Sporadic misfire

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dball

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Hey, Looking to get some help on tracing an issue on my TJ. If you know of another thread that already has this let me know but I wasn't able to find one or rather have already been through the ideas from them.

I am seeing it cough or almost a slight misfire every now and then around 2500 rpm typically while cruising in 4th gear but at times can pop up on other rpms and gears. I initially found a kink in the old plastic fuel line in the engine bay and replaced it, which helped but hasn't completely gotten rid of it. The issue seems to get worse when it is fully warmed up.

I have swapped spark plugs, MAP sensor, had the timing redone, verified 45-50psi fuel pressure and have checked all air leaks. What do you guys recommend next? I don't want to pull the fuel injectors but I am leaning towards that at this point.

Info: 98 TJ, 150K miles
 
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Timing redone? Timing is controlled by the PCM and the timing chain.

I chased mine for a bit. Cap, rotor, wires, spark plugs, checked wires.

Put a Napa Echlin crank position sensor on and it cleared up.

-Mac
 
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Thanks! Sorry my wording was incorrect. Had the distributor checked/tuned. Do not have a ton of confidence in the shop that did it but felt like it was a fairly straight forward thing for them to do.

I'll swap out the crank position sensor and see if that clears it up. Thanks for the recommendation. If that doesn't do it, I'll have someone else check the distributor.
 
Distributor doesn't really have any adjustments. It either works or doesn't.

Is it always throwing the same code or bouncing around on different cylinders...mine would fire off 301 to 306 or two, three or four of them.

-Mac
 
Interesting that this topic comes up now....
I have been thinking about posting my findings about this for a while.
The random misfires problem occurred on my '01 4.0L for quite some time and no matter what I did; the problem persisted....
Several months ago I needed to replace a badly worn clutch throw out bearing.
At the same time I replaced the clutch, pressure plate, flywheel and cleaned the transmission shaft.
While replacing the flywheel I noticed the CPS triggering blocks had a lot of surface rust and the CPS had rusty dust on the pickup surface.
I wiped off the rusty dust from the CPS and the new flywheel CPS triggering blocks were rust free.
Since that time the random misfires have disappeared other than a brief period when I found the #1 spark plug misfiring which I cured by replacing the spark plug.
When you remove the CPS inspect the pick up surface for rusty dust or metallic dust and try to inspect the triggering blocks for rust thru the CPS mounting hole (difficult to see, but it can be done).
 
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Thanks so much for all this info. I'll take a look at the CPS pickup surface tomorrow. The misfires got slightly worse the other day so I went ahead and swapped out the CPS and TPS that I had from Napa Echlin and ExtremeTerrain. Reset the computer through the battery method. However the following day it got much worse and even stalled 4 times on my 30mile drive home from work. Seemed to lack power, would idle low and stall at times as well as misfire. I have been reading up on various solutions people have had for this and so will swap out the Crankshaft Position Sensor tomorrow and verify my spark plugs and cables are good and happy (recently replaced). Kinda running out of things and nervous one of the sensors I bought is bad...we'll see this weekend.

I already verified fuel pressure at the rail so pump is ok and not getting any codes oddly enough. Had my O2 sensors checked recently and they were good too.
 
Hey, need some help. My misfire/occasional stalling has gotten so bad, it now won't idle for more than 5min without dying. I have been through almost every thread on here trying to find a cause but nothing has seemed to help. I must be missing something.

So far replaced:

MAP sensor
Camshaft Position Sensor
Crankshaft Position Sensor
IAC
TPS
Spark Plugs (NGK)
Cleaned throttle body
Reset computer
Never got any error codes.

98 TJ, 150k miles

I am some what at a loss here on next steps...what I'm I missing. Could one of the sensors I already replaced be bad?

Thanks!
 
YES... One of the replaced sensors may not be functioning properly.
What manufacturer did you use for the CKPS and CMPS ?
Forum members have had problems when using certain aftermarket ignition parts.
Perform a test on the CPS to determine IF it is working properly.

 
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I doubt this is your issue, but there is a TSB for 1998 about weak valve springs causing misfires. You have a 4.0L right?
 
Thanks for the replies. No I have a 2.5L 4 cylinder. Wish I had the bigger better engine. lol

I swapped out my new sensors, one by one with the old ones (this specific issue was not seen with the old sensors) and the issue persists. I'll look into testing my PCM.

Frustrating how something so small turns into a dead car and a long massive headache.
 
dball said:
Thanks for the replies. No I have a 2.5L 4 cylinder. Wish I had the bigger better engine. lol

I swapped out my new sensors, one by one with the old ones (this specific issue was not seen with the old sensors) and the issue persists. I'll look into testing my PCM.

Frustrating how something so small turns into a dead car and a long massive headache.
Click to expand...

I'm having a VERY similar issue with my 97 4cyl TJ. I don't have a ton of cash on me so I'm trying to test everything myself. Replaced the map and iac, but haven't reset the PCM yet. Also replaced gasket for intake manifold because this issue really, really seems like a fuel/air issue.

Good pressure on my fuel line and my injectors work too, although I'm curious, my injectors spray in more of a stream than mist, is it the same for you?

I had pulled the manifold off when replacing the gasket and cleaned the manifold, didn't see any cracks or anything.

I'm considering removing and plugging all the vacuum hoses from the manifold and plugging them, seeing if that temporarily fixes the issue. Then put them back on one by one to see if I can isolate any hose that has a leak. I've also seen online some people use propane around mating surfaces to listen for an rpm change but a bit nervous to try that lol.

Will be testing my O2 sensors and wire bundles, as well as map and iac bundles.

Lastly, my map sensor shows on my OBD2 live data as 32 kPa when engine is off. I know atmospheric is 101, so kind of concerned, and it's the reason I replaced it, but new one shows the same reading. May be the wires but does yours show 32 as well?
 
Any chance you know which cylinder is misfiring ? If you did you could swap that fuel injector with one from a working cylinder. Compression test isn’t a bad idea either.
 
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