Conversion of VW Jetta Diesel to Burn Vegetable Oil


What follows is a very detailed description of the conversion of a 1998 VW Jetta TDI to burn straight vegetable oil.  The oil could come from used fryer oil recycled from restaurants, or vegetable oil from various crops. 


Diesels designed to burn standard diesel fuel need some modification to burn straight vegetable oil:

This conversion was done using a kit supplied by  -- their website provides a lot of detail on how their conversion works and vegetable oil conversions in general.


Directory for this very loooooong page:

Thanks very much to Jeff for providing this detailed description of the conversion.  In the Frybrid forum, he posts as JeffNLisa, and is happy to answer questions or provide information.




1998 VW Jetta TDI (A3 Jetta) – Frybrid kit installed at 161,000 miles

OK – At long last (for me, that is!!) this thread has the pictures and descriptions of the Frybrid installation for my 98 Jetta. I have widely stated that my brother did this work AND that we plan to offer installation service. So I might as well offer up the evidence . . .

I’ll cheat and start with a pic of the finished installation!!



Converting VW Jetta to run on straight vegetable oil (WVO)



Tank in Trunk

Let’s begin with tank location. This kit was originally packaged and assembled for use in a Mercedes G-Wagon. The buyer of it ordered it for his G-Wagon, then changed his mind about the G-Wagon and sold it. He then put the complete and unused Frybrid kit on eBay. So the tank it has was intended for a different car.

The tanks for A3 Jettas are normally intended to run the long way on the passenger side in the trunk. However, the tank I have was intended to go side to side up against a back seat, which in the Jetta would block the fold-down back seat. But if we installed this tank long ways on the passenger side, either direction would place this fill neck in a virtually impossible-to-fill location.

So we located it snugged up against the back of the trunk:Converting VW Jetta to run on straight vegetable oil (WVO)

Anytime I am filling by pouring from cubies, I use a long funnel that hangs out over the bumper, and that keeps drippies out of the trunk.

Converting VW Jetta to run on straight vegetable oil (WVO)

This tank is temporary. I always wanted an under-car tank, so as to keep all the trunk space available. I am now shifting my thinking to the idea of having several tanks in the trunk, one in the left side as a vacuum collector, one in the right side as a heated vacuum-boiling de-watering tank, and a main tank that uses ALL of the space behind the seat. I never use the fold-down rear seat anyway, and this arrangement would still leave some space in the middle of the trunk.

I do want to perfect such a system in the garage first, then in a truck where there is more room to maneuver, and only then consider it in a Jetta. Maybe by then, Chris Goodwin will have already perfected it, and I can save a few steps! But for now this is workable. And it is adequate room to fit in luggage, or as I have often done, cubies of WVO . . .

Converting VW Jetta to run on straight vegetable oil (WVO)


HIH Lines

We left the HIH lines long so they could be easily moved, and we’d have length to get to other location(s) if needed. We used plastic ferrules for attaching the HIH to the tank, so it could be removed easily when that time comes. And when putting in the tow hitch, that time came much sooner than anticipated!! See THIS thread for a discussion of the plastic ferrules, and see THIS thread for a discussion of the tow hitch.

When we first finished hooking it up, I didn’t run a line from the vent to go outside the car. Found out right away, that some WVO finds its way out the vent routinely!! So we took a canola oil bottle and made an overflow bottle out of it, running the vent tube into it, and then venting that to the outside through the floor of the trunk:

Any overflow is collected in the bottle, and then as fuel is drawn out of the tank creating a slight vacuum, the suction pulls any WVO in the bottle back into the tank. I used a wire tie to hold the bottle secure, so I can move easily if needed.


HIH Routing Under the Car

They turn toward the front,

Route snugly under the stock tank (notice a piece of fuel line split down the length is used to protect the HIH from the edge of metal),

plenty of clearance over the axle,

and follow the same channel as the stock diesel lines:

We opened the holding clamps and stretched them across the lines and fastened them on either side.

Right at the firewall, they go into the engine compartment also along similar routing to the diesel fuel lines:

We considered that the heat from the HIH might hurt the diesel. But when running on diesel, the fuel is only in those lines for a moment, because it is flowing. And with no insulation or wrap to hold heat or transfer it to the diesel lines (like a HOH) we reasoned that not much heat would get transferred. When running on WVO, the diesel is stationary in that line, but only a few ounces of it would get any heat, and this is used immediately upon purge, replaced with cold diesel. When I tagged the picture I had 5,000 miles, an as of this posting I have over 9,000 miles on WVO, and have not noticed any issues with this. We intend to go back and insulate the HIH lines. When we do that, we will of course leave the diesel lines outside that insulation.

