Pulled my carpet and now I have a rough idle

97XJ, 4.6 stroker w/20k miles, 215k overall, AW4, daily driver,

I pulled the carpet out of my 97 XJ on Saturday. The engine had been generally running fine to that point. I say generally because i get an downstream O2 code occasionally, I think its the cat (rattles around alot, been that way for like 2 yrs). Put the seat back in on Sunday, started it up, and now its got immediate rough idle with occasional miss. Seems to accelerate normal, maybe a little off. At cruise I can feel it missing. All this continues after engine warms up. No CEL code for misfire though. Reset the computer to be sure.

So now I'm trying to figure out why my old beat up carpet plays such an integral part of the engine's performance. Or its coincidence.

While I was pulling the carpet, I had to have the key in "on" position to unlock the shifter lever, to move it back and forth to get the console moved out of the way for the carpet. Probablyt left the key in the on position for like 1 hour w/o starting it. Maybe I burned something out?

To me, it seems like its electrical. So far:

Vacuum is 16-17 whatevers
replaced the ignition coil
pulled injectors one by one, no apparent difference between each injector pull (that I could tell)

Other thoughts before I get into checking compression/pulling valve cover? next thought was O2, but I don't think that wouldn't cause the immediate rough idle (closed loop at startup), plus no CEL code. Or the cat converter? But it has power, just missing/misfiring occasionally. Maybe unplugged something by accident behind the dash/console?

A lot of stuff was replaced 20k ago when I did the stroker, inc. plugs, wires distributor, rotor, etc. Hoping its not the ol' aftermarket cam/lifter fail.

Thanks

Wouldn't hurt to take a look at the plugs after 20k miles. Should still be good, but might give you some more clues.

Maybe chase down what is triggering your error codes before suspecting the carpet?

The thing is there are no codes now ( I cleared them), and there are no pending codes either. I've since run it through at least 6 warm up cycles. When I had the O2 code it was running fine.

ok just checked a few more things, no change:

checked fuel pressure = 49 psi
ran it w/o the air filter for about 10 sec (to see if filter was clogged up)
checked map sensor voltage - ok

pulling the plugs and doing compression check next.

For idle vacuum - is 16 - 17 normal, or low??

Thanks

oh yeah, also checked all the underhood fuses. And still no CEL

ct97cherokee said:

ok just checked a few more things, no change:

checked fuel pressure = 49 psi
ran it w/o the air filter for about 10 sec (to see if filter was clogged up)
checked map sensor voltage - ok

pulling the plugs and doing compression check next.

For idle vacuum - is 16 - 17 normal, or low??

Thanks

Click to expand...

Vacuum is fine. Did you notice any unusual needle fluctuations while checking the vacuum?

vacuum fluctuated rapidly between 16 and 17, the needle was a blur. I had checked once when the engine had about 5k, right after I got a vacuum gauge, and it was doing the same thing, figured it was normal. But then I had read that thats not really good; but it ran fine, so i ignored it. maybe coming back to haunt?

Also check your plug boots and wires for holes or marks that might indicate arcing. Totally unrelated to carpet, but I've had 2 of 6 Magnecor boots spontaneously develop holes over the course of a couple years. Figured with a stroker you probably had same.

I would check for shorts or bare spots on the wiring that runs inside the cab and through the floor under the rear seat going to the fuel pump. I'm not sure about the 97 model, but it is in that location in older models.

Also i know on RENIX models there is a ground that runs under the back seat (driver side IIRC) if that isnt on, or if its bad im pretty sure it can kill/cause engine problems.

ct97cherokee said:

97XJ,
I pulled the carpet out of my 97 XJ on Saturday. The engine had been generally running fine to that point. I say generally because i get an downstream O2 code occasionally, I think its the cat (rattles around alot, been that way for like 2 yrs). Put the seat back in on Sunday, started it up, and now its got immediate rough idle with occasional miss. Seems to accelerate normal, maybe a little off. At cruise I can feel it missing. All this continues after engine warms up. No CEL code for misfire though. Reset the computer to be sure.

Click to expand...

Check your downstream O2 sensor. You removed your carpet, but it appears you also R&R'd your seats. If you Off-Road it, you may have smashed the O2 sensor into the floor a couple of times. Also, rescan for codes, you probably have been throwing an O2 sensor code. If the downstream O2 sensor doesn't work right by the time your engine goes closed loop, it will run like poop.

-Ron

Isn't the downstream sensor on the 97+ just to verify the cat is functioning? I was under the impression it would throw a code, but that only the upstream sensor would effect the way it runs. Correct me if I'm wrong on that one.

hubs97xj said:

Isn't the downstream sensor on the 97+ just to verify the cat is functioning? I was under the impression it would throw a code, but that only the upstream sensor would effect the way it runs. Correct me if I'm wrong on that one.

Click to expand...

that's what I thought and had heard. I've had a bad O2 before (upstream), and this rough idle doesnt seem like that. Then again, that was before it was a stroker.

update:
replaced IAC
swapped in 2 different MAP sensors that I believe were working
swapped in 2 different TPS sensors that I believe were working
checked coolant temp sensor, and air temp sensor. resistance within ranges in FSM
checked various grounds
checked for shorts in the fuel pump wire
replaced dist cap and rotor
checked for spark at all 6 cylinders with an induction timing light. Did not notice any spark misses coinciding with the misses I was hearing.

All this, no change.

I have not pulled plugs yet, maybe tomorrow night. Then maybe the o2 sensors.

So back to hubs97xj's suggestion - maybe its the cat thats partially clogged? All i have read on the cat going say no power to accelerate. I have power to accelerate, just a crappy idle and missing during cruise. How hard is it to replace a cat - any welding or is it bolt up?

another question - if I get 49 psi on the schrader valve on the fuel rail, one can presume that the fuel pump and fuel filter are ok?

Thanks for the input, keep it coming.

Did you try putting the carpet back in yet? You've screwed around enough with the other stuff. Oh, wait, never mind! You didn't check the spark plugs yet. Is that hard to do? While you're at it, why not check compression?

The cat inlet bolts up with a 4 bolt flange just above the crossmember. If the exhaust has been worked on before, often people will weld the outlet connection to the muffler. You need to crawl under your stuff and inspect it. Then you will know if yours is welded or not.

If you are suspecting the cat as plugged, you can remove the upstream O2 sensor and let the exhaust escape through the O2 sensor port as a test. I had a plugged cat in an S10 that would not accellerate or run over 15 MPH. I pulled the O2 sensor and was able to drive at highway speeds with it out. It will make a lot of noise and probably a loud whistle, so don't go far. You will know pretty quick if this is the cause of your bad lidle. Of course this will give you a CEL and code, but you have the scanner to reset that.

The vacuum test you did says the cat is OK.

I'll try to pull plugs and comp. test tonight - I wanted to do that in one shot. Didn't have a second person to crank the engine while I was watching the comp gauge.

So if the cat is bad, it will affect vacuum? what would it do to vacuum?

I'll try removing the U/S O2 to see what that does. looking like its not that, though.

A plugged exhaust should make the vacuum gage steadily go down. (As the exhaust pressure builds, I guess). Makes sense.