My dual fuel truck recently started to occasionally not run on CNG. About this time, I'd turn the ignition switch on, and there would be a loud pop under the hood that sounded like a back fire; however, the ignition switch would not be turned enough to even engage the starter. I thought on occasion that I was smelling some CNG, and I was concerned that the CNG runoff was somehow being ignited. A few times, I heard 3 rapid pops. A month later or so, I twist the valve to fill up, and there was a loud pop under the hood - it sounded identical to the backfires, and it made me jump. Every time I'd fill up, I was prepared to hear the pop, which it didn't do more times than it did, but it startled me so.

I got down to 1/4 tank about 5-6 days ago, filled up with CNG, and it hasn't run on it since. The green dash light goes on when I turn on the ignition, but as soon as the truck starts, it goes off. I've taken it to a local shop, who had it for a day about 1.5 weeks ago, but I'm scheduled to take it back on Monday. I have a feeling they are going to have it for some time, and I'm concerned of the costs. They think it's a wiring problem, but I think if I can track down the pops, I'd be a step closer to figuring out the problem. They are wanting to trace the wiring down prior to checking the popping sound.

The green dash light did start flashing on occasion - sometimes when driving after being solid for a while, and sometimes right on startup. At this time, the truck may switch over the gas, but sometimes it would continue to run on CNG.

This appears to be a Canadian system (sorry, but I don't know which one), and I've tested leaks from the tank to some kind of valve that sits at the passenger/front part of the engine. This valve has about a 1" vent hose that is routed to the bottom of the engine. This is just a gut feeling, but I wouldn't be surprised if the backfire sound came from the vent tube. I can see any other place it would come from, except through the inside of the engine and out the air cleaner. I've checked all fuses, and they seem to be OK.

I'd appreciate any thoughts - ready to get back on CNG after the artificially low gas prices jump up after the election.

I don't know if this is a possibility, but I've read there are some CNG filters need to be replaced - at least some people have reported the green light may flash if this is the case. I don't see any filters, and I was surprised to see 3600 psi all the way to the engine - I thought there were be a regulator at the tank to drop it down.

 

Google "Compuvalve". That seems to be the weak point in Ford's dual fuel system.

 

My understanding is that the 2003 and later years don't have a compuvalve. I thought I looked into this pretty thoroughly, and that's why I went with the 2003 when I found it.

 

Hello,

I believe I am having the same problem with my factory bi-fuel CNG 2003 F-150. The problem started with the engine switching from CNG to gasoline during acceleration past idle. Then, the truck started having trouble even starting on CNG, requiring several attempts at ignition to start on CNG. Finally, the engine stopped starting on CNG at all. The popping noise occurred in the "on" position just before ignition position. To me, it sounds like a compressor or pressure release valve. My popping noise often occurs several times in rapid succession though a single pop has occurred. As best I can tell, the popping noise comes from a 1" vent tube on the passenger side that comes out of what I believe to be the pressure regulator and goes to the bottom of the engine below all components where it releases into the open air below the truck. Recently, the truck began to release pressure through that 1" hose when I connected the Oklahoma Natural Gas public fueling station fast fill hose to the fueling port and switched the **** from the off position to the fill position. The 1" vent hose released a loud whoosh of air that stirred a bunch of dust off of the ground, and there was a natural gas smell. I even turned the pump on to see what would happen (no one was around, and I was prepared to hit the emergency shutoff), and a bigger burst of natural gas rushed from that hose. At the point I was experiencing the popping noise and the truck wouldn't start on CNG at all, I took the truck to the Ford dealership who adjusted the regulator/fuel rail pressure from 80psi to 110psi. They didn't quite understand how the pressure had gotten so low. I have yet to take the truck back to the Ford dealership now that it has done this strange whoosh during fueling. I have been running the truck on gasoline exclusively since it won't start on CNG at all and the popping and whooshing makes me nervous. However, it might turn out that the regulator/fuel rail pressure has gotten so low that some sort of pressure release valve has begun to vent natural gas out of that 1" hose. Finally, what dumbfounded me the most is why the whoosh would occur during fueling. It seems like the natural gas from the 3000 psi fast fill pump has been escaping out of that 1" vent tube. Why isn't the natural gas going into the cylinder? I might have a malfunctioning solenoid tank valve that causes intermittent availability of natural gas for the engine. Perhaps the natural gas cannot get into the cylinder during fueling because the solenoid tank valve is stuck shut and that tube serves as some sort of safety release system. However, it seems more probable that the solenoid tank valve is stuck open because a closed tank valve should prevent natural gas from getting to that passenger side of the engine, right? Does anyone know anything about the regulator/fuel rail pressure and any possibility that the 1" vent tube serves as a safety release during fueling when the natural gas cannot get into the cylinder?

 

Blumenthol's has had my truck for 3 days at this point, and they decided it might be a relay (I think they said it has something to do with PCM, but I'm not certain). They said they'd have to have the relay sent down from Canada. Well, it's ordered, but I found one at autozone for $18 rated for 40A. It hadn't run on CNG for several days prior to installing it, but immediately after the new relay was in, the truck ran a couple days until the next refuel. After filling it up, not surprisingly, it would no longer run on CNG. No pops or anything during the refuel or start process, but lately when it had been running on CNG, it would fail to run on CNG immediately after a fill up. I have this thought that a sensor sees the CNG pressure too high, and as some leaks out of the tank (I have smelled CNG - not all the time, but often enough), the pressure is in the normal range and the system starts.

I was told to keep the gasoline over 1/4 tank, but this doesn't help. There doesn't appear to be any useful computer codes or anything, and I can't find wiring diagrams or anything else to help troubleshoot. Please share with me anything you run across, and I'll do the same. Thanks

I might add that I've tried to remove and reinstall the relay, but this doesn't do anything.

 

EnergyFuture, my truck is following you on the symptoms. My truck has started swooshing on fillup and when the key is turned to the on position. However, my truck will run on CNG when it's cold outside (lower 40's or below). I've also noticed that when it is running on CNG, if I gradually increase speed on the Interstate, I can go to 70 with no problem; however, if I jump on the accelerator, the system will jump over the gasoline. I was able to pull the truck over and restart it with CNG. I'll have someone look at the fuel rail, but I beleive I feel somewhat like you in that I'm really nevous about the truck spitting out CNG. I can see it blowing up one of these days.

Have you been able to figure out exactly how the valve works or do you have a wiring diagram of the system?

 

I have been just running it on gasoline because I'm worried the Ford dealership will want to replace the Alternative Fuel Control Module (I think that's what corresponds with the Powertrain Control Module). I think I will go see them soon to ask them to adjust the fuel rail again if possible. I'm not certain if that procedure is entirely too dangerous to do myself. The Ford dealership charged $100 to adjust the fuel rail. I'm worried they'll charge me a $150 diagnosis fee. If anyone knows how to adjust the fuel rail or fuel pressure regulator, please explain it here or post a link to instructions.

I would say that my anecdotal evidence indicates that colder weather does improve the performance somewhat. It was the hottest summer I can remember in Tulsa, and I was turning the engine on and off frequently as a delivery driver when the issue first appeared. I haven't had the CNG on for a few months, but my truck seemed to run well enough on CNG during the cool down during autumn. However, it was quite frigid outside when I first heard the whooshing at the CNG station. Therefore, I do believe the weather greatly impacts the performance. Perhaps the cold weather hinders the solenoid fuel valve in the cylinder. I believe the valve heats up as part of the process, but I may be mistaken. The hot weather increases the pressure in the cylinder and fuel lines. I wish I could find some Ford reports on safety features such as a pressure relief valve in the front passenger side of the engine (I've had to assume that the rubber tube that points towards the ground serves as a pressure relief system since pops appear to come from that spot along with natural gas odor). I don't have any diagrams or official publications about the Fuel Pressure Regulator and the possible emergency pressure relief valve. I have however seen a Ford PDF publication that has pictures of the inside of the Fuel Pressure Regulator. I hope we can both figure this out. Good luck!

 

I thought a few pictures would help the discussion.

http://www.gficontrolsystems.com/pdf/GFI%20PDFs.pdf

I hope the link works.

 

I just received this manual on ebay:
2003 Ford F-150 Bi-Fuel CNG Engine/Emissions Diagnosis Manual Original
Paid about $20. It seems to have a lot of good troubleshooting information, but I've been under the weather for the past 1.5 weeks. When it warms up, I'll go through this. I don't recall if this gave the specifics for adjusting the fuel rail pressure, but I'll check it out. I know it does give the pressure range and what the resistance of the sensor should be.

The pressure regulator you sent the link for is identical on my truck. I've been running it on CNG lately, but when I went to fill up a couple days ago, I heard the loud pop when I lifted the handle valve. I have to think something in the pressure regulator is faulty. This truck is probably going to shorten my life by blowing me up one day, but I figured eventually we'll have the problem resovled.

 

This this out.
My 2004 bifuel regulator

This might be the problem for the pops/swooshing sounds. Just FYI, I emailed GFI a while ago and asked if they might have some input given the difficultly we've had in having a shop track down the problem.

I did find a replacement regulator (p/n 3Z-9C968-AB)
2002 Ford F150 - 1921517 Compuvalve vapor assembly - F150 - CNG/GAS