'96 4.0 destroying cats.... - JeepForum.com
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post #1 of 32 Old 02-14-2020, 12:12 AM Thread Starter
pinto_dave
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'96 4.0 destroying cats....

Not the animal, the converter breed of cat..

I have put five cats on this thing, all in different stages of of it's life, and now it has killed another one.

The first cat was replaced because the original PCM needed to be flashed with the E22 update, which reared the head of the bad cat.

A month after replacing that with an oreilly's high flow converter, it needed to be replaced. So I took it to the awesome muffler shop here and they replaced it with one of their standard replacements for a ZJ. That worked great for three months...

They replaced it again under warranty, that time it lasted 6 or 7 months, and I replaced the manifold with a better dorman one with the flex joints. That got me through the emissions test just fine, but it died two months after the emissions test.

I honestly just drove it as it was, until I was able to swap out all of the sensors (O2, TPS, CKP, CMP, idle air, MAF, coolant temp, transmission sensors, you name it). By the time I changed out all of the sensors, the motor had such a loud clatter that I ended up swapping out the motor for a crate long block.

After the motor swap, I replaced the cat again. I figured maybe it was the old motor that was just worn out and tired, killing off cats from poor combustion.

WRONG! I am getting a repetitive code for failing cat again. Every time I clear the code, it just comes back up through out the day.


This ZJ has a new motor, rebuilt trans, all new sensors all the way around, new connectors for every sensor on the rig, new muffler (twice), swapped PCM a few times with some spares I have, has gone through two manifolds, three different sets of injectors, running a 99 WJ horseshoe intake, and it still keeps killing off cats!


Is this normal? The guys at the muffler shop said they see the 4.0l Jeeps go through cats like candy, and ethanol free fuel "cures" it. I am skeptical of that statement though...I run Chevron 87 octane every fill up, I never run cheap gas anymore, since the install of the new motor.

The only thing I can think of is the engine harness is bad, or I just need to spring for a new PCM instead of using one of the four used ones I have.

If I need to do the harness, I would be pulling a complete engine harness from the junkyard this spring, test it wire by wire and visually inspect every inch of it, putting new coverings on it before swapping it.

Any other suggestions?? I'm stumped, and this doesn't happen often...


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post #2 of 32 Old 02-14-2020, 12:49 AM
Timo_90xj
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You're mentioning you are getting a code for a faulty cat, but have you actually verified this with an emissions test? What did the failed cats look like when they were replaced?
Motor burning oil or running very rich are common causes for cats failing very quickly, but with your new long block oil consumption probably isn't the issue here. How are your injectors, and what kind of MPG are you getting?

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post #3 of 32 Old 02-14-2020, 01:53 AM Thread Starter
pinto_dave
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The old cats were showing signs of running rich, with the 50-75% of the catalyst melted into a blob on the engine side, verified running rich by the visual, which they brought me out to see first hand. It never felt restricted, except for the last cat which was changed out just after the engine was swapped. Both O2 sensors changed, every time the cat is changed.

Those were all on the old motor, I have yet to do a cat replacement again, on the new motor, since I want to fix the issue before replacing the cat again. It does not smell like it runs rich anymore, like it did on the old motor, though, and not burning any oil like the old motor did, no coolant loss, haven't had to top it off since the engine swap (11 months ago).

I haven't flow tested the injectors, not entirely confident on that process, but would love to learn how. They do not stick open though. I rebuilt all three sets before installing them, and tested all of them for leaks and operation.

My MPG is pretty abysmal, but seems to be around "normal". 14MPG average, 70/30 highway/city driving, I can get up to 18 with strictly freeway trips.

Yes, fuel pump has been changed....three times. First time was because of a bad check valve, causing priming issues, second time was intermittent flow (sputtering not continuous), third time because the second pump was suspect, but no change. Fuel pressure is normal from gauge testing it for holding pressure, as well as operating pressures.
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post #4 of 32 Old 02-14-2020, 05:51 AM
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Quote:
Both O2 sensors changed, every time the cat is changed.

What brand? Do not use Bosch!
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post #5 of 32 Old 02-14-2020, 08:13 AM
zjosh93
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Agree with Timo, if you are melting cats you are running rich at some point. Might be worth data logging while driving if you have or can get a hold of a scanner with that capability. The first cat on my 93 fell apart, no melting. The second cat was fine after 4 or 5 years (still is I guess) I just swapped it for a bigger cat for less restriction. That third cat is still going fine after 3 or 4 years. It gets 13 mpg city and 18 highway so no help there.
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post #6 of 32 Old 02-14-2020, 09:18 AM
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I am hoping you are jumping the gin expecting a cat issue - I am thinking it is a NEW tight engine,I think I saw no more topping off fluids.

Idaho has emission testing? What a feel good regulation. ANYWAY

I have some thoughts - easy CHEAP things first.
This is a 1996, that should be OBDII can you confirm.
Post all codes that you have now and any history.
I am thinking FUEL, the engine was new I doubt the fuel pump, injectors, rail, regulator are new - yes no?

Your comment 'rebuilt' injectors? IDK anyway the OBII question above will get us a plan to check.

