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Unread 04-27-2015, 10:07 AM   #1
mhjo
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1996 XJ Cherokee 
 
Join Date: Jan 2005
Location: Oslo, Norway
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Rough idle after 3-4 seconds

After engine gets warm idle gets uneven. If I stop at red light the idle is smooth at first but suddenly it hesitates and I feel like something is bumping in the car. It gradually gets more frequent like bump,....bump,...bump, and then bump-bump-bump.

If I put it in Neutral it is less.

If I hit accellerator it goes away for a few seconds and eventually comes back.

Where to look?

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Unread 04-27-2015, 12:50 PM   #2
KelticTJ
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Does it feel like it's going to die while you are idle? Could be a bad IAC or vacuum leak. I would check the TPS first to make sure it is adjusted correctly since it was just replaced.
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Unread 04-27-2015, 01:24 PM   #3
mhjo
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It feels like its almost stopping but returns to normal after a split second .I will clean IAC when I get home on Friday. Which vaccum hose are you thinking of? Where is it located?
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Unread 05-01-2015, 01:25 PM   #4
KelticTJ
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Quote:
Originally Posted by mhjo View Post
It feels like its almost stopping but returns to normal after a split second .I will clean IAC when I get home on Friday. Which vaccum hose are you thinking of? Where is it located?
Check the vacuum canister (black ball looking thing) sometimes it cracks. It is mounted down by your bumper. There is a vacuum line that runs from that and it sometimes gets pinched by the battery. I would check the TPS and IAC first but it could be that too.
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Unread 05-01-2015, 01:41 PM   #5
mhjo
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Great, thanks. We will also replace CPS
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Unread 05-14-2015, 01:24 AM   #6
mhjo
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CPS repleced, no improvement
No cracks in vacum
IAC checked.

Still idle gets harsh after engine gets warm,but it still runs normal when cold.


Any other ideas?




Parts changed:

ECU
PLug wires
Distributor
IAC
TPS
CPS
Plugs (Champions)
Coil
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Unread 05-14-2015, 06:40 AM   #7
Kalali
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While you're throwing parts at it, you may want to replace your fuel pump as well....
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Unread 05-14-2015, 06:51 AM   #8
mhjo
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Fuel pump has been considered, injectors too, but why does the engine run fine when cold?
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Unread 05-14-2015, 07:14 AM   #9
CCKen
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When cold the PCM is in Open Loop and ignores the Oxygen Sensor inputs. When warm/hot the PCM goes to closed loop and uses inputs from the O2S.

Could be a faulty O2S.
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Unread 05-14-2015, 09:58 AM   #10
mhjo
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That was interesting (and very logical).

2 questios:

1.why is the trouble only occuring in idle and not when driving?

2. When in open loop, is the ECU not involved the loop?


We will check o2s!!

Thanks!!
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Unread 05-14-2015, 10:59 AM   #11
CCKen
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Quote:
Originally Posted by mhjo View Post
That was interesting (and very logical).

2 questios:

1.why is the trouble only occuring in idle and not when driving

I thought you said you have problems when driving (ref. post #1).

2. When in open loop, is the ECU not involved the loop?

Yes, in open loop the PCM is involved, it just ignores O2S inputs.

We will check o2s!!

Thanks!!
How many miles do you have on your O2S'?

I do not have 1996 XJ wiring diagrams but before changing O2S', check the PDC mini fuses for one that is blown. The O2S heaters are fuse protected but I don't know which fuse and where it would be.

Is this your '96 PDC?

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Unread 05-14-2015, 11:37 AM   #12
mhjo
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Thanks again.

The car drives fine, but when I stop at red light, it starts to hesitate after a few seconds and if I stay idle longer it gets more rythmic bump-bump-bump

We will check fuses too!! yes, same fuse layout



PS:English is not my native tongue as I live in Norway
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