The main reason for purchasing the Plantdrive kit was the doughnut diesel tank. I wanted to keep my full size trunk and not lose my spare to a second fuel tank. The doughnut tank doesn't have a fuel gauge, so I thought there would be some guessing involved.
The approach I took intially was remembering how much driving I did before refilling. Basically I work 5 days a week - that equals 10 start/purge cycles - 2 per day. I figured this would be fairly consistant plus or minus a few drives during the weekend. I thought this consistancey would give me a pretty accurate indicator of my diesel fuel consumption. Whether I drive 10 miles or 1oo to work once I'm on VO the amount of diesel being used is the same. It only changes if I purge more.
Without knowing it I started with the 2 gallon tank that Plantdrive offers. There was a mistake on their website as I thought I was getting the 3.75 gallon tank. After I confirmed that it was really the 2 gallon tank, Plantdrive offered to replace the 2 gallon with their 5 gallon (actually a 4.6 gallon I think). At the same time they gave me a great deal on a 17" rim and low profile tire they had available -as this is required for the system to work/fit. I believe the 2 gallon tank is standard with the Jetta kit and the 5 gallon is offered with the Passat - I assume because it's a bigger engine.
So how can I tell when the doughnut tank is low on fuel? Quite simply the car doesn't start on the first turn of the starter. It takes a second or 2 extra for the engine to start. When this happens I know I have about a gallon or slightly more in the tank. This is something I figured out myself when I thought I had installed a 3.75 gallon tank. After about a week the car didn't start on the first crank but would restart once it had started - this is before switching to VO and still on diesel. I thought there might be an airleak or something. I looked at the level in the doughnut tank and was surprise how much it'd level dropped - again thinking this was a 3.75 gallon tank. I refilled the tank and the car started fine when cold. After this happened a few times I realized that this was an indication of low diesel fuel. What I also happen to remember was a conversation with another Plantdrive owner who mentioned in passing that he new the tank was low because of slow starting.
I can not say enough how terrific Plantdrive has been with their customer service. This also was a major deciding factor in choosing their product. They answered all my questions promptly - sometimes at length and have been great with after sales follow up. Frybrid on the other didn't answer my emails or return my call. Frybird may have a great system but customer service is important too.
Another issue with short trips (& purges) - which in itself is not good for the engine -is contamination of the diesel tank with VO. Because the Plantdrive system uses a looped return the VO in a portion of the line is dumped into the diesel tank when purging the system of VO. According to Plantdrive this is not an issue because one doesn't use hydrogenated oils with this system, therefore the oil is generally always liquid and can not be clogged with solids (again I am in a mild climate).
Having said that, contamination with VO of the small 4.5Gallon doughnut diesel tank can not be good. Although the car starts fine the next time I use it after purging (ie; the next moring) I can still smell the oil from the exhaust.
Contamination of the diesel tank with VO
Since the tank is so small and I may draw from it until it gets low the ratio of oil to diesel has to be increasing. There are 2 & half soultions to this increasing ratio- one is to install a three port switch in the system that one would use when purging, sending the VO back to the VO tank. The down side is that during the process you are drain a portion of the diesel tank, the tank is already small and any removal off fuel requires you to replenish the tank more often. It is also is another step to follow during the purge cycle, this may or may not be an issue depending on how on top the whole thing you are.
The second choice and my current solution is to dump the near empty diesel tank into the veggie tank before refilling the diesel tank
In the picture above the bottle on the left represents diesel emptied from the doughnut tank after two tankfuls. The bottle on the right is diesel that has not been contaminated
As mentioned I have been going about 2 weeks before refilling the doughnut tank (this may change see beleow) compared with going about 3 weeks when the car was strictly a diesel vehicle with a 14g tank. Based on the assumption that there is about a gallon left in the doughnut tank when it's low (every 2 weeks) that means I'm using just over 3 gallons.
I purchased a 5 gallon diesel gas can from Home Depot with the idea I would fill it up and keep it at home so I refuel at my leisure and not be visiting a gas station every week or so. This works out well for the empting of the diesel tank before refilling. I fill both the doughnut tank & the gas can at the service station. I then drive the car until the doughnut tank needs to be refilled. At that time I empty the remaining fuel in doughnut tank, usually just over a gallon into the car's veggie tank. I then use the fuel in the gas can to fill the doughnut tank. When it gets time to refuel I simply refill the car & gas can and repeat the procedure. This works out to purging the doughnut tank every other tank.
I've since decided that I will just refill the doughnut tank more often. Instead of filling up every 2 weeks and letting the diesel get low - high ratio of VO to diesel - I will fill up about once a week. This should dilute the oil in the doughnut tank some.
Pulling the doughnut tank out to refill with the gas from the gas can has lost it's appeal. Mostly because of it's weight and the position it places my back during the procedure - the tank is not light when filled up. The other hassle is removing the trunk bottom cover. Usually there is something in my trunk so everytime I refill I need to unload the trunk's contents and remove the cover over the spare.
As a solution I've decided to cut a hole in the trunk floor cover. This will allow me to fill up much easier and keep items in the trunk. If I'm careful while filling up I can do it without any spillage.
With the 3-way valve installed I will not have to purge the tank and I will not have to remove the doughnut tank.