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Discussion Starter · #1 ·
I recently bought a 87 yj 4.2 only to have a valve spring break and bend the pushrods. So I replaced the parts in the head and gave her a tuneup. She ran great until the spring broke. After putting her back together I am having new problems. After warmed up the idle is spuratic up and down and when I put it in gear she wants to stall out. When revving up the motor at first touch of the pedal it's sputtering and sounds like its going to die. Like its starving for gas. Some sputtering out of the tailpipe too. But once it's rolling and pedal is past the hesitation point she runs great until I have to stop and then it happens all over again to get her going. Also have a little smoke possibly from the exhaust manifold to pipe gasket. Maybe it was damaged when the head was put on. Any help would be greatly appreciated. And as I said this all happened after the head was fixed. Dirty carb, vacuum leak, timing issue I'm just not sure
 

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how did you fix the push rod and valve spring? did you remove the head to do so or just the valve cover? there is a possibility that you could of jumped time on the timing chain. and there is a possibility that there is another bent push rod or valve. if this jeep ran fine until it broke a valve spring and bent a push rod then I would expect there is possible damage to something else inside the engine. maybe (but hopefully not) a bent piston rod. I would check all the push rods and valve. I would pretty much remove the head and have a machine shop check the head out for any bent parts. even the smallest bend in a valve or rod can mess the engine up pretty good.

And of course there is always a chance that something just wasn't put back together right. did you remove the intake manifold? there could be a big intake leak. those 4.2 are a nightmare when it comes to intake leaks. very common. especially with the old factory manifold. the 4.2 will run great when under a full throttle load but will idle very shitty with a manifold leak.

And of course if you have the factory emissions still hooked up on the 4.2 with the carter single barrel carb then there are 100 other things it could be. but start with those things first is my guess.

I run a 4.2(fully rebuilt bore 0.30 over) with no emissions and with an aftermarket intake manifold and 2barrel carb and I NEVER have any issues with the 4.2 but in my own opinion the 4.2 with all of it's factory specs (emissions and carb junk) they are a nightmare to keep running right and be reliable. If I was you. Strip that 4.2 of all it's junk and set it up to a very simple and basic engine.
 

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Discussion Starter · #3 ·
Thanks for the reply. I had the head removed checked and cleaned. Them replaced lifters, springs, push rods and has the rest cleaned up and inspected. We did have a little trouble getting the manifolds to bolt up to the head so it might be an intake leak. I surely hope nothing in the lower end is messed up. I'm new to jeeps so I don't have a lot of experience working on them but I'm not completely clueless about a motor. I didn't check the timing yet and I was gonna go through all the vacuum lines and also remove the carb and clean it. Not sure what carb is on it but the manifolds look to be original. Carb was recently replaced before I bought the rig
 

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Discussion Starter · #4 ·
It ran good after the head was fixed just a little blue smoke out of the exhaust which eventually went away. But it gradually began to run rough idling in gear. And when I hit the gas pedal as soon as I apply pressure the rpm drop and the jeep starts shaking and sounds like it is starving for fuel. Even get a backfire or two. But once it's rolling its good. If I'm cruising and let off the gas it's fine but when I resume acceleration the hesitation still exits and the first press of the pedal.
 

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Discussion Starter · #6 ·
No visible damage. Didn't really do to much on that side of the compartment. Just the plugs and wires so I don't believe it was damaged
 

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yes it sounds A LOT like a intake manifold leak or some sort of vacuum leak for sure. when I rebuilt mine it gave me so much trouble getting the manifold to seal up. I had to replace the bolts and washers and intake manifold itself and still have seal issues. I ended up using a good I think a steel or metal of some sort gasket and Black RTV gasket and letting it sit for 24hrs to dry before I quit having vacuum leak issues on the manifold. the factory manifolds are junk when they get this old. there is an offenhauser brand intake manifold that is highly recommended to buy when replacing the intake manifold. I have it and it's AWESOME! But it also removes the EGR valve. So be ready to have to adjust the tuning of the engine. You will have to do a modification called "NUTTER BYPASS" I can explain what it is but you can also google it very easily and find out cause it's a lot to kind of type down. If you have trouble looking it up then I will gladly explain it here for you.
 

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Discussion Starter · #8 ·
Thanks. I'm goin to get some stuff out of the way and start checking all the vacuum lines and ill also check the manifold to make sure it sealed tight. I'm putting a new fuel filter on it also so I will see what's doin in there and post back. Thanks for the help. Hopefully good results soon
 

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yeah good luck. also I think if you search the forum here and type in NUTTER BYPASS there is a lot of threads on this process
 

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Discussion Starter · #10 ·
I did see the bypass post. I'm not at that point yet. Gonna check all vacuum lines and intake manifold. Also a possible vapor lock in the fuel line to the filter and carb. So that will be replaced and see what she will do. I love this jeep, my first one, but it sure has been a headache getting her to run right. Hate seeing her sit and not getting topless and muddy. But when she's ready I know I'll love it. Gonna be as stock as possible
 

