Its source and extent remained unknown by mid-afternoon, although officials estimated it may have come from one boat, been as little as 30 litres or as much as 5,000 litres, and the cleanup was largely over.
Jeff Brady, a Canadian Coast Guard pollution response officer, said investigators focused on one vessel after an alarm on an inspector's personal air monitor went off earlier in the day.
He said firefighters trained in hazardous materials then tried to clean out the boat's engine space for government inspectors.
"Unfortunately, at this time, we can't give any solid numbers on how much has spilled," he said.
"Once Transport Canada finishes its inspection and identifies the source, we might be able to quantify or calculate how much was in the tank prior to the last time it was used and how much is left in the tank now."
The coast guard alerted the city to the spill around 2:30 a.m. Monday. A containment boom was set up near Granville Island and officials began skimming the water and using absorbent pads to clean up the fuel.
The city closed sections of the seawall walkway for hours and advised nearby residents to close their windows and turn off their air conditioning.
Vancouver Coastal Health also asked boaters and swimmers to stay away from the area and paddlers and swimmers in other areas to watch for any oily sheen.
Sadhu Johnston, the deputy city manager, said early in the day that between 30 and 1,000 litres of diesel had spilled into False Creek, apparently from a boat.
Brady said later that based on his personal experience no more than 5,000 litres had spilled, although he couldn't provide a solid figure. The fuel was likely diesel because of its smell and appearance, he noted.
Investigators ruled out a land-based source of the spill in the morning, after firefighters examined storm drains, he said. Officials still had to rule out possible leaks from pipelines under the dock and in the area.
Environment Canada was searching the area but had not reported any issues with wildlife.
The coast guard believed it had boomed off the right area, the cleanup was largely over and was shifting towards inspection, he said, adding officials would remain in the area throughout the night to ensure there was not another release of diesel.
"This is a very light product that's going to be evaporating, especially with a nice wind and the hot weather that we have today," he said.
Johnston said communication between the city and the coast guard had improved since a spill earlier this year, when a container ship leaked more than 2,500 litres of oil into English Bay.
"We're still working on our notification protocol, but it's gotten a lot better," Johnston said. "Clearly, the coast guard has recognized the importance of collaborating and communicating with the municipality. So we really appreciate that. And now we want to refine that."