Random Misfire, No Codes

Hello, all!

I have a random misfire with no codes, 1999 TJ 4.0 manual. You can hear it, the vehicle shakes a little when it happens, but I don't have, and have never had, a misfire code.

I've changed the crank sensor, TPS, chased down a vacuum leak and repaired it. Throttle body gasket changed. We tried pulling each injector harness with the engine running but there was no difference in performance degradation and the misfire continued. I tested fuel pressure a few weeks ago, and while I can't remember the values off the top of my head I had looked up what it was supposed to be and it was dead-on. Plugs are iridiums, installed about two years ago. Cap and wires as well.

I was driving back from out of town a week ago, it was bucking at high speeds, running a high idle (1200 RPM), though at idle it would sometimes try to stall. I was stuck on a long road trip and had to drive it about three hours that way. Replacing the CPS seems to have fixed the bucking issues, the TPS fixed the high idle.

I've searched a bunch of threads, but most involve a code, or have worse symptoms than mine. You can hear/feel it, but the engine runs and gets me where I'm going.

I'm at a bit of a loss here. Would anyone recommendation a next step? Is it safe at all to drive it like this?

The way I locate random misfires is to identify when it happens. IE 3rd gear, 35 mph, 20% throttle slight uphill etc . Try to recreate the misfire and when it happens hold it in the misfire even if you have to use the brake to keep the load on it. If the check engine light comes on, check the codes to see if it identifies a cylinder. This narrows it down to one cylinder.
 
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The way I locate random misfires is to identify when it happens. IE 3rd gear, 35 mph, 20% throttle slight uphill etc . Try to recreate the misfire and when it happens hold it in the misfire even if you have to use the brake to keep the load on it. If the check engine light comes on, check the codes to see if it identifies a cylinder. This narrows it down to one cylinder.

It happens all of the time, at idle, parked with engine revving, driving. It's consistent. That's part of why I'm at a loss, there's nothing I've found that makes it better or worse, there are no codes, but you can feel it and hear it.

4.0 timing chain tensioner is a curved black plastic spacer.

I don't think you'd have any issues turning the engine backwards.

-Mac

I'll see if my buddy can help me check that out, he's a bit more mechanically inclined.
 
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Another trick is to start it up in the dark and look for any shorts on the plug wires, you'll see faint blue sparks. I know you said the wires were only 2 years old but this is an easy and free way to check for leaks.
 
Another trick is to start it up in the dark and look for any shorts on the plug wires, you'll see faint blue sparks. I know you said the wires were only 2 years old but this is an easy and free way to check for leaks.

Hey, I'm on board. I'm already throwing parts at it unsuccessfully, I'm happy to try free.

I don't have a dark dark place to check, but I can drape a moving blanket over the hood tonight and run it.

Any other ideas for the daylight? Today is the last day of good weather, I'm willing to try just about anything. I'll get naked and dance around it if it would help.

I have a IAC motor and a MAP sensor I can put on, any possibility of those helping?
 
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Here's a fun thing: my number one and number two fuel injector harnesses were switched. However, switching them back to the correct orientation just made the misfire worse. Not a harder misfire, but much more rapid.
 
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Here's a fun thing: my number one and number two fuel injector harnesses were switched. However, switching them back to the correct orientation just made the misfire worse. Not a harder misfire, but much more rapid.

Makes me wonder about the entire injector harness.

I'd be continuity checking everything back to the ECU plugs.

Noid lights might help.

Try swapping the 1 and 2 injectors with the wiring correct.

How did the 1 and 2 plugs look?

-Mac
 
Makes me wonder about the entire injector harness.

I'd be continuity checking everything back to the ECU plugs.

Noid lights might help.

Try swapping the 1 and 2 injectors with the wiring correct.

How did the 1 and 2 plugs look?

-Mac

All six have been replaced by the previous owner, reasons unknown. I had to peel the tape back to see the colors below the splice.

How can I test this?

I've never heard of a NOID light until tonight. I googled, not sure what I'm looking at, but I might be able to get one from harbor freight tomorrow.
 
Might as well do both...if you have different resistance levels it might tell you something.

There's a common issue on the back of the valve cover when it turns towards the ECU...lot of times wires get frayed back there...could pull the loom wrap off and visually inspect.

And at this point I wouldn't assume the wiring to 1 is 1 or 2 is 2 until you pin verify it back to the ECU plug.

-Mac

P.S. Post pictures of the old spark plugs...what did they look like and did you note which one was in which cylinder?

P.P.S. Have you put a screwdriver to each injector and verified all of them are clicking?
 
Well, crap, I didn't mark them. I didn't see oil or anything on them, but here's a picture.

20231118_214007.jpg


I'll check the continuity and the wiring harness at the valve cover stud.
 
Question: I'm looking up NOID lights, they are all marked with different vehicle makes, but none of them are Jeep or Chrysler. Are there any others that can be used?
 
Some look darker than the others...or that's just shadows...wondering if we could equate the tanlooking ones to cylinders and that black one to the one not firing at the right time.

-Mac
 
Question: I'm looking up NOID lights, they are all marked with different vehicle makes, but none of them are Jeep or Chrysler. Are there any others that can be used?

The TJ uses siemens injectors... pretty common plug...I would imagine most kits have the connector.

Since the PO replaced your injectors...do they have a Mopar stamp?

-Mac
 
Some look darker than the others...or that's just shadows...wondering if we could equate the tanlooking ones to cylinders and that black one to the one not firing at the right time.

-Mac

It's hard to tell, but they're all the same pinkish color, barely any discrepancies between them. Just bad lighting inside.

The TJ uses siemens injectors... pretty common plug...I would imagine most kits have the connector.

Since the PO replaced your injectors...do they have a Mopar stamp?

-Mac

I don't know if the injectors were replaced, I just know the wiring harnesses are. I'll look at the injectors and look for a stamp.