For about a week now I have noticed at stop lights I am having to hold tighter on my brakes and seems my 1996 4.0L cat is idling fast sometimes if not most of the time, will also do it when in park (1100-1200 RPM even after fully warm). Today out of curiosity I shifted into Neutral while driving down the highway and the RPM's shot up to just about 3000 RPM with me not touching the gas at all. When I shifted back into drive you could feel the jerk as the engine was suddenly pulled back down to a lower RPM by the torque of the drive-train which was only going about 20 MPH. Tried it later at 55 MPH and the same thing. In fact if you don't hold on the brake the cat will do about 30 MPH without me even touching the gas at all.

First thought was a vacuum leak but during the time it's been doing it I have been all under the hood with a flashlight looking for cracked lines, listening for the sounds of a vacuum leak, etc. Even taking smooth jawed pliers and squeezing off one hose at a time listening for any change.

Then thought maybe a sticking TPS, etc. so tapped on the TPS and TB units with the handle of a screwdriver to see if it caused any fluctuation but no change.

From what I can tell the throttle is closing all the way (until the metal tab hits the stop) so it doesn't seem to be a case of binding and everything seems to be moving smoothly so that doesn't seem to be the issue.

I also tried plugging and unplugging every connector that I could think of and again no change.


Also checked the codes with my OBDII reader and only got a single code which was P1315 which on my reader translates as "No DTC Codes Defined", I cleared the code and checked again and it's still idling high and the code has not returned so I may have caused that code while unplugging and re-plugging things with the engine idling and so likely may not be related so the fast idle problem at all.


I'm at a loss though.... Pulling my hair out trying to figure this out.


Has anyone else had this problem (and what caused it) or does anyone have any ideas what could be causing it?

 

AJ16 engines are great, however they do have this one issue...weird idle problems.

1. Clean out the throttle body. Take it off and do it right. It's pretty straightforward and easy, just be careful not to spray carb cleaner into the TPS.

2. Check voltage at TPS. It may have slipped and rotates a bit and is reading throttle position incorrectly. Could be a bad TPS, but doesn't sound like it.

3. Reset throttle adaptations. You'll need to take it to a shop that has a PDU, WDS or something Jaguar specific.

4. Vacuum leak.

In that order.

 

Thanks, I will have to try those first two tomorrow. Have already done #4, and no one around here that I know of can do #3.

 

1. Like I said, pretty easy, make sure you have something to block the coolant lines that feed the throttle body. (You'll find it). I suspect this has a 50% success probability.

2. Do you know how to test the TPC? You will have to back probe the throttle body. I've drank too much to recall the settings exactly, but if you do a forum search, you should find the info you need. I think it was 0.5v at rest, but I could be wrong.

3. There must be a shop that has the right equipment...

4. I was unable to find a leak at my intake manifold gasket. One shop in three was able to find it and it solved an idle problem not similar to yours...just thought I'd mention it.

 

Block the coolant lines on the throttle body? You lost me on that one... Why would the coolant lines matter? They're not open to the inside part you would be cleaning as far as I know. Can't picture what your referring to with that one.

No I have never tested the TPC on one of these but can probably look it up somewhere.

 

https://www.jaguarforums.com/forum/x...econds-132409/

Check out that thread. I thought about one other possibility. Does your throttle body have an air bypass screw? My throttle body does not have one, but some seem to have it. I figured it'd be easier to have you read through this thread than to try to explain.

I'd like to add that the engine doesn't always immediately adapt to cleaned throttle bodies and repaired vacuum leaks. Give it a few days of driving once you do #1. The ecu has to relearn how to cope with the new values its receiving.

 

Wow that thread had a A LOT of replies, I read through all of them but don't think I am seeing what your wanting me to see, the bulk of it seemed to be about disconnecting your battery and people changing a whole lot of sensors only to have it not fix anything which isn't good. I didn't see anything about bypass screws, oh I also don't know if mine has one or not. Know where there is a picture of one? I tried a web search but didn't come up with one.

 

Sorry, wrong thread. See post #42 for the bypass screw that appears on some throttle bodies.

https://www.jaguarforums.com/forum/x...-126399/page3/

 

Yes mine has that screw, doesn't look like it's ever been turned since leaving the factory though also not rusted like the one in that picture. So since I have one what about it?

 

Oh, and I am about to have to make a trip in my truck to pickup something so I am disconnecting my negative battery cable and turned the ignition as though I was trying to start the car with it disconnected which usually drains any residual power from systems. I'm going to just leave it disconnected while I'm gone for a few hours.

 

I'd strongly suggest taking off the throttle body and cleaning it first. We can't begin to troubleshoot clearly without removing a clogged throttle body from the list of possibilities.

After that we can discuss the screw. It should affect your idle, but (a) since my car doesn't have it, I'm not sure it can solve your particular problem since you have an issue in drive and (b) if you decide to play with that screw anyways, make absolutely sure you mark its current position do you can undo anything unwanted.

