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Hey Moses,

I have a project I'm hoping to get started on and have a question.

I have an '83 CJ-8 with a worn 4.2 and plan on installing a 4.0 from a complete '93 Grand Cherokee.

The GC is 2wd with the AW4 trans, the engine has ~142,000 miles and has had regular oil changes with Full Synthetic oil. The Jeep ran great when parked. 😉

Now the problem is... it hasn't run for probably 15+ years.

I would appreciate some input from you on steps to take to get it running without possibly causing any internal damage.

I'm hoping to tap into your knowledge and experience to set me on the proper path.


P.S. A stroker is in my plans but I really want to get it on the road, enjoy it for a bit, and save up some money first.


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FLCj8...I'm sure your CJ-8 is a keeper!  The 4.0L will be a major improvement, even more so when you stroke it.

Good to consider how long the engine has set.  If it's in the chassis still, I would vacuum all debris from around the spark plugs and remove the plugs.  Fill a clean oil squirt can with Marvel Mystery Oil, and squirt a teaspoon or so into each cylinder.  Remove the valve cover and squirt Marvel Mystery Oil over the valve rocker arms and into the valve spring areas.  Not a lot, just a few squirts each.  

Remove the distributor cap.  Bring the crankshaft to TDC on its compression stroke (timing mark aligned, #1 intake and exhaust valves closed).  Mark the position of the distributor rotor, and remove the distributor so you can prime the lubrication system.  You can prime the engine's lubrication system with an expensive pre-lube tank or a simple priming tool.  With the priming tool, remove the ignition distributor.  (You can make this tool if necessary from an old Jeep distributor shaft.)  Be sure the crankcase is full of oil.  If it looks oxidized, drain and refill the crankcase.  Otherwise, if the engine oil looks okay, leave it for the priming and initial start-up of the primed engine.

A 1/2-inch, variable speed drill motor works best here.  Carefully and slowly rotate the oil pump drive, keeping the priming tool engaged with the oil pump.  (Do not damage the oil pump tang.)  You should feel resistance—that's oil pressure.  Continue rotating the pump and watch the valve train for oil.  After oil appears at the valve rockers, pause the priming and rotate the crankshaft 180 degrees.  Prime again until more oil appears.  Stop.  Rotate the crankshaft 180 degrees further.  Prime again until more oil appears.  Stop.  Rotate the engine 180 degrees further.  Prime again until more oil appears.  Stop.  Rotate the crankshaft slowly back to TDC on its compression stroke (180 more degrees).  By now, there should be oil at each rocker arm.  All parts should have oil.

If you can reuse the valve cover gasket do so.  Otherwise, replace it.  With the crankshaft at TDC on the compression stroke, reinstall the distributor with the rotor pointing exactly where it did when you removed the distributor.  You may need to align the oil pump tang with a large screwdriver to get the distributor to drop into the correct position.  Install the spark plugs and wires.

That's as good as it gets for initial start-up, you have oil everywhere and no resistance at the rings and pistons.  The engine will smoke at start-up as the Marvel Mystery Oil passes through...If you're starting the engine in the Grand Cherokee chassis, get rid of the fuel in the tank.  The fuel will be stale and potentially damaging.  Siphon it all out if possible to avoid the need to drop the tank.  Put five gallons of fresh fuel in the tank with a can of Sea Foam.  The fuel filter should be changed. 

That's it.  Start the engine and see what you have.  After running the engine long enough to warm up completely and sound good, shut it off and check the oil.  If it's still clean and looks good, being synthetic, I would leave it in the crankcase until after you get the engine running in the CJ-8.


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Thanks for the quick reply.

Yes, the GC is still complete so I have the ability to run it  for an extended period 🤞 to get up to temp and check for any other areas on concern before doing the swap.

Thank you for the very detailed oiling procedure as well. What time frame (min-max) are we looking at between placing MM oil in the cylinders and rotating the crank for the oiling procedure?

I hadn't really thought about the advantage of the synthetic oil for long term storage.

As far as other items (i.e. freeze plugs. RMS, etc.) should I just look for leakage and replace as indicated, or replace some items as PM?

Of course all belts, hoses, plugs, wires and fluids will be changed/replaced before putting into service.

Thanks again, I'll do a follow-up after I finish other projects and get started on this one. 


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FLCj8...Good question on the MMO soak time.  The advantage of an inline six (other than the vintage Mopar slant six) is that the pistons all stand vertically.  With a teaspoonful of MMO per cylinder, a few minutes would be plenty.  The engine isn't "seized", you simply want MMO to slip into the rings and coat the walls as you rotate the crankshaft (by hand with a socket and ratchet).  

