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Old 09-19-2008, 08:01 PM   #1
joey starling
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Default rough idle

my 1990 cherokee has a 4.0 and it is hard to start and idles rough, but it has
new tune-up hardware installed any help would be appreciated


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Old 09-20-2008, 12:13 AM   #2
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A bit more info would be great, what exactly was involved with the "tune up" that you did? What did you replace?

Thanks!
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Old 09-20-2008, 08:10 AM   #3
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Rough Idle can have some many different issues. Easy quick fix that is a likely culprit on Renix engines is the CPS. Try just unplugging and then plugging it back it. Usually makes a world of difference for a few weeks. If it comes back again after that I reccomend replacing it. If you need to know its location I can get a pic of it for you.
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Old 09-20-2008, 10:54 AM   #4
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In addition to CPS on Renix jeeps the O2 sensor and the TPS are also in the mix. Try XJ Strykers fix and if that does no good then let us know and we can help you do a further diagnosis. The only things needed for Renix diagnosis is a few hand tools and a digital multimeter.
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Old 09-21-2008, 04:48 PM   #5
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thanks for everyones help , i changed plugs,wires,air filter,dist cap, rotor cap,and the fuel filter. im sorta lost ---- whats a cps and where is it located? again thanks for the help!!
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Old 09-21-2008, 08:58 PM   #6
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Quote:
Originally Posted by joey starling View Post
thanks for everyones help , i changed plugs,wires,air filter,dist cap, rotor cap,and the fuel filter. im sorta lost ---- whats a cps and where is it located? again thanks for the help!!
CPS is the crank position sensor.

If you dont have one already I would suggest getting one, its a Haynes or Chiltons manual for your jeep. Just keep it under your seat but it has very detailed step by step instructions for most repairs on your jeep and costs less then 20$.
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Old 09-22-2008, 12:10 PM   #7
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make sure your air hoses are all good!
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Old 09-22-2008, 02:51 PM   #8
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All good suggestions. With my XJ - Renix mated to a '92 HO engine I had the same issue. I took apart my airbox and all the vacuum hoses and cleaned them all out really well. I have a slight blow-by issue and it was plugging up the lines a bit. Cleaning it all out helped quite a bit. I also put a K&N box filter in there which helped too.

If you see oil on your air filter when you check it I would start there as it is a no-cost fix if that is indeed the problem. If not I would look at the CPS too or possibly the ignition coil if it's misfiring at all.

Do the revs go up and down too or is it just a rough idle?
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Old 09-23-2008, 10:33 AM   #9
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Quote:
Originally Posted by joey starling View Post
thanks for everyones help , i changed plugs,wires,air filter,dist cap, rotor cap,and the fuel filter. im sorta lost ---- whats a cps and where is it located? again thanks for the help!!
There are only maybe two harnesses back there. Close to the firewall on the right side of the valve cover. With the engine off unplug it blow it out then plug them back together. If the vehicle starts running better you will know it is the CPS.
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Old 06-23-2009, 12:52 PM   #10
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Hi, I have a 99 jeep 4.0l. I just had this problem as well. I have come to the conclusion that the EVAP canister purge solenoid with the Leak dectection pump is intermittent leaving the charcoal canister open to the vacuume system durring open loop operation (cold) thus in turn creating a large leak in both the evap and vacuume systems. This results in codes PO442 and PO455 being thrown and rough idle durring closed loop operation. What it meant in my situation is the valve that opens and closes the evap canister to the vacuume and leak detection system was working intermittently. Since the leak detection pump and purge solenoid are one unit on this engine I just replaced it and a couple of hoses that looked supiciuos and the problem went away. I took the unit apart and confirmed corrosion on the purge solenoid valve, this is very common since it is exposed to the elements. But not bad for 170,000 miles.

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Old 01-06-2011, 01:11 PM   #11
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Check out your coolant temp sensor on the thermostat housing. The computer sets mixture (like a choke) as it senses coolant temp. A new one costs maybe 50 bucks, use anti-seize or some such, as sensor is brass and housing is alum or pot metal. Before this, try electrolit grease into the connector, and same into the TPS connector, they are subject to poor connectivity.
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Old 08-03-2011, 07:36 PM   #12
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Quote:
Originally Posted by joey starling View Post
my 1990 cherokee has a 4.0 and it is hard to start and idles rough, but it has
new tune-up hardware installed any help would be appreciated
Check your EGR valve.


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