96 4.0 had misfire while accelerating through 1500-1900 rpm and while in drive/reverse and foot on brake. No codes. Changed plugs. Changed fuel filter and checked each injector for drop in rpm when disconnected. Check. No change in misfires. Swapped a new Lucas coil into each plug and found #6 swap fixed 1500-1900 misfire but not low RPM misfire. So now no misfire while moving or idling in park/neutral but bad hickup in gear with foot on brake.

Where should my search go next?

Probably not related but car idles high. 1150 in park and 900 in gear and brake.

 

Your high idle issues are probably related.

You may have a vacuum leak on the intake side. I would first look for a leak.

if you cannot find a leak, you can either start throwing parts at it, start at oxygen sensor, then Coolant sensor then MAF sensor.

after that, you'll need to take it into a Shop with Jaguar diagnostic software, WDS, IDS, etc, and they'll be able to reset the idle parameters.

i guess before I said all of this, I should have asked you for your history with this car. Has it always idled high?

 

I don't have a lot of history with this car. I acquired the car after a large hole in the muffler melted the rear wiring harness and car didn't run. The car sat for at least 6 months. I cut off the muffler and replaced the rear harness and 25 fuses and it fired up. It ran rough with old fuel so ran it down and treated and added new fuel and it started to run better but I never really gave it a change to get hot. So now it runs fantastic when cold with the replaced coil, although with high idle, but when it gets fully warmed up things get progressively worse. It was hiccuping at idle but now it just cuts out and even stalls sometimes when coming to a stop. This "cutting out" now will happen at any rpm and it involves the rpms dropping 400-1000 instantaneously, the check engine light coming on briefly and as the car recovers light goes out. Still no codes.

So I pulled throttle body and cleaned all parts well but that didn't help. I've disconnected the secondary air pump, idle control valve, throttle position sensor, and all sensors along the air intake individually while running and the problem still occurs. Also disconnected battery cables and grounded positive in a attempt to do a reset. I'm not sure this exercise tells me anything useful.

The car idles at about 1200 rpms (my scan tool says 3% throttle position) and when the problem pops up the rpms drop to 500, the secondary air pump will always come on when this occurs and runs for a moment. Sometimes the car bounces back to 1200 other times it stalls.

I should also note that the rpms wander with the same throttle position. If you hold the throttle at 2000 it with jump to 2500 and then back to 2000. Then it will miss and drop to 1500 and then recover to 2000.

I don't want to start throwing parts at. I'm not even sure where I would start. What else can I check on the fuel side? My scan tool seems to give limited info for this car.

Thanks for any suggestions.

 

Does anyone have experience with Autolite 3924 plugs? When I started this troubleshooting exercise I installed a new set of these plugs. Is there a chance they are my problem?

 

I can't speak to any issues with the Autolite plugs, but there seems to be a general consensus that the X300 is very sensitive to plug selection and the best bet are the Champion RC12YC

.

 

Thanks. Looks like Champion 71 is another cross reference for RC12YC? The Autolite 3924 is supposed to be a cross reference to 71/71G.

 

Yep, lots of cross references for the OEM Champion plug, but many have found that anything other than the RC12YC, even if a cross reference, is problematic.

Do search here for "spark plug" or "Champion" and you'll have plenty to read on this much discussed topic

.

 

+1 Al

I can't guarantee that the Autolites are contributing to your continuing issues, but as Al says, many owners of XJ40s and X300s have found that using any plug other than the OEM Champions, regardless of how expensive and how high tech, may result in suboptimal performance. The original plug specified for the X300 was the Champion RC9YCC, but Jaguar subsequently revised the spec to the hotter RC12YCC. Since this "double copper" plug is no longer available, most owners use the nearly identical RC12YC. Rock Auto stocks them at USD $1.30 each, and there is no reason to spend your money on any other plug.

Regarding your hiccup in gear with foot on the brake, one possibility that comes to mind is the Idle Air Control Valve (IACV), which is part of the Throttle Body (TB) assembly. A stepper motor mounted on top of the TB has a pintle or plunger that opens and closes to allow more or less air depending on running conditions, such as whether the transmission is in Park or Neutral or any gear. The pintle tends to gunk up, which prevents it from sealing properly and can result in lean running and misfires. It might be worth cleaning the pintle and the bore into which it fits. Disconnect the electrical connector before removing the stepper motor, because with the power connected if you accidentally turned the key to ON the pintle could screw all the way out of the motor.

Cheers,

Don

 

Thanks for the suggestions. I now believe the misfires and high rpm are not related.

I abandoned the new Autolite 3924 plugs for the proper Champion RC12YC (71) plugs. That did not fix the misfire.

Coils. Rather that swapping a new coil around, like I had been, I just replaced all 6 with aftermarket. MISFIRE FIXED!

Now just dealing with high rpms so I'll start a new thread for this problem.

Thanks, again!