Gasoline to LPG or How to trick out your truck on sweet lady propane...

[Jason S]

I posted a question awhile back in the General Discussion section asking whether anyone would be interested in a posting about a bi-fuel LPG/gasoline conversion. There were a few interested parties. So here is my project.

After what being very satisfied with the LPG conversion on my 1973 GMC K2500, I decided to convert my 1974 Chevy K10 to be able to run on propane.  For this swap, I used the Impco 300 Propane mixer/carburetor with an Impco Model E propane regulator.  Southwest Oklahoma has numerous old farm trucks with propane systems. The parts I used for this conversion I bought used and rebuilt myself except for the lines and fittings.

For the most part the '74 is pretty much stock.  Here are a few photos of the truck. 







Under the hood resides a '76 4-bolt main 350 with TBI style heads, Edelbrock performer intake and an electric choke Quadrajet from a '85 Dodge D-350 dually pickup.
View of driver side


View of passenger side


Again, except for the lines and fittings, I rebuilt used parts.  If you were going with new parts, the installation would be similar.
Here are photos of the various parts that entail the conversion:

Impco 300 Propane carburetor/mixer, air cleaner and gasoline carburetor adapter assembly.


Bulkhead fitting, line pressure blow off valve and 90 degree fitting.


LPG liquid line. I used about 15 feet which was more than sufficient.


Example of a fitting for the LPG liquid line. I used two 90's, one straight and one 45 degree fitting. The 45 is shown in the photo.


Impco Model E regulator, filter and fuel lock-off valve.


Regulator mounting bracket


Heater hose y-connector.  There will be two, one for the 5/8" hose and one for the 3/4" hose.


Propane vapor hose. Connects regulator to propane mixer. Also seen in the propane mixer adapter photo above.


Fuel lock-off valve for gasoline.  This is a newer style, I used an older style.  They work the same though.


Fuel selector switch, primer switch, bowden cable and bracket. Here is a photo after installation. This bracket is incorrect in that it says in for gasoline, out for LPG.  It is actually the other way around.  The fuel selector switch is a simple three position toggle switch.  The primer switch is a momentary push type.


Last but not least, the propane tank.  I bought the tank used, cleaned it up and installed new valves and fittings.



There will be a few other brass fittings that may be necessary. When installing the gasoline lock off valve, I had to use several different brass fittings. The fittings necessary will depend upon the lock-off valve style.


The first thing I in the conversion process was to install the propane tank and bulkhead fitting into the bed of the truck.

Assemble the bulkhead with the blow off valve and 90 degree



Be sure to remove the protective cap from the blow off valve you hook up any LPG.


For the assembly, I used Ace Pipe Thread Compound on most of the LPG and gasoline fittings, otherwise I used teflon tape (per the propane supplier recommendation).



Then,

I cleaned and washed the front side of the bed.


When deciding where to mount this tank, I had to be careful and consider there is a crossbar under the bed that is approximately 1 1/2 inches wide about 3 inches back from the front of the bed.


I mounted the bulkhead slightly off center.  Pilot hole first then the 7/16" hole.



Then insert the bulkhead fitting and tighten the mounting nut.


View of bulkhead fitting from under the bed.


Next comes the actual propane tank. 


When deciding to mount this tank, I had to keep in mind the crossmember under the bed floor.  So I either had to mount the tank extremely close to the front of the bed or drop back to clear the crossmember.  I chose the later. A 2x4 will by the bed wall will show you about where the front edge of the crossmember is located.

 
Once I had the tank mounted and the fitting setup at the back of the truck, I moved to the front of the truck.  For the propane regulator to operate properly, it needs to be plumbed into the heater hoses.  As the liquid propane (or Liquified Petroleum Gas) expands to a gas, the ambient temperature drops.  So if the regulator is not heated in some fashion, it could freeze up due.

I decided to go ahead and replace the heater hoses and install a new thermostat. So I drained the cooling system and flushed it very well with clean water.  I installed a new 180 degree "heavy duty" thermostat and new hoses.


A comparison of the "heavy duty" thermostat versus the regular thermostat. 



Next, I assembled the propane regulator, fuel lock-off and filter.  I used the pipe thread compound referred to above on all the liquid connections. I used teflon tape on the vapor hose fitting.




Next, I mounted the regulator assembly to the regulator mounting bracket.  I put some rubber between the aluminum regulator and the galvanized steel bracket to try to keep corrosion to a minimum.
Front view of bracket, regulator side.


Side view of bracket


Side view with top brace installed.


