Renix blow-by

[hgeranium]

In all this tinkering that I've been doing with the crankcase, I noticed that there's a pretty steady stream of smoke from the CC with the oil cap off while it's running. It's not billowing smoke and clouding vision or anything, but I do see smoke coming out. I understand that 1. it's an old style motor and 2. it's an old (wear-wise) motor. I'm at ~188,000 miles. For similar mileage 4.0 guys, what does the blow-by look for you? Is what I'm seeing just typical ring wear from mileage/age or should I be more concerned? It doesn't feel like there's any of pressure when I cover the hole with my hand and no oil splashes out with the cap off either. The cap doesn't move at all if I take it off and set it on top of the hole.

 

Also, do y'all (renix guys) still get seepage from the PCV grommet/hose even with brand new parts? It's not a lot but I do see a fresh pool of oil on the grommet and around its spot on the VC after every drive. Granted, I haven't done cruiser's valve cover mod yet, but I would've thought the new stuff would fully seal it for at least a little bit.

[89 MJ]

I have probably 140k miles on my MJ, so a decent amount less than you. My truck has blow by. No leaks, but the engine is always just a little dirty from the blow by. In a 3,000 mile oil change, it burns a negligible amount of oil. I do get seepage around the CCV grommet, but I also have not done Cruiser's valve cover mod. Similarly, my Eagle engine has an estimated 85k miles on it by now, I went through and resealed it, but did not touch the rings or the bearings because the bearings look brand new. If you pull the PCV, the thing looks like a freight train when its up to temp. Its bad enough that it has a home made road draft tube made for it so it isn't creating puddles of oil on top of the engine. The thing drinks about a quart of oil every 150 miles and at a hot idle, you can see a cloud of oil smoke. My Eagles previous 110k mile 258 was no better.

 

Based on that, I would say your engine is about right for the mileage. Do you have any oil consumption issues? A quart between oil changes should be about right for these engines, in my opinion. A good portion of this also depends on how well the engine was taken care of by previous owners. 

 

That said, keep oil in it and it will run forever. These engines were designed in a time when vehicles were pretty much worn out by 100k miles. At this point, anything else you're getting out of it is a bonus. I think its also worth mentioning that these engines were from an old engine family even when they were new. The AMC I6 family that the 4.0 is apart of goes back to the 60s. Other than the cylinder head design and the fuel injection, a 4.0 is an engine from the 60s when odometers only had 5 digits because most cars didn't live past that. Sure, GM did the same thing with the small block, but those engines got a ton of updates to them throughout the years, including getting roller cams and one piece rear main seals, which do a ton to modernize an engine, but the 4.0 never got such.

 

You don't see it too much with the Jeep stuff, but in the AMC community, you see a lot of people swapping their 6s between 100 and 150k miles because they're just plain worn out. Everybody says that these engines cannot be killed, and for the most part, I agree. With that said, what those people fail to mention is that while these 4.0s will not die, you do need to treat them like an engine from the 60s if it is a vehicle you care about. And by that I mean you either need to check the oil at least once a week and expect blow by or you need to pull it out and redo it.

 

I think its also worth saying that quality control was not as good when these trucks were new. Sure, the quality of materials were better, but even super nice original vehicles had their problems with panel fitment and paint, so I'm sure that there were plenty of mechanical issues too. This was all automakers too, not just AMC.

 

I'm not saying this to hate on AMC because none of the other manufacturers were any better at the time. AMC did really well with what they had and did great things for the automotive industry as a whole. I'm also not trying to sound condescending here. I'm telling you this explanation the same way I had to hear it and piece it together myself. It is a very frustrating lesson to learn, or at least it was for me. It really isn't always very much fun throwing time and money at an engine that is worn out, but that's about all either of us can do right now. The light is at the end of the tunnel though and more money and hopefully more time are right around the corner. 

[Eagle_SX4]

After I swapped an H.O. block into my truck I will say that I would just swap the valve cover to a newer style as the newer valve covers do a better job at sealing both ports for the breather and CCV. Also the later stamped steel valve covers have way better baffles to keep the oil out of the air filter.

[hgeranium]

24 minutes ago, Eagle_SX4 said:

After I swapped an H.O. block into my truck I will say that I would just swap the valve cover to a newer style as the newer valve covers do a better job at sealing both ports for the breather and CCV. Also the later stamped steel valve covers have way better baffles to keep the oil out of the air filter.

