Hello everyone I'm new here and I have a problem with my x350 (2005 XJ8) that I just can't seem to figure out and maybe someone can help. Basically I have an idle problem. When I start the car in the morning to go to work it runs lumpy but not horrible untill it idles out at round 700rpms. It will continue to run rough but not really noticeably. Then if I sit at idle and rev the car to 1400rmps it feels like it's running very rough and vibrates the whole car and I can feel it in the seat. If I rev it higher just above that it gets smoother and smoother between 2000 rmps and 3000rpms it feels like how it used to run at idle. I have done everything I can think of as far as replacement. Oh and by the way I do not and have not gotten a check engine light so I am really in the dark on this one. So far I've changed mass air flow, intake manifold gaskets (lower and upper), map sensor, the whole throttle body and all of the gaskets that are associated, idle control valve that is attached to the throttle body, all new coils (Delphi) and plugs (plugs gapped to 0.40 from factory), canister purge valve (which is still making a clicking noise from time to time), new O2 sensors (denso). I have done a smoke test and found no smoke coming from anywhere that could be a possible leak. I must note that when in park and repeatedly stepping on the brake my rpms to fluctuate up and down but only a small but not enough to say there's a problem. The only thing I have not done yet is clean my fuel injectors but have ran plenty of injector cleaners though the system. Also had the injectors checked by pulling each plug off and seeing if it works or not. They all seem to be functioning. Is there something I'm missing here?? Please help thank you!

 

You have done everything I have done in your situation. But what about the catalytic converter. A blocked converter can bring the same symptom. You can check it with a camera thru the O2 sensor hole or check the exhaust gas with a scanner.

 

Yes I am working on the Cat converter now. I am currently trying some cleaners though the fuel system first. I have tried cataclean before but I don't think it gets the engine hot enough to burn off the deposits. I would think if they are that plugged up and causing a vibration that I would get a cel.

 

Have you tried an OBD II code reader? It really doesn't matter which way you're going if you don't know where you are.

 

I would think a clogged cat would be worse as rpm were increased. Has the fingerprint of a vacuum leak. What are your fuel trims at idle and higher rpm?

 

 

Just an update I did realize the other day that if tapping the break repeatedly the RPMs do go up 50 to 100 from idle which I believe is about 625 or so and then goes back down to normal but the idle does not fluctuate unless tapping the brake if I hold my foot on the break it does not fluctuate. I have had this rough idle for about a year or so and upon investigating more found this I am now thinking a vacuum leak but I have no idea where it would even be coming from after I have already used a smoke machine and various other methods to try and unveil this leak if it is a vacuum leak

 

As a matter of interest, check your trims at idle, then pull your oil dip stick up so it produces a vacuum leak, and check your fuel trims. I have not tried that however it is said that can cause as much as12% rise in trim values. That is not a really big leak to get that much change. It may give you an indication of just how big your leak is.
As you have found, pumping the brakes introduces a false apparent vacuum leak into the system. I am surprised that the ECU does not correct almost instantly to prevent RPM change. Possibly can't because of the 1 second O2 sensor swing time.
In reality there are no trim values that are far enough above or below zero fuel trim to cause a rough idle! The ECU should be able to handle those without any problem. Are your long term trims moving while watching at idle? Watch all of the values during rough idle and see if any are jumping around some.

 

They are definitely jumping around from 0 to - 14 to 5.5 and so on. What does that mean? The rough idle is very noticeable but does not seem to affect the drivability except for I can feel the rough idle in the seat and steering wheel when driving which is annoying. I also noticed that if I drive at high speeds which I never really do the vibration that I feel gets really bad

 

I have also checked my vacuum lines at least all the ones I could find. I went as far as changing my manifold gaskets at least 3 times thinking I was doing something wrong and tightening the bolts to the right torque specs so it can't be the manifold gaskets. I hope not anyways. I even bought a manifold used online with upper gaskets intact with only 30k miles and tried that so I'm thinking the leak is elsewhere.

 

Were the eight, internal intake manifold gaskets, changed out?

 

Are you saying the engine is running slightly rough at all speeds?

Does your reader let you monitor cylinder miss counts? Intermittent miss may never show up as a DTC since they clear pending if they get a short run without any. A miss lets unburned O2 into the exhaust and so fuel is added to try and use it up. When no miss it will again try to compensate in the opposite direction. Could be plugs, coils, injectors or MAF, temperature sensor etc.

 

Yes it is but rougher at higher speeds

 

I did change the upper intake manifold gaskets on the original manifold and did not make a difference that's why I went ahead and got a complete manifold with gaskets intact from a 2004 jaguar XJ8 with 30,000 MI

 

I agree with what has been mentioned already, it sounds like a vacuum leak, get some brake cleaner or even wd40 and spray it around the intake flanges (the gaskets between intake and head, between throttle body and intake etc) and any sensors injectors etc whilst the engine is running if theres a leak thus is the best way to find out it also pinpoints exactly where a leak is, if there is a problem you will notice immediately via a change in idle.

Ben.

 

You replaced the O2 sensors with Denso items do yo mind saying how much you paid and from where. I bought a "Denso" sensor from eBay and it had the same fault as the old one plus the car ran rough. I bought another from a proper shop for about £100 and problem gone.

 

The 2 before cat were $130 each and the 2 after cat were $100 each got them all from Napa

 

I agree that its a vacuum leak but I did already try this method with no success.

 

Grasping at straws here, they were what solved my issue.
You did check the brake system vacuum hose for leaks?
Didn't see if you said you checked there

 

I did spray some carb cleaner on the booster vacuum line and no change in RPMs