View of rubber insulator


View of regulator assembly installed on bracket.


I installed the regulator assembly on the fender well. As LPG will have some petroleum oils, I oriented the regulator as shown to allow any oil accumulation to drain to the bottom of the regulator.  Also I mounted the regulator somewhat low to prevent potential cooling system air pockets in the regulator. 






Next, actual plumb the cooling system into the regulator using the Y fitting.





 

[454Man]

Awesom

[bake74]

     I am saving this to my files for future reference.  I assume there is more to the installation, or is that all ?

[Jason S]

My posting got interrupted last night by a tornado.


The last step in the installation process is to install the propane mixer/carburetor parts.
First remove the air cleaner and air cleaner stud from the carburetor.



Next mount the propane mixer/carburetor adapter to the gasoline carburetor. Make sure there is a gasket between the adapter and the gasoline carb. I reused my old air cleaner gasket. Note the orientation of the adapter, as this will affect the rest of the assembly process.


Next comes another gasket.


Before the next parts could go on, I had to install the bowden cable for the dual fuel operation. As I was using the speedometer cable grommet to route the wiring for the Primer and LPG & Gasoline lock-off valves I wanted to add some insulation to the bowden cable. I used heat shrink tubing so the wire armor on the cable wouldn't chaff the electrical wiring for the rest of the LPG system.

Here is the cable without insulation.


Insulation installed from where the bowden cable mounts to the bracket to past where it goes through the firewall.


Close image of the insulation on the bowden cable.  I layered several pieces of heat shrink tubing to ensure there wouldn't be any chaffing on the electrical wiring.


After insulating the cable, I routed it through the LPG switch bracket.  Don't forget to install the lockwasher and nut that secure the cable to the switch bracket.



Once the cable has been installed and routed, the remainder of the propane carb/mixer can be installed on the engine.

Here is the underside of the Impco mixer mounted to the bi-fuel adapter assembly.  The end of the bowden cable is attached to the large plate (hold down screw and clamp on right of photo) and the brass swivel.  The center part is the actual propane mixer. The ring around the mixer, with the angled ramps, engages the spring loaded pins.


In this photo, the cable has been attached to the assembly.


This is a closeup of the ramp and how it relates to the spring loaded pin on the adapter assembly. The adapter is in the closed (LPG) position in this photo.


After the cable is mounted, I carefully turned everything right side up and installed the assembly to the carb adapter.  This was a bit tricky to line up everything and install the mounting bolts.


Closeup of mounting bolt.


Once everything was bolted down, I pulled the cable out to the open (gasoline) position. I checked to make sure the cable opened and closed the air diverter properly. If it didn't operate correctly, the cable would need to be adjusted at the end to get the diverter to open and close properly.



After I checked the operation of the air diverter, I installed the bottom plate for the air filter.


Then I installed the air filter and top plate. I secured the top plate to the mixer.  These type mixers usually have three brass nuts along with the three screws. I swapped the wingnuts on instead of using the brass nuts. 


Then I installed the breather tube and the propane vapor tube


Then I Installed the other end of the propane vapor tube to the propane regulator.


For the propane liquid lines, I used new hose and fittings. The liquid line is a heavy duty tubing with with wire and poly reinforcement.  I used a sawzall with a fine tooth blade to make the cuts.
Here is the liquid line again.


Here is one of the propane line connectors/fittings taken apart for installation


I installed the ferrule end of the connecter to the liquid line and used a ratchet and socket to tighten it down (counterclockwise). I wanted the end of the hose snugged down to the inside of the ferrule. Tight, but not too tight.


Then I installed the end of the connecter and tightened everything up.



The same operation happened for all the other liquid line fittings. As for connecting the tank to the regulator, my setup looks like this.
Connection at the tank.  I routed over the tank because there isn't much room under the tank.


Connection on the top side of the bulkhead fitting.  I slipped a piece of heater hose over the liquid line to help prevent sun damage and to keep the hose from rubbing on the tank.


I have to get an under the truck photo of the bulkhead fitting connected to the liquid line.

Connection at the fuel filter at the propane regulator


When checking the connections, once I opened the liquid valve on the propane tank and pressured the lines, I used soapy water in a spray bottle.  I sprayed all the connections to ensure there were no leaks. Leaks would produce bubbles when sprayed with the soapy water solution.  I had to go back a few times and retighten a few connections. Of course, the most difficult connection to get to would be the most problematic, that being the fittings on the bottom of the bulkhead connector.


