4.0L on last legs - should I call the hearse?

I think my 4.0 is dying, in need of a rebuild or replacement.

It's a '94 HO in a Sport model, with the AX-15 and NP231 behind it. I go through plugs about every six months - it just loses power on the highway and can't sustain speed. I usually end up with the first five plugs on the "slightly scorched" end of the spectrum, while plug number six always ends up oil fouled big time.

Pulling the oil fill cap while the motor runs results in a choo-choo effect, smoke chugging on out. I get a decent amount of smoke out the tailpipe when getting on the gas - it's like a li'l diesel.

I also leak oil like a sieve - I drove it back from school (~150 miles) to swap out for a friend's pickup that I'm going to change the clutch on, and I had a mosquito abatement cloud following me, must have lost around 3 quarts of oil on that short trip. The entire underhood is pretty much oil splattered.

So here are my questions - it's sitting at home, and I have a primary mode of transportation. What can I do to see if this motor is going to last me until the summer without any major work? I've been through a couple of oil pressure senders, for all I know I blew another one - I didn't even know the extent I was leaking until my mom called to tell me about the puddles underneath the jeep. What leaks are likely that I can fix and drive relatively leak free until I can pull the motor and rebuild it or replace it with another?

I'd be interested in a stroker if possible, but being that I'm a poor college student, I'm not sure I could shell out more than 900 bucks on the motor - will that even get me close? I saw master rebuild kits at $450, could I pull off a bore/hone and basic prep (Cam bearing install, new valve seats) for this rate? Would it be a good idea to buy a junkyard motor and do all the work on that motor, then sell my motor to be a big ol' paperweight?

Basically I need some direction to go in to salvage this motor, recommendations on what I can keep, reuse, and restore. Please, anyone that's been through a similar situation chime in! I'd love to keep the motor going, but a life of neglect before I bought it a year and a half ago has brought it to this stage.

Sounds like it's toast. Do a compression test, see what that says. If you have a pick and pull near you, you could grab one cheap. Take youre time
building the other back up to par. How many miles do you have on it?
Sorry to hear that another jeep dies so young. Good Luck

Yeah, I figured.

Has 168K on it right now, I bought it with 147K on it when it still ran strong. It really started to go downhill right after this summer, where I logged about 12k miles in 3 months, between round trips to AZ and OH for SAE Mini-Baja, and almost weekly round trips from home to school (I'm in Urbana right now, from Chicago originally). Funny enough, it was only after all this mileage that it started to act like this, not during the 6k mile round trip to AZ and back, and it ran like a champ out south of Tucson. When I got back home, it started to crap the bed.

I'll run a comp test on it when I get home next, but it won't be for a coupla weeks.

Someone is offering me a rebuilt renix engine for a decent price, what would it take to get that into my 94 with few problems? What kind of availability am I looking at here? Could I throw an auto trans motor on mine, just swapping the starter?

What's a good price to pay for a rebuildable motor? Car-Part.com lists several around the $450 range, but something tells me that's a motor I'd need to rebuild.

I definitly would do a compression test. From the symtems your decribing I bet It has exsesive pressure in the crankcase. I can't remember if the older 4.0's had a PCV valve. or use the CCV type system. I would look at it to make shure something isn't plugged. I just find it hard to belive that it would go all at one like that unless there was some outside force like a low oil, or an overheating to effect it.
Also IMHO if it does indeed be a lost cause. I would just score a parts yard motor, and not even worry about the old one.

It's a CCV setup, I did the madxj style update with the 2000-'01 parts toi reduce blowby in the air filter. It's been through a valve cover gasket and two oil pressure senders in the last four or five months, the major leak I have right now may be another sender. Also my oil filter adapater leaks, and I didn't have a torx socket to get that off.

that is very few miles for a 4.0 to crap out . Do as stated above , it does sound like you have too much crankcase pressure

I understand that it is relatively few miles for a 4.0 to crap out. However, this motor seems to have been poorly maintained during it's previous ownership (the reason I draw this conclusion is that the diffs and t-case either had the factory fill of fluid, or they were flogged) and I wouldn't be surprised if oil changes were a foreign thing. I cleaned the hell out of the ccv setup when I did the valve cover gasket.

