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93 XJ 4.0L H.O. trouble shooting help

Old 05-06-2019, 08:08 PM
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Default 93 XJ 4.0L H.O. trouble shooting help

LOOONG post short:
- What would cause the cherokee to sputter when applying/letting off the gas once operating temps, and then not want to idle/ re-start.
- Aside from spraying vacuum lines with starter fluid is there another way to check for vacuum leaks? Does anyone have a good vacuum line diagram? The ones I find online don't make a ton of sense to me or seem to really match-up with how my lines are routed

I am the lucky recipient of a $500 1993 Cherokee sport 4x4 with the 4.0L H.O. engine and manual transmission. The old girl has 270k on the clock, but I was assured it has good compression (I will be checking that once the rain lets up). The PO told me he had been driving it to and from school when it would suddenly start running rough and die and not want to start up for 20-30 minutes. He told me he heard somewhere that it could be an issue with the distributor.

Long story short he didnt know how to put the distributor in so I got the Cherokee for a steal. I installed the distributor when I picked it up, fired up the cherokee, and drove it onto a trailer for the 1.5 hour ride home.

While unloading I noticed a very stiff brake pedal and brakes that barely worked. I took it for a 15 minute spin (yes, not my smartest decision with questionable brakes) to see if I could replicate the problem he was having with it.

Sure enough, once it was warmed up it would sputter a bit when stepping on or letting off the gas. Then when I went to turn around it died. It did not want to start for about 10 minutes. I popped the hood as soon as it died and nothing seemed out of the ordinary (no steaming or hissing or anything to lead me to believe something was wrong).

When I pulled into the driveway I let it idle and it was running very poorly and it sounded like is was occasionally missing on a cylinder or two. I shut it off to do some research.

I waited about 30 minute and tried to start it again, it wouldnt start. I pulled the #1 spark plug and it was VERY worn. So, I ran to town and grabbed some new plugs (I read the 4.0L likes champion plugs so thats what I got).

New plugs in, and it fires right up and seems to idle ok, right around 900rpms. When I press the brake pedal it idles higher, but when I let back off it drops back down and starts to idle a little rough. The more times I do this the worse it seems to idle.

Can a bad brake booster be the cause to all my issues? I sprayed vacuum lines with starter fluid to see if I could find a vacuum leak somewhere, but had no luck. I pulled the booster check valve and tried to blow into it from the hose side and it didnt allow any air to pass through (I think that is a good thing?). The rubber it goes into on the booster is cracked. It doesnt seem like the crack is sucking or pushing any air out though.

Sorry for long post, just want to give as much info as possible to help with trouble shooting.




Almost forgot to mention there is a mystery wire hanging down by the pedals and it looks like it was plugged into the fuse area down there. Not sure what it does. But I followed it to a connector near the injectors. PO told me the person they got it from had a sound system in it before not sure why sound system's wires would need to go anywhere near the wiring for the injectors. Below are the pictures of the dangling wire and where it connects too.




Here are the nasty old worn plugs I pulled out of it in order from cylinder #1-6 left to right.




Last edited by Dutch42; 05-06-2019 at 08:43 PM.
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Old 05-06-2019, 08:14 PM
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What would cause the cherokee to sputter when applying/letting off the gas and once operating temps and then not want to idle/ re-start
That sentence is a bit tough to understand, but if I read it correctly, that has the hallmarks of a failed/failing oxygen sensor. It's not the only thing that can cause mixture problems, but when the mixture gets worse as operating temperature comes up - it implies that the mixture is getting out of wack after the vehicle goes into closed loop mode (using the oxygen sensor for air/fuel ratio calculations).
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Old 05-06-2019, 08:58 PM
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Yup, that sentence was pretty bad (I edited it, so hopefully it makes more sense now). I hadn't thought of O2 sensors. Honestly, I didn't even know it had any. I know they can make a vehicle run rough but I didnt think they would prevent it from starting at all.
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Old 05-07-2019, 12:17 AM
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After doing a lot of forum reading, I think maybe I am having a Crankshaft Position Sensor (CPS) failing. I think I will try that and see if it works. If that doesn't work maybe its an O2 sensor problem.

