How-to: Replace your cracked exhaust manifold -
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post #1 of 134 Old 07-04-2010, 05:26 PM Thread Starter
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How-to: Replace your cracked exhaust manifold

The exhaust manifold on my 91 4.0L YJ was badly cracked, making a nice loud popping sound and generally making it sound like I'm running a diesel. I'm a fairly amateur mechanic so I was fairly nervous going into this repair. If I can do it, any of you can do it.

My equipment list:

- Good selection of sockets & wrenches
- Flathead screwdriver
- Needlenose pliers
- Brake cleaner / wd-40 & shop towels
- rubber mallet
- patience
- pack of cigarettes

Time: 9.5 hours. Yes, really. I had a good 2.5 hours of screwing around, and had I a better idea in my head of what to do, it would have taken less time. If I had to do this again, I'm relatively confident I could do it in 4 hours.

I ordered my manifold from 1AAuto for $130 USD. It arrived in a couple of days. It was packed very nicely, and looked pretty good. It came with the gasket, donut gasket, and flange bolts/nuts. I gave it a light sanding and acetone bath, then gave it 3 coast of rustoleum high heat black paint. Follow the directions on the paint, this stuff said to leave the item for 24 hours before applying heat to it. If you do this outside, there will be a couple ants and flies that likely go and get stuck to it. Don't worry, they'll burn off quickly enough.

-Disconnect negative battery cable
-Remove the airbox hose - mine had 2 plastic clamps that were easy to release, then remove the airbox. There were three snaps on mine that came off with ease.

-remove all of the vacuum and electrical connections from your intake manifold, and be sure to label them. I used painter's tape and marked them numerically. This made re-assembly easy as pie.

-Pry the accelerator cable from the pivot ball, then unbolt the accelerator cable's bracket from the intake and swing to other side of the jeep.

-Remove the vacuum line from the brake booster. I found it easier to remove the plug itself from the booster rather than the hose from the plug. It will take some effort, and you'll be rewarded with a nice "whoooooshhh" as you pop it out and the vacuum escapes.

-Label and identify all your injector wiring harnesses after you've removed them. You'll need to get the little metal clip pried away from the orange tabs to do this. A small flathead screwdriver and a set of needlenose pliers will be your best friend here. Make sure to pop the clip back into place once you've removed the harness.

-Remove the four bolts from the fuel rail, and give it a good tug to get the injectors out along with the rail. I had a hell of a time, and was certain I was doomed to failure at this point. I gave it a few more tugs and I was not gentle, and out it came - but it left 3 of my injectors in the block and fuel puking up all over the place (yes, I did de-pressurize my fuel rail first). Not
the end of the world, but they had to be pulled out of the block, the O-rings removed from the fuel rail, installed back on the injector, and the injector crammed back in - which wasn't exactly easy either. The O-rings are very large so it took some poking and prodding with a small screwdriver to get them in. Had I foreseen this issue, I would have bought new o-rings and perhaps even new injectors just for the hell of it. You can set the injectors and rail along the spreader bar, it should seat nicely and not move anywhere.

-Loosen tension on the serpentine belt and remove power steering pump. My hayne's manual says NOTHING about this topic, so I had to figure it out for myself. I didn't get any pictures of this due to my photographer being uninterested in coming outside at this point. There are TWO bolts on the back of the power steering pump, one nut you need to loosen on the front below the pulley, and a supremely long bolt on the driver's side of the pulley that you need to loosen / remove in order to release the tension and remove
the belt, then get the power steering pump & pulley out of the way enough to remove the TWO bolts holding it to the intake manifold.

-Remove the bracket that holds the hard fuel lines. It has two bolts and is near the driver's side fender, bolted to the intake manifold.

-After that's done, you can go ahead and fight with all the manifold bolts. Take off the bottom ones first, but do not unbolt the exhaust manifold from the collector just yet. Get your top bolts off, then manhandle the intake manifold out of the way (watch your injector wiring harnesses), and set it down somewhere you can clean it.

-Unbolt the collector and remove the exhaust manifold. Take a look at how badly yours is cracked, you'd be amazed the kind of damage that can be done if these are left alone.

-Clean the mating surface on the block VERY Well, you'll be able to see on the block where the old gasket was left. I used brake cleaner and a scrubber at first to get the light stuff off, then WD-40 and a sharp exacto blade to scrape off all the miniscule stuff, as well as all the places where the old gasket was fused to the block. That took me a good while - don't write off cleaning the surface, this is a crucial step.

