Hi Team,

Not sure what is considered "normal" idle for my 96 XJS with the 4.0 in it? When I start it up, its around 1500 for a good few minutes. Then it veeerrrryyy slowly will come down to around 1100 (5 minutes?). There it seems to stay. I don't know a lot about these but hoping it's a turn of screw (like the good old days). Looking for any guidance on where it should be after its nice and warm.



Thanks all!

Mike

 

720rpm when cold.... 580rpm when warmed up.

Have you made any recent changes? The TPS will likely need to be reset. You'll need a device with Jag software on it to perform this. Maybe stick your hand under the throttle body and see if you can reposition the connection? Maybe unplug it and plug it back in?

 

Thank you Vee,

I’ll see what I can play with. If adjusting that does nothing and if I replace it, will the new one come “preprogrammed” with the correct idle settings? Or does it still need to be set somehow?

Thanks again!

Mike

 

Most likely reset. A new one is well over $400, so….

Yeah, finding a shop with a reset tool would be key.

There is a way to make manual adjustments. Search the forums about loosening the screws and manually adjusting the position of the TPS.

There is a possibility that you have a vacuum leak somewhere between the MAF sensor and the intake manifold.

There is also the possibility that the IACV is bad. This sensor is significantly less expensive. There is an o-ring that seems to be missing from most parts. Easy enough to get. I think a forum search will educate you on that too.

Lastly, the connection at the MAF may be dodgy. Maybe play with that connection with the engine running to see if that affects anything.

Actually one more thing, if you pull the ECU out from the passenger footwell, you may find some corrosion. It could be that as well.

 

Some folks have had success using a TPS from an older Mustang. Takes a bit of fiddling but seems to work. IMO $20 for the Ford part Vs. whatever the Jag part costs is worth a bit of fiddling. Search for more details.

 

Not for the 95/96 models.

If you are using JLM12074 you are SOL.

 

Vee - "Not for the 95/96 models."
I didn't realize they were different.
Do you know what the difference is?

 

The 95 and 96 models use JLM12074. No known substitute.

I’m not sure what model year they switched to that part. Maybe 1994, maybe 1993, but if that’s the TPS your car needs, then there is no known substitute.

I assumed only the AJ16 engine uses that TPS. The AJ6 use an older type? One that can collect fluids and such. It can be opened, vented, repaired. I don’t believe any of that applies to JLM12074

 

After recently spending nearly $3,000 trying to solve this same problem (can't pass smog in CA with a high idle in my 1996 XJS) I would recommend doing things in a certain order to avoid parts and labor costs as you hone in on the problem. My mechanic tackled the problem head on... replacing and cleaning all the possible causes before hooking it up to the software which was the expensive heavy handed way to go.
Yes it can be a leak, yes it can be a dirty throttle body or simply needing a TPS reset...it can be several things or a combination of things so my advice is start with the cheapest possible fixes first.

 

Good advice seacat...

Trying to find this special "computer" here in SoCal is proving difficult. Called a few of the local Jag dealerships with no luck. I called Rudy's Jag Service in Santa Ana and even he said they used to have one but it it stopped working and nobody could fix it. He said this would have been the easiest fix. He used to charge $50 and took 5 minutes to reset the TPS. Now nobody seems to have this proprietary computer to do these resets. I can buy a new TPS for $400 but then does that have to "set" somehow to? No idea...I'll keep looking. Did you get your idle issue resolved?

Thanks,

Mike

 

No. A new part will not fix the problem. There are manual ways to adjust the TPS. Search the forum.

 

I had this problem with my 4.0 coupe some years ago. High and erratic idle.
Took off the throttle body, associated air hoses, cleaned everything carefully and no more high idle.
Sometimes its a simple fix.

 

So an update on this...

I found the manual reset instructions which is the series of turning the ignition on and disconnecting the idle speed control connector. I left the idle speed controller disconnected the last time around per the instructions and started the engine...perfection! Idled right in between 600-700 rpm So I turned the motor off and reconnected the idle speed controller. Again, started up and idled beautifully and considered it fixed.

Took it go get gas and when I started it back up at the station, we're back to 1200 rpm idle. I tried the process again with the same results. Does this mean it's a bad idle speed control sensor or something else???

I was sure I fixed it but at least maybe I'm on the right track here. Thanks for everyone who has contributed with answers. Very much appreciated.

Mike

 

That's not as much of a reset as it is a test of the IACV. I think it means the IACV is good.

Been there, done that.

I am assuming you have taken off the throttle body and cleaned that already. Doesn't take much more than some rags, a can of carb cleaner, a 10mm socket and something to plug up the coolant hose that connect in and out of the throttle body. I use golf tees.

Again, search the forums for a manual TPS adjustment, or take it to a Jag shop for the TPS reset.

 

Yep, I've taken it all apart and cleaned it thoroughly but I'm about to do it again. Also checked for vacuum leaks but couldn't find any. Next is the manual TPS adjustment. Can't seem to find this holy grail Jag computer! Lol

 

My response is buried within the quoted text in bold.