Mazda B2600 Full Propane Conversion

I thought about it for a while and since many of the regular members here like to show off their mad engineering skills I figured would show off my latest experiment/just for the fun of it alternative fuels based hobby project.

As many here already know I have ran several of my pickups on propane over the years with custom built dual fuel systems I set up and install myself. What makes this one unique is that its a full propane conversion not a dual fuel system.

This time it's a 1987 Mazda B2600 extended cab 4x4 that I bought a while back for $200 with what the owner thought was a bad engine due to his having very little mechanical skills or knowledge. So here is my story of what I did to it so far!

What I started out with was a somewhat mechanically neglected 1987 Mazda B2600 extended cab 4x4 pickup. Originally it used an overly complicated and unreliable emissions compliant electronic carburetor system that was typical in the mid 1980's. That particular design Mazda used was impossible to tune properly or keep in tune and was an overall more of a waist of fuel and engine power than a gain to anything. What I did for the full propane conversion was to remove the entire emissions system from the truck and replace it with a simple single barrel vapor carburetor throttle body that came from an old propane powered combine engine I found at the scrap yard. All the old combine throttle body needed to work was some minor bolt pattern rework to make it physically compatible with a modern Woodward vapor mixer that attached to the top of it. It's the same setup as the vapor carburetor systems used on propane powered forklifts.

To start out I removed everything that was emissions related in the vehicle, the smog pump, evaporative emissions control unit, the multiple vacuum control units and lines, the crappy electronic carburetor, the catalectic converter, and the rest of the rotted out and undersized exhaust system. All of this was reduced down to a simple all mechanical propane carburetor and vaporizer unit and the now unrestricted 2 inch exhaust system.

For the full propane conversion of an engine there is only one moderate mechanical modification that needed to be done and fortunately due to the previous owners neglect and all thumbs maintenance attempts this became the perfect vehicle for this type of conversion. At some point the previous owner overheated the engine and blew out the head gasket between the #3 and #4 cylinders but apparently decided to try and keep driving it until it burned a large gouge into the aluminum head. Once that happened the engine could no longer be repaired just by putting in a new head gasket. He decided to scrap the pickup which is where I found out about it and bought it for $200!

The damaged aluminum head had a .065" deep blowout gouge that was only fixable by either welding it over or removing .065" of head face with a deep re facing job. The problem with that heavy of cut is it changes the engines normal 8.8:1 compression ratio and pushes it up so high that it can never effectively run on regular gasoline again.

This was perfect for me being that for propane a .065" cut would give me around 10.8:1 compression which would be a good start for a propane engine. I decided to go a little extra and had the head machined down .075" so that the compression would be in the 12:1 range with is super for propane operation!

After having the head work done I did run into the slight problem that occurs with chain driven overhead cam engines when they get considerable cuts taken off of the head. This heavy cut causes the timing chain to create a slight camshaft retarding effect due to having to take up the extra slack that is now present from the loss of material. To over come this I did a simple old school camshaft timing cheat and reworked the camshaft drive gear dowel pin hole into a short slot with my Dremel and a small carbide cutter.

With the .075" cut taken off the head the cam would have had an approximate 6 degree retarding effect from the added slack. But by re cutting the dowel pin hole it is possible to create an adjustable range from -6 to +6 degrees of crankshaft rotation. To make that modification work I threaded the dowel pin hole on the camshaft to fit a 1/4" grade 8 bolt in its place and cut that off just short of sticking all the way through the drive gear face. By doing this modification the camshaft still has a solid locator pin to keep its overall movement within the reworked gear slot where the original system had the fixed pin design.

On the Mazda 2.6L engines the distributor is driven off of a gear that goes on the front of the camshafts drive gear. It and the drive gear are all held in place by a single bolt that threads into the front end of the camshaft. To make sure the ignition timing stays correct I drilled a second dowel pin hole in the camshaft drive gear and relocated the distributor drive gears pin to that new position. This in itself does not pose a problem with the ignition timing as that is still determined by how the gear teeth line up when the distributor is reinstalled. By using a new fixed point for the drive gear pin on the camshaft the ignition timing will stay in a fixed relation to the crankshaft but the camshaft itself can be moved forward or backward in relation to the crankshaft and ignition timing.

Camshaft timing adjustment is very easy on this engine being the valve cover is held on with two bolts and the cam itself is easy to get a hold of from the back end when the cover is off. By loosening the front camshaft bolt I can then spin the cam slightly forward or backward in the range of the slot on the camshaft drive gear. By using the timing index on the crankshaft damper as a reference I can accurately readjust my cam timing up to 6 degrees forward or back from the stock position.