So since I already have an HO airbox, the HO valve cover would go right onto the air box with its appropriate CCV hose? That's good to know but I would feel so sad swapping out such a good looking valve cover, even if the HO design is functionally superior in every way.

[hgeranium]

33 minutes ago, 89 MJ said:

I have probably 140k miles on my MJ, so a decent amount less than you. My truck has blow by. No leaks, but the engine is always just a little dirty from the blow by. In a 3,000 mile oil change, it burns a negligible amount of oil. I do get seepage around the CCV grommet, but I also have not done Cruiser's valve cover mod. Similarly, my Eagle engine has an estimated 85k miles on it by now, I went through and resealed it, but did not touch the rings or the bearings because the bearings look brand new. If you pull the PCV, the thing looks like a freight train when its up to temp. Its bad enough that it has a home made road draft tube made for it so it isn't creating puddles of oil on top of the engine. The thing drinks about a quart of oil every 150 miles and at a hot idle, you can see a cloud of oil smoke. My Eagles previous 110k mile 258 was no better.

 

Based on that, I would say your engine is about right for the mileage. Do you have any oil consumption issues? A quart between oil changes should be about right for these engines, in my opinion. A good portion of this also depends on how well the engine was taken care of by previous owners. 

 

That said, keep oil in it and it will run forever. These engines were designed in a time when vehicles were pretty much worn out by 100k miles. At this point, anything else you're getting out of it is a bonus. I think its also worth mentioning that these engines were from an old engine family even when they were new. The AMC I6 family that the 4.0 is apart of goes back to the 60s. Other than the cylinder head design and the fuel injection, a 4.0 is an engine from the 60s when odometers only had 5 digits because most cars didn't live past that. Sure, GM did the same thing with the small block, but those engines got a ton of updates to them throughout the years, including getting roller cams and one piece rear main seals, which do a ton to modernize an engine, but the 4.0 never got such.

 

You don't see it too much with the Jeep stuff, but in the AMC community, you see a lot of people swapping their 6s between 100 and 150k miles because they're just plain worn out. Everybody says that these engines cannot be killed, and for the most part, I agree. With that said, what those people fail to mention is that while these 4.0s will not die, you do need to treat them like an engine from the 60s if it is a vehicle you care about. And by that I mean you either need to check the oil at least once a week and expect blow by or you need to pull it out and redo it.

 

I think its also worth saying that quality control was not as good when these trucks were new. Sure, the quality of materials were better, but even super nice original vehicles had their problems with panel fitment and paint, so I'm sure that there were plenty of mechanical issues too. This was all automakers too, not just AMC.

 

I'm not saying this to hate on AMC because none of the other manufacturers were any better at the time. AMC did really well with what they had and did great things for the automotive industry as a whole. I'm also not trying to sound condescending here. I'm telling you this explanation the same way I had to hear it and piece it together myself. It is a very frustrating lesson to learn, or at least it was for me. It really isn't always very much fun throwing time and money at an engine that is worn out, but that's about all either of us can do right now. The light is at the end of the tunnel though and more money and hopefully more time are right around the corner. 

Good to know our experiences are similar. And honestly, even with its minor leaks, I very rarely have to top up on oil. I'd say it is at about a quart every 3000 miles in total like you said. Much of this is surely exacerbated by God knows what else is leaking but if I have nothing to worry about other than keeping oil topped up (already good on that) then that's cool to hear.

[Eagle_SX4]

2 minutes ago, hgeranium said:

I would feel so sad swapping out such a good looking valve cover

The 91-95 valve covers look the same as the Renix cover but most of the time they are painted black. Sandblasting would take care of that real quick.

[hgeranium]

13 minutes ago, Eagle_SX4 said:

The 91-95 valve covers look the same as the Renix cover but most of the time they are painted black. Sandblasting would take care of that real quick.

Woah cool! Great to know. Thanks for the help

[75sv1]

The 93-95 ZJ VC is aluminum. It has the oil towers like the RNEX and HO. It has the CCV grommets like the later steel covers. 

[watchamakalit]

Anything is better than the original plastic valve covers amc used on the 258.  That said I would say what your seeing is normal 4.0 behavior.  My mj has been cruiser modded and all new ccv components, still insists on pissing oil out of the grommets and the oil fill cap. Other than that I have fixed all my leaks and use a less than noticable on the dipstick amount of oil between changes. As was said above, old engine design with old efficiency standards combined with being higher mileage engine means it won't be perfect by any means. just check the oil with with each fuel fill and add accordingly.  But most importantly enjoy the truck. 

