I have been meaning to write this for some time but have not sat down and taken the time to put it together. It is windy today so the ship I work on is staying put so I thought I would put this unexpected time off to good use.
I am currently restoring my 1970 280Sl (US spec) and I decided to clean up my injection pump and detail it rather than rebuild it. Well, truth be told in the process of bead blasting the pump housing I managed to peel off the tape covering the oil drain hole and glass beads got into the pump chamber. Not good. I knew that it now needed to be rebuilt. I have dealt with Hans at H and R fuel injection a few times and knew that if I sent it to him I could count on it being right when it returned but 2 things stopped me. The first was my own embarrassment knowing what Hans was going to say about what I did and the other was my own fascination for what actually was in there. I get the general principle but I think the thought behind it is pretty amazing stuff considering the basic plan was laid out in the early 20th century. I decided that in the interest of my own education and in order to avoid Hans's wrath I would rebuild it myself. Here is what I learned.
You probably don't need a pump rebuild. These things are very robust and once the adjustments are set they really don't fall out of adjustment. If it is leaking oil almost all of the gaskets and seals can be changed without upsetting the adjustments. If you bought a car that has been sittting for a while and it doesn't run you most likely have a stuck piston or pistons and that can be remedied without losing the calibration of the pump. If your car has a hot or cold start issue, you have tried everything and you have decided the problem is in your injection pump here is some bad news. Your problem is most likely leaking delivery valves. These come in two styles a pintle style and a ball valve style. Unfortunately, both styles of valves are currently not available from Bosch so unless you have a supply of used valves you are most likely going to have the same valves in the pump you get back.
The big caveat is if your dip stick equipped pump's oil level is rising or if you engine's oil level is rising you are probably leaking fuel into the oil and probably need a rebuild. Most likely a very expensive one.
When you are tightening the fuel lines on the injection pump end hold the fittings on the pump with a wrench. I bought a junk pump years ago and it was junk because someone had tightened the line at the pump without holding the outlet fitting and stripped the threads out of the pump body where the outlet fittting was.
I would never pull the outlet fittings with the pump in the car. I know it is more work to pull the pump out but I would pull the pump and lay it on it's side before removing the outlet fittings or more importantly the delivery valves. When I first bought my car I had some issues with warm starting so I replaced the ball style delivery valves. I knew there were no gaskets between the top of the barrel and the delivery valve (Sort of like an engine without a head gasket.) and sealing relied on near perfect surfaces. I cleaned the heck out of everything and blew it off with compressed air before and after I removed the old delivery valves and replaced them. Flash forward 12 years and when I took the pump apart I found tiny bits of aluminum that had gotten loose from the threads in the pump body and had been crushed between the delivery valves and the top of the element barrel. This could have easily created a bypass leak in the pump between the delivery valve and the barrel. At least with the pump on it's side anything that does get loose would drop in the threads on the opposite side rather than between the delivery valve and the barrel. In other words you run a risk of actually creating a warm start issue trying to fix a stuck pump or a warm start issue.
Be kind to the barometric compensator which looks like a little air filter sticking straight up at the top of the pump. it is basically a sealed can with the rod that extends at lower pressure and adjusts the mixture. If you bend it the rod get stuck in the compensator and it doesn't compensate anymore.
At the end of the day the one point I want to drive home is that if you have a runnning problem like an intermittant miss or stumble it is almost certainly not the injection pump, I would strongly suggest double checking everything like compression, Valve adjustment, Ignition timing, air filter, distributor advance and any thing else you can think of before sending your pump off for a rebuild.
