Hi there,
My 1998 XJ8 has had the S514 throttle body replacement suggested before on a previous post but the car is still stalling after doing some 75 mph cruising and then applying the brake to exit the freeway. The dealer is replacing the brake switch which was giving some "Fail Safe Mode" messages and cutting out the cruise but they cannot replicate or explain the stalling out.
Has anyone any ideas what might cause the engine to stall after some interstate speeds followed by application of the brake?
The car restarts once it stops, puff of white smoke after restart but no other abnormal smells or smoke or sounds, no caution lights, just stalling when I come to exit the interstate or patch of higher speeds followed by brake application.
Could the brake switch cause something else??
Any help as ever is greatly received, I really appreciate it.
Thanks,
Michigan.

 

See if you have that module in front of the spare tire in the boot. When you say it only happens under braking, I think of something being thrust forward, and that's the first thing I think of.

Are any other 'electrical' items affected? radio, lights, does everything go dark or just engine dies?

 

1998 XJ8 w/ 92k miles - My wife is having the same stalling problem on the highway. It has done it to her 3 times over the past few months. It has never happened with me driving but finally i was riding with her when it happened. She was stuck going 60 or so behind someone and then it just stopped. She was not driving aggressive (which I normally do). In fact she was kind of at an "in between" speed - wanting to pass the guy but just waiting for traffic to clear. Then, it just killed and she coasted to the side - started right back up and on we went. Don't think she was applying the break at the time. I didn't notice it this time as it was dark but she said it puts out a puff of smoke normally when it does it. I was getting a code for the knock senors and had both of them replaced before this most recent incident. Thought that might be behind this problem but obviously not. We are not getting any other codes and it runs like a dream otherwise. Have not had the throttle body replaced that I know of.

Would love to hear any other possible solutions.

 

Also - all of the lights/electronics stayed on - dash, head lights etc. Only the motor stopped.

 

This has recently happened twice to me. A short period of 60-70 mph and then a period of slowing down in anticipation of a junction is followed by a complete loss of power. I do not recall if I had applied the brakes before the loss of power. On both occasions I coasted still in "Drive" with the rev counter varying with road speed. I did not notice any coincident electrical failure. I was apprehensive about changing to neutral in case I should loose the power steering and servo on the braking. I do not know if I was being stupid about that, it may be that Jaguar had thought of that. I did not loose any steering or braking power whilst coasting with apparently no engine power.

On the first occasion I pulled over, came to a stop and the car started easily.
On the second occasion I was coasting nearly to a stop and the engine fired up spontaneously, presumably because I was still in drive and doing a sort of bump start.

I do not know of the throttle body problem mentioned here, I've only had my car a couple of months. I will do some further searching.

h20boy: I like your train of thought about the box in the boot. I have one and I assume that it is an petrol cut off switch for accidents?

I was told by the vendor that he had had some minor adjustment to the fuel injection which had improved fuel consumption slightly.

The problem is as if petrol just stops getting to the engine, but I appreciate that there could be many causes of this issue.

 

The car (original thread) has been in the dealer today but they can't replicate the problem, they think it could be the exhaust recycling valve or signal to it. As I understand it at higher cruise speeds some of the exhaust is returned to the intake to reduce exhaust pollution and leans the mixture but at lower speeds the exhaust return is shot off by a valve and an appropriate computer signal commands it. If it doesn't shut off when the car slows down the mixture is too lean and the engine stalls.
Does this jog any memories or sound feasible?
Thanks for your ideas folks.
Michigan.

 

the biggest cause of high vacuum stall (decel at freeway speeds) is plugged part load breather in the drivers cam cover. Also a dirty throttle body and Idle air passage in the throttle. This was always a service operation when doing fuel injection system cleaning. On the throttle clean with carb cleaner when the engine is at operating temp. Make sure to clean with spray down the Iadle air passage in the throttle bore opening. Carefully disconnect the part load breather and clean out oil sludge with a small 1/16" drill bit by hand. Theres an orifice in the cam cover once the line is taken off. clean it out. Youll probobly find it well plugged, do not enlarge it, it is a metered air bleed

 

For a picture-pages approach to that throttle body cleaning and part load breather check, go to the FAQ page in the XK8/XKR subsection of the forum, go down to the Engine sectin, and look for the link titled 'cleaning the throttle body, MAFS and part load breather DIY', same engine as yours so it will apply. good luck and report back!

thanks for chiming in brutal, we've had several threads lately about this highway decel stalling issue.

 

H20boy and Brutal: Thank you for your very helpful advice. Will set to and report back.

 

Can anyone explain why this happens? What is the mechanism? Why should cleaning the throttle body etc help? Also, is it possible to tell if I have the replacement throttle body? How did the replacement differ from the original? Questions, questions!

