Piles

Hey fellas, I recently purchased a 1997 Grand Cherokee and I have a few questions. First I will say it is an inline 6, with the Quadra Trac. Got about 256000 kms.

First off, there is a noise in the front passenger side, it sounds almost like a really warped rotor or something. But louder. I was going to change the rotor this weekend hoping it would help but as this is my first 4x4 I wanted to make sure I didnt waste my money. As I said it is quite loud and consistent with speed. Previous owner said it started about a year and a half ago.

Second, my front and rear diff are leaking, rear I know is the breather and so I will change that and the gasket. Front is puking out the seal as well. I have a hanes manual for the repair but I would like to know if there is anything that I could know beforehand that would help. like special tools, other things to check, and how difficult the job is.

Third would be the idle. My last truck never had a tach, and it was strait piped so It was tough to get a figure for the idle. The cherokee is around 750 upon initial start up, then after I drive about 550 or so. Is that right or a little low?

Lastly is mpg upgrades. I looked into this a bit, but is there anything preferably cheap that could give me a little boost? I have a guy with a chip for this as well. But I dont need performance so if that is a waste of money let me know.

Thanks for everything and if there is anything else I should check please let me know.

Bustedback

550RPM is normal for a fully warmed up engine. Boost the MPG? Yeah, good luck with that. There is no chip that will increase the fuel mileage and performance for the 4.0. Keep the tires aired up and keep your foot off of the skinney pedal, those are the only sure fire ways to increase fuel mileage.

Piles

Kinda what I figured for mileage. Just wanted to make sure. Wasnt positive on the rpms since I work on diesels exclusively and thats what I have pretty much always had. Other thanmy last truck with no tach.

What about the diffs though? Anything i should watch for or replace since I am gonna be in there? I am gonna check out the noise in the front. Hopefully its just a dustshield rubbing or something simple.

dave1123

Be very careful about replacing pinion seals. This is a very touchy deal and should be done by someone with experience. Getting the pinion bearings too tight can wind up destroying them. BEEN THERE, DONE IT!

Seal failure can be caused by worn pinion bearings. You can check this with a dial indicator. Apply pressure up and down on the driveshaft yoke and read the indicator. IDK what the wear limits are, but it shouldn't be much. I'd say around .001" to .003". Also check for a wear groove on the yoke. If it's grooved, it will cause the new seal to fail early. This is very often caused by dirt and mud on off road vehicles. Most vehicles have a flange or "slinger" on the yoke to protect the seal from this, but submergence in mud doesn't help.

On the brake rotor. If the brakes don't pulse when applied, I don't think it's a warped rotor. Check your hub bearings or maybe it's just the dust shield. On my Chevy truck, they were so rotted I just ripped them off. Never had a problem with the brakes.

Piles

Thank you, this is just what I was looking for. I am a HD mechanic apprentice so I can get my journeymens advice and help on the seal if I have trouble. I have heard shot bearings and this doesnt sound like it. I am thinking when I take a look it will be the dust shield. I forgot about that with rotors your right.

I gave the shaft a shake before and there was little to no play. I will check again though. The rear diff is ok its the front that is very ****ed.

KD3NE

Mileage Mod that works ....

Lighten up your foot!

Over the past year and putting about 160 miles on my ZJ every day I believe I have found the most effect engine/vehicle speed.

Slightly less then or about 60 MPH and engine speed under 2000 RPM

I'm averaging about 21 MPG .... If I lived right next to the highway and worked right next to the highway my average might be near 25 - 26 MPG.

If I get stupid and let the speed go over 60MPH/1900 RIM the MPG drops quite a bit.

Piles

Good to know. Damn city driving for me. Atleast I got an auto tranny now.

Got another question for you smart fellers, what oil should I use in the diffs. It says something in the haynes about additives for towing or limited slip diffs. I wont be towing and im not sure if this has a LS. I just dont wanna go through all the trouble repairing these and have one blow cause i used improper oils. Also the t-case as well.

Thanks again you guys have been very helpful.

dave1123

IMHO, adding a friction modifier can't hurt! Anything that makes the gears more slippery will make them last longer.

I once worked at Valvoline Instant Oil Change. We had a customer come in with a 1 ton Ford Diesel pickup and when checking his rear diff, the fluid was black as coal. He said "That's 'cuz I put Moly-D in it." He towed a low boy trailer with a dozer on it and needed all the wear protection he could get.

Molebdenum Disulfide is a dark grey powder used as a lube additive to reduce friction. I've used it to lubricate a new cam and lifters for break-in.

The standard LSD additive does the same thing.

On the mileage thing. Back in the 70's the National Speed Limit was 55 for the reason of saving fuel. The BEST mileage you can hope for is a constant 40 mph on flat ground. Wind resistance above 40 increases fuel usage exponentially. Stop and go uses a lot of fuel because of additional fuel needed for accelerating from a stop.

If you want my opinion on fluids, I'd use 75W90 synthetic in the diffs and ATF+4 in the transfer case. JMHO. If you live in the "Sun Belt" where the temp is above 70 all the time, you can use straight 90W.

Piles

I am in canada so in a few months it'll be real cold. Will 75 90 work for real cold weather?

dave1123

That's what it was designed for. In REALLY cold weather, 90W will "gel" causing the gears to cut channels in it like it was grease. It won't flow until it warms up. There is some even thinner stuff made for Antarctic use. It seems NOTHING flows at -70*F. Even diesel has to be diluted. They even use alcohol in the air brakes.

I live in upstate NY, but I've only seen -35*F.