Hello all

I have a 98 xj 4.0 auto that has a slight rough idle. No check engine light. Idle bounces between 720 and 750 rpm and I can feel a little miss every couple of seconds. Map bounces between 9.3 and 9.6 inHg. Spark advance bounces between 11 to 16 degrees. Throttle positioning sensor is steady at 14.5%. 02s11 bounces between .12
and .8 V. 02s11 bounces between .08 and .18. o2 sensors are new. Spark plugs are new, rotor and cap are new, crank sensor is new, fuel injectors are new. No check engine light. Compression numbers are tight around 145-155.

I need some help figuring this one out. Any ideas??

 

I'd start with cleaning the IAC (idle air control) and the throttle body, especially were the plate sits . See how that goes first.

 

I already did that as well. They are clean as a whistle

 

I have the same year (Classic) and had the same persistent stumble. I went through the same process you did and still had the stumble. Then, while checking for a cause for an intermittent cruise control function I found multiple cracks in the small capillary vacuum lines. Because they are very brittle at this juncture I dabbed silicone over the cracks that I could find and let it cure. The stumble significantly improved, but nothing is 100% I guess. I came to the conclusion that they were leaking, on and off, and messing with the idle stability. I found the small plastic capillary tubing at McMaster-Carr. I see tubing replacement in my near future.

 

Would be good to mention EVERYTHING that YOU have already done to try and solve the issue at hand so we can speed things up here..

 

sometimes it's hard to remember everything I've done. Been trying to solve this issue off and on for the past year and a half whenever I get the chance and have tried a lot of different things, that one just slipped my mind. And if you don't have any other suggestions don't waste my time with that kind of a comment. It doesn't help anyone

 

Jdean,

Do you always barge into a place and creating havoc with the regulars, mate?

I sense a bit of frustration. We are here to help, and you might need to take a breath here.

You want help and for us it's helpful if you, besides describing the issue, state what you have already done. It prevents members from starting to give you ideas to solve the issue that you might have already tried to no avail. THAT would be helping nobody.

I gave you a suggestions already earlier on, to help you on the way. You already tried that.you said. So I advised to mention everything you might have done already. Because that helps us to speed things up. You have been trying to solve this issue for over a year and I can understand the frustration. But reacting on a comment like you did there, will not be really inviting members to spend their free time here to help you out.

Now, if you feel I am wasting your time by just requesting more information for us, by all means, I wish you best of luck and leave it to others to meet your needs.

If not, don't be so quickly offended on a forum and we can work on this together. Choice is yours.

 

Couple of things
1) not trying to create havoc, all I said was that I slipped up and didn't remember that I had already done what you had suggested, I've been working on it for a very long time and just couldn't remember all I did when I made the original post.... Didn't realize that was such an unforgivable sin. Totally appreciate any help anyone can give me or or and other ideas, but just because i forgot one step that i have taken while trying to work on the car, doesn't mean I'm intentionally trying to **** you off.

2) I am not really frustrated with you. In fact I kind of am sensing the exact opposite. I welcome and am incredibly thankful for the knowledge of all of the members who have been working on these XJs for so long, that are willing to help. You just said you wanted to speed things up a bit, and I'm just pointing out the blatantly contradicting statement you made, as I really don't think this has speed things up... And that's why I said you probably just shouldn't have posted that as it really didn't accomplish any good

I guess I should have said "thank you" for your first suggestion, as I did appreciate it, but I forgot that I had already tried it about a year ago. Didn't mean to hurt your feelings by jumping in and saying that I had already tried it, without saying thank you.
So let me try again...

Thank you, for your initial suggestion. I however, have already tried that about a year ago, then went and checked on the throttle body after you had made your comment to ensure it was still clean. it did not fix the issue when I cleaned it. I meant to have that included in the initial post, but I totally forgot that I had done it, until your reply reminded me.

Now if we could move back to the issue I posted about that would be great. I'm sorry for offending you and if you do have any other suggestions I would love to hear them. If not that's totally fine.

I do think there is a vacuum leak as the other member suggested. Tried using the propane method to locate it yesterday without success, but there are several lines that appear to be starting to crack a little. I may go ahead and replace them for good measure and see if it makes a difference.

 

Bear with me, but have you tried turning it off and on again?

 

😂. You're a genius! It worked! Haha good one man.

 

ended up looking for more vacuum leaks and fixed another one. It definitely helped but I still have an occasional misfire. At this point I don't think it's much to worry about. Looked all over for more leaks and couldn't find any. All the hoses look pretty good now that I've replaced that last bad one.

 

Are you getting a misfire code?
How old are the plugs, wires, cap and rotor? 30K mile maintenance items.
Intake manifold leaking where it meets the head? Easily fixed BTW. I assume you know how to find vacuum leaks using carb cleaner.

 

Abit harsh mate

 

Roler just worked it all out. And, did a hell of a job.

 

plugs wires cap and rotor are all new. Fixing the vacuum line I found damaged did quite a bit to help, but I haven't found any other vacuum leaks on lines or the intake manifold.