Take a look for vacuum leaks first, since that's the least expensive diagnosis. Listen to the engine and feel around for suction or blow. If vacuum seems fine, I'd put my money on the alternator. However, change the fuel filter first. It really sounds like alternator to me, but you never know, and checking for vacuum leaks is soooo easy that I'd always take that route first. The fuel pump is the next least expensive repair, so that's second. Then, take the alternator off and check it at discount auto parts to make sure that that is the problem. If it is, let us know as I'm sure that Ken from Motorcars LTD can get you one. But, I'd definitely change the fuel filter(not the pump), regardless of anything because I wouldn't be surprised if in the 110K miles, it's never been changed. Good luck and keep us updated.

 

I know where you're coming from. Before you pay for anything, let us know what the mechanic is telling you that's wrong. Good luck.

 

I know I'm a few days late and I don't know if you got this fixed yet or not... but if you haven't, you can probably fix this yourself pretty easily and save some cash.

Your problem is not a fuse. The battery light and the tranny light come on when the car dies - it is completely normal. If you turn the key to the accessory position (not started, but one click before that) the lights will go on.

The problem is most likely not the alternator, as I'm sure your car would have died by now if it was. If you want to check it out though, take it to an auto store, they have the equipment to test it while it is on the car - you don't even have to uninstall it.

Your problem also doesn't sound like a fuel pump, as the problem is only at idle. If your fuel pump was on the way out, the car would buck under accelleration - not die at idle. IF you want to have this checked though, it is easy. Just have a shop hook up a fuel pressure guage to it and they can tell you right away.

Your problem is almost certainly a vacuum leak. If you pop the hood and look at the engine, you will see all kinds of little tubes everywhere. Some may be rubber and some may be plastic. They will be skinny hoses, not the really large ones. The plastic ones are probably more likely to be the issue (they are really skinny - roughly the thickness of a round toothpick), as they get brittle after time from the heat under the hood and break. Check for cracks in the tube or a completely broken tube all together. If it is one of the rubber ones, it was probably caused by rubbing against something - check for any tubes that are resting against or touching somthing. Start the motor, get out and go to the front and listen around the tubes. If you hear a hissing sound, you have a vacuum leak. All you have to do is find where the leak is (it will be easy because you will hear it - it is loud), and then replace that tube. They are all easy to replace. Just get a new one and put it where the old one was. This will most likely solve your problem for not much money at all.

If it isn't a vacuum leak, I would next clean out the throttle body. Find the air-box (the thing with the air-filter in it). Follow the tube into the engine. The metal piece that is about 6" long that is right before the top of the motor is the throttle body. It has a cable on the side of it. To make sure you have the right part, start the car and grab the metal piece that the cable is attached to. Move the piece back and forth and the motor should act like you are pressing the gas pedal. Unbold the tube that goes to the airbox, and unbolt the throttle body from the upper intake manifold (the metal piece that the throttle body bolts to - basically the top of the motor). then just disconnect any tubes and pull it aside. Clean it out with throttle body cleaner. It is a spray that is sold at the auto store for a few dollars.

Also, as was mentioned earlier, replace the fuel filter. Your fuel pump is fine, and I would imagine that the filter isn't the problem either, but it certainly could be and it is worth changing it at your mileage. The cost of a new filter is very cheap.

 

Would and idle air valve cause this same problem?

 

My XJ6 stalled at 70 miles per hour in the fast lane of a busy 6 lanes highway. Pulled over to the right shoulder, almost hit several times by cars and trucks flying around me, and thanking my guardian angels for sparing my life and car. After my heart got back in its place, I tried to restart the engine, but no luck, so I had to tow the car home.

After a month of troubleshooting and replacing parts, I figured the mystery that, I think, would help all Jag owners.

I'm an engineer, have three Jags and know what I'm doing most of the time, but after several days of troubleshooting I got nothing. During that time I figured that the engine would stall also after the cooling fans came on, which made troubleshooting much easier.

