4.0L rough idle after injector change?

hello, ive searched everywhere on 3 cherokee specific forums and 2 non xj specific, but jeep forums. ive not found anything that worked yet. heres my problem:

1988 renix 4.0. this thing is 150% stock other than routine maintenance things of course, but all was swapped with the stock spec when replaced.

since the very day i got it on february 3, 2016.. its had a very very subtle misfire. i changed the plugs and wires and hoped it would help but no luck. i didnt pay any mind to it since you really had to listen for it to hear it. its always been a slug, its a bone stock xj. not a racecar.. so i didnt care. but here lately, its hard to start, starts on first try, just turns over a good 3 seconds or so before it does. its also lost some power pulling the mountains. it runs rich, blows black smoke, etc.. not alot of smoke, but you can see it when the light hits it right. it would also randomly die after acceleration and pushing the clutch shortly after. mainly when it was a cold start, but sometimes on an already heated engine. i replaced the cps when i got it because it would die like it ran out of gas. i replaced it and it fixed it so i crossed that off the list to do it again this round. i replaced the fuel filter thinking it might help with the sluggish mountain pulls and foul smelling exhaust... nope. i got an idea to replace the injectors with some off another jeep i have for parts and it ran absolutely flawlessly before i pulled the engine, so without a doubt in my mind the injectors are good. i took my old ones out, noticed HEAVY restriction in each injector port in the intake manifold where you seat the injectors. i took a flat head and scraped all the gunk and buildup out off the holes to where they didnt have restriction. put the injectors in, buttoned it all up and...... took freaking forever to start. even longer than before. not a first try thing either, i had to try multiple times and finally by holding the throttle open it cranked. now it idles like complete crap, sounds like it has a beefy cam in it now.. thats how bad the misfire is. it smokes when given gas, but it runs perfect when i do that. so idle is crap but throttle is definitely improved. i took it for a drive and gunned it in the grass and it spun the tires, it wouldnt do that before it was so weak. both tb sensors look new. i unplugged them and nothing changed... but i unplugged the cps and it immediately died. so heres my question... if its a tps or whatever... why is it just now idling like crap instead of before? any help is appreciated. if you have any questions about anything, please.. feel free to ask. i will be glad to answer.

also to note, when given just a slight amount of gas, it sputters really bad like its going to die. push the throttle a little more and it runs perfect. let off, back to bad misfire. the rpms are normal, just the bad misfire.

You may have more than one problem and need to take some time and systematically go through a series of steps before you buy or swap any parts.

First is to clean the wiring connections and contacts at the battery posts and the ground connections and block contact surface at the passenger side of the block near the oil dipstick connection to the block. By clean you want bright shiny metal on both contact surfaces. A wire brush and sand paper work well.


Check all the vacuum lines for any disconnect hoses, loose leaking hoses, etc, especially the one Vac line that runs from the throttle body to the MAP sensor. It comes loose of the Throttle body very easily, sometimes blows out. It is a common source of issues like yours.

Also pull the Vac line off the male nipple on the Fuel Pressure Regulator FPR, to see if there is any sign of gas leaking out it when you cycle the fuel pump (turn the ignition switch to run for 6 seconds, then off for 10, then repeat) Also borrow a fuel pressure tester and have it connected to the fuel rail while cycling the ignition switch on and off. Do not crank yet. If there is any sign of fuel at that male nipple on the FPR is leaking gas into the intake and by passing the injectors, thus it is the source of the running rich, black smoke. If it is OK, then reconnect the vac line to the FPR and crank in while taking the fuel pressure readings, before you crank (ignition cycled into run mode/off/run), then check the pressure while cranking, and then while running and then while accelerating. Write then all down on data table and post the results here. Lastly check the fuel pressure at idle with the Vac line disconnected and post it too.

Get an ohm meter and check an entire list of things for bad grounds on the sensors, then Test the TPS with the volt and ohm settings, etc and post that data. Check the O2 sensor wiring for 12 volts to the O2 sensor heater and the heater inside the sensor. Check the ground at all sensors with the sensor disconnected and checl between the ground wire at the sensor and the battery negative post. The Renix jeeps frequently have ground issues in the harness and other places related to the engine controls.

Also check and see if you criss-crossed any spark plug wires or fuel injector plugs, as that is real easy to do. Just disconnect one at a time (use insulated tools for the plug wires at the dizzy cap) while the engine is running to see if the engine smooths out are runs worse. If there is no change, chances are that is one of the problem cylinders, hopefully just a criss-crossed wire?

There are more tests to run later if needed that are free to do, or very cheap-cost effective to do, that do not require wasting money on parts in guess-a-thon to find the problem. Replacing parts will not fix criss crossed wires, bad grounds, loose vac lines, etc. Many tools can be rented for free from the various parts stores nationwide.

There are many old threads here and write ups by Cruiser54 and me on the Renix 87-90 jeeps on testing and fixing these problems the easy way, systematically at little cost. They cover the TPS, MAP, IAT, CTS, CPS, O2 sensor, grounds, sensor ohm and voltage tests and normal readings!!!!

