Need some help from the forum.

[Mrbeemernc]

Hello all, time for my 1st post.

I recently picked up a 1998 XJ with 4.0 and Auto trans that looks really nice on the outside, but it needs some help under the hood.

The engine starts right up cold or hot and settles in a nice 1,000 rpm idle for about 30 seconds. After 30 seconds it starts to miss a little but it still idles.
In fact it would idle for 20 minutes if I let it and I can live with the small misfire. The real trouble starts when I put it in gear and try to accelerate away. The mis gets more pronounced as I apply progressively more throttle. Once up to cruising speed the misfire is so pronounced that it unloads the drivetrain accompanied by clunking from the slack being taken up abruptly.
Backing out of the throttle doesn't do much to correct the issue.
For such a smooth running motor at idle it sure doesn't like to accelerate,

When I purchased it the CEL was illuminated and I pulled a bunch of codes before erasing them. Multiple cylinder misfire, lean operation bank one, throttle position sensor implausible value, extended low temp operation.

With all of those present I figured it just needed some tune-up parts - the PO hadn't done much other than change the oil occasionally.
New autolites, wires, dizzy cap, rotor, 195degree thermostat, TPS, and air filter are all in place. I changed the oil & filter as well as new coolant while I was putting it back together. Well guess what, nothing about the engine operation has changed and I'm still throwing the lean running bank 1.
On top of the poor engine operation the engine is spewing gas rich carbon from the exhaust pipe bad enough to stain the grass in my yard.

Any ideas?

I've checked the distributor indexing and the plug wires are on correctly. The fuel pressure reads 47-50psi with the engine running and it holds that pressure for 30 minutes after shutting it off. The fuel that was in the tank when I bought it is only 1 week old and I've added some fresh ethanol free to it. Battery is in great shape and charging system output is 13.6 at idle. Throttle body is clean, idle bypass idle controller is working perfectly, and there are no vacuum leaks. Any ideas on the uniform rich running condition and extremely rough operation under load?

Many thanks in advance.

 

[Green Mesa XJ]

Probably a stupid suggestion but I'd check the plug wires are in the correct order and maybe see if the timing is correct.

Other than that o2 sensor would be my best guess.

Hopefully someone will have a clearer idea of what's happening.

Sort of reminds me of when a pal of mine would go around buying cars. There were a few old 300zx. There were running rough so the owners only saw huge expenses on the horizon so he'd buy the zed for a song, drive them down the road until he was out of sight swap the mass air flow and it would be right as rain. Don't know why that story came to mind.


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[Mrbeemernc]

Probably a stupid suggestion but I'd check the plug wires are in the correct order and maybe see if the timing is correct.

Other than that o2 sensor would be my best guess.

Plug wires are installed correctly and the distributor index is spot on.
Hard to believe the O2 sensor could effect the fuel curve that much.

 

[xCWolf]

This is going to sound dumb but it happened to me sooo... Did you recheck the plugs? one time I did my plugs didn't notice it but I must have tapped it on the way in and closed the gap on the plug. worth checking if you haven't

 

[Mrbeemernc]

This is going to sound dumb but it happened to me sooo... Did you recheck the plugs? one time I did my plugs didn't notice it but I must have tapped it on the way in and closed the gap on the plug. worth checking if you haven't

Yep, all six plugs are gapped at .030" and all are equally carbon fouled after less than 30 minutes in the engine. I've pulled each one and inspected it when I went back to confirm the distributor index

 

[D.I.T.A]

Do an o2 sensor as they are cheap (rockauto) and I had same issue and for me it was the coil .

 

[trippled]

If you are getting a lean code and the exhaust is that rich, I would suspect the o2 is stuck lean and the ECM is trying to compensate

 

[Ou2mame]

This sounds like a similar problem I had with a bad injector

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[Mrbeemernc]

The uniformity of carbon buildup in the spark plugs leads me to believe that the catalytic converter might be blocked or clogged up.
It's a factory mopar converter leading me to believe that it's the original one installed at the factory 217,000 miles ago.
The muffler and tailpipe are old and appear to be aftermarket.
Anyone have any opinions on cat converters?

 

[old_man]

Things that come to mind since all cylinders appear the same:

1) plugged CAT (check with vacuum gauge)
2) bad O2
3) bad MAP (measure with meter)
4) possible TPS issue (measure directly at TPS and then at the computer and make sure they are the same)
5) check compression (not likely, but possible)

 

[Transpoquick]

O2 sensor's.
And the cat. if it's original w/ 217k.
Wiring could be a issue too, delete the o2 plug's clear the codes and see what else pops up.

 

[gradon]

I agree with the o2 replacement.


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[Transpoquick]

yeah.

 

[Mrbeemernc]

So I cut the muffler off last night and dumped out what must have been half the catalyst honeycomb. The jeep sounded no different without a muffler so I knew the cat had to come the rest of the way off.
Well I ditched the cat this evening and went for a quick spin with nothing but the header and down pipe installed. Well it runs much much better now, noises, but better.

I just ordered a complete cat-back system from Iron Rock Off-road including a magnaflow high flow catalytic.

 

[SleeperXJ]

How many miles do you have on the jeep? I'm surprised since my 99 has 387K and cat still seem good,
no red hot header or anything indicating a bad header.
I did have similar running condition, it turned out to be a dirty IAC valve.

 

[Mrbeemernc]

Jeep has 217K on it and was completely stock minus the roof rack & trailer hitch.

One of my prerequisites when I was looking was a stock vehicle. Is always easier to start a build form a blank canvas. This one is going to be very mild, in fact I'm going to keep it at nearly stock ride height.

 

[SleeperXJ]

Sorry, meant to say no indication of bad cat.
It sounds like you found your problem.
I would still check iac there has to be a
reason the catalytic went out so early.

 

[Mrbeemernc]

So it turned out almost all of the catalytic honeycomb was in the muffler!
I pulled the cat and turned it on its end and small slivers of metal started pouring out of the cat body.
I noticed that the muffler seemed awful heavy when I pulled it so I grabbed a sawsall and chopped it in half.
I forgot to take a pic but to say the muffler was clogged would be quite an understatement.

I ended up sourcing a 2.5" mandrel bent cat-back system from Iron Rock Off-road. While I was at it I had them send me a magnaflow high performance cat and a set of their boomerang shackles.

I stopped by carquest and grabbed a new front and rear O2 sensor while I was waiting on the exhaust system to come in.

I got everything installed today (including a set of 1.5" front coil spacers and some extended bump stops that I picked up locally on Craigslist for $40).

The Jeep sits a touch higher, the ride quality is as good as stock, and the exhaust system sounds very nice (without the dreaded drone that you sometimes get with flowmaster mufflers).

Still a host of things to do, but at least I can drive the Jeep around while I sort out the remaining items I want to do to it.