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Idle Speed Specification for 4.0L Automatic?

soyjer

NAXJA Forum User
Location
California
The following is the only information I can find in the 1999 XJ FSM regarding idle speed, but it doesn't seem to match at all what others are saying their idle speeds are in this forum...generally around 750 RPM.

Why would the FSM give an oil pressure spec at 600 RPM and call it "Idle Speed" when it doesn't seem that there is any way that the vehicle would normally ever be able to achieve that low of an RPM?:

I'm trying to diagnose a "lurching clunking when put into drive" problem, but first I want to make sure that my idle speed is correct... Has anyone ever found the actual factory specified figure for what idle RPM is supposed to be?:

FSM page 9 - 93:

4.0L ENGINE SPECIFICATIONS:
Oil Pressure
At Idle Speed (600 rpm) ..(13 psi)
At 1600 rpm & Higher . . . (37 to 75 psi)
 
It should be listed on the emissions info sticker under the hood. 600-750 is about right. In Colorado, they may fail you if it is too high.

As far as clunking, have you checked your ujoints?
 
It should be listed on the emissions info sticker under the hood. 600-750 is about right. In Colorado, they may fail you if it is too high.

As far as clunking, have you checked your ujoints?

The U-joints seem solid...the clunking seems to be coming from the Chrysler 8.25 differential. I'm just surprised that the differential would be shot to hell with just 97,000 miles on this stock XJ that's never been off road nor abused, and has never towed anything.

Another symptom of (I guess) differential failure is that when I grab ahold of a rear wheel, when the rear of the vehicle is jacked up, and I push and pull on the wheel, there is a definite play and associated smaller clunking noise when I do that. As I mentioned, part of the problem here is that I don't know what "normal" is on an XJ. I haven't been able to find anything on the internet yet regarding a clunking noise that only happens when you put it into drive and never when you put it into reverse, and so I will probably have to take the XJ into a Chrysler mechanic so that he can tell me if the clunking that I am hearing is normal or not for that Chrysler 8.25 differential.
 
A few things will cause clunking, grab the pinion and try to move it sideways, not rotate. I usually slap it hard sideways with my palm or you can try prying the pinion in and out. If it moves, your collapsible sleeve (which has many names) is out of spec. or your pinion bearings are likely toast. Or remotely possible your pinion nut is loose.

Most common is carrier bearings. Pop the cover off and pry under the carrier and see if it lifts. If you can detect anything but a tiny (.003 thousandths) movement your carrier bearings are likely toast. It is remotely possible the the preload adjuster lock fell out for your carrier bearing and the bearing loosened. I'm pretty sure you have an adjuster lock, some do, some don't. There was a TSB about the adjuster locks, I forget which year.

Less likely is the spider gear thrust washer worn. There has been some spider gear failure issues, a bad batch of spider gears. Thrust washers wear out, but usually with way more mileage than you have, unless you ran the differential dry (no Oil).

The axle moving in and out doesn't really mean anything, I've seen them way loose with no issues.

If you have it fixed don't let them soak you, not really that hard a job. One good thing about the 8.25 is they are relatively easy to fix. It's usually not necessary for a total rebuild unless it grenaded.

I always replaced the collapsible sleeve, but they say you can try re-torquing the pinion nut.

Last tip, save the oil that comes out in a clean container. Strain it through a cloth and see what you have for metal flakes and metal grit. A tiny bit of grit is to be expected, flakes usually indicate a bearing is slowly disintegrating.
 
Another symptom of (I guess) differential failure is that when I grab ahold of a rear wheel, when the rear of the vehicle is jacked up, and I push and pull on the wheel, there is a definite play and associated smaller clunking noise when I do that.

How much in-out play? It's a c-clip axle so the axles float a bit. I'm not sure what the spec is, but 1/32-1/16" end play with a slight clunk as you yank back-n-forth is normal.
 
I finally got around to measuring the in-out play...it is > 3/32" (.094") and < 1/16", and equal for both rear wheels.
An ex Jeep dealership mechanic said that the specification is .070" max, which if true would mean that mine have about twice as much play as they are supposed to have.

I thoroughly cleaned my throttle body and idle air controller off the vehicle, and the idol stayed at about 730 RPM when warmed up, which I believe is the correct RPM. Since my clunking only happens when putting it into drive and never in reverse, idle speed seemed an unlikely suspect, but now at least I've eliminated that as a possibility.
The ex Jeep mechanic checked out my clunking problem and verified that it is most likely the transmission because the rest of the drivetrain seems tight and because it only happens when going to drive and not into reverse.
 
Oops...I meant to say that the in-out play on both rear wheels measured >1/32" (.031) and less than 1/16 (.063), so in fact it does fall within the .070" max spec.
Amazing how loud the brake drum acting almost like a dull bell can make that little bit of movement sound.
 
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