I would say that it is possible to replace the cold start O-ring without removing the pump, however, you would have to have skinny long fingers. Just find a torx bit & get a 5/16" wrench, and see if you can get it that way. Then try to pull out the old O-ring replace with your new one and put the cover back on and make sure both torx head bolts are equally tight. you do not need to retime anything if you go that approach. I removed my pump to do this job. It was a good time to change my timing belt also. Another reason I removed my pump was because I did not want to drip diesel all over my coolent hoses because it will deteriate them. If you have had this leak for some time I would suggest changing the coolent hoses in that area. you can get a whole set for 30 dollars from the parts place in aurburn hills. It is really not that bad to change/retime the timing belt, and you do not necessarily need any special tools. It should be changed every 50K or so. If you want to go the approach of removing your pump. Then the procedure is as follows...
1. Remove The injector lines (put some small caps over the tops of the injectors so nothing falls into them), remove upper timing belt cover, remove V-belt, remove the pully on the crank, remove the water pump pully, then remove the plastic lower timing belt cover. There is a idler/tensioner pully that you need to loosen. Now you can take of your old timing belt.
2. The next step is removing your pump. Remove the sprocket on the pump (please use a wheel puller). Then remove The shut off wire, cold start cable, accelerator cable. Then there are three bolts on the pully side of it, and one or two bolts on the other side of it that hold it in place. With the pump out look at your coolent hoses replace if needed.
3. The next step is replacing your seal. First clean of the exterior of your pump very well you dont want any dirt to get inside. I got a whole gasket kit for my pump it ran me $18. replace the o-ring in question and tighten back down the cover I would say 7.5 ft*lbs
4. Putting your pump back in. Is as reverse as step 2 EXCEPT make sure you line up the little line on the pump with the line on the mounting plate as close as you can get it. Then tighten down the bolts. Put your sprocket back on. Torque it down.
5. The next step is timing your engine. You must have your cam, crank and pump all running in sync with each other. It will blow up or not run if they are not...so it is very important to get this step right.
A. You need both your valves on the number 1 piston to be closed. First remove your valve cover. Next rotate your cam shaft until both lobes on the shaft are pointing upwards. As you can kinda feel the cam shaft snaps into 4 different positions and it takes quite a considerable force to move it from one to the next so it is not necessary to lock it in place, but if you feel the need. On the end of the shaft by the number 4 cyclinder there is a slot to wedge something into to hold it.
B. The next is timing your crank shaft. Here is where it will help to have someone help you. There is a plug on the bell housing. It is hexagon shapped, and with a diamater of approx 2 inches. you need to unscrew this plug (not only pull out the thing in the center of it) but unscrew the whole hexagon. Next have someone spin the crank shaft slowly by putting a socket and rachet on it. And you get a flashlight and look down that hole (there is a triangle on the fly wheel and there is another triangle on the hole where you pulled the octagon out at. Line these two up as best as you can.
C. The third thing to time is the pump which is the easiest. There is a line on the sprocket of the pump and there is a line on the pump (the one you had to line up eariler in step 4) line these two up. Finally you have to slip on the belt with all three of these things lined up(easier said than done). The way I do it is, wrap it around the crank sprocket, then up around the idler/tensioner pully around the crank sprocket. Then wrap it around the intermediate shaft pully. So in essence you are slipping it on the injector pump sprocket last. It may take a few trys, but each time you do it make sure your crank is still lined up. Once you have it where you want it tighten the tensioner pully. To test the tension on the timing belt grab it with your index finger and your thumb, you should only be able to bend it 90 degrees from its starting position.
6. Hook up your injection pump (cables, shut off wire, and injector lines. and fuel lines). Hopefully not too much diesel leaked out of your pump if it did prime your pump or your battery will have to do some work priming it for you. Now see if you can start up your car. It may not start up right away because of the fuel leaking out of the pump. If you cant get it (chances are pump needs to be primed better) you may be a tooth or so off. retime it again and see what happens
I have replaced 3 timing belts all with this same procedure, and one time I was a tooth or so off and my car didnt go above 45mph.
7. As soon as your car is running good shut it off, and hook everything else back up. Also it is very important to make sure your upper timing belt cover is not rubbing against your timing belt. It will eat up the belts. So be extra careful putting this back on making sure you use all the bolts that came out of it.
In conclusion if your belt does not need to be replaced then first try to replace the seal with the pump on the car. But if you were in the situation I was in I replaced my belt, headgasket, valve adjustment, coolent hoses, O-ring in pump, exhaust and intake gaskets, block heater. all in one time and it worked out nice for me. Also get the Bentley manual it will have pictures and help more.