As for the wiring, everything is pretty simple.  A power wire to the three position toggle switch and primer switch at the dash. Then run wires from the switches to their respective positions.  I used a dedicated ground wire for each valve to ensure proper operation.  You can color code the wiring however you wish.  I used white to the propane lock-off valve, yellow to the primer and red to the gasoline lock-off valve.

I have never used the primer before, but when the primer button is engaged it energizes a little valve within the propane regulator.  When the valve is open, additional propane vapor can be fed through a vacuum hose a vacuum fitting on the gasoline carburetor.  It provides a little extra fuel for cold starts, if needed.  Like I say, I've never had to use it and didn't hook it up (yet) on this setup.

In this photo, the primer is located on the right side of the regulator (with the yellow wire).  Some regulators have it, some don't.


On my gasoline fuel lock-off valve, I used an older style with compression fittings to adapt the valve to the fuel line.  Then more fittings to make a fuel delivery setup that wouldn't pinch itself closed.  It's not the best and definitely wouldn't be ideal for full on racing, but works well for regular driving while using gasoline. A newer style lock-off would probably be a better setup if one was planning on using gasoline more, but I think this will be adequate for my needs.


Additionally, my gasoline carburetor had a working choke, so I disabled it from operating for now by wiring the choke open. At least for now I'm sure the choke won't set while the truck is on propane.


As for the propane mixer/carburetor; when I rebuilt my mixer I used the silicone diaphragm as it is supposed to last longer than the rubber diaphragm. It is the yellow piece in this photo.


To adjust the air fuel mixture of the propane mixer, remove the plug on the top of the mixer.
This is what the little plug screwed into the top of the propane mixer looks like.
 

In the mixer is a little slot screw.  Adjusting this screw will allow fine tuning of the air/fuel ratio delivered to the engine.  While tuning, if you allow the weight of the screwdriver to rest on the screw, it will affect the ratio. So I used a light hand and tried not to allow the screwdriver to rest on the screw.


When everything was hooked up, there's propane in the tank, no line leaks, etc. I started the truck on gasoline.  So toggle switch in gasoline position, cable pulled out and start. Let the truck warm up on gasoline.  Switch the toggle switch to off and allow the carburetor to run dry. When the engine dies, crank it again to make sure there is no more gasoline in the carburetor.   

Then push the cable closed, position the toggle switch to propane and crank the engine over.  It should start pretty quick if the air/fuel mixture is close. If not, try screwing the mixture in or out until it fires off. Then fine tune the mixture.

[bake74]

     Jason S, first I want to thank you for the write up, taking the time to document it with pictures.  I know I appreciate it and will probably refer to this in the future.
     The hose you used to hook up the propane lines look like pressure washer hose or hydraulic hose, R2 as referred to in the industry, only if I saw it right it is rated at 350 psi, while the others have much higher and variable rating pressures.
     I do have a couple of questions, this is your 74 you put this on correct ?  Are you using the mechanical fuel pump for the gas or a inline fuel pump ?  If you are using the mechanical fuel pump, my 74 does not have a return line, what happens when it is on propane ?  Where does the fuel that it is trying to pump go.  (Is this what the gas lock out valve is suppose to do, in the pic it looks like it is after the mech. fuel pump, I can see if it was before the pump ?)
     As far as starting it on gas and running it dry, I assume that is only the first time to get rid of the gas, or after switching from gas to propane correct ?  (You do not have to do this every time you start it is what I am getting at.)
     Last question, why do you have to disconnect the electric choke ?  Will it not shut off on propane after it heats up ?
     Again, thank you for the time you spent, I am looking forward to learning from you.
                                                                       Tom
     

[Jason S]

Hey Tom, thank you for the compliment.

The propane liquid line actually has L.P. Gas molded in.  It's molded in the hose before the part no. E-7915.  I don't know for sure what other hoses might substitute. I have actually seen earlier systems that had the whole system plumbed with copper line that had a protective sleeve.

The system did go on the '74.

I failed to mention in the posting that both the LPG and the gasoline lock off valves are normally closed.  Activation with 12V (or 6/12 on the lock-off valves I used) will open the valve and allow the fuel to flow through. 

Both of my trucks have a mechanical fuel pump without a return.  The gasoline lock-off valve works the same way as the seat in the carburetor would. It simply shuts off the fuel supply to the carburetor. If you were to have an inline electric fuel pump, you could simply delete the lock-off valve and have the pump switched on/off. 

As for the fuel, it doesn't go anywhere and a stock mechanical pump simply won't build up enough pressure to cause any problems. 