In the Engine Compartment

In the engine compartment the routing of the lines is hard to see, and so the pics under the hood go in the order the components are placed, rather than the order of flow.

To begin, under the hood we removed the stock air box. It takes a ton of room, and is only somewhat efficient. The TDI Club, I’m told, suggests against the use of K&N filters, but I have used a lot of them, and they are the best.

In the space left from removing the air box, we located the heated WVO filter low in the front right corner, and the HE about where the edge of the air box was:

The K&N we used is model RU-1460. No dealer or supplier we could find stocks this filter. There is even a K&N warehouse in Riverside, and they do not stock it there either! This was only a concern because for special order items, you have to pay first, and it’s a one-way ticket. If it doesn’t fit the space, you’re stuck with it. However, we measured, checked and measured again, and according to K&N’s catalog, the measurements we took exactly matched this unit. Most parts stores can order it, and we got in about 2 days. It is a perfect fit for the space.

There were two drawbacks to removing the stock air box and using a K&N as we did in this application. One is that it eliminates the cold-air intake. (Placing the filter in the location of the cold-air intake led to another problem we had to solve, discussed in a moment.) And second, the un-insulated HE is here, and air coming to the K&N must pass by it if not over it.

But I have noticed no degradation in performance, and the greater surface area of intake with the K&N may also reduce or eliminate any associated problems. I plan to insulate all the Frybrid lines anyway, to keep heat in for WVO purposes. By the summer, if performance degrades, we have contemplated an air dam and tunnel that will more directly feed cold air to the K&N.

In locating the valves, we were concerned with appearance, but only to the degree that we could hide them under the cover. Aside from that, functionality was the primary criteria in locating them. We fabricated brackets to hold them in the desired locations. For the supply valve, as close to the HE as possible was important, so we would have a minimum of unheated line after the HE and before the valve. Then we wanted that whole assembly to be as close to the IP as possible. We got about 4 inches and less than 2 inches for the final result!!

Next came the problem of mounting the K&N. The stock air intake feeds through a plastic elbow, around the back of the valve cover to the turbo. Fortunately it is bellowed for about 10 inches, and we were able to cut it at one end about 3 bellows from the MAF, and cut off all but about 3 bellows on the other end, just before the bend. We then fit several bellows from the one end into the other, smeared some sealant in and wallowed it around to make a good seal, and achieved a perfect shortened intake!

For maximum efficiency and speed of the purge cycle, it is important to have the shortest possible return line from the IP to the return valve. We achieved just over 3 inches here:

When setting up the purge cycle we put the sight glass in the joint at the end of this line, and then from the sight glass into the valve. We timed the cycle for fuel-change to be 12 seconds for the fuel to “completely” wash thru to the replacement fuel at this point. I have contemplated putting a delay relay into the switchover process, so as to give the return valve about an 8-10 second delay when switching over to WVO, just to save the few ounces of diesel that normally feed to the WVO tank at switchover.

The next most important thing in setting a good purge is to minimize unheated line after the valve and before the entrance to heated line. We could have located the terminus of the HIH for the return line right next to the valve, but it was much cleaner to locate it where we did:

But to be sure we didn’t leave any VO in unheated line; we originally timed the purge cycle to run diesel all the way to the entrance of the HIH. An additional 12 inches. By placing the sight glass there in the line, we timed that to be a total of 20 seconds. So an additional 8 seconds purge time to have the HIH terminus where we wanted it, for that additional 12 inches of unheated fuel return line.

However, Chris Goodwin later told me that it was safe to purge only to pass diesel thru the switch valve, and leave WVO in that short of a line. He said that by the time the car has warmed up enough for switchover next time, sufficient heat would travel thru the ounce or two of WVO in just 12 inches of line to make it flow, and flow is the only concern. So we reset the purge cycle to 12-14 seconds, and it has worked perfectly.

To get the heat to the HE, at first we teed into the heater hoses that feed the cabin heater. When buying a kit off of eBay, I got one with parts and instructions for a different car, that IS supposed to be plumbed that way!! But we found when running it that virtually no heat goes thru. When I posted a question to ask, Chris Goodwin said that either an aux water pump or re-plumbing in series was the correct way to do a TDI VW. We had already tried it the other way and found that out for ourselves by the time I saw his answer on this forum . . .

