Been having an issue with the idle on my jag recently, whether the engine is cold or warmed up some times when I start it up the the engine idle will jump from about 1200 then immediatly plumit to 500 then back to 1200. I've had this issue in the past but seems to be apearing more frequent now. At idle it feels like its slightly missfiring because the car sort of feels like your bouncing up and down in it. On the road it feels fine no issues just start ups when it acts up sometimes. No CEL, even scaned it with my elm scan tool to see if any are stored I don't know where to start, tried looking for vaccum leaks but didn't find any I could have missed some places though since i'm not to familliar with the SC V8, I'm lost at the moment.

 

Use the elm to check (in this order; in P or N will do):
1. that the OBD monitors are Complete
2. that the car is running CL
3. LTFT values for each bank, at idle and at 2500rpm

Please post the results.

(#3 will likely reveal whether an air leak is the issue.)

You should have a (pending) code P1111 or else P1000. If not, you're not using the elm properly or it's not working right LOL

If you like, also check the most obvious sensors for plausibility. Things like ECT & IAT.

See below:

 

Should the engine be warmed up before monitoring the fuel trims, or does it matter? thanks

 

Either, but it'll be harder to figure out what's happening if you start from cold because everything should be gradually changing due to warming up. My feeling is you'll be doing it a few times, maybe more than that, trying to figure out what's happening / looking for clues. You could do with seeing what it does when it's behaving OK then seeing what is different when it's being naughty.

It will be a bit away from cold before it goes CL. (It's the rough equivalent of going from choke to non-choke if you know older engines. OL is choke and CL is roughly non-choke, but it's more intelligent due to all the sensors, fuelling maps, the computers, and so on.)

 

Sorry for the long delay. My dog got to my elm scan tool so I had to buy another one off ebay. Did as you said up top but I don't know what the heck Im looking at all I see is a whole lot of numbers rapidly changing I don't know how to read this stuff. I did notice that bank 1 seems to say lean beside the percentage on LTFT. Can you school me a bit on fuel trims or post a link to a thread that explains it? Also the pending codes you mentioned are nonexsistent.

 

See post #2

 

LTFT at Idle: 2.34 Lean bank 1, Bank 2 0.00

At 2500 RPM: 1.56 Rich bank 2, Bank 1 0.00

 

Not obviously an air leak. Could be all manner of things but sounds like you'll need a competent tech for it unless anyone posts good ideas.

 

I hear a loud hissing sound that is comming from around the passenger side of the engine, by the crankcase breather. Is this normal or is it the boost bypass that i'm hearing?

 

Loud hissing could well be an air leak. Find it

An OBD tool would let you monitor for it (check LTFT values).

 

I hear a hiss on my 2005 STR and it has no problems so it may be normal. Without codes it gets a bit tuff? Several STR's have had vacuum leaks from the black plastic canister on the driver’s side near the brake booster. But that problem does set a code so I don't think that's the problem.

Left field suggestion. When was the fuel and air filters last changed? Have you run any injector cleaner?

You may need a dealer for this one. Be sure and post back as I have not heard of this problem before.
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I would start with LTFTs because an OBD tool is far cheaper than one visit to a dealer and will reveal things such as whether it is an air leak, even if there are no codes.

 

again sorry for the late reply, Did monitored the FT again still got the same reading as the ones listed above. I did solve the start up issue. It was because when I installed the new battery I covered the battery post with the anticorrosion grease, I didn't now it was only supposed go over the terminal once it was on the battery post, my mistake. That being said my car still hunts under low loads just before shifting it seems to back on and off the throttle before the nexts gear. Some times the the rpm would just drop with my foot steady on the accelerator when cruising could it be a slipping supercharger belt? I've ordered one and just waiting on it but wanted to ask in the meantime.

 

I doubt it's the belt.

 

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What else could it be?

 

Er... any of many things. A/C dragging on the main belt. A bad sensor (O2, for example). TCC. And the list goes on...

 

Guess i'm off to the dealer then...