[pizzaman09]

My 277k mile Renix 4.0L absolutely pukes oil out of the valve cover.  If one were to place an oil catch can in the system l, where would you put it?  I was thinking the vacuum line between the manifold vacuum source and the back of the valve cover.  

 

I have replaced my rear main seal, oil filter housing and valve cover seals.  I really would like to not have a 4" puddle or oil under my truck everywhere I park.

[hgeranium]

55 minutes ago, pizzaman09 said:

My 277k mile Renix 4.0L absolutely pukes oil out of the valve cover.  If one were to place an oil catch can in the system l, where would you put it?  I was thinking the vacuum line between the manifold vacuum source and the back of the valve cover.  

 

I have replaced my rear main seal, oil filter housing and valve cover seals.  I really would like to not have a 4" puddle or oil under my truck everywhere I park.

Where does the oil tend come out of? I know @Muncherinstalled a catch can on the front CCV hose. He bolted it up to the old ballast resistor point.
 

I think it’s pretty smart. Eliminates any chance of oil on the air filter plus the catch can will trap/filter that nasty oil smell that someone like me gets for venting it out into a water bottle lol. 
 

And SX4 recommended switching to an HO valve cover with the improved PCV/CCV system. That could be worth your time too. 

[hgeranium]

Awful picture but I noticed that the gasket on my few month old oil cap was starting to warp and wasn’t seating properly anymore. Since I bought it at O’Reilly, it was under a one year warranty. I pulled the old gasket off, warranty swapped the oil cap, and double stacked the old gasket with the fresh one on the new cap. It’s on there so tight I felt like I was going to break the thing when reinstalling the cap. A few heat cycles should break it in though. 

 

I’m pretty confident about this version of cruisers oil cap mod though. I think it’s a lot easier than the cruiser recommended option of having/buying some innertube and cutting it to fit the cap or outright buying a sheet of gasket material. 
 

This should help with my crankcase pressure and VC leakage woes.

[pizzaman09]

9 hours ago, hgeranium said:

Awful picture but I noticed that the gasket on my few month old oil cap was starting to warp and wasn’t seating properly anymore. Since I bought it at O’Reilly, it was under a one year warranty. I pulled the old gasket off, warranty swapped the oil cap, and double stacked the old gasket with the fresh one on the new cap. It’s on there so tight I felt like I was going to break the thing when reinstalling the cap. A few heat cycles should break it in though. 

 

I’m pretty confident about this version of cruisers oil cap mod though. I think it’s a lot easier than the cruiser recommended option of having/buying some innertube and cutting it to fit the cap or outright buying a sheet of gasket material. 
 

This should help with my crankcase pressure and VC leakage woes.

I stacked an extra layer of gasket on mine too, it helps some.  

[llhat]

11 hours ago, hgeranium said:

Awful picture but I noticed that the gasket on my few month old oil cap was starting to warp and wasn’t seating properly anymore. Since I bought it at O’Reilly, it was under a one year warranty. I pulled the old gasket off, warranty swapped the oil cap, and double stacked the old gasket with the fresh one on the new cap. It’s on there so tight I felt like I was going to break the thing when reinstalling the cap. A few heat cycles should break it in though. 

 

I’m pretty confident about this version of cruisers oil cap mod though. I think it’s a lot easier than the cruiser recommended option of having/buying some innertube and cutting it to fit the cap or outright buying a sheet of gasket material. 
 

This should help with my crankcase pressure and VC leakage woes.

bought a new stant brand filler cap from ebay,  while still new and un-used,  put a very thin bead of red silicone between the bottom of the cap suface and the top of the gasket and allowed to cure,  in effect making the gasket thicker...   was very difficult to tighten up also thinking i would break it...  been 'ok' for a few thousand miles now, but do not know if long term solution for this... 

 

maybe one day the later valve cover... but  Felpro VC gasket not leaking... 

 

 

[75sv1]

I'd put a catch can to the rear tube. I'd also 'upgrade' to a latter XJ "big Tube' CCV. If oil is puking into the air filter, there is a leak in your system. You could put one of the 'Hot Rod' type Valve Cover filter. 

[llhat]

not sure a catch can on rear tube would assist...    vacuum flows from the orifice and to intake manifold... this tube must be kept clear for the CCV to work.