The crankshaft rotation should be slow and easy.  This will have the added effect of allowing the MMO to seep further into the rings.  The spark plugs are removed, so there won't be cylinder pressure to push MMO out the exhaust ports.  Some MMO will find its way there, so expect blue smoke on start-up.  The goal is to protect the cylinder walls and lube the rings for initial engine start-up.  There will be oil throughout the engine from priming.

As for the cooling system, if the engine ran synthetic motor oil, I would guess the antifreeze/coolant got changed regularly.  Confirm this and look for any residue in the cooling system.  Freeze plugs are zinc-coated steel or brass, so I wouldn't expect bad plugs if the coolant looks clean.  Your call here.  Freeze plugs usually last between engine rebuilds.

On crank and first start, watch the oil pressure.  It should pop up immediately with the oil primed.  Note the pressure, I want to see at least 50 psi on a cold Jeep inline six at start-up.  It can be as high as 65-70 psi at a fast idle.  OEM gauges are not perfect and can be off some.  The '93 Grand Cherokee has full instrumentation and not an oil light.

The rear main seal is elective but prudent.  AMC/Jeep inline sixes with two-piece seals are notorious for rear main seal seepage/leaks.  At mileage, they either seep now or will seep soon.  When you have the engine drained and on a stand, it's not difficult to remove the oil pan and change the rear main seal.  Turn the engine upside down for this work.

Fel-Pro makes a terrific one-piece oil pan gasket kit for the Jeep inline six.  (It's actually not a chore to change the main seal in the chassis with this pan gasket.)  Read the Fel-Pro steps for installing the main seal.  Follow Mopar guidelines for use of RTV and any other kind of sealant.  It must applied at the main cap and block points according to the Mopar and Fel-Pro guidelines.  If so, you will have a non-leaking, properly sealing rear main—for at least some time.

I did my '99 4.0L rear main seal at 175K or so miles.  At the same time, I installed a Sealed Power oil pump and new Melling screen.  There is limited risk of an oil pump failure, but if you decide to change the oil pump for PM, use a Melling tool to install the screen.  Make sure the new screen is installed correctly and riding at the correct angle.  I did this in the chassis.  It's much easier on the engine stand with the engine upside down. 

If you don't have the '93 GC FSM, get a CD or download FSM for the two-rail MPI 4.0L engine (1991-94).  You'll need the FSM for the engine and chassis wiring harness diagrams, tuning and troubleshooting.  Make sure you preserve the donor engine's wiring harnesses.  On the '93 Grand Cherokee, this includes the full engine bay harness and wiring hook-up to the 60-Way PCM.  Of course, you will be using the 60-way PCM from the GC.  Good thing you have the entire vehicle.

When using the '93 factory EFI/MPI system, you will want all wiring and connectors intact.  You will also need the right inline fuel pump and a PCM power signal for the fuel pump.  You can follow the Mopar wiring guidelines.  In my Jeep CJ Rebuilder's Manual:  1972-86 (Bentley Publishers), I cover a 1991-94 Mopar two-rail EFI engine conversion using the Mopar EFI Conversion Kit.  The kit's cost and parts availability (HESCO remains the source) have most owner/installers using the recycled donor engine's EFI components and wiring. 

You pore over the FSM wiring diagrams at the kitchen table and sort out how to mate the donor engine's EFI and wiring to your CJ chassis.  HESCO can be helpful here, they understand that many owners use 4.0L donor parts.  HESCO can provide some direction and parts like the correct inline fuel pump and filter.  With the two-rail MPI/EFI system, you will be using an unrestricted fuel return line to the fuel tank.





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Thank you for the in-depth steps for doing this swap, it is very much appreciated.

This swap has been a long time coming, life just got in the way. 

I actually have well used FSM hard copies for both vehicles, like you said I need to get back into them and create a game plan.

In my research I had come across a post that stated the GC fuel tank (with internal pump) would fit into the CJ8 because of the additional chassis length. The site is no longer active but I figured it's worth investigating when I get to that point.

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FLCj8...You're welcome...If the fuel tank fits, that would solve the fuel pump issue.  You could follow the GC factory wiring schematic and wiring color codes to bring a power feed to the fuel pump module.  (Power is provided via a PCM signal.)  Was there a hint that the module/fuel gauge sender could also provide the right fuel gauge reading in a CJ?  Since you have both manuals, compare the fuel gauge ohms resistance for "Empty---1/2 Full---Full" at the CJ gauge and the GC gauge.  If they happen to be close or the same, you may have that resolved as well.  Be sure you have fuel tank skid plate protection if this all works out.

