DIY Oil Catch Can for Jeep 4.0L Engine

By Dino Savva

When combustion occurs, small amounts of combustion gases leak past the piston rings (blow-by) and down into the sump. These gases accumulate inside the crankcase and unless they are scavenged, the pressure inside the crankcase increases causing oil seals (especially the crankshaft rear main seal) and gaskets to leak. Vapours of unburnt fuel from the combustion process also accumulate and unless these are removed, they contaminate the engine oil and cause it to break down.
The oil/air mist is sucked out from underneath the valve cover by the vacuum in the intake manifold. It passes through a pipe that communicates between the grommet at the rear of the valve cover and the manifold, where it is directed into the cylinders and burnt. Filtered, fresh air is drawn into the crankcase from the airbox through a pipe that joins onto the grommet at the front of the valve cover.
The passage of oil/air mist into the intake manifold produces two problems. Firstly, the contamination of the combustion chamber with oil increases the probability of the engine detonating. The addition of oil to the air/fuel intake charge reduces the fuel's effective octane level. Secondly, the inner surfaces of the intake manifold and cylinder head intake ports get coated with a thin layer of oil. The oil contaminates EFI system sensors (e.g. manifold air temperature) that are attached to the manifold and causes them to malfunction.
Preventing the oil vapours from getting into the intake manifold clearly has some benefits. Directing the crankcase gases straight into the air by pulling off the pipe from the rear valve cover grommet will cause the inside of the engine bay to be coated with a thin layer of oil after a while and pollute the atmosphere. Attempting to solve that problem by substituting a filter in place of the grommet will soon cause the filter to become clogged with oil so that doesn't work either.
The answer is to build a catch can and plumb it in between the rear valve cover grommet and the intake manifold. The idea is that oil/air mist from the crankcase will pass through the catch can, where the oil is condensed onto some filtering medium, and only clean air is then passed on into the intake manifold.


Here's a list of materials that you'll need:
One old soup can
Three lengths of 1/2" diameter copper pipe
Two packets of "Scotch Brite" scouring pads or sponges
One nut & bolt
One piece of aluminium or stainless steel plate
Two metres of 5/8" diameter rubber heater hose
Five hose clamps
Tubes of fast-setting epoxy and hardener

They're all cheap and easily obtainable.
Is there a catch (pun intended)? No. As long as all the connections are secure and the can is properly sealed, there'll be no problems with vacuum leaks. If there's a leak, you'll hear a hissing sound and the engine idling speed will be too high.


1. Drill two 1/2" holes in the side of the soup can at a 60 degree angle to each other, and epoxy a length of copper pipe to each hole.
2. Cut the piece of aluminium plate to size so that it just fits inside the can, leaving the lower edge 1" off the bottom.
3. Epoxy the plate inside the can as shown. The plate will act as a baffle.
4. Drill a hole in the bottom of the can that's just big enough to admit the bolt. Epoxy the nut over the hole and thread the bolt though the nut and hole. The bolt will act as a drain plug.
5. Stuff the Scotch Brite pads or sponges into the can until it's full. The pads will be the oil filtering medium.
6. Seal the soup can lid to the top of the can with epoxy. You can then spray paint the can in your favourite colour.

7. Select a place under the hood that's away from any heat source to mount the catch can. I selected the space next to the brake master cylinder and made a sling using two long zip ties to hold it in place.
8. Disconnect one end of the original factory pipe from the rear valve cover grommet leaving it attached to the intake manifold, rotate it ~150 degrees clockwise, and keep it to one side.
9. Cut a 3" length of copper pipe, attach it to the free end of the factory pipe, and secure it with a hose clamp.
10. Cut a short length of heater hose, attach one end to the free length of copper pipe, and the other end to the catch can, using hose clamps to secure it in place.
11. Cut a second length of heater hose, attach one end to the valve cover grommet, and the other end to the catch can, using hose clamps to secure it in place.

Now you can enjoy the benefits. All you have to do is periodically remove the bolt to drain any oil that's accumulated in the can.