Common problems with 90 yj, 4.2. Manual tranny - JeepForum.com
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post #1 of 12 Old 05-15-2019, 09:08 AM Thread Starter
Lineman85
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Common problems with 90 yj, 4.2. Manual tranny

Hi everyone I'm new to this site. J just purchased my 90 yj 4.2 and looking to see common problems/issues (besides typical rot, frame is actually in good shape)I should look for or address. Currently going over it to get it to be reliable driver. So far have found a few vaccum lines that are pulled off and plugged. one comes from charcoal canister. Also harmonic balancer rubs on timing cover after it warms up. And the nipple comeing off the bottom of the distributer looks like it should have a vaccum line but there isn't an abandoned one anywhere close to it. Any help is greatly appreciated.

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post #2 of 12 Old 05-15-2019, 10:24 AM
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The '90 was the last year of the 4.2L, and it got the Aisin AX15 tranny, so they pretty much had the bugs worked out of it all. The late '89 and the '90 were about as good as it got with the carbureted engine.

But it still has the electronic carb and all the ridiculous emissions stuff that goes with it. Emissions got so much better once fuel injection came on the scene with the 4.0. Most guys end up getting rid of that Carter BBD carb and going to either a swapped in carb, or a throttle body fuel injection kit like the Howell. It sounds like it is likely that someone has been doing something with yours, since you've noticed missing vacuum lines. I think you should start with some research on what carb you have. Some photos with the air cleaner removed posted here will get you some help on that.

That Howell system is very popular, especially here in CA where it is the most reasonably priced system that is smog certified in our state. I have it, and highly recommend it, but it isn't cheap and obviously you will want to see where you stand with all the other mechanical stuff before jumping into an expensive swap. Besides, you might already have one of the carb swaps.

Welcome to Jeep Forum. Where we cheerfully spend your money for you.

Bill
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1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #3 of 12 Old 05-15-2019, 01:01 PM
Beachcruiser
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Yes, the vacuum advance on the dizzy should have full manifold vacuum.

As suggested above there are several carb options that are pretty cheap if you're mechanically inclined & don't live in an area that requires stock equipment & a smog check.

There is also a quick way to disable the factory compute controls over the carb/ignition timing, if permitted.

These steps, plus a very cheap mod to your factory distributor (needed if you disable the computer) are well with the skill set of an average shade tree mechanic & REALLY wake up the YJ 4.2. It also gets rid of the rat's nest of hoses & such under the hood.

I've done all this & and my rig has been super-reliable for 10+ years. Tuned well, on 31" tires...I get 14 or so MPG in town and 20 on long (albeit slow, I drive like a Granny) highway trips.

probably out breaking stuff.
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post #4 of 12 Old 05-16-2019, 10:53 AM Thread Starter
Lineman85
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Ok. I will go double check the carb. It's starting to look like the P.O. did a bunch of messing around under the hood. (None of which was right). Any opinions on the balancer rubbing the timing cover after warm up? Stops rubbing when I apply pressure to the clutch. Possible bad bearing on the crank?
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post #5 of 12 Old 05-16-2019, 11:06 AM
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Quote:
Originally Posted by Lineman85 View Post
Ok. I will go double check the carb. It's starting to look like the P.O. did a bunch of messing around under the hood. (None of which was right). Any opinions on the balancer rubbing the timing cover after warm up? Stops rubbing when I apply pressure to the clutch. Possible bad bearing on the crank?
Crankshaft thrust bearing worn? It can probably run that way for a long time without really hurting anything. But be prepared for a bottom end rebuild before too long. Or if you want to get after it, pull the pan and check bearings now. If the thrust bearing face is worn to the point of rubbing, the main bearings are likely worn radially too. Our thrust bearings are built into one of the main bearings.

Your clutch release bearing pushes the whole crank forward when you step on the clutch, so that's why it stops rubbing.
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Bill
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1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #6 of 12 Old 05-16-2019, 11:13 AM
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Quote:
Originally Posted by Lineman85 View Post
...the nipple comeing off the bottom of the distributer looks like it should have a vaccum line but there isn't an abandoned...
The vacuum advance doesn't come off the bottom of the distributor, it is a dash pot sticking out the side of the distributor body. So I'm not sure what you meant here. Maybe you just meant it comes off the side near the bottom, which does sound like the vac advance.

We like photos here. Would be good if you could post some. You can't upload them directly here until you've got more posts, but many of us don't upload them here anyway. Host them on a sharing account somewhere, and then link to them. I use Google Photos, others use Photobucket, and a bunch of others.

Bill
Jeepers drive high!
1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #7 of 12 Old 05-16-2019, 11:16 AM
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Sorry for the multiple posts, but I think you will find this site helpful. He has a CJ, but the info is very relevant to the 4.2L YJ too. It's the best source I know for carb and emissions related stuff.

http://www.gleebledorf.com/

Bill
Jeepers drive high!
1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #8 of 12 Old 05-16-2019, 04:05 PM Thread Starter
Lineman85
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Yes Bpounds, you are correct. It's coming off the distributor body. And thanks to all for all the info. !!!
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post #9 of 12 Old 05-16-2019, 04:10 PM Thread Starter
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And also. It's looking like the p.o. did do the nutter bypass. Not sure if he advanced the timing tho. Also as far as the mod for for the distributer I'll post some pics to see if you guys can tell me if he did that one also. It would be so much easier if the the p.o. would just respond to messages i leave just asking about what mods he did. I guess he knows he sold me a hacked up jeep. But I'll give him credit. He hid it well
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post #10 of 12 Old 05-18-2019, 01:28 AM Thread Starter
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Update. Stock Carter carb. With a functioning stepper motor. Also while idleing at operating temp stepper pins are fully extended. When shut off and started at operating temp the pins are in the middle holding steady then after about 10 seconds extend all the way forward. Any thoughts guys?
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post #11 of 12 Old 05-18-2019, 08:47 AM
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Originally Posted by Lineman85 View Post
Update. Stock Carter carb. With a functioning stepper motor. Also while idleing at operating temp stepper pins are fully extended. When shut off and started at operating temp the pins are in the middle holding steady then after about 10 seconds extend all the way forward. Any thoughts guys?
So that tells us it is at least trying to control the carb. I think that means it is not nuttered. What made you think that it is?

Whether it ever goes into closed loop emissions control is a different question. It sounds like some unknown portion of your emissions controls have been disconnected. Maybe you should just not touch anything until you know how it drives? I'm assuming you don't have to deal with emissions testing in your area.

Bill
Jeepers drive high!
1988 Wrangler,4.2L I-6, Howell EFI JP-1,CRT HEI Ign,AX15 ext slave conversion, Zone 2.5" lift + 1/2" booms, BFG 31 x 10.5, Bestop.
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post #12 of 12 Old 05-18-2019, 09:08 AM Thread Starter
Lineman85
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Yes, I believe that tells us that the 02 sensor and stepper motor are working properly since the pins are moving and adjusting. As they should. As far as the pins being all the way extended at operating temp I dont know if that indicates a lean or rich condition that the stepper is trying to correct.
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