So on my car, the coolant travels into the firewall to the heater core thru the hose on the driver’s side, and then out the firewall back to the engine thru the hose on the passenger side. (Apologies to Aussie Bloke, Tony from West Oz, WVO Downunder, et al. for the wrong orientation for them!) We spliced into that return line, and fed that coolant to the WVO system. In TDI VWs (mine at least) coolant flows thru the heater core at all times, and heat is diverted into the cabin or not, simply by controlling whether the fan blows air thru it or not.


Later we also found that we could still decide on-the-fly whether to take heat in the cabin first or send heat to the WVO system first. If you select heater on in the cabin, the air will transfer all the heat it can from the coolant to the air flow, just as if the WVO system wasn’t there. If you select heater off, virtually all the heat travels right thru the heater core and on to the WVO system. This is remarkably efficient. I really can select it on the fly and take all the heat, or give it all to the WVO system, or any combination I feel like depending on how cold it is!!

The coolant travels over to the HE and enters there. The temp sensor is supposed to go in the line right before the HE, but we wanted to be safer at the beginning, so we located it after the HE and before the filter. So coolant flow is HE, temp sensor, then it goes into the filter. From there it goes into the HIH back to the tank, through the in-tank HE, then thru the other HIH back to the terminus where the fuel return line is. It then is routed back to the engine head.

The WVO comes from the tank, thru the HIH, thru the filter, thru the HE, the supply valve, the IP, the return valve, the other HIH, and back to the tank. The lines are routed tightly and closely, but fit in very efficiently.


Under the front of the right fender is the access to drain or change the filter element:

This is real nice, because the drips that leak when changing it all land on the ground, not in the car. Also, when changing the filter, I simply fill the new one to the top with diesel or fresh filtered WVO, and at the next switchover, there is never any hiccup from any air in the fuel line.

I was concerned about having the filter this exposed. I have contemplated putting either insulation around it, or even an electric or coolant heated wrap. However, as it was engineered, it stays blistering hot, even with cold wind that might hit it. I have had a few problems which may be related to cold in this filter, but have not been able to conclusively determine that to be the cause. Still, when I go to insulate all the lines, this will probably get an insulated cover for it.

As I said already, it sits in the space where the cold-air intake was. To protect it from direct air blowing across it we welded in an air dam to block the original path of the cold-air intake.

It is also visible in this shot, which shows a better view of some of the lower hose routing.

And from this angle you can get a good perspective of the filter location.

And another perspective:

This orientation of the filter and HE led to a problem of how to flow the fuel from the filter to the HE. The path it must take exits the fuel outlet of the filter, travels several inches, then must make a 180* turn and into the HE fuel inlet. Heater hoses are made that have 180* bends in them, but we could not find any fuel line that does. But heater hose would not hold up to fuel. We considered that it MIGHT hold up to WVO, but there would certainly sometimes be diesel or BioDiesel in the line. I asked on the Frybrid forum, and several including Chris Gooodwin responded that even WVO would break down heater hose over time. Chris suggested bending a piece of aluminum line and using that. And that’s exactly what we did!

Here you get the best view I can show of how that line goes:

With the K&N removed, you can see the routing of the diesel lines:

There were several electrical and structural arrangements that were designed for the location of the MAF. When we relocated it, at first we thought that several of those electrical and at least one mechanical assembly would have to be relocated. But in the end, they reached from the existing locations, and nothing had to be moved:

Here’s a final view of the routing of the lines and flow of coolant and fuel. The first shot is straight with no diagram, then the same shot with a diagram of flow drawn on it:

All in all, I think it’s a clean looking installation!


The location of the valves and components was designed not only for efficiency of operation, but also for cleanliness of appearance, and to appear either hidden or as close as possible to stock with the cover fitting back on:

And again, this is the “final product” view under the hood!!

In and through all this, I am not suggesting that these are the “best” locations or the only logical locations for these components to be located. I am simply showing where we decided to locate them, and why; and what we did to solve any associated problems.

In the Cabin – The Frybrid Controls

And then into the cabin for the Frybrid controls.

I did not want any extra gauges in the cabin. I wanted everything to remain as stock looking as possible. So to begin with, we did not mount the provided VDO fuel guage. Instead we wired it in to the stock fuel guage, which I will show in a moment.