-Clean the intake manifold. You'll likely be better off without an exacto knife for this part. Plenty of brake clean and a scrubber / shop towels should do fine. Some q-tips as well. Make sure to clean out the injector ports and scrape out as much gunk as you can, then get the mating surface nice and shiny.


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post #2 of 134 Old 07-04-2010, 05:30 PM Thread Starter
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-Put the exhaust gasket on your new manifold, and the gasket on the block. There should be only one way the gasket fits the block if yours doesn't have a "MANIFOLD SIDE" label as mine didn't. Look at the area I circled on both gaskets, that notch goes towards the front of the jeep,and you can just barely make out a "MANIFOLD SIDE" on the old gasket. Aside from that little notch, the gasket is exactly the same flipped around.

-Now is the time! Set your exhaust manifold on the block, ensuring you catch both studs front and back. Now would be a good time to go and connect the collector. Once that is in, start the bottom four bolts for the exhaust manifold. This will make it a lot easier to torque everything down once you get the intake back on, as getting those bolts started once the intake is on will be difficult. You could even go and put the nuts back on the front and rear studs as the intake manifold doesn't use them. Hand tight only though. Now get your intake and fanagle that back on. Since you started the four bottom bolts, it will take a bit of handling to get it to sit, don't forget to make sure it connects to the two alignment studs. For the reinstall, I tucked all the injector wiring and the throttle body wiring up over the black rail. I also used some anti-seize on the bolts just in case I had to do this again at some point.

Make sure you have either an assistant or keep your bolts nearby as I had a hoot of a time keeping the intake and exhaust manifolds flush with the block with one hand while reaching for a bolt with the other. Get the rest of your bolts started and barely tightened, then begin torqueing them down.

From the FAQ section...

Intake and Exhaust Manifold Retaining Bolts/Nuts 1989 Only
Nos. 1 and 2 - 30 ft-lbs
Nos. 3 through 12 - 23 ft lbs

Intake and Exhaust Manifold Retain Bolts/Nuts All Others
Nos 1 through 5 and 8 through 11 - 23 ft-lbs
Nos 6 and 7 - 17 ft-lbs

Please view your Haynes or Chiltons book for torque sequence. I don't have my scanner set-up right now, so I cannot add that in.

I forgot to torque down the nut at the front (#11) of the manifold, and later on when I did - I broke it. It looks like it was already half busted apart as it was fairly well rusted halfway across the bolt. I don't have any bolt recovery tools, so I'm running without it for now.

Now that you've got everything torqued down, go ahead and get your power steering pump and pulley back into place (don't forget to bolt it back to the intake before you move the pulley), make sure your belt is routed properly and tensioned appropriately. I used the markings from where the bolts used to be bolted down to to gauge for tension, then gave the belt a good test and it seems okay.

Now you can plug the injector rail back in. Gentle, even pressure will do the trick here. I ended up using a rubber mallet to get them seated properly. You'll have a good idea they're seated when the tabs sit flush with the bolt holes. Bolt it back up, reconnect the injectors. Bring your accelerator cable back over, hook the bracket up and pop it back onto the pivot ball.

Reconnect all the little vacuum and electrical connections you removed earlier and [hopefully] marked. Replace the fuel line bracket on the manifold, plug the brake booster back in, hook up the air box and make the connections to that.

Reconnect your battery, do one last check for any missed items and fire it up.

If you painted yours like I did, then it's likely your manifold will smoke for a while. Do not freak out like I did. This is perfectly okay, and will probably stop in 5 - 10 minutes depending on how many coats you gave it. I ran my jeep for about 20 minutes while I cleaned up, with my girlfriend in the driver's seat keeping an eye on the RPM's, temperature, and looking for a check engine light. I was lucky and didn't have any problems aside from the single stud I busted.

I cannot believe just how amazingly quiet my jeep is now. It's a completely different vehicle. I haven't noticed any noticeable power gains, but I will be tracking my mileage and see if I get anything out of that. I'll post my gain if it's noticeable.

