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Hey guys, about a month ago I bought a 1989 4.0 inline 6 that's 2WD. It sounded and seemed to run well when I test drove it, so I bought it and trailered it home. Once I started driving it around, I noticed the idle sounds worse, like it's stuttering. RPM barely changes, though. It also started to occasionally stall when coming to a stop or accelerating from stopped. I got stuck in road work and every time traffic came to a stop, it'd shut off and I'd have to start it back up. I've done Cruiser's tips 1-5 and so far nothing has made a difference. I did a smoke test in several spots and the only leak I've found so far was at the crankcase breather gasket at the back of the valve cover - I have a new one ordered. I've read that the exhaust manifold commonly breaks at the bottom weld, so I was looking to see if any smoke came out there, but I never saw any. Is there a better way to test this? Also I hear a quiet knocking that doesn't change with RPM, I'm going to get a stethoscope to try and figure out where that's coming from. So far I've replaced: Spark Plugs + Wires Distributor Cap + Rotor ICM + Coil MAP Sensor + its vacuum line Both Vacuum Harnesses O2 Sensor IAC valve Big 7 Wire upgrade I'm pretty new to wrenching and I'd greatly appreciate any help or insight. I love this little truck and hoping to get it to a place I can confidently daily drive it.

In my 88 pioneer, i6 4.0L that I’ve recently purchased I’ve changed out the fuel filter and fuel pump/assembly after it sat for a year-ish. It ran decent with the old gas that was in it but since I’ve added new fuel in it runs like crap. I’ve added seafoam and got the oil changed. When it runs anywhere above 3,000 rpm and then I let off the gas it drops below 750rpm, stutters and almost/sometimes dies. I’m not sure what to check next since I’ve done the fuel pump and filter. Thanks in advance!

So I recently get my Comanche back on the road after 18 years (yay!) but, I’m having a reoccurring issue. If I cold start it, it will rev up to about 2k then drop to about 500ish, then it will stumble and jump to 750 where it will idle consistently. If you rev it up past 2500ish, when it drop back down, it will stumble then die. I’m not sure what would be causing this, as it runs great at higher rpm, and had plenty of power. I do, however, think it’s running rich, because it smells a tiny bit. things I’ve done so far: new fuel tank new fuel pump/sender new fuel filter removed the thermostat (old one corroded closed) new air filter. refreshed the main grounds on the block. it has plenty of pressure to the rail, and I’ve put about 50-ish miles on with no avail. I’ve also ran sea-foam through it, which helped a tiny bit. I’ve already ordered new injectors, and will be installing them. I might also check the TPS, but I think it’s fine. any help is appreciated.

Just got my Comanche (1988, 4.0L inline 6 cylinder, 2wd) a couple months ago, and have been working through the various quirks. It drives has been driving well, but two days ago I had to get towed home. After driving for about 20 minutes, it died at a stop light. Turning the key would crank the engine, but no fire. I tapped on the fuel filter, poked about in the engine, and tried to start it for a while, but didn't want to run the battery down. When my tow got there and pulled me into a parking lot, it started, but it died again after a few blocks and had to be towed the rest of the way home. Looking around the forum here, I see it could be an O2 sensor, the ERG valve, or the IAC. I pulled the IAC and cleaned that with some Carb cleaner. I killed it by pulling on the ERG, but it started right back up again. I let it idle until it died (about 20 minutes?) and it won't start now. I will be checking the reading on the O2 sensor once I can get it running for a bit again. I have only a little experience working on cars (thanks for the pinned Acronym list!), so any advice you can give me would be great. Thanks in advance.

Hello all, I am a proud new owner of a '87 2.5l 4X4 4-speed. I recently picked up this MJ in Albuquerque, NM, and immediately drove it home to NH coast without a single problem along the way. Well, there was one thing...the valve cover leaked a little. Today I replaced the valve cover, using a little RTV and one of those rubber gaskets. I scrubbed the hell outta the cover, getting all the years of gunk out of the two pathways where the two hoses plug in. When I started it up after replacing the gasket and letting it sit for 6-7 hours, It sounded terrible...a low stumbly kind of idle, with very strong exhaust fumes coming from seemingly everywhere. I visually checked for leaks, but i couldn't find any apparent ones. I also changed the oil today, and the truck took about 4.5 quarts. Is there something I could have knocked loose or something else that needs to be cleaned after clearing those ports on the top of the valve cover? I am at a loss here. I;m gonna let it sit over night and check all the seals again tomorrow. Any advice would be much appreciated. -Frollix

So 3 weeks ago i bought an 89 comanche short bed 5 speed with 91 thousand miles on it clean frame and good body. I paid 1000 doallars at police auction. I finaly got it home changed the oil and filter, drained the fuel and added new tried to start it, i had no spark. So i started to check things out so far i have replaced the CPS, Coil, Cap, Rotor, Wires and ICM was going to change the plugs but after i pulled one and i looked brand new i decided not to, after all that got it to start, but it would stall while driving. I replaced the the TPS and it was good to go or so i thought. It drove fine for three days. my brother had it on friday and said it ran great, actully better then his 90 cherokee when it was renix (we just swapped in everything from a 95 so i have an extra renix engine and majority of sensors) so saturday i go to drive it and it was running like total garbage and got worse till it stalled. now it starts runs so rough that it seems like i have muitple cylinders missing and there is no throttle input. I checked my fuel pressure its at 38 pounds. I'm going to change the spark plugs to see if maybe i fouled one or mutiple out while trying to get it to start and I'm going to change my fuel filter because i wanted to do it any way. So what should i check after i do the plugs and filter. I'm leaning towards something fuel related because it got so low (less then 1/8th) and i don't know the quality of gas that was in there. As far as service records or any history i don't know any. The only tring i know is that it sat for at least two years at the impound.

I have done my research before asking you guys on this but I can't seem to put my finger on what the issue is. Latly only at idle after coming to a stop, the rpm's would go really low and want to stale then jump back up a little. Today the same thing happened but worse and there was a little hesitation on take off. But after gaining speed I am totally fine. Shifts fine and cruises fine at all speeds. SO I was thinking either a vaccum leak ( I have checked the hoses but who knows) or a O2 sensor going bad or the MAT sensor is going bad (engine temperature sensor that determines air-fuel ratio I do have a small intake/exhaust manifold gasket leak but I don't see how that would create such a hesitation. I had the gasket go about two years ago and was running fine. I have upgraded all my grounds, I have a mopar CPS and drilled out the top mounting hole, cleaned all connectors and relays, plugs and distributor are fine, air filter is fine. I would appreciate the input.

Hi all, i know for you more experienced MJ owners this may be a dumb question, but here goes. I have an 86 XLS with the 2.5L with a high idle problem, recently replaced every vacuum tubing i can find even the one on that ball under the passenger side headlight, replaced MAP, recent tune up still no relief. I ordered the only replacement that looks a lot like it, since I can't find one for the Comanche listed anywhere, so I ordered part from amazon its AC Delco 19160423 Idle Speed Control Motor it states that it will fit 1986 cherokee 2.5L now my question is this did the two,the MJ and the cherokee have the same engine that year. I read somewhere on the forum that another member used same part from chevy which will be AC Delco and it worked. :dunno: by the way did clean TB but no change Thanks for any help on this :bowdown: