Idle stutter on the XJ - what's goin on??

[Kramer]

OK.

I'm super happy with the 96 XJ that I've just procured and the vehicle behaves "just like it should" according to sir Gojeep but there's one thing that has stumped me: a rough idle.

The rough idle is constant. It is in time, i.e. every second engine revolution or whatever - can be felt as thumping when you hold the hand near the exhaust. Once you lift off idle it is smooth.

Already checked the following:

1. Cylinder head gasket - presumed good cause no bubbles in water, no smoke from oil filler cap after a long run and no power loss - the vehicle canes, yes sir.
2. Vacuum leak on inlet manifold - gasket was replaced by the stealership to weld the cracked manifold (taken off) and there are no other hoses loose (to my eye anyway).
3. Checked the brake booster - pinching the hose does not fix the problem so the diaphragm in the booster is good.
4. Cat/exhaust obstruction - eliminated cause I've put a hiflow cat and muffler on her 2 days ago (sounds sooooo sweet).
5. Spark plugs and leads, new
6. Dizzy cap + rotor button - cleaned up and replaced.
7. Put some injector clean into the fuel but have only done 100 kays since then.
8. Checked the error codes - no sensor failure (e.g. map sensor)

So there you have it. The only thing I haven't done is removed the injectors and have them cleaned professionally.

What are your thoughts?? Is there something else I should look at?

Kramer

[ 22 August 2002, 11:44 PM: Message edited by: Kramer ]

[dxj]

HI,

Sounds like one cylinder not quite firing cleanly.

Try a dynamic compression check, removing one injector cable, so the injector doesn't fire, at a time to see if all cylinders have the same fall off effect on idle speed. You may get ECU fault codes saying injector fault when you do this. Similarly you may get mixture or other faults. So don't do it for long.

If it is one cyl running low it will have less effect when you cut off its fuel. Sometimes you can hear the uneven double "beat" in the exhaust when you disco a good cyl, the beat goes single and even when you have the suspect cyl.

Once you know which cyl, you can concentrate on finding the problem with it.

Try swapping plugs/leads ( at both ends) to see f fault follows the plug.
Try swapping injectors around to see if the fault follows the injector.

I had a similar prob with a falcon 6, was sticky injector that didn't fix after cleaning. A new injector solved the problem. Of course check all other things first, sounds like you have, but most likely idle-only problems are small manifold leaks.

To check for manifold leak, get an oil can with long squirter. Squirt engine type 20-50W oil along the intake manifold gasket all around the suspect cylinder. The oil will clog the hole and slow the leak to fix the problem temporarily. Dealer already replacing it means just as likely to leak as not in my experience.

Worst case could be a slightly burnt or sticky valve ( or hydraulic lifter ). To check this do a compression check the traditional way using guage in spark plug hole etc. One cyl down indicates mechanical problem.

Idle problem is generally not fuel pump related as fuel delivery is slow, and pressure required is low. Pump problems usually occur at high revs/load.

Disclaimer: Of course this is non-professional advice only. Check with a professional if you are unsure or not confident this advice applies to you.

[Jonesy]

maybe throttle position sesor? i know you checked your error codes...but drop into the closest jeep centre and get them to plug the the hand held computer in ( they never charge me ) and see whats going on.

[Gojeep]

Who else out there with this problem and a 96 as had another around this week and it sounded worse than yours Krammer at idle but was told it has always been that way? :confused:
It was the first year they went to the OBS II system and maybe there were a few bugs? graemlins/dunno.gif
Just drawing at straws now I guess but at least it pulls well with no break down at speed or load which usally means fuel and electrics are good.
I think injectors could be it like Ron said but you have to use the right brand of cleaner as the cheap ones are no more than detergent. :( I have used the Nulon brand with good results every 5000 kms a full bottle. Added a bottle two tanks in a row once when the economy was bad and it picked up again tongue.gif

[Kramer]

Thanks for the advice.

Will try all of that over the w/e.

[Gojeep]

Originally posted by Gojeep:
Who else out there with this problem and a 96 as had another around this week and it sounded worse than yours Krammer at idle but was told it has always been that way? :confused:
<font size="2" face="Verdana, Helvetica, sans-serif">Turned out he had been running two spark plugs that had a different electrode length. :eek: