2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

[jeepzz]

I'd like to know if anyone with one of these factory bi-fuel GMC Sierra/Chev Silverado 2500 (Quantum Tech) know how to manually switch back and forth between gas and CNG?
I have a lot more towing power using gas but since the factory setup doesn't have switch, I have to plan ahead to run out of CNG before hooking up my trailer. I've tried pulling fuses, relays and even disconnecting the main electrical plug going to the CNG tank all on the advise from several local CNG shops and the GMC dealership, but all I get is a check engine light and a truck that will not idle. I'd like to put a manual switch in but all the local help has no clue how to do it.

I even wrote to Quatum Technologies and here's the very short reply:
This fuel system on this vehicle is a GM factory installed and certified option. All questions regarding this system and its functionality should be directed to your local GM dealer or GM vehicle engineering.

It would not only be improper but illegal for Quantum Technologies to advise you on ways to defeat primary system functionality.

First of all I'm not trying to "defeat" any system, I'm trying to manually switch over from one COMPLETE fuel system to the other. No emission devices are being tampered with or circumvented.
Any help would be greatly appreciated.

[Highmarker]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I know with the '03 Cavalier (Quantum system), there is a fuse in the fuse box by the battery that is labeled 'CNG'. If you pull that fuse, the car runs on gasoline. I believe that the idea of having the car default to run on CNG is Chevrolet's idea and not Quantum's, because most if not all Chevy factory bi-fuel systems operate that way. There has got to be a fuse for the CNG somewhere.

I did think about putting a toggle switch on that fuse so I could turn it off and on when I wanted to, but here's the catch. You can pull the fuse while it is running on CNG and it will switch over to gasoline (on the fly), but if you put the fuse back in while it is running on gasoline (not recommended - small sparks will fly) it will not switch to CNG until you turn the car off (reset the computer) and then re-start the car. So if you're going down the road and you want to switch to gasoline that's fine, but if you want to switch to CNG, then you have to turn the car off and then re-start it.

[siai47]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

One of the best ways to do this is to interrupt the signal from the fuel pressure transducer. The Vehicle thinks it has run out of NG and switches to gasoline. When you restore the signal, it thinks it has been refueled and switches back to CNG. The only problem with this approach is that if you are low on CNG (below 600 psi in the tank) it will not switch back to CNG until the transducer sees 600 psi. There is a way to overcome this if necessary. I can detail how to do this "mod" if you want to try it.

[jeepzz]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I know with the '03 Cavalier (Quantum system), there is a fuse in the fuse box by the battery that is labeled 'CNG'. If you pull that fuse, the car runs on gasoline. I believe that the idea of having the car default to run on CNG is Chevrolet's idea and not Quantum's, because most if not all Chevy factory bi-fuel systems operate that way. There has got to be a fuse for the CNG somewhere.

I did think about putting a toggle switch on that fuse so I could turn it off and on when I wanted to, but here's the catch. You can pull the fuse while it is running on CNG and it will switch over to gasoline (on the fly), but if you put the fuse back in while it is running on gasoline (not recommended - small sparks will fly) it will not switch to CNG until you turn the car off (reset the computer) and then re-start the car. So if you're going down the road and you want to switch to gasoline that's fine, but if you want to switch to CNG, then you have to turn the car off and then re-start it.

We tried pulling the fuse for the CNG but the truck dies instantly. When restarted, it does run on gasoline but it will NOT idle. Pulling the fuse also trips the 'check engine light'. Once the fuse is plugged back in the truck will now idle on gasoline but the check engine lights stays on for a couple of days and then will finally shut off on its own. Once the check engine light shuts off, the truck will once again switch over to CNG (but not until the check engine light resets).
I have tried this twice and both times had the same result.

Hopefully there's some other way similiar to the Cavaliers.

[jeepzz]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

One of the best ways to do this is to interrupt the signal from the fuel pressure transducer. The Vehicle thinks it has run out of NG and switches to gasoline. When you restore the signal, it thinks it has been refueled and switches back to CNG. The only problem with this approach is that if you are low on CNG (below 600 psi in the tank) it will not switch back to CNG until the transducer sees 600 psi. There is a way to overcome this if necessary. I can detail how to do this "mod" if you want to try it.

When I was down at Lancer Automotive, they had the same idea. They disconnected the wiring under the truck that leads into the tank. They thought for sure that would do the trick.
Check engine light comes on but this time the truck did not stall while running on gasoline. Now the bad news, after reconnecting the plug I still have to drive it for 2-3 days until the check engine light resets itself, then the truck will switch back over to CNG. We tried this just several miles after a CNG fill up, but even while pressing the fuel gauge button (to check the level in the tank opposite of what you are currently running) the CNG shows empty. It will continue to register empty until the check engine light shuts off, then will automatically run on CNG once again.
Surely someone out there has figured out a way to easily toggle back and forth even if it requires a simple restart like the Cavaliers.

Thanks again for the input guys.

[tntw]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I have a 1998 Chev pick-up with the IMPCO factory set up on it that my son uses at college. It came from the factory with a shut off valve located under the drivers side door and any time we need to tow with it or prefer to run on gasoline and save the CNG we just close the valve. The Check Engine light comes on but after reopening the valve it will clear after a couple of starts.
Maybe Lancer could install one.

[jeepzz]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I have a 1998 Chev pick-up with the IMPCO factory set up on it that my son uses at college. It came from the factory with a shut off valve located under the drivers side door and any time we need to tow with it or prefer to run on gasoline and save the CNG we just close the valve. The Check Engine light comes on but after reopening the valve it will clear after a couple of starts.
Maybe Lancer could install one.

For now I'm just crawling under the truck and unplugging the main electrical connection going to the tank when I tow. The check engine light immediately comes on but when I plug it back in, the truck STILL stays on gasoline until the light resets and shuts off after a few days.
Sounds like about the same as what you do when closing the valve only I have to wait to run on CNG again 'til the light shuts off.
I'm going to continue to use this (unconvienent) method until something better comes along.

Thanks everyone for your input .

[Highmarker]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

It must be a big enough power difference between gasoline and CNG to make up for this inconvience. I am considering getting a '03 or newer factory CNG Chevy 3/4 pickup. With my '03 Cavalier, I don't notice too much of a power difference, so I "assumed" that the Chevy trucks were about the same...I guess not.

[CNGfamily]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

It must be a big enough power difference between gasoline and CNG to make up for this inconvience...

Sample of three bi-fuel vehicles that I have driven for long enough to see a substantial difference in power: '95 Sonoma with a V-6, '00 F-150 (5.4), and '00 Tahoe (5.3). Of the three, the factory converted F-150 had the least impact but was still significantly less power on CNG. The V-6 mini pickup had enough power to start with to live with CNG even towing (a small pair of waverunners). The 5.3 Tahoe is pretty marginal at best when towing ANYTHING with regular gas when there is any type of grade, and it's even worse with CNG.

[kirkbenge]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I think the easiest way to switch from gasoline to CNG is to stop the car and unplug the power to the Low Pressure Lockout (LPL). Doing this has never given me a check-engine light, but it does require that you pull over, stop the engine and lift the hood (you may be able to do this with the engine running, but I've never tried). When you want to switch back to CNG, just plug it back in.

In the cavaliers this is very accessible and easy to do. It's easy to find to, thanks to it's blue color.

I don't know about your vehicle. I have attached pictures of an '01 cavalier engine with the LPL labeled. In each picture the connector plug that you want to disconnect is marked by a yellow box.

[jasonjsimon]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I know this is an old thread, but I'm really interested in accomplishing the same thing with a 2003 Silverado. Has anyone been able to easily trick the system without a check engine light?

[Chris Sullivan]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I have seriously been considering purchasing a CNG 2500 Chev/GMC truck with a 6.0 engine in order to have more power pulling a trailer than a 5.3 gasoline engine. According to some of the above comments there is a very noticeable difference and loss in power when pulling with cng verus gasoline. This sounds like the loss in power will defeat the purpose of getting 6.0 rather than using a 5.3 gasoline.

Does anyone have experience from using both a 6.0 cng compared to a 5.3 gasoline? Thanks for the info.

09 GX

[jasonjsimon]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

I have seriously been considering purchasing a CNG 2500 Chev/GMC truck with a 6.0 engine in order to have more power pulling a trailer than a 5.3 gasoline engine. According to some of the above comments there is a very noticeable difference and loss in power when pulling with cng verus gasoline. This sounds like the loss in power will defeat the purpose of getting 6.0 rather than using a 5.3 gasoline.

Does anyone have experience from using both a 6.0 cng compared to a 5.3 gasoline? Thanks for the info.

09 GX

I may not be the perfect example since my new to me 6.0 liter 2500HD has oversize tires and the gearing may not be the greatest for the setup, but I did own a 5.3 liter Tahoe before this. I haven't had a ton of experience pulling with the newer truck yet, but I can definitely say that on CNG it is a lot more solid and in better conrol of the tow than the Tahoe was on gasoline. I definitely think that it was a worthwhile upgrade, but I'm just trying to plan ahead for some tough pulls this summer. I would like to be able to switch off CNG on demand, even if I had to turn off the engine to go back to gasoline.

[siai47]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

You might want to try this if you are adventurous . CNG switchover occurs when the fuel level drops to a certain level and will restart when the fuel level is raised above a certain level. Changing the fuel tank pressure sensor circuit might allow you to change fuel type on the fly without getting a DTC. The fuel tank pressure signal is sent from the tank to the FCIM as shown in the drawing. When the tank is full the sensor has low resistance causing the voltage seen at pin 25 to be nearly 5 volts. When the pressure is low, the resistance is high causing the voltage at pin 25 to be close to the low reference level (not ground). By cutting the Dark green with a white tracer wire and installing a two position three pole switch you should be able to either send the correct pressure signal from the transducer to the FCIM or a false 0 pressure signal making the vehicle switch to gasoline. The remaining pole on the switch needs to be attached to the grey wire. I am not sure if returning the switch to the normal position will restart CNG operation or an ignition cycle wil be required. Again, TRY THIS AT YOUR OWN RISK !

[jasonjsimon]

Re: 2003 GMC Sierra 2500 factory bi-fuel: How do I switch back and forth?

You might want to try this if you are adventurous . CNG switchover occurs when the fuel level drops to a certain level and will restart when the fuel level is raised above a certain level. Changing the fuel tank pressure sensor circuit might allow you to change fuel type on the fly without getting a DTC. The fuel tank pressure signal is sent from the tank to the FCIM as shown in the drawing. When the tank is full the sensor has low resistance causing the voltage seen at pin 25 to be nearly 5 volts. When the pressure is low, the resistance is high causing the voltage at pin 25 to be close to the low reference level (not ground). By cutting the Dark green with a white tracer wire and installing a two position three pole switch you should be able to either send the correct pressure signal from the transducer to the FCIM or a false 0 pressure signal making the vehicle switch to gasoline. The remaining pole on the switch needs to be attached to the grey wire. I am not sure if returning the switch to the normal position will restart CNG operation or an ignition cycle wil be required. Again, TRY THIS AT YOUR OWN RISK !

So the next question is where these wires can be found on the truck. I may get brave and try it if time allows. It seems pretty logical.