Project MJ: Four Point Oh No

"Justin Hughes" (justinhughes54)
12/21/2015 at 16:03 • Filed to: jeep comanche

The Jeep inline-six 4.0 liter motor is a classic AMC design, dating all the way back to 1964 in its original form. You’d think that by 1988, all the bugs would be worked out, right? Well, not exactly. And a couple of them are rearing their ugly head in Project MJ.

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When under the hood there arose such a clatter

I sprang from the cab to see what was the matter.

OK, now I’ve made the obligatory Christmas reference. Moving on. The 4.0 is not a quiet engine. Almost, but not quite, entirely unlike the NA and NB Miata, the hydraulic lifters are known to make a tapping noise. Many times at autocrosses I’ve had other drivers warn me about this noise, yet at a Miata club event, every single car is doing it. It’s completely normal, especially after it’s been driven hard. But in recent driving I’ve noticed a rhythmic tapping, connected to engine speed, getting progressively louder under the hood.

When the oil light turned on and wouldn’t turn off, I shut off the engine and panicked. Fortunately this happened in my driveway, so I didn’t have to worry about getting anywhere, and I could panic in the privacy of my own home. I checked the oil and found it had plenty, which was good. But that doesn’t mean the oil is necessarily getting where it needs to go.

My panic spread over to OppositeLock and !!!error: Indecipherable SUB-paragraph formatting!!! . I got a wide variety of feedback, ranging from “it’ll be fine,” !!!error: Indecipherable SUB-paragraph formatting!!! style, to “that engine is toast.” Several people suggested that one or more lifters might have failed, a common issue with this motor, but the tapping was definitely a cause for more concern. It seemed to be coming from the top of the motor, not the bottom, which meant it was likely not a rod bearing. So I decided to pull the valve cover and see what I could see. Also, it’s likely the valve cover gasket has been leaking, so since it needed replacement anyway I figured I might as well open it up now.

The first thing I noticed is that one of these things is not like the other. One of the rocker arms on the #2 cylinder had rotated so far that it wasn’t even pushing the valve anymore. The rocker arm on the other valve was still in place, but loose. This means that both lifters for the #2 cylinder have failed, with a unique twist on one of them. (See what I did there?) A sharp eyed Comanche Club member pointed out that in the video he could see the #2 spark plug vibrating a bit. This led him to believe the plug might be loose, which I know isn’t true because I replaced all of the plugs myself. But despite being wrong about the cause, he was absolutely right about where the problem was. I had overlooked this hint earlier.

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As you can see, underneath the valve cover isn’t the cleanest place in the world, either. The oil is old and dirty, and though I have everything I need to change it, I’ve been waiting to diagnose where it’s been leaking first. The most common place for this is the valve cover gasket, and the seepage I noticed around it as I removed the bolts leads me to believe this is the case here. So replacing this gasket will happen when I put things back together regardless.

My main question right now is whether the lifters failed on their own, or whether it is a symptom of A) sludge buildup, and/or B) a failing oil pump, something else this motor is known for. Unfortunately my dashboard has the oil idiot light, not a real live oil pressure gauge, so I took a drive to the local parts stores to find a test gauge. Nobody had one. This is sad. I don’t know if it’s a cause or effect of more people not working on their own cars and/or ordering online these days. So no instant gratification for me, but thanks to !!!error: Indecipherable SUB-paragraph formatting!!! I’ll be able to test my oil pressure later this week.

It’s quite possible that the switch has gone bad, especially considering the gunk it’s covered in at the moment. It’s conveniently located next to the oil filter, and I’ll be removing it to attach the pressure tester in its place. If the pressure is good, I’ll first try cleaning it up and reinstalling it, but if that doesn’t work they’re pretty cheap to replace. I hope this is the case. It’s an easy fix, would mean that my oil pressure is fine, and would mean no major issues besides the lifters.

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If oil pressure is actually low, that’s a more complicated problem, but not impossible to fix. It involves dropping the oil pan and replacing the oil pump inside – another reason to not bother changing my oil just yet, since I’ll have to drain it to do this anyway. Fortunately, from what I’ve read there’s just enough space to remove this around the front axle when the suspension is at full droop, and !!!error: Indecipherable SUB-paragraph formatting!!! would certainly lift it high enough to achieve this.

Regardless of the oil pressure situation, it looks like I’m going to have to pull the head to replace the lifters. This is a big deal, and certainly more than I’ve ever disassembled a car engine before. But the age of this design makes it easier. I’ve watched a few videos on the job, and I think I can handle it. (How did DIY mechanics do this stuff before YouTube, anyway? The main thing I’ve learned from numerous Haynes manuals is that contrary to what they say, installation is NOT the opposite of removal.) I’ll need to clear my workbench to keep the parts organized as I remove them, but my garage came with two workbenches, so this won’t be a problem. It’ll also be a good chance to give all of these parts a good cleaning to remove any sludge that might have built up over the years, especially while the Jeep has been sitting. Unfortunately there’s no way for me to fix the motor by the time my fiancee needs it to move out of her place (basically, now), but at least that means I can take my time to do the job properly instead of trying to rush through it. The last thing I want to do is cause more problems later.