Feel free to ask any questions. :-)
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post #3 of 134 Old 07-08-2010, 09:23 PM
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Sleuthound, thanks for the great write up. I've had my new manifold just sitting in the shed for about a year now waiting for me to get motivated to change it out. Also good idea on painting it (never would of thought of that). Thanks again, Dave.
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post #4 of 134 Old 07-08-2010, 09:52 PM Thread Starter
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Thanks Dave! It was fairly daunting going into it, but it was fairly simple all in all, just tedious really. Good luck with your manifold, please post if you run into any problems during or any questions beforehand. :-)
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post #5 of 134 Old 07-08-2010, 10:20 PM
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An excellent write-up!

I hope to be doing mine in the next few days, as my cracked manifold has decided to get much worse recently.

The only difference between the way you did it & the way I'm doing mine is that I will be disconnecting the fuel lines & setting the rail out of the way, and I have a 90 degree air grinder with a scuffy pad for quick gasket mating surface cleanup. If I weren't lucky enough to be in a shop environment, I would be doing it EXACTLY as you have documented here.

Congratulations on a job & write-up well done!


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No one here knows what they are talking about. You should try Pirate 4x4 they will be happy to help you.

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post #6 of 134 Old 07-09-2010, 06:24 AM
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It is a great writeup. When I did mine I replaced the studs with bolts. Those middle bolts were a real *&^&*&* to put on. I wish I had this when I did mine! I added this to the bookmark list! Thanks.
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post #7 of 134 Old 07-09-2010, 06:39 AM
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excellent write-up. I have done this job. Mine experience was 6 hours one day and 3 the next.

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post #8 of 134 Old 07-09-2010, 06:41 AM
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many thanks for the write up!! i need to replace mine also, but i thought about taking the fender off so i could sit on the tire!!! its only a few extra bolts!

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The weird thing is that I thought I hear a minor bang, almost like a fart sound coming out of my Jeep, but I'm not sure what it was. It has never happened before the glue incident.

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post #9 of 134 Old 07-09-2010, 06:56 AM
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Awesome write up man!

A lot of us have the cracked manifold. I had no idea it was so involved.

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post #10 of 134 Old 07-12-2010, 11:58 PM
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Thanks for your efforts on the write-up, and great photos. I ordered my new manifold just yesterday, and as soon as the boat makes it here, I'll be doing this repair. I have yet to read thru my FSM on this to see their comments on replacing injector O rings, but I'll let everyone know what I come up with.

Hope this gets made a Sticky in the FAQ section.

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yes I'm a some guy and have always put gasket sealer on my surfaces before mating
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post #11 of 134 Old 07-13-2010, 12:40 AM
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saving the link, thanks man.

^^^thats a lie.
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post #12 of 134 Old 07-13-2010, 01:45 AM
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Originally Posted by Mean Max View Post
The only difference between the way you did it & the way I'm doing mine is that I will be disconnecting the fuel lines & setting the rail out of the way, and I have a 90 degree air grinder with a scuffy pad for quick gasket mating surface cleanup. If I weren't lucky enough to be in a shop environment, I would be doing it EXACTLY as you have documented here.
A Dremel and wire wheel made short order of the cleaning part of this fix for me. Definitely worth it over the scraper and hand brush.

I also disconnected my fuel lines. It's not hard at all and gets the fuel rail out of the way since where he has it is right where I was most of the time.

Overall a very good write up though. Sorry about your snapped bolt problems OP. Hopefully you can get that remedied soon.
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post #13 of 134 Old 07-13-2010, 07:49 AM Thread Starter
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Thanks for the praise, folks! I'm happy to contribute to our fantastic community here. So many of Max's write-ups have come in handy for me.

Yeah, my tool selection is relatively limited. I really only started wrenching heavily when I got my POS cherokee last November (sold it in March and got my YJ), so it's slow going collecting tools and the like. I'll keep the dremel and wire wheel in mind for any future gasket surface issues.
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post #14 of 134 Old 07-13-2010, 09:41 AM
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My YJ sounds like a diesel as well and I know for a fact that it has to be a cracked manifold but Ive just been putting it off over and over again because I'm fine with not dealing with it.

Now that you put out this write up though I think I will reconsider once I have an entire free weekend to get it done (something always goes majorily wrong in ANY wrenching project of mine).

Awesome write-up...this needs to be added to the FAQ/Write-Up section!

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post #15 of 134 Old 07-13-2010, 09:45 AM
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That shirt is awesome man.....haha
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