Let us know- do you have access to a fuel pressure gauge, vacuum gauge and a SCANNER?

PS: you donot have a MAF - it is a MAP. A less expensive a part..

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #7 of 32 Old 02-14-2020, 09:53 AM
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Quote:
Originally Posted by pinto_dave View Post
WRONG! I am getting a repetitive code for failing cat again. Every time I clear the code, it just comes back up through out the day.
Hmm... Now I'm wondering. Has the new cat you put on after the motor swap actually gone bad like the previous ones? Or is it just codes. Be a weird coincidence but stranger things have happened. What code are you getting specifically?
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post #8 of 32 Old 02-14-2020, 11:23 AM Thread Starter
pinto_dave
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The only O2 sensors I have ever used are NTK. The jeep had a bosch on it when I got it, but I replaced both sensors with NTK units after the first cat change.

The code I am getting is P0420. When it first reared it's head again, I got a P0351 for the ignition coil, and a P1282 for the fuel pump relay circuit, WITH the P0420 code, all at the same time. The car didn't hesitate, buck, stall or anything. I just got the CEL and checked the codes, those three came up...SINCE then, all I get is P0420. Every time I reset it, a 5 mile drive brings the CEL back on, with the P0420 code.

The fuel pump is new, actually the second new one since the new motor. The first time I replaced it (since the new motor) was for concerns of a bad check valve because of some quick bleed off from the fuel system. Then that pump started to cause some stalling because it was "spurting" fuel, rather than flowing fuel. Since replacing the pump the second time, I have not had any random stalling, and the fuel pressure has been validated with gauge.

There is no separate fuel regulator, it's built into the pump, which I think was wrong with the first fuel pump that I replaced.

Fuel rail has been cleaned out every time I replaced the injectors as well.

Rebuilt injectors, as in thoroughly cleaned, new o-rings, filters, and tested for operation and flow.

I have fuel and vacuum gauges, as well as a scanner with logging.

I did a few drives today to record some short trips, I didn't see anything odd with the operation besides the O2 sensors behaving badly, consistent with the bat cats I have had previously.


The jeep runs great BTW, a little sluggish,....maybe...but doesn't really give me any issues besides this CEL

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post #9 of 32 Old 02-14-2020, 11:35 AM Thread Starter
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I forgot to add, I was playing with the scanner and noticed a note about the o2 sensor heater circuit is not passing. Perhaps it's just a bad sensor?

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post #10 of 32 Old 02-14-2020, 02:10 PM
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Quote:
Originally Posted by pinto_dave View Post
The code I am getting is P0420. When it first reared it's head again, I got a P0351 for the ignition coil, and a P1282 for the fuel pump relay circuit, WITH the P0420 code, all at the same time. The car didn't hesitate, buck, stall or anything. I just got the CEL and checked the codes, those three came up...SINCE then, all I get is P0420. Every time I reset it, a 5 mile drive brings the CEL back on, with the P0420 code.

I did a few drives today to record some short trips, I didn't see anything odd with the operation besides the O2 sensors behaving badly, consistent with the bat cats I have had previously.
Misfiring will kill a cat. It dumps unburned fuel into the exhaust. But the PCM should pick it up if it is. Maybe worth checking the ignition system though. How do the plugs look?

"O2 sensors behaving badly" Both? A bad cat will give you some funky sensor 2 readings but it shouldn't mess with sensor 1. What is sensor 1 doing?
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post #11 of 32 Old 02-14-2020, 02:47 PM Thread Starter
pinto_dave
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I just pulled the cap and rotor so I could double check the cam sensor condition, just to be sure it isn't chewed up or something. Cam sensor is good. Found this in the cap though. Odd that carbon tracing is only on three of the six terminals....
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post #12 of 32 Old 02-14-2020, 05:41 PM
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You said you replaced the pcm with one you had.

There was a recall many years ago that replaced the cat and reprogrammed the pcm to prevent further cat failures.

Perhaps your pcms still have the old firmware?
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post #13 of 32 Old 02-14-2020, 08:59 PM Thread Starter
pinto_dave
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The original pcm I had needed the e22 update after I purchased the vehicle. That pcm fried and had to be replaced with a different good junk yard unit.

I didn't realize there was an update that was supposed to correct engine management to prevent car failure...

Is this normal ignition advance behavior?
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post #14 of 32 Old 02-14-2020, 09:40 PM
zjosh93
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Depends on what the engine was dong during that log. If it was idling, then that's not normal. If you were blipping the throttle a bit, then it looks fine.

That ignition rotor looked normal. What did the cap look like?
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post #15 of 32 Old 02-14-2020, 11:02 PM Thread Starter
pinto_dave
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Quote:
Originally Posted by zjosh93 View Post
Depends on what the engine was dong during that log. If it was idling, then that's not normal. If you were blipping the throttle a bit, then it looks fine.

That ignition rotor looked normal. What did the cap look like?

It seems those didn't actually upload. Here are the pictures of the cap. I did replace it, and the rotor. The old rotor seemed to have the blade protrude slightly less, so the new one has the post and the rotor blade closer now
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