I never planned to crack open the engine on Project MJ, but it looks like you’ll get to see that happen. All of the other tasks are on hold until the engine is dealt with.

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DISCUSSION (19)

Dogapult > Justin Hughes
12/21/2015 at 16:11

This was interesting, but the article requires editing, possibly due to copy/paste.

crowmolly > Justin Hughes
12/21/2015 at 16:13

The retainer on that one valvespring looks damaged.

Definitely looks like you have a collapsed lifter.

CalzoneGolem > Justin Hughes
12/21/2015 at 16:22

After you get this sorted you may want to toss some atf in with the oil and run it for awhile then change the oil and filter. This should flush a lot of the gunk out.

Justin Hughes > Dogapult
12/21/2015 at 16:30

Thanks for the tip! Should be fixed now.

Kinja'd!!!

Justin Hughes > crowmolly
12/21/2015 at 16:31

I’ll have to take a closer look at that retainer. It’s quite possible the rocker arm was banging on the edge of it as it rotated off of it.

Justin Hughes > CalzoneGolem
12/21/2015 at 16:34

I was thinking Seafoam, but I’ve read that ATF works too. After I get an oil pressure tester on there that’ll determine the order of operations here.

EL_ULY > Justin Hughes
12/21/2015 at 17:43

crud could get stuck in the lifters causing them to get stuck or wear out

That Bastard Kurtis - An Attempt to Standardize My Username Across Platforms > Justin Hughes
12/21/2015 at 18:17

Are you a Douglas Adams fan? Your comment about the sound of the Jeep being “almost, but not quite, entirely unlike” the Miata reminds me of a line from Hitchhiker’s Guide to the Galaxy.

Justin Hughes > EL_ULY
12/21/2015 at 20:30

Yes indeedy.

Justin Hughes > That Bastard Kurtis - An Attempt to Standardize My Username Across Platforms
12/21/2015 at 20:31

“A substance almost, but not quite, entirely unlike tea.” Why yes, I do know where my towel is. You’re a hoopy frood to understand that.

That Bastard Kurtis - An Attempt to Standardize My Username Across Platforms > Justin Hughes
12/21/2015 at 21:03

I just used frood earlier today. Your response hangs in the air in the same way that bricks don’t.

Man, I haven't read that book in about 2 years. Been way too long.

Rock Bottom > Justin Hughes
12/22/2015 at 00:54

I would have thought a dead hole would be pretty obvious from driving it, regardless of sound. It would feel lazy and have a lumpy idle. Also, once you fix it expect it to keep clattering a bit. It’s a 4.0 feature! My 91 XJ has been clattering for years. Hot, cold, idle, highway, always. I have a real gauge showing 50 to 55 psi over 1000 rpms and about 35 psi at idle, all good numbers for a 4.0. It also leaks... everything. Oil, PS fluid, trans fluid, transfer case, antifreeze, both difs. I like to think of it as cheap rust inhibitor!

Justin Hughes > Rock Bottom
12/22/2015 at 09:14

I’ve had three Miatas, so I’m used to noisy lifters being normal. I have a real gauge cluster that came with the Jeep waiting to go in. I just need to finish other tasks first, like fixing the motor. As for rust, this thing needs all the prevention it can get!

Rock Bottom > Justin Hughes
12/22/2015 at 16:06

That’s a lot of Miata! Good luck with that MJ. There are too few of them around these days!

Brad D > Justin Hughes
12/24/2015 at 10:26

I was certainly unaware of this. Sorry! Seems like no matter which way I went I would be working on engine internals. The car I replaced this with needs crank bearings :(

Justin Hughes > Brad D
12/24/2015 at 11:02

I don’t think this even became a problem until after we took it off your hands, so you couldn’t possibly have known about it. Fortunately the repair seems a lot less extensive than what you’re looking at. No worries!

Brad D > Justin Hughes
12/24/2015 at 11:15

It still pains me to see. I hate even feeling dishonest! Good luck, let me know if you need anything.

Justin Hughes > Brad D
12/24/2015 at 11:17

Thanks! I’ll look you up for a gas tank later as we discussed, but getting it running again comes first. :)

greasemonkey235097 > Justin Hughes
12/26/2015 at 17:53

Kinja'd!!!

If you were in North Jersey, i had this 89 4.0 for sale..