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Jeep 4.0L Engine Oil Pump Priming - Take 2

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Hi all,

Just rebuilt my first motor, left bottom end/block alone. My son and a friend both talked me out of doing the bottom end - and a leak down test showed only air coming out exhaust and intake (valves), nothing coming out of oil dipstick, etc - so rings/cylinders seemed fine. Head job, new cam, lifters, pushrods, oil pump, timing chain set, FMS/RMS, exhaust man., water pump, thermostat, freeze plugs, gaskets, etc.

I filled it w/fresh oil yesterday, excited for the milestone. Found a 11" piece of 3/8" threaded rod, ground it down to replicate the bottom of the distributor, chucked it to a drill and spun the oil pump - valve cover off.
Soon, oil squirted vigorously out of most of the push rod holes - except #5 exhaust. #2 and 4 also weren't nearly as "gushy" as the others, but 5 was bone dry - nothing.
I spun the motor several times with a socket on the crank/balancer bolt, then spun the shit out of the oil pump for another 20 seconds or so - nothing out #5 push rod.
I'm very worried. What could have I done wrong? I just saw someone recommend that you remove the spark plugs while doing this?
I'm hoping there's something I can do to get oil flowing out all push rods equally - very hopefully without tearing apart this motor.
My initial thoughts:

  • Do a leak-down test, just to get pressure into the system somehow?
  • drain the oil, re-fill with a very lightweight, viscous oil to see if that can worm its way through
  • buy one of those $170 pressure tank systems to more forcefully push oil thru
  • Pray that you gurus out there tell me I'm over-reacting and somehow everything is fine
  • Tear apart the motor and start over (childlike sobbing)

Many thanks for your ideas/expertise!


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zakleeright...I use a pressure tank or drive the oil pump with a tool (like you're doing, basically) to charge the oil system.  When you installed the oil pump, did you use a screen driver tool to seat a new oil pickup screen into the pump?  Is the screen setting at the correct angle to the pan floor?

Do you have the engine in the chassis?  If so, turn the key to the On position (do not crank the engine) and have someone observe the dash oil pressure gauge while you spin the oil pump drive.  You should reach normal range oil pressure at the oil sensor/sender.  If the engine is on a stand, get a mechanical oil pressure gauge, attach it to the oil sensor/sender port on the block, and read oil pressure while spinning the oil pump with your tool.

The alarm bells would be ringing if you changed the cam bearings or handled the camshaft roughly during installation.  A spun or misaligned cam bearing could block oil flow, though likely to the crankshaft bearings and not the lifters.  Since you did not touch the lower engine but did change the cam and lifters, I am guessing that you were careful with the lifters, cam installation and bearings. 

Did you attempt to "pump up the lifters" with oil or let them soak in oil?  This would be a mistake.  If you filled the lifters with oil, the plungers were full extended with you attempted to turn the engine over.  This holds the valves open and could impact oil flow while priming.  Worst case, it causes the valves to hit the pistons, which is highly unlikely in your case, as you can turn the crankshaft without restriction.

To make it easier to rotate the crankshaft while priming the block and head, do remove the spark plugs.  As for cylinder pressure with the plugs in place, that has no impact on oil pressure or oil flow whatsoever. 

Is this a "hybrid" engine with a mix of 4.2L and 4.0L parts, or strictly a 4.0L AMC/Jeep inline six?  Essentially, the engine ran okay before the work (no knocks or rattles)?  The only machine work concern would be surfacing the head.  Head surfacing does alter pushrod length and lifter preload.  Did you confirm the lifter preload to make sure the lifters and pushrods are still within correct preload range? 

Since the block was not decked, minimal head surfacing usually does not alter pushrod length to a great degree.  If the head was "milled" significantly to boost compression and the block decked, you would likely need new pushrods to correct lifter preload.  We have covered pushrod length checks and solutions thoroughly at these forums.  Use "CompCams" as a search keyword.  The CompCams pushrod check tool will come up for 4.0L and 4.2L engine building.

Other than that, you should be able to flow oil through all pushrods unless there is an oil restriction somewhere in the #5 lifter or pushrod passageway.  A restriction might exist at the pushrod or lifter.  However, I would not  "borrow trouble" or panic until after you perform the basic checks I described.  The engine flowed oil before the cylinder head and valvetrain work, it should flow oil still. 

Perhaps you can improvise an oil pressure tank, there are good examples at YouTube, using a purged propane tank, etc.  The advantage of a tank is that you can apply steady oil flow/pressure while slowly rotating the crankshaft by hand.  This may improve the oil flow while priming, the reason for this device.

Below is an engine oil flow diagram for a 4.0L Jeep inline six.  Note that the lifters get plenty of oil flow directly from the pump.  #5 lifter and pushrod should be getting oil if #6 does.  Keep priming while slowly rotating the crankshaft (two person job).  Let us know if you get flow.  I would suspect a lifter issue before anything else.  If so, fortunately a lifter(s) can be removed with the head in place.  I'd continue priming before considering any other issue.





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