With the engine now set up for high compression propane operation I am now running with 9 degrees base ignition advance and +2 degrees camshaft advance. Advancing the camshaft timing puts more torque on the low RPM end while retarding it will put it toward the top end. A rough rule of thumb is 1 degree moves the peak torque point 100 RPM up or down. With the original emissions cam a slight advance of +2 degrees from stock is beneficial and has the effect of putting the peak torque slightly lower in the power band.

The old single barrel combine throttle body did take some minor modifying and intake manifold rework to get it to fit but it was not hard. I made an adapter plate out of 3" x 4.5" x .5" aluminum stock. I drilled and tapped it to fit the old four bolt pattern the tiny two barrel electronic carburetor used then drilled a larger hole through the center that was just slightly larger than the throttle body throat. The throttle body I used only has two bolts holding it down so I drilled and tapped two holes in the adapter plate so it had a solid point to attach to.

After that it was just a matter of using a die grinder with an aluminum cutter to reshape and blend the transition from the single barrel throttle body down into the intake where the original two barrel carburetor once sat. This big single barrel vapor carburetor gives it about 50% more intake air flow capacity over the undersized two barrel which works well for getting more power due to simply having far less intake restriction.

The intake manifold does need several slight modifications for this engine which involves plugging several ports that are no longer needed or beneficial to a propane fuel system. The first is plugging EGR passages either by welding it shut or using JB weld epoxy. The same goes for several coolant circulation passages that where intended to heat the intake manifold and the old electronic carburetor.

What I did was clean up the ports and passages and block them off with healthy slugs of JB weld epoxy. There are two coolant feed ports and two coolant return ports that need to be blocked off. Also the EGR valve assembly needs to be removed and its related ports and passages need to be plugged as well. One of the EGR passages goes up through the intake manifold and connects to a passage in the head that goes directly to an exhaust port and the other passage goes into the intake just below the carburetor inlet.

Electrically this propane conversion did not require changing anything other than the removal of all of the emissions and vacuum control wiring. After the 20+ control wires are removed I was left with two wires, one for the oil and temperature sensor and one for the temperature gage sensor. The only other remaining wires going to the engine are the ones for the alternator and ignition which are in their own harnesses so they do not need to be changed.

The vaporizer unit for the propane system only needs one 12 volt power lead from the ignition for it to work. Being the 1987 Mazda B2600's have an electric fuel pump control system I just re routed its output line from the pump to the vaporizer fuel solenoid which was an easy one wire cut and splice job. The vaporizer does need to be part of the coolant circulation so I plumbed it into one port near the engines thermostat (a now unused emissions sensor port also where one of the intake coolant passages fed from) and returned it to the return line where the intake coolant return originally went. By doing this it keeps the engines coolant circulating as it needs to while also keeping the vaporizer heated at the same time.

With all of that done then it's just a matter of finding a good propane fuel tank that will fit on the truck. For me being the old fuel tank is no longer needed and the simplifying of the exhaust system left me with loads of underside room I am going to get a set of tanks that can fit underneath the truck and plumb their filler ports into the original gas cap location.

There are some other little things that also help on this type of conversion is that I have found to be worth the added cost and time as well. Bosch +4 spark plugs work very well with propane fuel and are well worth the added cost along with changing to a lower resistance sparkplug wires and a higher powered ignition system. Propane burns considerably leaner than gasoline plus with the much higher compression added ignition power is beneficial as well.

Power and performance wise the truck is very snappy now and the fuel mileage seems to be stable at around the mid to upper twenties! By figuring the fuel mileage as a cost per mile ratio comparison of propane Vs gasoline that works out to a cost per mile equivalent equal to having a pickup that would get around 45 - 55 MPG on gasoline!

Financially here is how it breaks down for the conversion.

Propane conversion system $180

Head milling $105

Engine gasket kit $68

Timing chain kit $89

Chilton manual $25

Bosch +4 sparkplugs $28

Misc parts & supplies $150

New exhaust system $180

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Total to date $ 825

As you can see I am using some old propane tanks for the test and tune until I get the tanks that will fit underneath.
Officially this truck is now TCMTEK2 and my new license plates should be here in a week or two.

Eventually I am going to remove the pickup box and set it up with a flatbed and my Bobcat welder generator and gas powered air compressor along with some tool boxes in order to make a service vehicle out of it. But for now it just a really fun and dirt cheap way to get around!

I think I ended up with a pretty good result for having started with $200 beater pickup and some spare junk I had laying around.