Welcome to Naxja!!!!

http://cruiser54.com/

Is where the easiest to find, most condensed and best laid out tech tips for troubleshooting the Renix 87-90 jeeps are right now regarding stock engine running problems.

Bandito said:

also to note, when given just a slight amount of gas, it sputters really bad like its going to die. push the throttle a little more and it runs perfect. let off, back to bad misfire. the rpms are normal, just the bad misfire.

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That is usually a vac line problem to the MAP sensor (you may have knocked it loose changing the injectors), bad TPS, bad grounds on those sensors (wiring, and or ground contacts) or bad FPR.

Ecomike said:

That is usually a vac line problem to the MAP sensor (you may have knocked it loose changing the injectors), bad TPS, bad grounds on those sensors (wiring, and or ground contacts) or bad FPR.

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The MAP line issue happened to me the firct time I swapped injectors in my XJ.

what year engine did you pull the injectors from?

Johnnie Walker said:

The MAP line issue happened to me the firct time I swapped injectors in my XJ.

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Mine came loose a good 6-7 times on my 87 every time I did a lot of work in that area, on the TPS, IAC, FPR & injectors at the fuel rail. I think I finally fixed it for good. The problem is the connection at the throttle body after 30 years.

Used injectors that have sat for a while can be problematic. The fuel dries in them and they clog up. The new gas does little to unclog them. A can of BG44K fixed the problem for me when I went with some used injectors on my stroker. It ran like crap, I filled up with gas and put in a can and within 5 miles things were better.

Now for buildup where the injectors go into the intake, this should not happen. The only thing there should be air and fuel. A bad intake can allow exhause gasses to back up into the intake. I would do a vacuum meter check for the valves.

old_man said:

Used injectors that have sat for a while can be problematic. The fuel dries in them and they clog up. The new gas does little to unclog them. A can of BG44K fixed the problem for me when I went with some used injectors on my stroker. It ran like crap, I filled up with gas and put in a can and within 5 miles things were better.

Now for buildup where the injectors go into the intake, this should not happen. The only thing there should be air and fuel. A bad intake can allow exhause gasses to back up into the intake. I would do a vacuum meter check for the valves.

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Interesting,... or a bad EGR valve!!!! Since it is a Renix.

Ecomike said:

Interesting,... or a bad EGR valve!!!! Since it is a Renix.

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You are correct. That could also cause the crappy idle. Hell its Monday, your lucky I can type.

old_man said:

You are correct. That could also cause the crappy idle. Hell its Monday, your lucky I can type.

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LOL. It doesn't matter what day it is for me now, with my severe Rheumatoid arthritis I feel like I won the lottery when I can just type.

Everyone else brings up good thoughts about the MAP/EGR etc. I'll second one of Ecomike's ideas since some of your symptoms match some old ones I had: did you label or otherwise mark your injector pigtails as you disconnected each one? I did a similar job awhile back and neglected to mark stuff: ran for several months like that until I failed smog and had to figure out that I had hooked up 4 out of the 6 injectors to the wrong plugs (that's like a six page thread somewhere on these forums:confused1)

I have an '88 as well; if you peel back the ancient rubber boots on your injector plugs you want the wire colors to match (front to back):

1-light blue
2-light green
3-tan
4-yellow
5-white
6-brown

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After I got them in the right order it ran really weirdly (making me think the renix ecu does store at least some prior data) and I had to disconnect the battery and things have been great since then. Definitely clean and check the condition of the main grounds like the dipstick stud and firewall braided cable, they're likely either corroded or soaked in oil which can make a huge negative difference.

1-light blue
2-light green
3-tan
4-yellow
5-white
6-brown

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Is this wiring the same for a 2001 xj? I'm having super similar issues but also got the codes P0432 Catalyst 2-1 efficiency, P0175 Fuel system bank 2 sensor 1 lean, and P0204 Injector 4 control circuit. Runs super rough at idle and died once, but when i step on the throttle it runs great and has no issues. I'm Incredibly stumped. Any other suggestions for me?

nmerrion93 said:

Is this wiring the same for a 2001 xj? I'm having super similar issues but also got the codes P0432 Catalyst 2-1 efficiency, P0175 Fuel system bank 2 sensor 1 lean, and P0204 Injector 4 control circuit. Runs super rough at idle and died once, but when i step on the throttle it runs great and has no issues. I'm Incredibly stumped. Any other suggestions for me?

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it is not the same colors for your 2001. but you are throwing a code for injector 4 so the wiring to that injector is bad. the connection to the injector or the injector itself is bad. to further diagnose the issue i suggest starting a new topic.

I just replaced the injectors with new 4-hole ones. That was after i was getting codes for a multiple misfire on cylinders 1 and 5, plus a p0201 injector 1 control unit. After i switched the injectors, all my problems went away and about a week later i got the new codes.

I just replaced the injectors with new 4-hole ones. That was after i was getting codes for a multiple misfire on cylinders 1 and 5, plus a p0201 injector 1 control unit. After i switched the injectors, all my problems went away and about a week later i got the new codes.

i also checked the injector connectors with a noid light and they seemed to be fine. i guess ill try that again on all of the cylinders

try wiggling the harness. but focus more on cylinder 4