In the end, see whether you can save and use the OBD (not OBD-II) port plugs.  This would provide a diagnostics access for your 4.0L engine.  It would be pre-OBD-II and require an older scan tool software and hook-up like DRB, older Snap-On or equivalent, but either way, that would be better than nothing.  Otherwise, I would run diagnostics with an oscilloscope and/or a VOM.  I have enjoyed the detail that my Autel MP408 with OAK accessories provides.  You would be testing sensors and devices, not scanning, so the VOM tests would cover a lot of diagnostics.  The FSM has those tests.

Please let us know how "smoothly" (or not) the conversion goes.  Share photos of your progress and solutions...You have some motor mount bolt-up differences between the 4.2L and 4.0L engine blocks.  The block brackets from the 4.2L need slight modification to fit.  (You will be using the 4.2L engine brackets to work with the OEM CJ motor mounts.  The fit is otherwise stock.) 

The CJ-style engine driven fan is the usual approach unless you want to adapt the 4.0L GC fan plus its electric auxiliary fan system.  (This is thermostatically PCM driven and requires correct wiring.)  I've always used a CJ water pump, fan and radiator with shroud, but it's worth taking measurements and considering.  The radiator shape and mounting system would dictate your choice.  If you use the CJ radiator, make sure its A/C capacity for the added 4.0L horsepower.  (Horsepower equals BTUs.)  Keep in mind that the water pump on the 4.2L and 4.0L have opposite rotation direction.  You will need to pick the right water pump for the fan and belt system.

Whether or not you want/need to run the CJ drive belt system will be a question.  (Do you currently have V-belt(s) or a serpentine belt on the '83 engine?)  You can easily work your way through most of this.  The timing cover bolts may need swapping to accommodate belt and power steering pump bracketry.  Your manual transmission (presumably not an automatic) will require a crankshaft pilot bearing.  The bigger focus will be wiring and emulating the GC's chassis/engine wiring needs. 

The PCM is hypersensitive to voltage measurements and needs clear voltage signals.  Solid grounds are crucial.  I strongly recommend that you rosin core solder and double heat shrink shield any wire splices between the EFI engine harness and the CJ wiring—no crimp connectors.  The starter motor solenoid wiring is easy to adapt if you keep the 4.0L starter (better design than the older Motorcraft/CJ style).  You have options in any case. 

You'll want to use the GC MAP sensor and other engine related peripherals.  There will be the upstream O2 sensor for GC/4.0L, which you must use in conjunction with the GC's tubular header/exhaust manifold.  Study the engine bay vacuum circuits for the Grand Cherokee (basic, nothing difficult here), the EVAP system, the O2 sensor, crankshaft position sensor wiring and so forth.  You need to mate the 4.0L/4.2L EVAP systems to meet the CJ's fuel tank/filler neck EVAP requirements.  This is not difficult but needs to be done safely and correctly.

You have the advantage of holding onto the GC until the engine swap is completed.  This provides access to donor parts.  There won't be any surprises or need to buy missing parts.  It's all there.

Even in stock form, the 4.0L performance will be substantially better than the 4.2L.  You may miss the 4.2L bottom end torque until you do the 4.6L stroker build.  You'll be holding onto your 4.2L crankshaft and rods for that update.  Pistons must be right for the rods that you decide to use (4.0L or 4.2L) when you build the stroker.  You need the piston and rod combination that will provide the right piston crown height for the block's deck height. 

Crankshaft snout length is a concern when using an "early" (V-belt style) 4.2L crankshaft that has a longer snout length than the short snout (serpentine belt) 4.2L or 4.0L crankshafts.  This can be remedied with a special spacer from HESCO...It's all part of a 4.6L stroker build, which has been done myriad times now.  


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My plan is to try and use as many of the GC's parts as possible, along with any engine accessories that can be reused. I hoping to have it where the entire engine "systems" are pretty much '93 factory for ease of servicing so it's not too much of a Frankenstein's monster. 

Unfortunately the fuel sending unit resistances are not the same but I'm looking at the SpeedHut CJ cluster that allows custom settings for the fuel sending resistances (along with other nice features).I would just need to work out something for the fuel fill location.

Anyway, thanks again for the additional thoughts and input, it will help keep me from bothering you too much in the future as I move along with the swap.


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