When I had the valve cover off I noticed I had smoke coming directly out of valves - the valve seats are probably gone. The way it smokes while running (out of the tailpipe) I would venture to guess that the piston rings are gone. Especially since I get at least one completely oil fouled plug every six months or so, enough that the engine misfires and makes power like a hyundai.

Plus the way it's leaking right now, I wouldn't be surprised if I wound up spending more money on oil than gas.

So what would ya do in my place?

You could sell it and start over, or just get the renix motor and swap out. Just use all youre sensors, intake and exhaust manifolds. not hard to swap out at all. As for the trans swap, I have no idea what it would take.

I wanna keep my AX-15. Would a renix block with a HO head be what I'm looking at doing? I hear the major difference between the HO and the non HO is the head, so that would be a more or less duh answer.

The difference between the AW4 and AX-15 blocks may be the starter, however a visit to parts house websites can't confirm this. I figure if I bought a complete motor from a Renix, that it would be easier to just change out the ECM.

I'd like to keep the body and drivetrain on this vehicle, which are very solid for a Chicago Jeep. I've also put some speakers / radio upgrades in (including a CB) and I doubt I'll get much for a barely running XJ. Plus it's a two door, and I'm 6'5", and I had a helluva time finding this one.

Mateo94XJ said:

So what would ya do in my place?

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The high crankcase pressure (and the resultant oil leaks) is a sure sign of blowby due to worn piston rings (combustion gases blow by worn rings and pressurize the crankcase). You probably also have worn valve stem oil seals and possibly worn valve guides.
The engine definitely needs a full rebuild. The problem is that it's so tired it'll need a lot of prep work so the down time will be lengthy. I suggest you buy a low mileage used long block from a junkyard (preferably with 7120 or 0630 head casting so that means not newer than a '99) and swap it into your XJ. That'll get you going with a good engine until you either build your old engine into a stroker or break it for parts.

Dr. Dyno said:

I suggest you buy a low mileage used long block from a junkyard (preferably with 7120 or 0630 head casting so that means not newer than a '99) and swap it into your XJ. That'll get you going with a good engine until you either build your old engine into a stroker or break it for parts.

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Ok, this is actually what I was thinking. So I guess I wasn't wrong then, cool.

What would it take to swap in a newer (for example a '97 OBDII) engine into a '94? Would this require an ECM swap, or would I just reuse sensors? How would the ghetto rigging work for the electronics?

To perform a simple test, try pulling all of your CCV lines out. Disconnect the rear line from the intake manifold, and the front line from the airbox.
Then pull your spark plugs, clean them up and reinstall them careful to avoid getting oily crud from the spark plug wells on the plug's side electrodes.

if she runs better afterwards, you may have a sludged valve cover from all those hard miles. You may need to pull the valve cover again, and desludge it.

Though a parts washer or hot-tanking is the best, try a high power carb/throttle body cleaner. Spray into the baffles and let it soak a while. You ought to see a lot of dissolved sludge run out with repeated spraying.

I agree it might be time for a swap. While it's a fairly low mileage for a 4.0 to crap out, it isn't so low for a very badly maintained one to fail, and I suspect you're right that it had a hard life before you got it. I'm guessing a bad or broken ring on the cylinder that's fouling. When you get up to that mileage, especially if the engine is badly worn, you're looking at a lot of rebuilding, possibly even reboring, oversize pistons, reground crank, etc. etc. Unless you really want a major rebuilding project for the fun of it, a healthy boneyard engine will almost certainly be cheaper, and if it's a good one you have a chance that it will last just about forever under good care.

Depending on the mileage of the replacement, you would probably do well to replace the rear main seal before you even put it in, which will give you a head start on leaks, and also allow you to have a good look at how clean or dirty the crankcase is.

But of course, first a compression test, or at least a sniff with a vacuum gauge, to make sure it isn't something else. Bad valve guides and seals can suck a lot of oil too.

92DripCherokee said:

To perform a simple test, try pulling all of your CCV lines out. Disconnect the rear line from the intake manifold, and the front line from the airbox.
Then pull your spark plugs, clean them up and reinstall them careful to avoid getting oily crud from the spark plug wells on the plug's side electrodes.

if she runs better afterwards, you may have a sludged valve cover from all those hard miles. You may need to pull the valve cover again, and desludge it.

