funny thing is it's got a pretty good throttle plate gap at the stock setting as-is. So???
I have to scratch my head on that.
I have no idea what the base settings are supposed to be for throttle plate and TPS, I would set it up if I did, just to be sure.
I also tried the hard re-boot on the ECM yesterday and checked all the ECM and trans control connections.
That was a treat. :-0 ...Some pretty weird shiffting on the first drive afterwords.
I would rather not pull the trans apart if I don't have to...but I will if that's what it's going to take to make it right.
I'll do some further throttle body checking today.

 

im having the same troubles on my 98 VDP. installed new hoses under the intake and by the firewall. had to take off the throttle body and cleaned at the same time. however, after reinstalling i had the high idle and jerky downshift problem.

took the car to the dealer on another issue, but asked them to look at the idle problem. they said it was a shot throttle body. i took the car home and did the soft and hard reset, but to no avail. currently, i am looking for a good used part. seeing them for about 150 to 200. i have been out of town the past two weeks but will try to install a new throttle body and hope it will fix.

will advise.

 

I had similar issues - perhaps not exactly the same though... Initially, I had some high idle speed events, then later some throttle surging accompanied by limp mode (no codes thrown), and lastly a code thrown during the surging which pointed to the throttle position sensor signal (TPS).

There is a Jaguar TSB about the TPS connector and its pins - the TPS pins are gold plated, and the connector's pins are tin plated, causing, I assume, some galvanic corrosion. Eventually the contact between the two sets becomes intermittent causing incorrect TPS readings at the ECU and my accompanying symptoms. Permanant solution is replacement of the tin pins with gold plated ones.

I found that by unplugging the connector and cleaning both sets of pins with electrical contact cleaner/lubricant (from Radio Shack) that all returned to normal. I will be replacing the tin pins with gold, but the cleaning has given me a temporary solution.

This would be an easy thing for you to do - clean both the TPS and connector's pins, to eliminate this as one possibility. They are simple to get to, just on the right side of the throttle body.

 

I do recall reading about the tin/gold connector pin issue, I didn't realize that was the connector in question at the time though.
Yes my symptoms are very similar to what you describe, even though I've jostled that connector a few times recently - it wouldn't hurt to give it closer look.
I am a bit miffed that the idle returns to normal when I shift to neutral - Every time.
Not one to look a gift horse in the mouth though, If can get it sorted out by repairing those pins, I'm happy.
Thanks for the tip - I'll post back with any results.

 

bob:
I think you are on to it with the speed sensor, or some other sensor, since the ecu is certainly adjusted to feed forward a higher throttle position for "in gear", and AC on, and probably others. Considering that you have decided to tame one of these beasts (and I understand that- I'm up to four Jaguars!) I recommend you either find a factory scanner (big $$) or get AutoEnginuity with the Jag extensions ($500.??). You can read many of the sensors, including the transmision speed sensors, valve commands, and status bits, and it will pay for itself pretty quick with parts you don't need to replace to troubleshoot!. Good luck and keep us posted.

 

My scanner situation could certainly be upgraded. Thanks for the ideas on those units sparkenzap. It sounds like the AutoEnginuity is a bit like the Vagcom for VW.
I think the newer Snap-On scanners are able to read Euro cars now too, I'll have to consider something though.
There's just too many pieces on this thing to do resistance and output checks on all the little bits, plus harness continuity - well with four of the beasts I'm sure you know how that goes.
I found these guys online a few days ago,
http://www.jaytanindustriesinc.com/jaguartb.aspx
they're within driving distance for me and they picked up the phone and answered my questions. I may pay a visit to have them check mine over and see what advice they can give on basic service and set-up of throttle bodies.
It seems really sketchy to me that just cleaning the throttle body can cause all the problems I've been reading about.
I have an pipe dream to ditch the servo throttle altogether for a staged 2 venturi TB. That would free up some ECM memory for other things perhaps. Of course it would need a whole re-map. Well, it's a pipe dream :-)

 

yea- unfortunately there is very little legitimate tribal knowledge of the Jag ecu for remapping. There are some european tuners who make claims, but most are adding external signal modifiers to either move the maps into a different range, or by pulse extenders.

There are legitimate concerns about throttle bodies, but cleaning the body only fixes a stuck butterfly. Cleaning the pins is apparently a very legitimate endeavor since it invloves throttle position feedback, which is a primary control signal. Same for cleaning the MAF which can give some help to gas mileage and so on.
Keep us posted.