2004 XJ8 runs rough when cold. It idles rough around 1300 RPM. After a minute idles falls below 1000 and it runs fine. Plugs are new, MAP is new, cleaned throttle body. 170,000 miles don't know if coils have every been replaced. Any suggestions?

 

Remove and Clean the MAF sensor with MAF sensor cleaner..Just spray it well and let it dry before reinstalling.

 

I agree with toyman1001. I too had this problem a few months ago and cleaning the MAF sensor with cleaner did the trick for me. Do not touch the elements in the MAF sensor. Just spray the cleaner into it and allow to fully dry before reinstalling the MAF sensor. It is easily accessed from the top of the air intake pipework. Good luck.

Thanks,

Chris

 

Though the MAP was fairly new I did clean as suggested. It starts to idle about 1300 then speeds up to 1800 and hunts up an down for a little bit and finally settles down below 1000 and seems to run OK. I was letting it warm up to drive over to winter storage and it blew codes P300 through P308 (except 307) and P1316. So it is misfiring. This seems to have occurred with the colder weather, coincidence, maybe. So something changed that effected all the cylinders at once except 7???
So it couldn't be coils or injectors.

1. Low fuel pressure, so check that.
2. Faulty ECM, ??? how do you test a ECM.

Any ideas?

 

MAF (Mass Air Flow) not MAP (Manifold Absolute Pressure). There is nothing to clean on a MAP as it's an air pressure sensor Is there even a MAP sensor on these cars?

 

I misspoke, it was the MAF I cleaned,

 

Start with the basics by first confirming the battery voltage of 12V+. Strange things happen if the voltage falls off.

 

With that many miles, if you are on original coils, I wouldn't be surprised if they are the problem. I have had multiple cylinder misfire due to bad coils (check for the slightest hairline crack on them, sometimes regular wear and tear will do it). I have also had misfires because my catalytic converters were clogged. I'd start with checking the battery voltage as mentioned, although I think if you were able to start up fine, your battery is ok.

 

Just remembered you could switch your coils with cylinder 7's coil. If all of a sudden you start to get a misfire in 7, and don't get a misfire in the cylinder you swapped coils with, there's your answer.

Also, how long has it been since you put a fresh tank of fuel?

 

I have been learning about using my scanner and fuel trims. The long term fuel trim at idle was high and would approach normal with higher RPMs. So it would suggest a vacuum leak. I drove 30 miles to put the car away to be worked on this winter. Never triggered any codes and fuel trims were close to normal. At idle both short and long fuel trims went high and got a P0171 & P0174. lean bank 1 & 2. Had those codes once before and found the vacuum take off for the brake booster at the back of the engine was loose. I wonder if the previous P300 codes are related to this or could be a separate problem. The brake booster vacuum takeoff has a broken clip so I have a good one to replace it with and see if that solves the problem. I should be able to tell immediately by using the scanner to measure the fuel trims.