On a side note, I don't know much about brake boosters so any advice would be greatly appreciated. Here is what I have tried.

- I pulled the check valve and blew into the part that connects to the hose. No air passed through, which as far as I understand it shouldn't. I am assuming the check valve is good.
- I searched high and low for a vacuum leak. I sprayed all the vacuum hoses I could find with starter fluid, but no change in engine RPMs was detected.


- With the engine off, I pumped the brake pedal 6 times. Keeping steady pressure on the brake pedal the 6th time, I started the engine. The pedal did not seem to depress at all. I read online this procedure indicates a bad booster.



Is there anything else I can check before I spend the big $$ on a new booster?
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Old 05-08-2019, 05:48 PM
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Not that anyone is paying any attention to my thread but just in case, I changed the CPS last night... what a PITA... Unfortunately it did not fix my issue.

I was told by the PO that he did a compression test and he found it to have good compression. After the CPS didnt fix the problem I decided to do my own compression test here is what I found.

Cylinder 1: 90, 100 wet
Cylinder 2: 102, 120 wet
Cylinder 3: 105, 120 wet
Cylinder 4: 102, 120 wet
Cylinder 5: 95, 130 wet (might have accidentally put a little more oil into this piston)
Cylinder 6: 90, 100 wet

I bought a new O2 sensor but have yet to install it. I'm wondering if its worth spending anymore money on? I may just turn around and sell the rig. I bought it as a quick fix for a cheap daily driver. It is quickly turning into a throw money at it and see if it works.
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Old 05-08-2019, 06:15 PM
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Originally Posted by Dutch42 View Post
LOOONG post short:...
I’d hate to see the long version. JK, it is helpful to provide as much potentially pertinent info as possible.


Originally Posted by Dutch42 View Post
...Can a bad brake booster be the cause to all my issues?
No. For example, the brake booster has nothing to do with the spark plugs you replaced, which look as though they could be the originals. This vehicle has been neglected, but it doesn’t mean you can’t fix it. You aren’t going to find any single remedy. There are several things you can do to improve it. Yes, a vacuum leak at the brake booster can cause it to run poorly. You know the brakes need attention. Fix them, and do some basic maintenance. Give it a full “tune up”. Check grounds and clean them. Clean the IAC. Google it if you don’t know what it is. Also google “Cruiser54’s Mostly Renix Tips”. You will find some good basic information and procedures there.

I’ll chime out now, but hopefully others will chime in.

Good luck!
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Old 05-08-2019, 07:19 PM
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First off, thank you for the reply! I will certainly be reading over cruiser54's tips especially about the Throttle body/IAC cleaning.

Originally Posted by ElMartillo View Post
I’d hate to see the long version. JK, it is helpful to provide as much potentially pertinent info as possible.
I meant for the top two bullets/questions to be the short part. Everything after is the LOOOOONG part lol. Whenever I bring my boss an issue I always give him the problem and my recommended solution up front, then I explain all the background info. In essence if he doesn't have time to read all the background he can quickly look at the problem and my recommendation and decide off of that.

I am reluctant to fuss with the brakes until I get the jeep running well. A new booster is pretty expensive and, as you pointed out, it probably has nothing to do with the XJ running poorly.

I got the XJ hoping it would be a quick, cheap, and easily fixed daily driver. I am glad to do routine maintenance/easy fixes on it. However, I am not looking for a big project, because I have one sitting in the garage already and not enough time to dedicate to it.

Work has me out of town frequently so I really have to manage my time. If I find this cherokee is eating up too much of my time it will probably find its way back on craigslist.

Again, thank you for the advice. I think cruiser54's website will be super helpful!
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Old 05-09-2019, 05:30 PM
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I was gonna say either clean or replace the IAC valve...among other things. To my knowledge(which is not perfect, mind you) the IAC valve compensates for any draws on the engine by adjusting the the air intake I believe...and helps maintain the correct idle. Say you are sitting in park with the engine running and you turn the wheel,,,this would put a strain on the engine which the IAC hops into action to compensate for. A dirty one can get stuck and no longer function correctly. It basically bypasses the throttle body and lets in more air to correct the idle. So yeah just my 2 cents!
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