ToothMechanic

Howdy fellas,

Owned my Cherokee for a year. It developed a rough idle/misfire issue after I bought it and have really fought it. Two shops looked at it and told me it was ok -- it's not.

I'm at my wits end with it and probably going to sell the car if I can't fix it here in the next month or so.


The car idles rough. It shakes around noticeably at idle. It's not bucking or anything but as my dad put it 'feels like it needs to clear it's throat.' It will occasionally throw a cylinder 1 misfire code that goes back away in time. If you idle it for a while, especially after running it hard, it will SOMETIMES break into a more violent rough idle that throws cyl 1,3, and random misfire codes. When it does this the throttle appears to really struggle and it appears to want to die. After running for a bit again it clears it up.


I have replaced:

Plugs, cap, wires, rotor, fuel pump, IAC valve, upstream O2, added heat shield to injectors, and TPS.

I have:
Checked compression (150 +- a bit on all cylinders)
Checked oil pressure (a bit low at hot idle, around 11-13, normal when driving)
Block checker (for head gasket leak) got normal results
Vacuum check (low (15ish) but appeared to function normally)
Checked fuel injectors (nothing leaking, normal resistance on all, can hear all firing with stethoscope)
Checked fuel pressure (49)
Checked for excessive buildup with scope in cylinders
Seafoamed throttle body and fuel
Oil change
Cleaned air filter
Checked for vacuum leaks with carb cleaner and visual inspection of lines
Cleaned grounds
Tried higher octane fuel


The things I have noticed: It has burned a slight amount of coolant while I've owned it, not a lot, just enough that I have had to top off the reservoir a couple of times. The AC has stopped working (compressor works, recharged it with dye, didn't see any leaks, so not sure). The exhaust system looks a bit rough, cat appear to have been welded in/replaced at some point, and the exhaust smells bad.


I've read every thread on this board and others about this and haven't gotten anywhere. Besides shooting parts cannon I have no ideas.


Thanks

Roler

I've got a few ideas but: Besides the rough idle, is it driveable at all? Or does all this continue when you go for a drive?

my 2 cents in the meantime: On those occasions I had rough running, both idle and driving, the CPS and CAM sensors were the cause. The shakings being caused by the misfires.
(Especially the CPS is common to fail at some point during its life in the XJ) and... I miss those in your parts list...

fb97xj1

You say the exhaust smells bad. How so? Fuel, or rotten eggs? Does it struggle uphill at all? If the same cylinders are are at fault, the injectors or the wiring could be acting up.

BlueRidgeMark

Some things I don't see on your list, in no particular order:

• O2 sensor. If your tailpipe is sooty that's a good clue.
• TPS. Easy to test. Search YouTube.
• Loose connector into the coil. Had that on my 97. Drove me nuts until I found it. Start here, because it's easy.
• IAC. Basic for a rough idle condition.
• Points, plugs, and... oh, wait. We don't have condensers anymore. But the basics. Rotor, cap, plugs, wires.
• Worn distributor bushings.
• Cam Position sensor. It lives in the distributor.
• Crankshaft Position Sensor. Not the prime suspect, but don't rule it out until you have tested it.

Roler

Half those items, he actually mentions in his first post

ToothMechanic

It's perfectly driveable, unless it is one of the rare occasion is 'breaks out' into one of those serious misfires. Smooth while driving. Plenty of power. It's just rough at idle.

ToothMechanic

To be honest I don't know how to describe the smell... out of the 3 (oil, fuel, eggs) I would choose eggs

Does not struggle uphill

Dave51

Dave51

Been at this for a while, huh?

The first image you posted in another of your threads. It showed the O2 sensors were awful. However that was with a 25 cent analyzer.

The second image is a slightly better analysis with a $3.50 data program. Little more detail.

The third image is from an oscilloscope and shows what's really happening. It's all about sample rate. At 750 RPM and 6 cylinders firing (or perhaps 4-5) you've got like 12.5 pulses/second. If your device is sampling once/second the data you get may not be very helpful.

I would look at better tracings of the O2 sensors fuel injectors, ignition and CkPS/CPS relationship.

Dave51

If O2 sensor 2 looks like the cat isn't doing anything you might want to take a look at that and see if it's occluded (and in your image it is suggested that the cat is dead, but again that data isn't reliable enough to shoot the parts cannon at a cat).

Can you get a free emissions test without getting a ticket if it's bad?

ToothMechanic

Hi Dave,

not sure about the emissions test, would have to look into that.

Can you suggest a more reliable scan tool that I could purchase to check the cat?

Can't remember what picture I posted in the other thread but here is the best one I got with the cheap scan tool I have

https://i.imgur.com/JBihoB9.png

Dave51

Or is it 6.25 pulses/second. Need some help with my adds and gazintas.

Anyway it's a lot.

Dave51

Final guess--- 37.5?

Dave51

Sure, I soon as I figure out how many pulses are coming through.

So at 750 RPM, 6 cylinders firing, but only exhausting every other revolution...

Dave51

So 2 things:

If the cat is dead and/or O2 sensor 2 is giving bizarre readings. Prior to 2000 we really don't care from a performance point of view (we do care from an emissions standpoint).

If the cat is falling apart and occluding the exhaust. That would need visual examination.

For a scan tool, besides the O2 sensors, even more important(ly) would be to test those other components. To do that you need at least an entry level oscilloscope.

You can better a better view of the injectors:



Lemme see if I have one of the ignition. IDTS cause I have COP and it's somewhat of a pain. Will be easy on yours.