We considered a custom panel for the LED lights and button and switch. But we found an unused blank in the dash board that we could very tightly fit the lights and button, and although this was a difficult fit on the back, it looks very clean on the front:

I did not want the switch in view in the cabin. In my “previous life” with Mustangs, it is common for the cooling fans not work well in fox-body Mustangs, particularly the turbo charged ones. And so in nearly all of my Mustangs we had wired a fan over-ride switch, and located it just under the dash to the far right side. I was very familiar with how easy this location is to reach, while at the same time being hidden from sight. This is where we located the master power switch for the Frybrid system:

And the Frybrid controller is located just up under the dash, hanging right under the steering column. That area does not lend itself well to photographs at all, so here’s a “ghost” drawing of its location:

Use of the stock in-dash fuel gauge for both fuels

In a PREVIOUS “previous life” I used to have a fleet of Ford Econoline vans. The ones that had 2 tanks had a switch up under the dash on the left side of the column, that you would switch tanks, and the fuel gauge would change to register the level of whichever tank you were currently using. Since I got quite used to that, this is how I wanted it in my Jetta, even way before I thought about how to accomplish it.

Then I saw where Brian (ourwebstop) had posted pics of his conversion and setup. Go to and click the link to pictures. In his pictures is a pic and link for directions for the fuel guage modification that he did. If you follow the link in Brian’s pictures, you can read it.

The sender unit that came with my Frybrid tank didn’t work exactly right that way, and we started with several resistors and tested the resistance of the circuits. In the photos to follow I describe how we accomplished it instead.

Even though it worked out differently for my car and my Frybrid tank sender unit, I wish to extend SPECIAL thanks to Jamie Braman (whom I’ve never spoken to) for letting Brian post his how-to document. And SPECIAL thanks to Brian for posting it!!

For the fuel gauge, the first thing we did was open the hatch door to access the sender unit, which in the 98 Jetta is in the trunk under the carpet to the right of the spare tire well.

(Picture of Fuel Sender hatch in trunk -- to be added later here)

We then spliced the live wire that comes from the stock sender and attached it to the default closed position on a SPDT relay. We brought the live wire from the Frybrid tank sender, and attached it to the normally open position on that relay.

(Picture f wires coming from stock sender, and splice of sender wire from WVO tank -- to be added later)

We wired the trigger power to the relay to come from the power to the supply valve. That way, whenever the supply valve is not powered (fueling the car with diesel) the relay trigger is not powered, and defaults to its #1 position, which closes the link between the gauge and the stock tank sender unit. Whenever the supply valve is powered (fueling the car with WVO) the relay trigger is powered, and switches to the #2 position, closing the link between the gauge and the WVO tank sender unit.

(Picture/schematic of relay.  This is a SPDT relay, with default to postion 1 -- to be added later)

We got some resistors and an electric multi-meter. After we tried several resistances, we determined that even NO resistance still would not allow the gauge to read all the way to “full” with the float ball all the way up in the tank. This was easy enough to check with the sender still in the tank; we could use a piece of wire to reach into the tank and gently lift the float ball to the ceiling. That only took the gauge up to about 2/3 full reading. But I drove it this way for several tanks, just to make sure that it was moving up and down, and that it was switching to read the 2 tanks.

So we had to remove the sender unit from the tank, and adjust it. We determined that with the wire of the float ball all the way up to the max position on the sender, that the gauge would indeed read full. But then the ball would be WAY higher than the top of the tank! We found the spot where the “fuel” warning light would come on, and that would have the ball floating at over half full in the tank.

We emptied the tank and put in exactly 2 gallons. We used a stick to dip into the sender hole, to mark the level of fuel for 2 gallons, and then set that alongside the sender unit. By trial and arror, we adjusted the height of the sender up and down on its rail, and a bend in the wire that holds the ball; and located it so that when the ball was where a full tank would be the wire had the sender all the way up, and when the ball was at the level of 2 gallons, the wire had the sender at the point where the “fuel” light came on.

This alignment works perfectly, and gives 10-11 gallons of usage before the “fuel” light comes on, and gives a reserve of 2 gallons of WVO when the light does come on.

For testing, it was necessary to turn on the ignition to the run position, but not start the car, and short pins 3 and 4, so the green light would come on, and adjustments to the tank sender would register immediately on the gauge. (When the car is moving, changes do not register instantaneously, as described below. But when the car is still, they do.) When the ignition was on, the “door open” buzzer was irritating, so I facilitated a temporary disabling device, with the help of a golf club and a cubie of WVO . . . .

In the cabin, the fuel gauge works perfectly with the exception of a significant delay in changing readings when switching. In this photo, the car has almost warmed to switchover temp, but you can tell by the temp gauge that it is not quite there. And you can see the RED light on the Frybrid controller to indicate diesel. The gauge shows the level of diesel I have in the stock tank:

I resumed driving and once it warmed enough and switched over, the gauge then moved to show the amount of WVO in the Frybrid tank:

You can't read the clock on the dash board very well, but the elapsed time is about 10 minutes.