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mine was pain in the ass when I first got my 1990 4.2 and it had all the factory junk on it too. I tried so hard to keep as factory as possible but it was so unreliable. left me stranded about 5 times. so that's when I started to remove all the vacuum lines and did the nutter bypass and changed out my carb. then it ran way better and always started and no more breaking down. the ignition module was always getting hot and shutting the jeep down and that's when I did the nutter bypass. then I went to HEI distributor which made it even more simple to keep up with and keep maintained. then it blew up and then rebuilt the engine and did away with EGR and did the new manifold and so not the engine is pretty much a 2barrel carb and electric choke and HEI distributor and it only take a single hot wire to run the distributor. it's a very easy setup and very easy to diagnose.
 

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Discussion Starter · #12 ·
That sounds like the road I'm gonna be going down. I don't want to drive this rig too far from the house because I'm not sure ill make it back. Haha. My only problem is time right now I don't have enough of it but at least the jeeps paid for and has a nice parking spot. Ill get her running good soon. Hopefully it's just a vacuum or intake issue and not lower end. Thanks again for the help. Ill reply back when I have some news on the progress
 

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Def do the bypass. I got a 90 with the 4.2 fathers day weekend, it's still sitting in the driveway. My first Jeep and it's the only vehicle I've ever owned that I'm excited to own even though it didn't run right. Mine bogged down really bad during accelerating. I knocked my head against the wall for a while. Just did the bypass today, took about an hour and it's well with it! Granted, I do have a weber carb so the bypass was mandatory and apparently never done. If you have the stock Carter, junk it, get the weber and do the bypass. New weber 32/36's go for $229 on Amazon or Ebay. This may NOT solve your current problem but is with doing none the less. Happy Jeeping!
 

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yeah I used to run a weber 32/36 and it puts a lot of power to the engine but mine kept on stalling under full throttle accelerations but was great if you just eased on the throttle but I ended up just selling it and buying MC2100. Much simpler for me to keep tuned up right. I couldn't figure out very well the weber. But I do know that webers are very nice carbs.

And it took me over 2 years to get my jeep all done and running right and reliable so I know what you mean by not having enough time and money. It's so frustrating seeing that jeep sit in the driveway and not able to drive it and have fun with it.
 

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Discussion Starter · #15 ·
I knew I should have joined WF sooner. All this feedback is great. I'm taking notes like a champ from you guys and appreciate the knowledge you give. I'm trying to do as much of the work myself as I can but my good friend is a mechanic so he helps me and shows me how to do it. I'm just confused with all this bypass talk and different carb models. I'm not sure what carb is I'm it but I know it's new. Ill look at it more to see what brand and model it is. But if the jeep ran great before the bent rod and broken spring, with no bypass done or heavy tuning of the carb why do it now. I know you guys say your jeeps are running better with bypass done but I'm kind of stumped as to why it isn't running the same after I fixed the head parts. This is why I think you we're right about the intake leak or vacuum leak so I will address that before and bypass is done or carb swap. Hoping its just something we did when getting the head on and off. Keep the input coming guys ill let you know how I make out. My girlfriend is moving in this weekend and between her and the dog my jeep gets no love haha. Well I'm tired of looking at the jeep sit so my gf is gonna be one pissed of lady watching me work on it all the time.
 

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oh I do believe your jeep is fine and there is just something that wasn't put back together quite right. I don't believe you NEED to do any bypass to make your jeep run right but I do recommend doing it because once you do you will understand what we mean but I totally understand only doing one thing at a time and if your jeep is running like it should with it's factory parts on the engine then why mess with something that isn't broke I totally get. definitely just figure out what happened this time and then once it runs right again then if the jeep makes you happy with the way it runs with this setup then keep on jeeping.

to figure out what kind of carb the best way I can help is if you post a picture of it. take the air filter off and see how many barrels it is. And I mean when looking down inside the carb how many openings do you see the lead into the intake manifold. That's a start as to figuring out what kind of carb or at least the size.
 

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Discussion Starter · #17 ·
Okay just an update. I replaced the fuel filter and modified a few line that needed to be shorter and it helped reduce the hesitation when I press the accelerator. But the rough up and down idle still exists. Found a few issues with the vacuum lines so I'm working on that now. Still have some sputtering in the exhaust. Trying to find a one piece exhaust and intake gasket because I believe there may be a leak at the intake manifold. Ill update with more progress soon
 

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I had the same problem, rough idle, stalling in gear at a stop. But then once you get driving it drive great, then once you get to a stop it wants to stall out.
I had only one vacuum hose leak but it didn't make a difference. my stock carter carb finally gave out.
 

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Discussion Starter · #20 ·
Same issue I'm having. But the carb is almost brand new and ran great until I bent a push rod. It sat while we fixed the head and now that's the problem I'm having. But when it's moving its runs real strong but start stop and idle are just bad
 
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