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Help! 1990 cherokee-what maintenance should be done?

I am about to begin servicing my new/old jeep.
I am unfamiliar with fuel inj.

it is beginning to run a bit rough idling in drive with brake applied
it is beginning to start less easily, turning over longer before firing


That could be a lot of things but if you’re looking to keep it around you should start with the basic obvious things. Spark plugs, wires, I’d say coil but figure they’re pricey and it’d be nice to diagnose this as faulty before doing that, and from there your fuel system. Take off your air intake and throttle body and use carb cleaner to burn out all the carbon build up, mass air cleaner for the sensor, etc. maybe a can of sea foam as well?

Depending on how bad the symptoms are you could be looking at a fuel pump as well. But start with the obvious tune up stuff and cleaning. Should be pretty simple on a straight six (if that’s what it has?)

yes straight 6, thank you fender.

I was most curious about different sensors, my old truck is pretty basic, and has little modern tech.

I do have the Haynes repair manual, and I ll probably open it soon, but nothing beats experience

i did find a broken sway bar bolt, and will be taking it in for alignment after fixing that, and putting on new brake pads this weekend.

@Fender1325, i think the fuel pump is ok, and i have some sensor cleaner, i ve changed the oil previously, and the new oil has stayed clean. i m having a problem with the tranny switching gears, my son says the bigger tires contribute to this and there is something you can replace that will help if you drop the tranny pan, but he can t remember its name

Compression test

Fuel pressure test . . . does this engine have a fuel pressure test port?

@wesw some kind of solenoid? Beyond me. You shouldn’t have too much trouble getting it back in shape. Luckily it’s such a popular vehicle and engine there’s a lot of info online from other jeep enthusiasts. My family went through two jeep Cherokees with the 4.0 I-6. One of my favorite cars ever.

Switch the key on and off 3 times then back to on immediately and watch the CEL light. It will blink any codes in memory. 5-5 means end of codes. 1-2 is irrelevant as best I recall. Your problem could be a dirty throttle, a vacuum leak or possibly old plugs,etc. The early model 4.0L had a common problem with the throttle position sensor that often caused an extremely fast idle at start up but never showed up a fault code. It would be worthwhile to service the transmission. Some had a Toyota transmission, some had the Mopar transmission. Both were good but I saw better service from the Toyota.

I have owned several. The first was a 1990 that I finally junked at 300,000 miles when the transmission died. All in all it was one of the best vehicles I ever owned.

thank you!

don t know yet db, I ll check. engine has plenty of power. will be changing plugs soon. money is a big issue so buying much testing equip. or paying a mechanic is problematic. I should have most tools I need tho. my son is a jeep enthusiast and works at a shop that builds specialty off road vehicles for customers worldwide so that helps. but he is busy and travels a lot, his company also does millwright work, and monster truck shows occasionally. he just got back from some shindig in vegas. I ll probably change the plugs check and clean the sensors, change tranny filter and fuel filter, and see where I am then.

this is gonna take me awhile. I m not the man I used to be. which is good in some ways, but I can t do the work of three guys anymore, more like half a guy, and even that is a stretch.

Just take your time. Should all be very straightforward stuff.

You say “beginning to run a bit rough idling in drive with brake applied” does it idle ok without the brake applied? if so I would look at the vacuum hose that goes to the brake booster if applicable.

i will check that, but it is just more noticeable in drive I think, and I m hoping it just my plugs.

rainy day today.

ok… I found two vacuum problems

  1. a small plasic tube (about 1/8 in) that connected to a black plastic fitting (about 1-1/2 in dia. by about 2in tall) that connected to a junction of 4 water lines one of which went to my heater thru my firewall. was broken. repaired that

2)a set of 5 vacuum lines (blue, white, red, yellow, green, and blue) they go down to my front axle on passenger side, except for the blue one it has a black plasti disk on the end, about 1-1/4 in dia., with an empty nipple on the other side, I can t find where it is supposed to connect. ???

blue vac line may be air vent filter and not go to anything, my manual is not clear and does not have the color codes

the small vac leak I repaired actually seemed to help

Good to hear. keep plugging away at it. Tough to say for those vacuum lines. I’m just a shade tree hobbyist type, but, while its running do you feel a vacuum at the end of any of those lines? might help to put some tape over them for the time being I dont know. I wonder if any of that would relate to engaging 4 wheel drive or something along those lines. Outside of that I’d stretch the lines in every direction seeing what they mightve reached to. I’m pretty sure the axles have a vent line that doesnt connect to anything. One time my dads 4x4 expedition had a small hose hanging down from the rear axle and we couldnt figure where it went. Turns out it was in fact just a vent line that doesnt connect to anything so we shoved it back upward and its been fine ever since.

the back end of my air filter is covered in oil. I see where it s coming in, but why???