Odd rough idle (1989 YJ 4.2 automatic) - JeepForum.com
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post #1 of 9 Old 08-09-2020, 10:53 PM Thread Starter
ScotL_1970
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Odd rough idle (1989 YJ 4.2 automatic)

hey guys...


need some help.... today my jeep started doing an odd thing
it just started today..... So, I am kind of baffled



it is a 1989 Sahara.... 4.2 automatic


When I first put in in D or R.... it goes from a normal idle.... to a somewhat rough idle..
If I put in it P or N... it goes back to a pretty normal idle



its only really noticeable til about 5 to 10 mph... at a stoplight... it shakes a bit...


after 10mph... it drives like a champ for being 31 years old... ha ha



it doesn't seem to have a problem going up the gears automatically... no grinding or skipping that i can tell


any idea? driveshaft at a low RPM? Clutch issue?

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post #2 of 9 Old 08-10-2020, 05:18 AM
jkbrereton
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Wouldn't have anything to do with the drive shaft because that isn't spinning at idle. It's probably a carb issue. Perhaps your idle jets are clogged

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post #3 of 9 Old 08-10-2020, 05:45 AM
cas6969
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Any idle issue on anything, the first thing I do is clean the butterfly/throttle body. What you're describing at lights was a pretty common occurrence on an '86 Oldsmobubble I had. I'd get a can of carb cleaner, spray the crap out of the throttle body's butterfly and matting surfaces and the problem would go away.



That aside.. If it's only doing it in gear, on most cars that might point me to a vacuum issue.
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post #4 of 9 Old 08-10-2020, 06:11 AM
Wabatuckian
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How much does the engine rpm drop when you go from N to D?
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post #5 of 9 Old 08-10-2020, 08:17 AM
jbolty
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usually a little different symptoms from clogged idle tubes but look at that anyway

https://www.jeeptech.com/engine/carter.html
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post #6 of 9 Old 08-10-2020, 09:21 AM
mark1305
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I would definitely go down the throat of the carb with carb cleaner. Don't however introduce it into the float bowl - I did that on the nearly new non-stepper Carter BBD I installed and it dissolved the rubber plunger in the accelerator pump. Had to do a rebuild kit and the carb was only 7 months old.

Where I am in Florida we have two seasons. Hot and not as hot, and the electric choke on both the original BBD and the newer one are finicky about adjusting the bimetal spring to match the prevailing temperatures. (I have a manual choke kit that is going in real soon). This hot season my YJ started to bog then lurch up to speed when accelerating. Readjusting the electric choke and the idle stop screws on the accelerator linkage/choke linkage helped a little. Then backing out the idle mixture screws 1/2 turn from where I previously set them pretty much fixed it.

Whenever I adjust idle stops or mixture I go by both rpm and vacuum gauge.

Hope some of that helps.

89 YJ Parts Bin Special titled as 84 CJ7; parts ranging from 1984 -2000; 4.2L with AX-15. Also a 2000 XJ Sport 2WD with a troublesome AW4.
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post #7 of 9 Old 08-16-2020, 10:00 AM
Boojo35
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Quote:
Originally Posted by jbolty View Post
usually a little different symptoms from clogged idle tubes but look at that anyway

https://www.jeeptech.com/engine/carter.html
I agree with this. One I tore down to rebuild had them completely clogged.
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If you cannot fix it with a hammer then it has to be an electrical problem.
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post #8 of 9 Old 08-18-2020, 01:09 PM Thread Starter
ScotL_1970
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Hey guys... thank you for all the help... it was a couple of issues... mostly a vacuum line had split in such a way i couldn't see it.. and a small carb adjustment.
thank god it wasn't anything to serious... just a bit of detective work ..... (as you can see by the cardboard underneath.... I am still chasing a small oil leak ;-) )
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post #9 of 9 Old 08-18-2020, 06:42 PM
mark1305
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Glad you found it and it was a straightforward fix. Don't you just love when it's multiple issues as you start to troubleshoot?

89 YJ Parts Bin Special titled as 84 CJ7; parts ranging from 1984 -2000; 4.2L with AX-15. Also a 2000 XJ Sport 2WD with a troublesome AW4.
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