You are correct about switching from gasoline to propane. You have to let the carburetor run dry. Otherwise you'll be burning gasoline and propane.  If you have your system running on propane and then shut off the truck, the next morning you simply start it on propane again. You don't have to start on gasoline then switch to propane.  You mainly want to ensure the carburetor has run dry before switching to propane. It's not exactly a switch on the fly system (like switching L to R on dual tanks), although I know people that have switched on the fly.

The purpose of the choke on a gasoline carburetor is to damper the air going into the carburetor to enrich the fuel so that the engine can warm up to operating temperature on an enriched fuel mixture and properly atomize the gasoline being siphoned into the engine. A portion of that enriched fuel/air mixture in a cold engine is never burned.  With a propane setup, the fuel is vaporized (i.e., changed from a liquid to a gas) at the regulator.  The regulator is really a regulator/convertor.  So, when the liquid propane gets to the regulator, it expands from a liquid into a gas within the regulator.  The propane gas is pulled into the mixer, with vacuum supplied by the engine, and mixed with air and then pulled into the engine. The mixture going into the engine is a completely mixed fuel/air mixture. There is no need for a choke to enrich or alter the air into the engine.

I wish I had taken photos of the mixer and regulator when I was rebuilding.  I have another set that I need to rebuild, so maybe I can get some photos of that process.

However, to go into more depth:  Starting with a cold engine that's off; you have the system set on propane, choke on the gasoline carburetor is wired open.  You crank the engine over, a vacuum is pulled through the carburetor, then through the mixer and into the regulator.  Within the regulator is a vacuum operated diaphragm that opens a valve to allow the propane gas to be pulled through the propane vapor tube into the mixer.  The propane vapor is mixed with air at the mixer and then drawn into the engine through the gasoline carburetor. If the choke were set, it would simply choke down the process possibly even preventing enough vacuum signal to be received to operate the system.  If an enriched mixture is needed for starting, that is where the primer comes into play.  You simply push the primer button while cranking to allow additional straight propane directly into the engine without being mixed with air at the mixer. 

I have started the '73 GMC on propane when it was -10 degrees outside and simply walked back in the house. I didn't touch the foot feed, simply three cranks. The engine fired, I gave a little rev, made sure the oil pressure came up, set the heater on high and went back in.  No need to set the choke or fast idle, etc.  It's very simple.

One other thing, the engine cooling system needs to be in good working order.
    Firstly, as propane vaporizes at something like -34 fahrenheit, the antifreeze mixture needs to be able to accommodate the low vaporizing temperature. This isn't a big deal in the summer, but if it's very cold in the winter it could lead to regulator freeze up.  The coolant flowing through the regulator simply freezes in the regulator and then the whole regulator ices over. If that happens, the liquid propane won't vaporize very well.
   Secondly, with an engine running on gasoline there is a certain cooling effect as the gasoline atomizes from a liquid state.  As the propane fuel is a vapor when it reaches the mixer, there really isn't going to be any cooling effect upon the fuel/air mixture going into the engine. However, there is some cooling effect at the regulator upon the engine coolant.

I have noticed that on days that aren't extremely hot (i.e., 80's and 90's), the engine does not get quite as hot while on propane. Last summer, we had over 100 days of 100 plus degree weather. Many of those days were 111-115 degrees.  While on propane, the engine ran hotter than on gasoline (with the A/C on, of course), but it never did overheat.

I'm still in a learning process on this myself. There is quite a bit more to be said on the subject and I'll try to help as much as I can.  I'm in the process of installing an Impco 425 mixer setup on the '73 GMC so I'll post photos of that when I'm done.

 

[bake74]

     Thank you for the answer's.  Very informative, I believe I understand the process now enough that if I decided to do this I would at least be able to do the basic's.  Thanks again.

[howlinwolf]

now my setup doesnt have the cable handle like yours. its a valve and an electric fuel pump. (only swapped to the electric pump because i rarely use gasoline and the mechanical pump went out.  so what exactly does the cable do?

[Jason S]

Howlinwolf: If you have an Impco mixer on your truck it sounds like yours may be the Impco 425.  I was going to post photos of the procedure for adapting that particular mixer as well.

With the Impco 300 setup as shown, the cable operates a cam under the mixer/air cleaner assembly which in turn opens a "valve" to allow outside air to bypass the propane mixer and go straight into the carburetor.  This would be for gasoline operation.  The 300 with an adapter is normally set to propane, you pull the cable for gasoline.  There is a photo that shows the mixer without the air cleaner and the "valve" in the open position for gasoline operation.  I guess I didn't get a photo of the valve in the closed position. 

