About two months ago, I installed a Greasecar vegetable oil conversion kit in my 1983 240D. This involved splicing into and adding coolant hose. After installation, the car was run, coolant added, run again, coolant added again. About two weeks ago, I noticed my coolant level significantly low, filled it up again. Last night, I topped it off again. I don't drive too often, maybe 50 miles a week, maybe less. I can't find an obvious leak, I was told maybe it could be a leaky head gasket. How would I be able to diagnose that?