 

The front tube and fitting on the valve cover 'should' have negative pressure, supplying clean air from air box to engine.   When the 'flow' is disturbed by poor performance of the rear orifice and by excessive crankcase pressure,  then the engine pushed air into the airbox,  along with oil vapor in suspension where it accumulates on the filter.

 

this side is where most put the catch can....

but as spoken, not a lot of room..  and the appropriate size hose does not conform well to the limited space.

 

the later design is just a bit better,  with a larger hose on the rear and a straight shot to the HO airbox...   I think the large rear HO hose would be an improvement, but 'I' have yet to try it...  

my 89 has the case valve cover,  I think it looks better than the later stamped steel... but that stamped steel valve cover has its advantages for certain...

 

( mainly IMHO) the larger rear hose 

[watchamakalit]

Has anyone tried double orifice tubes?  I'm thinking along the lines of removing the front breather tube that runs to the air box and replace it with a second orifice tube to match the rear and connect to intake vacuum?  Worst you would do is burn off the excess oil, right?  Seems with the crankcase pressure created by the blowby, a second vacuum orifice might mitigate the positive crankcase pressure and help keep the engine blood inside the engine instead of puking it all over the outside of itself????

[llhat]

Double vacuum?   where's the engine block to get its air?

 

i also think these engines are a bit temperamental when vacuum leaks are introduced. 

[watchamakalit]

10 hours ago, llhat said:

Double vacuum?   where's the engine block to get its air?

 

i also think these engines are a bit temperamental when vacuum leaks are introduced. 

Presumably from the blowby? 

 

I didn't think about air in when I opened my mouth. Il just shut up now. 

[75sv1]

All engine have a bit of blow-by. It is 'excessive' blow-by that might cause issues. My though on why oil gets into the air filter box, is leaks in the system. Or the CCV tube is blocked or restricted to much. So, the engine is drawing air into the airbox, instead of drawing air from the airbox. I'd check Dino's 4.0 page on how he set up his catch can. 

[watchamakalit]

My valve cover has been cruiser modded with shortened baffles and all my ccv system is new and clear.  I have owned a fair number of 258\4.0 family motors over the years and it seems to be common among them all. Catch can is probably the cleanest easiest solution. 

[hgeranium]

2 hours ago, watchamakalit said:

My valve cover has been cruiser modded with shortened baffles and all my ccv system is new and clear.  I have owned a fair number of 258\4.0 family motors over the years and it seems to be common among them all. Catch can is probably the cleanest easiest solution. 

Do you typically see the catch can routed through the CCV hose? I have an HO air box where the CCV is a straight shot, so I wonder how I would set up a catch can while also plugging the hose into the air box.

[Htchevyii]

On 12/10/2025 at 4:12 AM, 75sv1 said:

I'd put a catch can to the rear tube. I'd also 'upgrade' to a latter XJ "big Tube' CCV. If oil is puking into the air filter, there is a leak in your system. You could put one of the 'Hot Rod' type Valve Cover filter. 

I did exactly this! My engine runs great and doesn't seem to have much blowby, but it's always got oil in the air filter, even when it was just 4 years old. I tried the Cruiser mod, but didn't notice a difference. I now have a 1996 valve cover and it has a much improved breather design. It's larger hoses made it easier to install a catch can, (on the rear which should be the hose sucking into the intake). I don't think that the catch can was needed as it barely collects any oil. I went from burning a quart every 2000 miles to essentially burning no oil after the valve cover change.

[75sv1]

I am thinking of cutting out the baffle from a 96+ VC and adapt it to a 93-95 ZJ. Just thinking, haven't looked at both side by side. 

[hgeranium]

I talked about it in my build thread, but I’m currently using a spare PCV grommet and hose in the front hole because I figured, until I get a new vacuum harness to steal the CCV hose from, a fresh grommet that can actually seal oil was preferable to my beat up CCV hose that wasn’t even being held in the hole. 
 

I was worried about a buildup of pressure from the smaller orifice opening, so I watched over the next few days.  Normal city driving is fine. It actually holds oil pretty well. Extended driving on the highway proves to build too much pressure because little puddles of oil will be on the valve cover instead of just light seepage. 
 

This is probably pretty obvious to most of everyone, but I felt it was worth it to mention that going with a smaller orifice size from stock as an alternative will lead to undesirable crank case pressure during highway driving. All the more reason to switch to an HO valve cover with the improved breather design rather than buying a whole vacuum harness to cut up for one hose.