'01 Cavalier - pressure sensor problem?

[nttrainer]

I just bought a 2001 Cavalier running on gasoline and went to fill it with CNG for the first time. It wouldn't take any fuel. I thought it was a problem with the pump but got the same result on 3 different hoses. I decided that the tank must be full but the gauge is showing empty. As I was leaving the pumps it actually started on CNG and the fuel gauge shot up to almost 3/4 full and I drove about 50 feet and the gauge went back to empty and it switched to gasoline and is showing empty on CNG again. Try as I might I cannot get the red light on the switch to go out and there is no pressure in the line. I have a "donor car" that I can pull parts from to test out any problems. Where should I start?

[siai47]

Re: '01 Cavalier - pressure sensor problem?

Most likely a bad fuel pressure transducer. However, they usually get stuck. It could also be a bad connection at the transducer (unlikely because of its location) or at the firewall connectors or at the AF ECU behind the headlight. I hate to always harp about the Tech2 scantool, but with it you could read the fuel pressure that the ECU is reporting and troubleshoot from there. If you have access to the CNG service manual supplement, there is a chart showing either resistances or voltages from the transducer at various pressures. If I was just shooting from the hip, and had access to a known good transducer, thats what I would change first. It's easy to change even with a full tank. With the ignition off, pull the orange wire single connector at the fuel tank to prevent the High pressure lockoff from opening. You could also remove the battery cable and do the same thing. Next, relieve the pressure in the CNG fuel lines by "cracking" the tube nut at the Low pressure lockoff valve under the hood. This is an intermidiate pressure line so you won't get the 3600 psi blast that you would get if you tried to remove anything from the tank area first. After the lines bleed off, you can safely remove and change the transducer which is attached the high pressure lockoff valve after pulling back the bagging around the valve. Watch for "O" rings and lube them with Vasoline before re-installing them. Completely remove the tube nut from the low pressure lockoff valve and inspect and lube the "O" ring before re-tighening it. Reconnect the orange wire connector and/or the battery cable. If the car then runs on CNG, inspect any fitting area you worked on for leaks. If that didn't solve the problem, the next place I would go is the connectors at the AF ECU. It is very easy to remove after taking the drivers side headlamp assembly out. Make sure they are clean with no corrosion on the pins. Look at the connector area of the AF ECU and be sure there are no flakes of corrosion hanging around the connectors. If your donor car is a 1999 through 2001 you could swap out the AF ECU and see if that corrects the problem. Have fun--I hope this helps .

[nttrainer]

Re: '01 Cavalier - pressure sensor problem?

Thanks for the detailed info. Actually my donor car is an '02. Any difference in that? I've noticed that '02s tend to turn on the check engine light more often

If your donor car is a 1999 through 2001 you could swap out the AF ECU and see if that corrects the problem. Have fun--I hope this helps .

[siai47]

Re: '01 Cavalier - pressure sensor problem?

You can use the pressure transducer from the 2002 for a test. The removal and installation is the same for either year. The AF ECU cannot be subsituted from the 2002 to the 2001. You will notice the engine ECU (PCM) is located on the radiator core support in the 2002 and is a different style then the earlier Cavaliers. The 2002 AF ECU could be re-programmed (flashed) for the 2001 if necessary, but you wouldn't want to go through that for a test. At that point it would be cheaper and easier to get the 2001 scanned with a Tech2 and find where the problem is. You would need a Tech2 and the GM SI system to reflash the AF ECU anyway .

[nttrainer]

Re: '01 Cavalier - pressure sensor problem?

Actually the problem turned out to be less complicated than we thought. Somehow the tank valve was shut. All that was needed was an Allen Wrench to open it back up and everything is working fine.
That fits all the symptoms I described in the initial post - it wouldn't take any fuel (maybe 2 cents worth). It started on CNG after I filled it because that put a little fuel in the line but when it burned that off it switched to gasoline because nothing was coming from the CNG tank. The fuel gauge must be on the line somewhere and not in the tank because it showed CNG pressure right after I filled it but no pressure the rest of the time even though the tank had CNG in it.
Thanks for the input.

[siai47]

Re: '01 Cavalier - pressure sensor problem?

Actually, the pressure transducer for the operation of the fuel gauge is located on the High pressure lockoff valve assembly which is screwed into the tank. There are several ports on the valve including the fill port, discharge port and the port the transducer mounts into that are all connected together. When you fill the tank, the area where all these ports are located is pressurized by the fueling station compressor. The high pressure lockoff valve operates like a check valve allowing fuel into the tank but doesn't allow it out of the tank until the solenoid in the valve is energized electrically. As you found out, the operation can be overridden by the manual valve. If the manual valve is closed and/or the electric valve fails to work, the car will run for a few seconds on the residual CNG trapped in system between the fill point and the low pressure lockoff valve. Glad your problem was easy to fix---I seem to remember someone else on this site that had a similar problem. I don't know why people want to mess with the manual valve unless the vehicle is being serviced .