 

when the part laod breather is plugged it starts pumpping alot more oil vapor into the intake tract and gums up the throttle opening and the idle air control solenoid passage. when the throttle is dirty there is a partial to total blockage with No air gaps between the throttle blade and bore. The IACS normally can comphensate(thats its job to increase idle speed for ac, power steering, dirty throttle etc.) problem is that as it gets dirty and when your take your foot off the throttle going down the freeway, the rpm is high, speed is high and you suddenly close the throttle. The IACS cant keep up and respond fast enough to maintain air opening to the engine, and the throttle is so dirty that it doesnt have an air gap, effectivly killing the engine in many cases

 

Thanks Brutal, that's very helpful. It's so much better when doing a job to understand why you are doing it. It means you do a better job.

Now, if you could explain this to me...(cheeky)

Why, when you make fuel mixture available to a normally aspirated internal combustion engine, does it rotate faster? Similarly, why when you increase the pressure in the intake with a turbo or supercharger does the engine rotate faster? Is it this: One cylinder fires and therefore speeds the compression of the next and so on? It's always something I've never fully understood.

and this...

MrTexasDan's excellent DIY guide to the cleaning procedure recommends "Throttle Body Cleaner" as opposed to "Carb Cleaner". Can I do any harm if I use Carb Cleaner or Brake Cleaner? Why doe he/she recommend Throttle Body Cleaner? I won't have it available this weekend. Should I wait until I can get the right stuff?

Best wishes
Phil

 

your first question all has to do with combustion pressure. the more air and fuel you can get into each cylinder, the more EXPLOSIVE power (horse power and torque) you get out.
you want to use a product to disolve the oil deposits which is what they are. Carb or throttle body cleaner and an old tooth brush is fine. Brake cleaner is not really gonna clean the deposits as well. The differance on TB or carb is probobly that the TB cleaner wont remove protective coating on the TB. Some have this others do not. Or it may just be the same product in the can with a differant name since no cars use carbs anymore as of about the last 22 or so years... Just read the label, products are all differant. 1 may be formulated for both, and another may not

 

just make sure you do it when the engine is hot and then restart right after your done to clear out the little cleaner in the intake. If you do it on a cold engine youll have a very hard time getting it to restart. And if you let it sit and dont restart youll have the same problem

 

I have performed the procedure discussed and the fault has gone. Thank you for your excellent advice.

My Model had a different MAF unit, which was a sealed unit and so I left it alone.

The part-load breather was slightly occluded.

I disconnected the battery for an Engine Management reset as well.
It may be my imagination but the engine seems to be running smoother, sounding better and performing better.

Hurray!

BTW..I used Wynn's Air Intake and Carb Cleaner. It did leave a small amount of stubborn dirt behind but basically did the job.

 

I had this happen today, and a couple weeks ago as well. Thanks Brutal, I will clean things up and see how it goes. Of course it had to fail in the worst spot for miles, just entering a tunnel. On both occasions I popped it into neutral, switched the key off and on again (quick so the steering lock doesn't engage!) and carried on, baffled.
Gotta love this forum!

 

Years ago Alex at Jaguar Tech Line explained to me that after several (5?) accel/decel cycles the ECM commands the throttle motor to momentarily close the throttle completely (bore wipe) and opens it again. If the throttle has deposits, the throttle will be pulled closed by high engine vacuum. After the engine stalls the pressure normalizes and the throttle operates.

Some recalls were to reprogramme the ECM and some were to replace the throttle assy.

I have heard that resetting the throttle stop screw more open and cleaning the bore cured the fault. I clean them but I have not changed the throttle stop setting. I might change my own throttle but I would not do it to a customer car.

bob gauff

 

I cleaned the throttle body, checked the breathers (clear) and the problem went away.
For about a year and 3 months. It just did it twice, each time coming off of cruise control at 110 KMh.
Today I cleaned the throttle body again, and it was pretty darned dirty. I think the oil from the breather is the problem. In the future I will clean the throttle body once or twice a year.

 

what do I need to do to get it started. I ran it up to operating temp and did the checks and procedure. I did use electrical cleaner to blow debris from the little hole then routed it with a very small Allen key. I shut the car off. Disconnected the battery to reset the faults and hooked it back up. I can't get this car to start! What have I done? Help.

 

What year is your car? Maybe bore wash.

But, hold it all the way to the floor and crank it. Stop, do it again. Then again. It will most likely clean out and try to start; as soon as you hear it trying to start, let up slightly on the accelerator (which will provide fuel).

That doesn't work, search bore wash and follow the instructions.

That doesn't work, review each step you did . . . You probably forgot to reconnect something. And if that doesn't work, call a tow truck . . .