Though a parts washer or hot-tanking is the best, try a high power carb/throttle body cleaner. Spray into the baffles and let it soak a while. You ought to see a lot of dissolved sludge run out with repeated spraying.

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Could be, but I did that only about 4k miles ago. Used carb cleaner, and I really didn't get that much sludge out. That much have built up in such a short amount of time?

I read your thread entries again and read smoke is coming out of your exhaust. She may be on her last legs.

The compression test is great idea. You nee to determine how the oil is entering the cylinders, through worn valve guides, a cracked head, or worn piston rings

A sludged up pair of BC baffles can cause all of your symptoms, did you see any excess sludge buidup when you pulled the VC?

92DripCherokee said:

A sludged up pair of BC baffles can cause all of your symptoms, did you see any excess sludge buidup when you pulled the VC?

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Negative, it cleaned up real easy, I used pipe cleaners and really went to town on it - there was no sludge buildup.

Matthew Currie said:

I agree it might be time for a swap. While it's a fairly low mileage for a 4.0 to crap out, it isn't so low for a very badly maintained one to fail, and I suspect you're right that it had a hard life before you got it. I'm guessing a bad or broken ring on the cylinder that's fouling. When you get up to that mileage, especially if the engine is badly worn, you're looking at a lot of rebuilding, possibly even reboring, oversize pistons, reground crank, etc. etc. Unless you really want a major rebuilding project for the fun of it, a healthy boneyard engine will almost certainly be cheaper, and if it's a good one you have a chance that it will last just about forever under good care.

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Well I was looking at doing the DIY stroker on this motor at some point, somehow now I figure it would be easier to score a parts yard motor and rebuild that. I was thinking a healthy .030 or .060 overbore with a 258 crank, rods, and pistons. Possibly a new cam, though the stock grind would probably be fine for what I'm doing. I could always keep her with the stock internals (242 crank, rods, etc.) and just go overbore - a local machine shop will do me a nice bore / hone for relatively cheap, and they said they'd put in new cam bearings for free if I had them do the other work.

So, how long does it take you guys to swap a 4.0 in an XJ? Be serious, now.

Mateo94XJ said:

Well I was looking at doing the DIY stroker on this motor at some point, somehow now I figure it would be easier to score a parts yard motor and rebuild that. I was thinking a healthy .030 or .060 overbore with a 258 crank, rods, and pistons. Possibly a new cam, though the stock grind would probably be fine for what I'm doing. I could always keep her with the stock internals (242 crank, rods, etc.) and just go overbore - a local machine shop will do me a nice bore / hone for relatively cheap, and they said they'd put in new cam bearings for free if I had them do the other work.

So, how long does it take you guys to swap a 4.0 in an XJ? Be serious, now.

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I've never done an XJ swap, but I've done a couple of others, and it shouldn't be too bad if the engines are wired similarly and you have all the pieces. A really good engine hoist (rentable) and a floor you can roll it on help a lot. If you're working outside, get some plywood. Being able to move the hoist a tiny bit without having it rock and stick and change angles is a very big help. I think if you're going to do a stroker and want it to last you're probably still better off starting with a known good block rather than your own, for two reasons. One is that it's a good block, and the other is that you can keep your old one chugging along until you're ready for the swap, saving downtime. Then if you do find the old block has a busted ring and a deeply scored cylinder wall or something as bad or worse, you're not hung up waiting for a block.

Yup, that was my concern (the block I have being FUBAR). I might buy JUST a block, but I would like to limit downtime as much as possible - buying a junkyard motor, rebuilding / stroking that, and then swapping 'em. Possibly turning around, tearing down my old motor and reselling it once rebuilt (if possible) to recoup some investment.

The last engine swap I did was on a minivan, so something tells me the XJ swap should be easier - and the minivan was pretty easy.

I’m with the rest, I bet you its bad rings. Get a good junkyard engine and rebuild that. If you decide to turn it into a stroker; do a search there’s tons of articles to help you out.