The only indication of a problem, that I got with all the fancy instruments I have, was code 335 and 404; crankshaft position sensor (CPS) and EGR circuit malfunction, which codes were actually activated during the high-speed stall. That called for changing the CPS and EGR, which was done promptly, but the stalling continued. This is when I knew that there is a mystery to be solved in my future.

After several more days of testing and trying, I almost gave up. The engine would stall as soon as the cooling fans came on--whether idling on the driveway, or running on the streets.

I then continued the mystery solving game by unplugging different components and noting the changes in the engine cycles.

Then finally, Eureka; while the idle control control valve (IACV) was unplugged, the engine did not die. Not at idle, not at high speed; never! It appears that the wicked, almost insignificant in its function, IACV would misbehave--probably at the last days of its life--and would send random error messages to the ECM while it is dying. In my case it was very likely rapid temperature increase slightly above normal that triggered the cooling fans to come on, also causing the IACV to send error codes.

And so, while slowly dying, the old IACV sent several error messages to the ECM, blaming other components for its misfortune. That made the ECM to respond faithfully and shut the engine, trying to protect it. I almost killed my self, coasting from lane 6 to lane 1 in heavy traffic, and then spent a month chasing the wind, while the wicked IACV was sitting there quietly, pretending that it knows nothing of the problems it had caused.

Now the car runs perfectly with the old IACV disconnected. Although I bought a new one, I will keep the old one in there to just show it how useless it is, as a punishment for all the troubles it caused me.

So, Jaguar owners, any time your car dies, check the old wicked IACV first. It is easy to do and might save you many, many problems.

Anco Blazev

 

Quote
"and would send random error messages to the ECM while it is dying. In my case it was very likely rapid temperature increase slightly above normal that triggered the cooling fans to come on, also causing the IACV to send error codes.

And so, while slowly dying, the old IACV sent several error messages to the ECM, blaming other components for its misfortune. That made the ECM to respond faithfully and shut the engine, trying to protect it."


With all due respect to your engineering background, I think the conclusions you have drawn from your diagnosis are wrong.

The IACV does not send any signals to the ECU, it merely receives output from the ECU. Also, to the best of my knowledge there is no "Limp Mode" or other shut down capability in the X300 Engine Management system.

I can't work out what role the IACV would be playing at 70mph. It's an idle valve. There may be some kind of short circuit in it that could cause other issues?

I think you need to replace the IACV with your new unit and plug it in to finally condemn the old IACV as the root cause of your problem. You are perhaps just compensating for another issue.

 

Well, I have been wrong before…so you be the judge.

Facts:
- Engine starts and runs well until it gets to normal operating temperature,
- Shortly after that, however, the engine shuts off
- After such a shut down, the engine will crank, but not restart for 15-20 minutes.
- Unplugging the IACV makes all this go away. The engine starts and runs indefinitely at idle or high speed.
- The engine shuts off immediately, when I plug the IACV back in, regardless of RPMs.
- The ECU also generated two error codes—both of which were wrong-- after the high-speed shut down.
- The engine would not restart for a long time after that shut down too.

Conclusions:
Going through dozens of tests, during many hours of idling, driving, and analyzing the results thereof, my findings and conclusions are as follow:
- The IACV is the only malfunctioning part in the entire system.

- It forces the ECU to shut the engine off by sending some sort of a ‘signal’.
Note: This ‘signal’ could be a simple short circuit or parasitic ground—any of which would confuse the ECU and make it do weird things, as those I’m describing.
- No engine shut off occurs when the IACV is unplugged, which means that there is no misinformation sent to the ECU
- The ‘signal’ sent by the IACV is confusing the ECU to where it shuts off the engine, and does not allow it to restart for a long period of time, in order to give it a chance to recover from the fault.
- The solution is a new IACV, or unplugging it all together. This allows the engine to run normally at all times and speeds.

Hope this helps to clear the confusion.


Anco Blazev

 

IACV where is it located in the engine?

 

On the throttle body. It is a an electrical solenoid/motor that moves a plunger to open/close a bypass in the TB that allows for smooth idle when the butterfly is closed.

In the parts catalog they call it the "stepper motor"

Troubleshoot Idle Speed Control System



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