Obviously I'm still a huge proponent of eliminating the TB before we try anything else..

Unplugging the car won't change anything, throttle adaptations are recorded into RAM, not some temporary memory database that needs power to retain data.

 

I had the same kind of high idle problem. I ended up replacing the throttle body, the idle air control valve, the exhaust gas recirculation valve and the crankshaft position sensor. I finally took it to a nearby Jaguar dealership, where they reprogrammed the idle adaptations. It's still not perfect, but it's much better.

 

The 96 XJS does not have an idle air screw. The screw you are seeing is for setting the butterfly valve (located fwd side of throttle body).

Idle is controlled by the idle air control valve (located on the aft side of the throttle body, side closest to the fire wall) via the ECU.

As Vee has stated cleaning the throttle body can help this situation as they are prone to coking up.

Most times the answer is to reset the TPS, which can only be done with a computer for these years, as the TPS is in a fixed position unlike older years.

I have experienced all the symptoms you have stated, and this was the only way I could reset the idle.

Craig.

 

The Mongoose is capable of resetting the TPS. I have a clone and have been successful.

I believe it would also be achievable by rotating the TPS while monitoring the voltage with a multimeter. I've replaced a TPS using that method.

Adjusting that screw did help others, but I don't have it, so I can't claim to be knowledgable.

I will repeat what I have been saying all along. Spend two hours and take the throttle body off and give it a thorough cleaning. If that doesn't work, we can move from there.

 

I had this issue on my 94 4.0 and a new IACV cured it.

note: I had to replace it twice, as the inexpensive part I bought the first time only lasted two months. I then splurged on the more expensive one and its been good for about 3 years now. the cheaper part is $18 and the more expensive one is about $200. you will see it on ebay. you could always go with the cheap one to see if its your problem and maybe you will be lucky with yours. if not, you are only out $18.

 

Finally had a chance again to look at it for a couple of hours today and it's still doing it. If it's idling in park it's running around 1500 RPM, if I'm driving and shift it into neutral while coasting it jumps up to almost 2500 RPM.

It's not a clogged or dirty throttle-body. It was clean to begin with (looked like a new car) and now it's spotless. Sprayed every sensor with the appropriate cleaner also, and then sprayed the electrical connector with electrical contact cleaner as well.

After doing all that I also tried the Seafoam treatment pouring it down into whatever that hole is in the top of the elbow (in the old days it would have been the PCV hose but since this car doesn't seem to have a PCV valve I'm not sure what it does now days) as it idled varying how quick I was pouring including pouring so fast I killed the car. Let it soak in 5 mins, then took off driving aggressively while the white smoke poured out the tailpipes, which went on for about 2 miles, and then drove it for another 20 mins. No change.

Checked again for vacuum leaks, listening, unplugging and capping things, squeezing lines closed with smooth jawed pliers. Nothing changed it. One thing that did confuse me was I squeezed the vacuum line going to the back of the fuel pressure regulator on the front of the engine and that also didn't change the idle speed any holding that hose closed a couple of mins. Assumed it would after a short time, but don't know if that's normal or not since I have never tried it before on one of these.


Is there anything down low on the engine that could cause this, or cause a vacuum leak? This started about the same time I had a local shop with a lift put on a temporary hose on my transmission line while I try to find a new factory metal line. Which I still haven't found one and still need one but just being a rubber line instead of rubber/metal on a transmission cooler line should not affect idle. Though I wonder if he could have knocked something down low (that I can't see without having the car on a lift) loose. Problem is he has his lift tied up now so will be a week or more before I can get it back on his lift to look under there. Anyone know of anything down low he could have knocked loose? He was struggling pretty hard with that original metal line getting it out...

 

I would take the idle air control valve out (Two screws and electrical connection) pull slightly on the plunger to increase the distance that it sits in the orifice when fitted back in place.

Refit it with out the electrical connection, start the car and see how it idles. If it is better remove again and adjust a little more until the idle is where you want it.

Leave the electrical connector disconnected. If the car idles properly this eliminates vacuum leaks and other items. IACV is controlled by the ECU, with out the electrical power and a fixed position it then points to setting up the TPS with what ever computer
you choose.

This is how I manually controlled my high idle speed until I reset it with my Genrad 3500.

Craig.

 

Is there supposed to be a gasket between the throttle body and the intake manifold on these cars in 1996?
(I noticed yesterday that mine did not have one)

 

Nope. Metal on metal, no gasket.

 

That's bizarre, Before you responded and after some searching I found a part number for a gasket which says it's for 96 model 4.0 throttle bodies.

OEM Part # EAC9822.

But my car is low miles and no sign anyone has ever had the throttle body off before which would indicate there never was a gasket like you said, yet I found all those references to one online.