Jeep 4.0 Oiling System Upgrades

By Dino Savva

The Jeep 4.0 I6 engine is equipped with a conventional wet-sump oiling system and an aluminium case oil pump. The oil pump is mounted to the block inside the crankcase midway between the front and the rear of the engine opposite the no.4 main bearing cap. The oil pump shaft fork meshes with the distributor shaft and the distributor drive gear meshes with the camshaft gear, effectively making the oil pump camshaft-driven.
The crankcase does not have a windage tray nor a crank scraper. The only form of oil control is a baffle welded at the front of the sump to prevent oil from surging forwards during hard braking or when going downhill. Without the baffle, the oil pump pick-up would be starved of oil and the oil pressure would drop to zero, resulting in catastrophic engine damage.
The stock oil pump has been known to last upwards of 200,000 miles without causing any problem but diminishing oil pressures (especially above 1600rpm with a near-normal oil pressure at idle) in a high mileage engine will be indicative of a worn oil pump or a weak oil pressure relief spring. The cure in both cases is to replace the oil pump with a new unit.
Normal oil pressures in the Jeep 4.0 engine when the engine's hot are minimum 13psi at idle, and minimum 37psi from 1600rpm upwards (10W-30 weight oil). Most new 4.0 engines will run oil pressures comfortably higher than those values but a low oil pressure (especially at idle) will be indicative of worn rod, main, or cam bearings (rod bearings usually fare worst).
Fortunately the Jeep 4.0 engine doesn't have any reliability issues with the oiling system but there's always room for upgrades.

High Volume Oil Pump

A popular upgrade in 4.0 engines and strokers is to install a high volume oil pump. I chose the Melling HV pump (not high pressure) that carries part no. MEL-M167HVS for my 4.6 stroker engine. You will see in the attached photo that the HV pump has 25% larger gears than the stock oil pump so the oil pressure is increased by 25% across the rpm range. After installing the aforementioned pump in my engine, oil pressure was 25psi at hot idle, 50psi at 2000rpm, and 55psi maximum (Mobil 1 0W-40 weight oil). Excellent!
Is a HV pump really necessary? In most cases no, but a HV pump is beneficial if the engine is likely to experience prolonged periods at high rpm, and it also helps to reduce oil temperatures by flowing oil at a faster rate over friction-bearing surfaces. If the engine has been built with loose bearing clearances (rod and main bearing clearances of 0.0020" are preferred for the Jeep 4.0), a HV oil pump may be necessary to provide adequate oil pressure. The Melling HV oil pump has a heavy-duty cast iron case rather than aluminium so it should be more resistant to internal wear than the stock pump.

Oil Pan Modifications

The HV oil pump body is 7mm larger (98mm v 91mm) than the stock oil pump so the oil pan must be "massaged" with a ballpeen hammer to provide the necessary clearance from the pump in order for it to fit properly to the block. The hammer marks inside the oil pan give you an idea of where the pan must be worked.
My modifications to the oil pan didn't stop there though. I did some research on oil pan technology and found that a second horizontal baffle at the rear of the sump might be beneficial. The idea was to prevent oil surging up the back of the oil pan and overwhelming the rear main crankshaft seal under hard acceleration or when driving up a steep hill. The baffle would ensure more oil remained in the sump around the oil pump pick-up screen thereby preventing oil starvation (even more critical with a high volume pump). There was also the potential for prolonging the life of the rear main seal and reducing the risk of oil leaks.
I made a paper template, used it to have a piece of steel plate cut to shape (2.75" front to rear), and the plate was welded to the oil pan as shown in the photo. The new baffle sits ~5.0" above the bottom of the sump (same height as the existing baffle) with enough room between the baffles for the oil pump pick-up and the dipstick to pass through.
I had considered adding a passenger-side kickout to the sump to increase the oil pan capacity (in similar fashion to some racing oil pans), but there was only enough space below the starter motor for a very small one so I didn't bother.

Reducing Windage

The second oil pan baffle should reduce horsepower-robbing oil windage to a small degree but can anything else be done to reduce windage?
As I mentioned earlier, the crankcase doesn't have a windage tray nor a crank scraper. Unfortunately the oil pan isn't deep enough to leave any room for a windage tray. The no.1-no.5 main bearing caps are already close to the bottom of the oil pan and if your 4.0 engine is fitted with a main bearing cap girdle ('96+ models), the girdle actually touches the bottom of the pan.
Ishihara-Johnson manufacture crank scrapers for the Jeep 4.0 and stroker engines. The crank scraper fits in between the block and the oil pan, removing oil from the crankshaft counterweights and preventing it from being flung around the crankcase. In theory this should reduce parasitic horsepower loss, reduce oil temperature, and produce a small reduction in fuel consumption. Results will vary between different engines but the scraper has been dyno tested, with measureable HP/TQ gains and a reduction in oil temperature above 3000rpm.
Windage can also be substantially reduced by using a low viscosity synthetic oil. In a back to back dyno comparison of 11 different oils, (Mobil 1 0W-40) proved to be the best for making horsepower and still performed very well in wear tests.
Finally, I'd like to touch on the subject of oil coolers. Oil temperatures in the Jeep 4.0 engine will seldom reach 107*C (225*F) in normal driving so an oil cooler isn't needed unless the Jeep is racing offroad or wheeling at very slow speeds for long periods on a hot day. An adapter is sandwiched between the oil filter and the oil filter housing, and two oil lines (one supply plus one return) are run to an external oil cooler. Flexalite manufacture two oil cooler kits for the Jeep 4.0; part no. 3951 is for the standard duty cooler kit while part no. 3953 is for the heavy duty version.