 

[Jason S]

Here is a photo set of the installation of the Impco 425 with an adapter to a gasoline carburetor.  The 425 can also be bolted to a Holley 4-barrel baseplate, preferably a double pumper, and bolted in place of the gasoline carb.  The Impco 300 can also be used as a stand alone, but it seems like the 425 is preferred.

When running the 425 in a gasoline/propane setup, it's about the same as with the 300 listed above.  It will need the gasoline lock-off valve with a mechanical fuel pump setup (or wire an electric pump to be off when in propane mode), but won't require the cable for switching from gasoline to propane (or vise versa).   

Here is a top view of the Impco 425 mounted to the carburetor adapter. 


View of the silicone diaphragm


View of the fuel/air mixture adjustment screw



For installation of the 425 with a carburetor adapter: remove air cleaner from gasoline carburetor, make sure the air cleaner to carb gasket is in good shape and set the mixer assembly onto the carburetor. The mounting stud (old air cleaner stud in this case) needs a rubber washer to prevent vacuum leaks.


I then added a flat washer over the rubber washer and then the old carburetor wing nut.


For the air cleaner, there are offset base air cleaners from Impco as well as some other styles. I decided to try my hand at a custom assembly.  It works fine for now, but I may rework it in the future.  I started with a Mr. Gasket 6 1/2" air cleaner assembly for the upper and lower air cleaner plates, an air cleaner base gasket and the K&N filter E-3346 filter.  I couldn't find anything locally with the dimensions I was shooting for, so got the K&N.  It seems to fit the setup I had in mind.





I installed the air cleaner base gasket to the air cleaner base using Permatex #2, mainly to keep things from sliding around since the air cleaner would be in the vertical position instead of horizontal.


I used the air cleaner mounting stud supplied with the Mr. Gasket air cleaner.


Then I installed the base plate with gasket


Then I installed the K&N filter and the air cleaner "top" or end.


There are no crankcase vent ports on the 6 1/2" cleaner so I added an inlet to the top of the air cleaner. I set the filter on the air cleaner "top" and marked the inside diameter of the filter



Then using a air cleaner fitting from another propane setup, I marked the hole in the top of the air cleaner.



After I drilled out the hole, I installed the fitting and snugged it down.


As there is quite a bit of weight cantilevered off the carburetor mounting stud, a reinforcement bracket at the front of the assembly can help keep the propane mixer from breaking out the mounting stud area on the carburetor.  This is a very simple and crude bracket I made as a support.  When I get my shop back together, I'll refine this one or make a new one.


Finished shot of the Impco 425 with gasoline carb adapter and a custom air cleaner setup. To complete the install process, you attach the propane vapor line (from regulator to the mixer) and hook up a crankcase vent hose. 


This view shows the air cleaner with front bracket.

[howlinwolf]

mine looks more like that setup, when i get my truck home ill take pictures. i need to replace the filter. its pretty old

[howlinwolf]

heck forget the pictures right now, that exactly what i have. a 425.

[bake74]

     Once again thanks Jason S, I have really enjoyed reading/learning from you.  One question.

 
     The back hose is for the propane vapor inlet correct ?  Is that front hose hooked up to the PVC valve ?
     I love that air cleaner set up, it looks great and very clean.

[Jason S]

     Once again thanks Jason S, I have really enjoyed reading/learning from you.  One question.
     The back hose is for the propane vapor inlet correct ?  Is that front hose hooked up to the PVC valve ?
     I love that air cleaner set up, it looks great and very clean.

Thank you very much Tom.  I'm still learning about propane myself, but if I can save someone from having to retrace their steps then that will make my day.

When I assembled the parts for the air cleaner, I didn't want to invest a lot of money in them just in case I didn't like the 425. I mainly wanted something inexpensive that would work.

The K&N filter I used (E-3346) was the right size for what I wanted, I just wish it had some more rigidity. I spent a lot of time in the Wix catalog at O'reilly's looking for something else, but either they didn't have it in stock or it wouldn't work...

The back hose is the propane vapor hose.  The PCV valve and hose is still connected to the gasoline carburetor.  The front hose is the breather. I could've just stuck a breather in the valve cover, but I chose to run it to the air cleaner.  For now, this setup works. I'm thinking that I may re-engineer it when I get my shop setup after the move.

Because of getting ready to move, house hunting, new baby and all I really haven't had much of a chance to run the 425 and compare it to the 300. 

[howlinwolf]

My only problem with the 425 is when i shut off the truck it floods it out