1) i assume you drained the old oil when the engine was still hot so you could get most of it out... (2) remember that even when you drained 'hot', as in (1), there is an unknown quantity of oil lying in the oil pan beyond reach of your pump. . the best pumps won't get all that oil out of the oil pan, so there is x amount of oil in the oil pan after you've pumped dirty oil out. then you add exactly the amount of oil that it says in the owner guide , and unfortunately you have now over filled the engine, BECAUSE the owner manual figure for say 4 quarts is the total lube oil capacity of the engine, including the oil in the oil pan. and you now have 4 quarts plus x quarts left in the oil pan. this 2 Gm20F relies on splash lubrication. overfilling the splash-lube engines with oil is is VERY BAD because such engines are not only lubricated from an oil circulation pump, but are also splash-lubricated. in this engine design , the oil in the oil pan splashes up to lubricate the cylinder walls. the engine would prefer to burn pure diesel with a proper air mix. . with adequate lubrication of the cylinder walls from splash lubrication with the right amount of oil in the pan, the fuel
burns properly . when the oil is overfilled, there is excess oil in the oil pan. more oil then splashes into the cylinders, and too much oil in the cylinders will adversely affect the ability of the engine to combust the diesel fuel it is trying to burn. (and give black smoke and rough running.) so your poor engine is trying to burn the diesel at a known air-to-fuel ratio as set by the injector pump, but its precision fuel-to-air ratio coming out of the injectors is now screwed up because there is excess oil splashing up which the engine has to combust . so the engine runs rough or not at all, until the excess oil is gone. so GET THAT OIL OUT!!
now the questions are: how do we get the oil out of a cold engine without running it to get the oil hot.. well there are some good vacuum pumps that will get enough oil out 'cold' , though it will take longer. i am guessing that the 2Gm20F takes about 4 quarts as stated in the owner manual, so that is probably how much oil you put in .. but this is too much oil, as no pump can get all the oil out of your oil pan. the Rx: i'd take out about a 1/2 quart with the vacuum pump. get the oil dipstick between low and high. then start the engine, see if it runs better. you can now 'top up' the oil about an ounce at a time to get it to the 'high' mark; and remember that even with the engine hot it will still take ten minutes or so for the oil you've just put in to drip down so that you get an accurate measure on the dipstick. How to avoid this next time : remove your oil when the engine is hot, suck it out with a good engine oil change
pump like the topsider MVP vacuum pump or the west marine
oil extractor. if you were pumping with a little manual pump like the brass 'little pal', you might not be getting as much oil out as with the more powerful pumps. now the last trick : we assume that you kept your oil topped up to the 'full' mark on the dipstick : as you pump the old used oil out, pour it into a few clear plastic quart milk jugs.(you'll need 4 or 5..) measure what you took out so you'll know how much oil you successfully removed.. you now put back the same quantity of clean new oil that that you removed, and you'll never overfill the engine with oil again.
by the way it took going to yanmar school
and overfilling my own engines to figure this out.
if it is too cold in your area to get enough oil out with a good pump, there is usually an oil pan drain bolt in an obscure location . loosening the drain bolt with a large pan to catch the dripping oil will let you get a lot of oil out, but it's hard to control the flow, you will get all oily, and you don't want oil in your bilge
i congratulate you on your effort to do your own maintenance
on your engine. this forum is a good source to check for little clues and tips when you are doing these jobs so that we can all benefit from each other's mistakes
... if you will be doing more work, you can get the yanmar service
manual and parts
manual for your style engine, and these books
about 44$ will be the best $$$ you spend as they tell you all kinds of stuff the owners manual does not. (for example if you are tempted to change your # 2 fuel filter , the parts
book shows a 'crush washer ' and filter. remember that you should also replace the copper 'crush washer' that maintains a good seal on the filter. most parts places won't tell you you also need the washer.. ) i could go on and on.....