I have a MF 255 that would not start. Everyone pointed their fingers to the lift pump so I replaced the lift pump. It starts....but now it has fuel POURING out of the injector pump. Why? Is there something that I did that could have caused the pump to leak. There is a piece in top of the injector pump that is approximately 4" long and 2" wide that has a gasket under it and that is where it appears to be leaking. Is this something that I can fix myself or does it need to go to the shop to be fixed?

Is your pump a CAV? If it is a CAV, I have been there done that. The CAV is prone to leaking around the throttle shaft and the cut-off shaft. First you will need to get 2 or 3 cans of engine cleaner. Clean, clean, and clean again the whole area. After each spraying, hose off, let dry and do it again. Cleaner the better, if you drop a ball of grease/dirt into the top while the cover is off - big trouble, one piece of crud dropped into the bottom of the pump will cost you about $600-700. Now using a paper towel, start looking for your leak; it will show up real easy on the dry, clean paper towel.
1. MOST important thing is to have some paper and pencil BEFORE you take off the cover.
2. Remove shut-off linkage and throttle linkage.
3. Remove screws or nuts holding lid on.
4. DO NOT just lift up the top. Lift it just a little, very carefully so you can peek under it. A spring is attached from the lid to the bottom part of the pump. One end of spring is hooked on a wide post looking like a tombstone. The tombstone has several holes in it. Write down which hole the spring is hooked in before you remove it. The other end is through one of the holes in the flattened end of the control rod. Again write down the hole it is in before you remove it. This step is VERY IMPORTANT. If the spring is not reassembled to EXACTLY the same holes on both ends, the tractor will never run right again. The same pump can be used on different model tractors with the only difference being how the spring is hooked. My MF dealer mechanic can tell the proper holes by using the 3 numbers on the CAV. Now remove the spring, and lift off the lid.
5. Write down the model/year of tractor, and the model of the CAV pump and serial number. There are 3 different numbers on my CAV pump.
6. Take the cover with you and head to MF dealer and purchase a new gasket and four (4) tiny o-rings that fit around the throttle shaft and the shut-off shaft. Each shaft has 2 o-rings. If the o-rings are not leaking now, they soon will so replace all of them. Total cost of gasket and 4 o-rings is about $5 or $6.
7. You will need a little patience and two extra hands to get the spring attached on both ends and the new gasket fitted. I did this complete procedure on my MF 690 and now it has NO LEAKS.
Tom

Tom is right on the money. When you remove or replace the cover, I found it easier to remove the spring by removing the nut and sliding the throttle lever out of the cover. Opposite to install. Hook the spring then slide the shaft back into cover. Also, be sure the shutoff lever engages the fuel shutoff rod when it goes back together.

There is a guy selling a top seal kit for these on ebay if you don't have a dealer nearby.

So, am I correct in thinking from what you have said, that I don't have to totally remove the injector pump from the tractor, just the top piece that is on there????????? is that correct? Also, do you think I should try to do it myself or have the MF dealer do it.

Do you think that I could have caused this to happen when I primed the tractor?

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