My 2001 XJR with 144K miles on it has decided to display a tendency to idle roughly. It doesn't seem to be missing, just rough, and only at idle. There are no codes. Here is what I have done so far:[LIST]

• Checked for vacuum or air leaks in the obvious places. Found none so far.
• Pulled all the plugs and checked the gaps - nothing fouled or wrong there; coils and boots all look fine.
• Part Load Breather - open as it should be
• Throttle Body - clean
• MAF - Clean (recently replaced)
• Hard reset for the hell of it - no difference.
• Fuel filter - replaced last year

So now I am thinking about fuel related issues. It is almost like I got a bad tank of fuel, but I have re-filled and have the same rough idle. Fuel injectors? How do you get to them? Any other ideas?

Thanks in advance for the help!

 

You need to read the OBD II codes.

Are the secondary tensioners the new 3rd generation metal units? If not, then I would suspect a timing problem.

Some of us have had luck with using fuel system cleaners to clean the injectors.

If you buy an OBD II reader, get one that will read both the short term and long term fuel trims.

 

Hi Jim,
Thanks, but there are no codes and no CEL. Tensioners are 3rd generation. I may try some injector cleaner; I guess it couldn't hurt.

 

good; then what are the fuel trims. Go back over the fittings and plugs from the work of cleaning the MAF sensor and throttle plate and bore. I would start to suspect an air leak.

Second, are you having trouble starting in cold weather? Your temperature sensor may be bad.

 

Not sure that I can get fuel trims out of my scanner, but I will check.

This being So. Cal. it doesn't get too cold usually, but the car always starts up fine cold or hot.

Jim, I was told that there is a module of some sort in under one of the the front fender wheel liners? I have no other details. Do you know what this is and if it could be causing the rough idle if it has failed?

Thanks!

 

Sorry

 

I am not familiar with any module in the fender. I also think you have an air leak. Use a can of carb cleaner to spray all around the entire intake system, including underneath the intake hose at the corrugations, and listen for a change in the idle.
If you can monitor short term fuel trims then it is even easier. When you spray at the source of the air leak the trims will go crazy.

 

Great suggestion! I will be traveling for a few days (not with the Jag) so I will give that a try when I get home and report back.

 

I had a rough idle problem, especially in the morning. After trying several of the fixes you originally mentioned, my mechanic suggested to replace the intake manifold gaskets. I purchased them on line ($25.00 for the set of 8) and my mechanic replaced them for $150 labor...the car is now running perfect. Just a thought!