The only problem with this operation when driving is that the gauge doesn’t move to the new level instantaneously, unless you are sitting still. The ECU takes readings from the sender every few seconds. When the car is still, it displays those readings directly to the gauge. This way, when you stop to fill up your empty tank, when you turn on the car, it senses the new level and displays the new level immediately. But when the car is moving, the float ball bounces around in the tank. Even with momentary readings, this would cause some erratic behavior on the gauge! So while moving, the ECU takes these readings from the sender every few seconds, and holds them for a period of time, and averages them for the reading it sends to the gauge. This smoothes out and eliminates the erratic behavior of the needle when driving, and the level of the tank drops relatively slowly, so that in the stock design each reading gets slightly lower as the fuel is used, and the overall average does go down so the needle on the gauge does go down as the fuel is used.

This functionality though serves to foil the switchover. When driving, if the ECU has been detecting readings near half let’s say, and then at switchover the readings are now at full, let’s say, it will add a full reading into the average, which raises the average a little, but not a lot. As more full readings get included into the average, the average gets to be larger and larger, and the needle gradually moves to the new level.

In practice, this takes about 10 minutes. I have gotten quite used to it, but does take about 10 minutes for the needle to reach the correct level after switchover when driving. After the switchover photo, I purged and the needle went back to the diesel level:

Then I switched off the Frybrid controller and switched it on again so it would switchover to WVO again, and the needle went back to the level of the WVO tank:

Notice that each of these has about 10 minutes elapsed time.

I was already very accustomed to this 10 minutes of the needle “crawling” when one day I had driven several miles of a warmup, but I was sitting still at an idle when the system switched over. This happened to me while sitting at a stop light.

If the car has been sitting still for a minute or so, then switchover will make the gauge needle move instantaneously to the new position. I have tried to time it, and do not yet have conclusive results, but the car must have been sitting still for somewhere between 30 and 60 seconds for it to stop “averaging” the readings, and start displaying the exact reading instantaneously. I have also tested it on the purge cycle, where if I stop and sit still for 30-60 seconds before purging, the gauge needle will move instantaneously back to the diesel level.

Brian (ourwebstop) and I were discussing this on the GC forum, and TDIguy posted that the speedometer is of course the device that the ECU uses to control this. So if there were a way to trick the ECU into thinking that the car is not moving for 30-60 seconds before switchover, it would then register the correct reading on the fuel gauge instantaneously. TDIguy mentioned that one of the fuses in the fuse box controls the speedometer, and that if you pull it momentarily (or rig a switch) then the speedometer will shut off, giving the exact desired effect to the fuel gauge. However, if driving at that moment, the gauge apparently will start jumping around erratically a little. I also really do not want to turn off the speedo for 60 seconds, but I may experiment with it and post results.

I wish there were a way to trick the ECU to think the car was not moving, but still display the speedometer reading in the dash. However, I assume the ECU controls the speedometer. If anyone knows a way to keep the speedometer reading correctly on the dash, but trick the ECU into thinking it’s at zero (for 30-60 seconds) I’d really like to know about it!!!


As of February 6, 2006 my brother is now officially offering installation service for WVO kits.

We do NOT profess to be professionals at WVO, I do not think we have nearly enough experience for that.

However, my brother is and has been a mobile mechanic for most of his working life (he's 40). As of this writing he does not want to begin doing installs at the customer's location yet, until he has several more under his belt. This is because there are too many things he might need to stop and go get, such that he is not yet confident he can offer it in a timely manner at the customer's location.

However, his location is Mentone, CA. This is near San Bernardino, CA. Anyone wanting to have a conversion kit installed can bring the vehicle, and I would be happy to provide WVO-fueled "shuttle drop-off service" back to their place as long as the distance is not extreme, and then pickup service to come get their vehicle when it is complete.

Basic kit installation is priced at $895, which will include light custom bracketry that may be required.

Please e-mail me off-forum for specific questions regarding hiring for an install.

Any questions about this install in my Jetta, please go to thread in the Frybrid forum and post there, and I will answer!!

Jeff Stephan

You can email Jeff at:


One further comment from Jeff:

It should be noted that the Frybrid kit comes with rubber fuel lines designed for fuel that will withstand hot WVO.  However, those lines are NOT designed to withstand BioDiesel, and it is important to know that if you install ANY WVO conversion kit, if you also plan to use BioD you need to install viton fuel lines instead of the ones I used, which are supplied normally by Frybrid.  When they have stock, I believe Frybrid is willing to supply you with BioD-safe lines as an option, if you request and pay the difference.