88 Mj questions

I stumbled upon a 88 MJ, 2wd"big ton",4.0,5 speed,D44, 23 gal fuel tank, rear diff with a little over 100K on it. Its a bone stock plain wrapper and never seen a days work or towing. I cant belive the guy sold it to me for $1000 bucks.
I was hopin a few were familiar with a few things on it or maybe a 88 XJ has the same stuff, as my 95 dosent. There is a little arrow in the left dash area where the warning lights are. The arrow comes on at odd times, it always points up. I thought it meant shift but dosent seem related. It also is hunting for a idle speed, it varies from 500 up to 1300 or so? any ideas? I need to smog it and i seriously doubt it will pass with the idle issue.
It runs pretty good on the road but i also have a 95 HO Cherokee, and man what a difference in power.
Any help of other sources would be greatly appricated.
Thank You.

bumpity bump.
anybody?

I have an '88 also. The light with the arrow is a shift indicator. Supposively is you shift when the light tells you to, you will save gas milage. You truck runs indepented of the light.
The hunting for idle could be a vacum leak, the idle control in the Throttle body or maybe the EGR valve. The throttle body would need to be cleaned. These a guesses at best. Might check Cherokee America for cleaning of the throttle body.
Tom

the shift light operates off vacuum and I never found it to be helpful. ignore it, or remove it.

BrettM said:

the shift light operates off vacuum and I never found it to be helpful. ignore it, or remove it.

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Close, the shift light is controlled by the ECU. It takes into account MAP sensor data, TPS data and such. I actually drove using that light for a few tanks. You'll drive like an old man, but you will get better gas mileage. I got about 19 MPG mixed city/two lane highway when using the light. I've also noticed that the light only works on mine when I'm under very light throttle acceleration. Anything that's more than 1/4 of the pedal and it just doesn't bother coming on.
Liek I said, driving like an old man is how the light wants you to go.
Pretty much everything firewall forward is the same as an 88 XJ, so if you go junkyarding for parts you can find plenty of donors.
I agree that the idle hunting sounds like a vacuum leak.
Likely the power difference comes from your motor running lean with a vacuum leak, and the difference in gearing because it's 5spd.

Thanks for the info. I`ll look for vacume leaks. Seems to run smooth when its running. It will idle at the right speed for a couple minutes then race at 1300 rpm. Today it dosent seem to wanna idle at the 500 mark. I checked the diff for a tag but see none. curious what the ratio is. The original owner did a special order on it so who knows. I dont think its 3.55`s as suggested in all the Long bed "big ton" models. comparing this to my 3.55/auto equipted 95 chero it barely runs going 65 in 5th.
Thanks for the info.

^^^yeah, what they said^^^

DYNA said:

Any help of other sources would be greatly appricated

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www.comancheclub.com/forums

Stop over and introduce yourself...

Jeff

Ah - welcome to RENIX. It's a fun place, but it does call for a little more skill in troubleshooting, because the computer doesn't tell you what's wrong (you have to figure that out on your own...)

The most common 4.0 vacuum leak, in my experience, is the manifold bolts working loose over time. So common, in fact, that I just automatically retorque them all once a year - it's not wasted effort.

Remove the bulb for the shift light - I did in my 88XJ. It's supposed to give shift points for "economy" - but it's full of crap. I get better economy when I don't use fifth gear, and cruise at 2500-3000rpm.

Once you retorque the bolts (they should torque to 21 pound-feet. Get a 9/16" socket with inbuilt universal joint at Sears - you'll be glad you did!) the idle should settle down. If not, get a can of carburettor cleaner and "fog" it around all the vacuum seals - where it gets drawn in will be the other leaks you might have.

The CCV/engine management vacuum lines can be had at NAPA - about $50 the set. I don't have part numbers handy, but I've posted them here, so search for my post on the subject (I posted right after I'd gotten a few sets for myself...)

If it ever gives you too much trouble, go ahead and bring it up here (just make sure you have something to drive back - I've been looking for an MJ, and I don't mind working on the thing...:loveu: )

5-90

dyna said:

I checked the diff for a tag but see none. curious what the ratio is. The original owner did a special order on it so who knows. I dont think its 3.55`s as suggested in all the Long bed "big ton" models. comparing this to my 3.55/auto equipted 95 chero it barely runs going 65 in 5th.

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It's 3.07.

ALL '88 XJ and MJ 4 liters with 5-speed were 3.07. You could get a Dana 44 rear axle as an option, but there was no option for the ratio. (Note: If the truck has the Big Ton package, the Dana 44 axle was included as part of that package.)