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Old 06-23-2011, 05:40 PM   #1
Join Date: Jun 2011
Location: Putnam, CT
Posts: 3
4.2l running issue

I just bought my first 89 YJ and having problems left and right. First it wouldt start so replaced the ignition module and did a nutter bypass and now it starts. Then it was just dumping fuel into the motor tried adjusting the carb but failed to do anything so i bought a weber carb. That sort of fixed my problem now i have the idle adjustment screw all the way in and it idles at 600rpm (smoothly though). And when you go to give it alot of gas it will just about die. I put an electric fuel pump with a fuel regulator so i know its getting the fuel. I also checked the timing its right on the money. ive eliminated alot of the vaccum lines thinking that was the problem but it wasnt. Im willing to try anything at this point.

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Old 06-23-2011, 06:45 PM   #2
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Join Date: May 2009
Location: Las Vegas, NV
Posts: 180
i have an 89 4.2 and i had a lot of those issues. is your an auto trans? mine is and for some reason these motors and auto trans have tons of issues. only thing that kind of fixed mine was a TBI conversion. paid 1200$ for it and its reliable now. 132K miles later i need the motor rebuilt. just the way things go sometimes.

if you've elminated vacuum, timing, and all that other stuff as being the issue make sure the electrical is good. ECM bitches at wrong voltages.

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Old 06-23-2011, 09:33 PM   #3
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Join Date: Oct 2010
Location: Las Cruces, NM
Posts: 130
If your idle mixture screw is all the way in and its still idling, that points to a major issue and likely the cause of all your problems. You need to check everywhere for vacuum leaks. Intake manifold to head, carb to intake and ALL vacuum lines. Something is leaking somewhere.
1990 YJ Islander, 4.2L MC2100 DUI HEI, 2.5" RE lift, 31x10.50 DuraTracs, HandenFab rear bumper / swing-out, Tough Terrain off-road camp trailer.
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Old 06-24-2011, 04:30 PM   #4
Join Date: Jun 2011
Location: Putnam, CT
Posts: 3
No its a 5 speed manual tranny. im goin to try spraying everything down again but havent found the problem yet. been thinking of taking the head off of a 4.0l to eliminate valve issues but want it to run good first though. also would a bad seal say on a gas cap or in the filler hose pose a problem cause i was thinking of checking there next.
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Old 03-07-2012, 04:36 PM   #5
Join Date: Jun 2011
Location: Putnam, CT
Posts: 3
so i gotn the head remanned put it back on and it runs a whole hell of a lot better. ( the head was warpped and had a couple burnt valves). also when you hook the carb on the jeep make sure you have the vaccum advance on the manifold vaccum and not the ported vaccum cause that was one the problems i was having also. now its like a brand new jeep. next project is paint
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