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90 xj idle issues

low_life

NAXJA Forum User
I just picked up a 90 4.0 xj. It does not idle well, (it idles at about 200rpm)
When you give it gas it hesitates real bad and smokes a bit out the tail pipe.
Any ideas on my limited amount of info?
Thanks for any input

Scott
 
A good place to start on most any early XJ is by checking the air filter to see if it´s oil soaked. Cleaning the small plastic line running to the top of the valve cover (unhook it from the intake manifold and blow through it, if you can). Then cleaning the oil and baked on junk out of the throttle body and the idle air controller and the seat. Be carefull not to spray much of any kind of liguid on the throttle position sensor. Some people clean the TB on the vehicle, I´ve always been a bit shy of washing all the crap down into the cylinders and always remove the TB. Reusing the TB gasket isn´t advisable, but if you do, a light coat of grease and making sure it lines up exactly, usually gets a good seal. Be carefull of the plastic vacuum lines, they get old and brittle. The TV cable (to the tranny/AW4 auto) adjuster can be knocked out of wack, a little mark on the sliding adjuster, will tell you if it´s moved.
 
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I checked the jeep out a little tonight(quick glance under the hood) someone
pretty much eliminated all the pcv system, plus no air filter. Also it needs a harmonic balancer. The guys who had this before me installed a high volume oil pump to try and cure a low oil pressure problem(right now the pressure drops after a few minutes of idling) also a little noise in the bottom end, thought it could maybe be the balancer(yeah right!). Is the high volume pump a good idea in an older engine? Anyway thanks for the input

Scott
 
1990 is the last year that uses an Exhaust Gas Recirculation valve (EGR). If it is stuck partly open, then it lets exhaust gas back into the combustion mix at too low an RPM, which causes low idle and hesitation (or stalling if fully open). My 1990 is on the third EGR since new. You can check the EGR operation by verifying that the plunger moves up and down freely, although it is easier to just pull the vacuum line off the condenser (flying saucer thing at the top of the EGR) and see if this solves the problem (not as good a check).
Don't disable the EGR- it does two things. One is to help keep emissions down, the other is to lower combustion temperatures (= longer engine life, less cooling hassles). It is easy to replace if you remove the airbox first. The two bolts go into the aluminum intake manifold, so don't overtighten or they will strip. Every low idle situation I have had has turned out to be EGR replacement time.
 
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