View Full Version : lengthy high idle thread...

May 26th, 2006, 19:20
hi guys,

ok, it's the 91 xj 4l aw4:

idle is always (cold or hot engine) at least at 1k, more often at 1.25k - 1.5k
when starting up, it goes up to 1.75-2k then dropping to the said rpms.

it's really annoying me - too loud, too much fuel / km, too much pull when on the brakes...

i checked the tps, it has exactly 5v supply, it starts & ends at the right voltage.
i can't find a vacuum leak. the iac is cleaned and works.
the plugs are rather brownish - indicating lean & hot running. the engine seems to get hot really fast (2-3 min to thermostat opening) but runs at the right temp. under all conditions.

can the engine management think it has to raise the idle (i.e. for the ac)?
switching the a/c on is making the rpm stumble for a fraction of a sec, so it is NOT running all the time...

something is fooling the ecu to raise the idle. any ideas as to what this might be?

thanks, guys,


May 26th, 2006, 21:26
I had the same issue with mine doing the same thing, I checked everything and could find nothing wrong. But i did have a exhaust leak that i thought was a cracked manifold. During my attempt to install a new exhaust manifold i discovered that it was not cracked but the bolts holding it on where extremely loose as well as the intake manifold bolts. When i put it all back together my high idle was cured. So i am thinking that it was either loose connection somewhere or it was sucking in air from the loose intake bolts. That might be something for you to check. Good luck.

May 26th, 2006, 21:29
hey! i think i have a cracked manifold but can't see or smell anything. you might be onto something there...

i will check and report,
thanks, friend

May 26th, 2006, 21:34
I have heard that spraying some carb cleaner until it fogs near the suspected leak will make it easier to find and confirm!

May 26th, 2006, 21:39
As I've said so often, it's a good idea to check torque on your manifold bolts every year or so.

Since the intake is aluminum and the exhaust is steel, there are two different expansion rate at temperatures - aluminum expands twice as fast as steel (12 millionths of an inch per degree Fahrenheit vice only six for steel.) This results in a sort of "lever" action, which translates into a turning of the bolt (due to the thread helix) and eventual loosening.

I check my manifold bolts yearly, and I'll usually have one or two that want tightening.

I replaced the bolts with silicon bronze last time I replaced a gasket on one of my rigs, and put new Belleville spring washers on, and haven't had trouble since. I therefore suggest replacing the bolts with brass or bronze if you have to remove the manifolds for any particular reason (it's enough of a job you don't want to do it "just on account of 'cause.")

The bolts are 3/8"-16 x 1-1/4" w/Hex Head, and are available in both brass and silicon bronze from Fastenal (www.fastenal.com.) Belleville spring washers are also available through Fastenal - you'll want to get about 10 of each.

The studs are best replaced with cut-off bits of brass allthread rod - also 3/8"-16. That should be available in your local hardware store (that's where I get it,) and cut the new studs to about 1-3/4" or 2" long. Screw in finger tight.

If you want to use Loc-Tite, use #272 ONLY. It is the only one that is formulated to handle elevated temperatures (it's actually designed for exhaust manifolds,) and will hold up well. #222 and #242 just aren't going to happen - they'll fail at the first heat soak.


May 26th, 2006, 23:36
thanks, 5-90

will consider this.
any tricks on how to get to the bolts with little hassle?

May 26th, 2006, 23:50
Pull out the airbox.

Get a 9/16" short socket with an inbuilt universal joint, and an assortment of extensions for 3/8" drive.

If necessary, bubblegum will work for holding bolts in the socket - it works better than packing the socket with grease. The socket goes in the freezer overnight after you're done, and the gum will come out rather easily as long as it's frozen.

It is usually easier to start the bolts by hand and put a socket to them, rather than start them with the socket already on. I've done this job a few times, so I've got a pretty good idea... The lower back bolts are the most difficult - a hand mirror may help you here.

Have plenty of work light, you'll need it!


May 27th, 2006, 13:31
all right - update!!!

i just did what some of you suggested and tightened the bolts holding the manifolds. they were not exactly loose but definitely not tight enough. of course, the one low in the back was the one needing most tightening :-)

that brought the idle to a stable 950 rpm, as far as my parking lot experiments tell me. i have to wait if it stays that way over time.

i also noticed, when the throttle body lay open 'cause i had to remove the airbox, that the throttle was not COMPLETELY closed. there was a cap of apr. .5 mm on both sides. so i took my vise grips and turned the stopper in a bit more. actually quite a few turns.
now the trottle is really closed.

right now it idles at about 750 rpm

if it stays like that i'm more than happy

May 27th, 2006, 15:34
ewww dont use bubble gum, put a piece of a napkin over the head of the bolt, then into the socket.....works insanly well, and isnt...well bubble gum

May 28th, 2006, 00:19
There's nothing wrong with using gum - I've been doing it for years.

If it's not a "blind" hole that I can reach easily, I'll use paper instead. However, when I REALLY need the bolt to stay put, I use gum.

It comes out easily when you freeze it, so what's the problem?

Packing the socket with grease can also work, but I'm a little leery about using grease around exhaust hardware. Gum doesn't seem to cause as much trouble.


June 1st, 2006, 10:02
I've had the high idling problem for over a year now (usually when the weather is warmer). It was embarassing at the very least- having a friend hop in for a ride and, upon cranking the engine, your vehicle sounding like you're about to blast off for the moon! Tweaking with the vacuum lines seemed to help, but it wasn't until I found this thread that I was able to finally solve the problem.

After checking the manifold bolts, I found one (#5) that had worked itself half way out! Some of the other bolts were only finger tight. It was difficult getting at some of the lower ones. A smaller torque wrench would've been nice! By the way, for those who might want to take shortcuts, how important is it to obtain the exact torque/sequence?

Thanks all/5-90! My engine idle is now back to normal!

June 1st, 2006, 11:42
Considering you want an even clamping load, proper preload/torque is fairly important. There are spring washers under most bolt heads, so there's room for a little variation, but don't get silly. Ideally, you'll also want the torque value you're trying to get to be somewhere in the middle third of your torque wrench's scale.

Sequence is also fairly important - if you're installing the thing. If you're just "touching up," the bolts should already be fairly close to spec, and it's less important then (but proper torque/preload is still important...) When I'm doing the annual "touch-up," I'll usually just work back to front, bottom first, then top.

Now, if I can just get to sort out this intermittent RENIX high idle I've been having for the last month or so...


June 3rd, 2006, 23:25
i still havent gotten my high idle issue fixed either, new iac, new tps, manifold bolts are tight, no vac leaks

limited 90 xj
June 4th, 2006, 03:15
same here i've check everthing it is all good
and still idle at 2000 what a pain

June 4th, 2006, 12:55
Yeah - but my problem is intermittent! If it would do it consistently, I'd have an easier time troubleshooting...

Not only that, but it's inconsistent as well. Sometimes it will idle about 1200, sometimes up around 2800. Sometimes it drops within a minute, sometimes it will do it all day. Sometimes... Well, you get the idea.

Damned irritating.