Sometimes the engine in my 1996 XJ6 races (RPM in excess of 3000) when it should be idling. For example, the engine may start and idle fine and I'll start down the road, and when I come to a traffic light I'll have a difficult time stopping the car because the engine wants to race. If I then put it in neutral it'll sometimes "idle" at over 3000 RPMs. If I continue my trip this condition may persist for a while or, moments later, it may be normal again. The condition comes and goes, and I haven't been able to establish any pattern to it. I don't think it's mechanical, as it comes and goes so quickly without my having done anything to it. About the only clue I have is that it seems to be related to cold weather; it doesn't happen on warm days. I live in New England and, prior to this year, always put the Jag away for the winter. This year I've had to drive it all through the winter. It can be a really exciting ride when there's snow on the road and your car wants to apply power to the wheels while you're trying to stop. Any ideas (other than 'don't drive the car in the winter")? Thanks.

 

Your problem points to a potential issue between the Idle Air Control Valve, the throttle angle and the ECU. Is the check engine light on?

 

The four best possibilities, in no particular order:
1) Intake leak- the throttle is being bypassed by air and the engine management is correcting the fuel to match
2) The ISSC valve is sticking. Check archives here or on jag-lovers.com but dont cycle the power to the car with it connected but dismounted. It will eject the pintle!
3) The TPS is going bad and does not let the ECU know the throttle position is closed and the ECU therefore does not attaempt control the idle
4) The throttle spring either did not get the update in the early recall or the throttle plate bore is gunked, so the throttle is not getting fully closed.

If you need troubleshooting help for any of these, check archives, or check back.

 

I have an OBDII reader, but there's no CEL and no codes. It seems unlikely to me that an air leak would come and go as suddenly as this does - no? I have tried manually moving the throttle while the engine was off and it seems to move (and return) freely, but I have not tried it while the racing condition existed. I'll do that and report back.

 

Sparkenzap: I think we can probably rule out #4. I just went downtown on a quick errand, and on the way it happened. I pulled over and got out and manually operated the throttle, in particular applying extra force (beyond the spring pressure) to closing it. NO EFFECT. The engine continued to race. After parking the car and doing my errand it started and idled a little high (around 1200, but not 3000+ as it had been) and continued like that for the entire return trip. Thanks.

 

I does sound as though you've ruled out Ross' #4, and also a sticking accelerator cable, which I was going to add to the possibilities.

To me, the symptoms don't seem consistent with #1 either - I can't recall hearing of an intake air leak that was intermittent to the degree you describe. If your car is equipped with a carbon canister, it might be possible that the purge valve is sticking open,

Another possibility is that the diaphragm in the fuel pressure regulator is failing and allowing fuel to be inhaled into the intake manifold, but the resulting rich condition might be more likely to choke the engine at idle rather than cause it to race.

Ross' ideas about the TPS and Idle Speed Control System are still looking good...

Cheers,

Don

 

If it's idling too high and you push/force the throttle back, with no effect, that suggests that the butterfly valve inside the throttle body is gunked up and sticking. So, while the external part of the accelerator has returned back to its' "Stop" position, the internal part is still partly opened. If you remove the air intake hose/bellows, where it goes into the throttle body, I think you'll find a heavy deposit of gooey material inside. If so, it's an indication of what's gotten into the throttle body itself.

Removing the throttle body itself is easy, just unhook accelerator cable/ cruise if fitted, a couple of bolts (no gasket fitted) and you will see the butterfly valve inside. A build up of gunk prevents it closing fully and so, you get the high idle. As a follow up you can fit a stronger throttle return spring, or adjust the present one, as described and covered in numerous posts here. A little search ( high idle) will uncover lots of info.

Hope this helps.

 

When I got my Jaguar the cruise bellows was bad. It idled normally. When it got fixed it started to high idle, 1200-1500 Rpm.

When I looked at it it just didn't look right so I pulled the pin and disconnected it. Problem solved. A little bit of looking and yes the pull was connected wrong. When corrected it was no longer a problem.

 

I agree that it does sound like an intermittent electrical problem. Is your OBD-II tool able to monitor live data? If so it will be a simple matter to drive around until the problem occurs. Shift into neutral and look at the readout for Throttle Position. at a normal idle of around 700 rpm, you should see a TP value of about 12-13%. If it is reading higher than this and you are sure the throttle plate is closed, then you have a problem with the TPS.

If the TPS shows normal but the engine is still idling at 3000 rpm, then it would suggest that you have a an idle speed control valve that is malfunctioning.

 

Tsfo2,
Beware how much time you spend in Neutral or Park at those speeds. There is a TSB somewhere (It's in the stickies for TSB's but I can't remember which one and didn't download it - I was just perusing them and reading the ones that looked interesting) that says extended operation above 2000 rpm in Neutral or Park will damage the transmission.


Does the engine accelerate to those values on it's own? or does it just never come down from operations in that range? I understand if you have brought it to a stop and shift to N it may be at 1200 and balloon to 3000 - but you can likely "feel" or "sense" that it wants to be much higher whilst holding it with the brake. I'm asking if it is erratic and speeds up uncommanded - like you have no control of it? Or is it rather just a failure to decelerate from higher-speed operation? What actions have worked to restore proper idle momentarily in the past? (Punching the throttle swiftly is a pretty common reaction, as well as a frequently successful one) Just curious how you get it down when it is doing that?

 

The throttle spring will normally close the throttle when the engine is NOT RUNNING.
The throttle may 'hang' when the engine IS RUNNING due to 'impact air' against the throttle plate. There seems to be enough airflow to affect the 'butterfly' with the engine running.

I could duplicate the fault in the shop with the engine running but NOT with it OFF.

Increasing the spring tension AND cleaning the throttle fixed MANY of these faults when I worked at the dealer and the TSBs were being complied with.

You might have an IACV issue but I would address the throttle first.
It is easier and cheaper than the idle air control valve.

I have also had to replace throttle assys because the shaft was bent trying to open the throttle when stuck (cold) (see TSB 5.1-23AM2)

The bent shaft will also cause the engine to 'race' and the throttle to 'hang-open'.

bob gauff

 

It's a nice day here in Massachusetts so I've pulled the car into the garage and removed the throttle body. It's definitely got crud built up on it. I'm going to give it a good cleaning. I CAN feel it sticking a little as it first begins to open from being fully closed. I CANNOT feel it sticking as it's returning to fully closed, but we'll see. So far two questions have come up: 1) what's that black plastic square box that hangs off the plastic elbow between the MAF and the TB for? It's just hollow and doesn't have any connections. It doesn't have any obvious purpose. And 2) since there's no gasket between the TB and the intake manifold, should I use a little bit of gasket shellac, or something like that, for re-assembly?
Aholbro1: yes it will rise unprovoked; not just fail to come down. Blipping the throttle doesn't do anything. And re: "but you can likely "feel" or "sense" that it wants to be much higher whilst holding it with the brake", yes, absolutely. Like I'm power-braking it to begin a drag race (or a short-field takeoff procedure, for you pilots out there). It sometimes takes a lot of braking power to bring the car to a stop. I haven't found anything I can do which will bring it down; I just have to wait until it feels like it. Again, thanks everyone.

 

If your scanner will display throttle position, it would be interesting to see what that shows!

 

Thanks for the clarification. Based on your initial post, I was going to suggest you revisit the throttle spring, as it sounded EXACTLY like my experience, right down to the cold-wx onset and randomness - although mine would hang up at 2,500 - cruise speed. After your clarifying remarks, you're on your own! Nothing at all like mine, I can't help.


Good luck, and keep us posted on your progress!

 

Well, I finished cleaning the throttle body and put everything back together. Went for a ride to buy some gas. (Station about 2.5 miles away.) At first the car ran beautifully, and idled just as it should. But when I restarted it after filling the tank, it raced (3000+ RPM). By the time I got back home it would idle at about 1200, but nowhere near 650. So, I have the satisfaction of knowing I have a nice clean throttle body, but I still have the problem. As I was re-assembling everything, the EGR valve caught my attention, as it was right there above the TB. Does anyone think that a sticking EGR might cause symptoms like mine?

 

No, not under any normal scenario. If it is just sticking and not leaking air into the housing, it will reduce engine speed when it sticks.

So, I say you are back to the TPS, which, by the way, is what caused my high idle. Again, can your scanner display TPS?

 

My code reader will not display sensor values - only codes. I agree it would be very interesting to see the TPS readings under both "problem" and "non-problem" conditions. Buying/borrowing a better diagnostic tool will probably be my second-next step (recommendations welcomed), but first I've decided to do the following: wait until the engine is warm and idling normally, and unplug the idle air control valve. I'm thinking that the valve should then remain in the same position forever. So later, under similar environmental conditions, the engine should always idle reasonably well (certainly, at least, not at over 3000 RPM as it's been doing). What do you think? BTW - what would you think the life-expectancy of a CPS should be? I think mine is the original, and the car has 135,000 on it.

 

You may find that it is more difficult to start the engine with the IACV disconnected, and it may also throw a DTC (I'm not sure about that).

I would say that, on average, owners begin to experience CPS problems around 100,000 miles, so you're over-due for a pre-emptive CPS replacement. I would suggest buying and installing a new one, and keeping the known-working old one in the "emergency spares" kit you keep in the trunk/boot.

Below is a link to photos of the kit I keep in our XJ40. I'm assembling a similar kit for our X350.

Cheers,

Don

Welcome to Jag-lovers - Members Photo Viewing Page

 

I would say the strategy of unplugging the ISSC while the engine is "behaving well" is a good plan. Note the ambient temperature, whether the AC is on and so on.

Goosd luck, and keep us posted!

 

Drove the car over 200 miles today with the IACV unplugged - both city and highway driving. NO ENGINE RACING! I'm pretty well convinced that we've found the problem. I see some new valves for sale on ebay, with prices ranging from $27 to $359. I'll probably order one of the $27 ones and give it a try, although the car ran so well today it hardly seems that you need one at all (except to keep the CEL happy; it did come on). Again, thanks to everyone for their good advice.