I have a 1999 XJ8 Vanden Plas that I am having a little trouble with. It has a rough idle and seems to have a random misfire. It has set the code for random misfire before. If you unplug the MAF, it smoothes out and runs great. So you would think it's the MAF right? Wrong. I have installed two different new ones and taken one off a car that runs great and installed it. Same thing. Runs rough until you unplug the MAF. It has 2 new coils, all new plugs and all 4 new O2 sensors. It doesn't set any codes other than that. But the Restricted Performance will set if it runs for a while at idle like this. And once you get it over the stumble crap while driving, it runs fantastic. Any help would greatly be appreciated.

 

You might have an air leak? This can give a lumpy idle. Have you got an oily plug boot? This can give random misfire.

 

I've checked for air leaks and vacuum leaks but dont' have any. I unplugged each coil at a time while running trying to see if it's a cylinder misfire. But that shows all cylinders firing but still having the rough idle.

 

If you get a PCM 174 Code , those can be triggered either by PCV Hose , EGR Valve as well . How is your fuel consumption , poor or normal ?
My dad's ride threw that code along with poor consumption , we were thinking about O2 sensors but it turned out to be EGR Valve and PCV Hose failures

 

It did have lousy gas mileage but two plugs were broken(you could hear them arcing). It has a new PCV hose I haven't checked the EGR valve. I have noticed that the car is idled lower now than ever before. So low it nearly dies. It did this after the hard reset of the ecu. I'm gonna clean all the plugs on the throttle body and clean the butterfly. This may be it's problem.

 

I can say that I had a random misfire caused by an oily plug boot. I read the codes and got a misfire on plug #1. So, I thought I would swap the coils from #1 to #2 to see if it would move the misfire to #2. I pulled the plugs and they looked fine, but there was some oil on the boots especially #1. Cleaned them up, reset codes, and problem gone. Hard to believe I know. It wouldn't give me any trouble until it warmed up.

 

When the ecu is reset, it takes awhile for the computer to relearn. During that time, the idle may be unusually low. It may sag especially when accessories like AC come on. But it should get back to normal after a few driving cycles.

 

Just a suggestion - to carry out remedial work before the reboot, i.e. cleaning the MAF sensor with an aerosol electrical contact cleaner. Fully wet each sensor, and let evaporate before fitting it back. Next would be the throttle body, a carb cleaner on a clean rag works well. Part load breather is another thing to clean out. Next the throttle's electrical connection and contacts cleaned and checked. Then perform a reboot. The idle will start and climb to around 1500-1800rpm, then it falls back almost to stall as the ecu re calibrates the throttle. At this point the engine will benefit from a few journeys to settle everything down.
Hope this helps.

 

Hi All,
Just for future reference and if anyone has this problem. Here is what fixed it. I did everything that has been suggested in the replys to my thread with no change. And I don't know why I did this but I did. I unplugged both pre 02 sensors and swapped them from bank to bank. It fixed it. No rough idle, no random misfire, no restricted performance. It runs better than it ever has. All I can figure is that when I was trying to diagnosis the 02 heater circuit, I swapped them to see if it flagged the code for the other bank and I forgot to put them correct once I replaced all the O2 sensors. Glad it's fixed. I was pulling what little hair I had out of my head. Be seeing you.

 

If all the O2 sensors are new, why would swapping them around cause a problem or make it go away? I'm confused!

 

You are not the only one confused. I can't understand what did this or why it did this. What got me thinking about it was reading on here about the wide band and close band 02 sensors. And that Bosch just sells a generic to fit both. Those are Bosch that I installed in it. But the last one I purchased from a different supplier than the others and that's when all this crap started. I would like to hear anyone's take on why this fixed it so I can know what 02 sensors to buy in the future.

 

Good to see you found the smoming gun anf fixed the problem and thanks for keeping us updated on this score . However, I hope it's not by fluke .

 

I have a 98 XJR which had similar symptoms. Tried most of what you have done with no results. Found oil in the spark plug wells. Repair shop pulled the valve covers which had small cracks around the spark plug wells allowing oil and probably air to leak. Also replaced one coil and spark plugs. I think the cracks in the valve covers were the reason it ran rough. Runs smooth and with more power now.

 

so, my 2000 XJR started exhibiting a bunch of the same symptoms and throwing a bunch of coil codes. I replaced the 3 coils it said were bad and threw in 8 new spark plugs, only for new codes to show up including 1316, etc. BUT if I do hard reset (undo + battery terminal overnight or so), when I start it up cold it runs great on all 8 cylinders for about one minute. Then RESTRICTED PERFORMANCE and rough running starts in again. Any ideas?

 

Couple of things: your location 'thee OC' sounds cute but where is it? Your personal data says you have a 97 XJ6; you are commenting on a XJR V8.

I would look to a broken or grounded wire that might have been damaged as you removed the plug and coil covers while changing plugs and coils. Those wires are thin and frail.

 

​These are the possible causes for that code.

• Cylinder compression low
• Low fuel pressure
• Clogged fuel injectors
• Faulty / worn out spark plugs
• Faulty ignition coil
• Faulty fuel injectors
• Faulty Engine Control Module (ECM)