Thanks Cajun. I've been lucky to be involved in some very unique projects over the years. As for this tip, if you do end up replacing the oil pump here is how I did it . . .
Considerations =
1) Is your pan gasket going to need replaced? If so you will need to remove the pan entirely. Not sure if that can be done in the bay on an S12 but it is impossible on a WD21 Pathfinder. If the gasket in question is not new enough to still be supple just plan on pulling the engine anyway.
2) Do you have the mechanic skills and tools described below? Not every tool box has some of the stuff I used (but they should) and without some of them attempting this is folly.
3) If you find out you are fucked halfway through, do you have still the ability to pull the engine after all and get it done the traditional way? If not you probably shouldn't start this in the first place.
Prep =
1) Disconnect battery.
2) Drain the oil.
3) Remove the radiator, shroud and fans if equipped.
4) Remove the accessory drive belts, crank pulley, timing belt and timing gear from the crank.
5) Unbolt the motor mounts.
6) Loosen the transmission mount bolts.
7) Raise the engine as far as possible making sure you disconnect any lines/wires/hoses/exhaust that pinch/bind/stretch and write down where they are so you don't forget to re-connect them.
8 ) Block the engine/transmission with jack stands, large wood blocks or some other adequately strong and stable substitute.
9)
Removal =
1) Remove all of the pan bolts. Under the car this may require creative use of socket/universal/extension combinations and various lengths of box, open and angle head wrenches to get at every one of them.
2) Gently drop the pan making every effort not to damage the gaskets.
3) Carefully stuff clean rags or paper towels in the opening between the block and the dropped pan to ensure no debris falls in during your work. COUNT THEM and make sure the same number come back out before re-assembly!
4) Disconnect the oil pickup tube from the bottom of the oil pump. Before you get started with this make sure you have at least 2 magnetic pickup tools, several different length/configuration end wrenches, and several socket/universal/extension/ratchet combinations of the appropriate size (10mm???) within arms reach of where you are working. That or a second person who can bring you tools.
4a) If you have a good strong magnet, stick it to the bottom of the oil pan just below and behind where the oil pickup bolts are. If you drop a bolt (you should plan on not dropping them), washer or gasket (some of the pickups use a metal gasket) this magnet may prevent it from getting lost in the pan. Loosen both of the pickup tube bolts. Stick one of the pickup magnets onto a bolt/washer. Stick the second pickup magnet on the gasket (if it is a metal gasket). Depending if you are using wrenches or sockets you may need to get a little creative with how you hold the magnet onto the bolt as it turns. Be particularly careful the last couple of turns and make sure you don't drop them into the pan. The pickup magnets are worth their weight in gold here. Once the two bolts are out you should be able to flex the pickup tube just enough to slide the gasket out.
4b) Time to use more rags/paper towels to fill in the front portion of the gap between the pan and the block. COUNT THEM for removal later.
5) Remove the bolts securing the oil pump to the block. This should be fairly easy.
5a) Removing the oil pump itself will require some wiggling of the pump and the oil pan to make enough room. I ended up rotating the pump about 90° around the crank to align the shortest end of it with the dip at the front of the pan, pulling it part way forward, then rotating both the pump and the pan to walk it through. The S12 may have more room up front but that's what it took on the WD21.
5b) Make sure the towels are in place and clean the oil pump gasket surface.
Installation =
1) Prep the gasket surfaces on the block and pump to your preference. Depending on where you sit in the yes/no RTV on gaskets camp you can either use a thin film of RTV to stick the gasket in place or just a couple of spots to stick it in place and prevent damage while installing the pump.
2) Rotate the flat portions of the cam/gear inside the oil pump to be roughly aligned with the crank once installed (much easier than trying to turn it once it is on the crank shaft). Install the pump using the reverse motion you figured out to sneak it past the pan.
3) Bolting on the pump should be fairly easy. Just make sure the gasket was not damaged.
4) Remove the correct number of towels from the front of the pan/engine gap. You did count them right?
4a) Locate any towels that are unaccounted for.
5) This is where finger dexterity is an absolute must. Get your tools from "Removal" step #4 above again and/or your helper handy. The magnets will prove invaluable for securing parts before they fall and retrieving them if they do fall. You may have plenty of room with the S12 but the WD21 only allowed 1-2 fingers at a time.
5a) Put the washers for the oil pickup bolts onto the bolts and attach one of them to one of the pickup magnets. Attach the oil pickup gasket (if metal) to one of the pickup magnets. Align it as best you can between the pickup tube and the pump, leaving the magnet attached. If it is not a metal gasket use extreme caution not to drop the gasket as you move it into place. Get the bolt with the magnet on it started a couple turns then remove the magnets. Attach a magnet to the other bolt/washer and get it started also. Once they are both started remove the magnets and tighten them to spec.
5b) If you do drop a washer or bolt that you can not reach take your strongest magnet and rub it around the bottom of the oil pan until you hear the part stick. You can use the various magnets to slide/walk the part back up the pan wall to a place where you can reach it with your fingers or even another magnet extended into the pan. It is frustrating and can be time consuming but it is not the end of the world.
6) Pull the dipstick out. They can get pinched between the pan and block if you don't.
7) Remove all of the rags from around the pan/engine gap making sure the count is correct.
7a) Locate any that are missing.
8 ) There will likely be oil drips that developed along the pan/block gasket surfaces. You need to wipe these down and ensure the gasket surfaces are clean and dry.
9) I don't care where you sit in the yes/no RTV on gaskets debate. When you are re-using a gasket it is best to give it a little help sealing back up (appropriate types of gaskets only of course). You want to apply a thin film of RTV between the re-used pan gasket and the opposing gasket surface that it is not still stuck to.
9a) If you have not done much work with RTV before you may want to practice a bit before you get under the car. You will need to apply it to places that you can't quite reach with your hand so you may need to get creative. A few things to remember about RTV, it sticks to ANY dry surface, it does not wash out of cloth but it does wipe away clean from hard surfaces, and most importantly . . . it does not stick to saliva. Practice putting some RTV on something and smearing it into a thin film. Then put down another glop, then spit on your finger and smear it. Depending on how much spit and how far you pulled your finger will come away clean. Now put some on the end of a screw driver and use that to place and smear it. Then spit on the end of the screw driver, put the RTV glop on the spit and try again. With the spit more of the glop should have stayed behind.
9b) Put on grubby cloths (because you WILL get RTV on them) and get a roll of paper towels. You will need them to clean your fingers and various reaching tools several times. Using the basic methods mentioned above apply a thin layer of RTV around either on the gasket or the surface it will match up against. Thin RTV dries fairly quickly so make sure you are going to do the entire surface and replace the pan in one sitting. Wipe off any excess on the edge inside the engine/pan.
10) As soon as the RTV is applied double put the pan back in place and secure it finger tight with a bolt in each corner.
11) Install and tighten all of the pan bolts.
12) Replace the dipstick.
13) Give the RTV a few hours minimum to dry. Shouldn't be a problem with the rest of the re-assembly to finish!
14) Re-assemble the rest of the car in the reverse order you disassembled it in.
15) Don't forget to put oil in it before starting it up!
Summary =
This worked for me on my pathfinder and I still have no leaks almost 3 years later. The only real tricks to this are creative use of the magnets where you can only get one or two fingers in to start the bolts, and working with one of the messiest substances ever created (RTV) artistically. If you have spent much time working on aircraft firewalls or baffle then you likely already have the RTV skills required. If not be ready for a mess and some frustration.
If I forgot anything, you guys point out any glaring errors that I mistyped or request more detail I will update this as needed. Until then happy reading and good luck with your leak man!