2000 Cavalier CNG shows empty when full -- won't run on CNG

[jimcive]

I recently purchased a dual-fuel 2000 Cavalier. it has about 74,00 miles on the odometer, so a 8 year 80k mile warranty on the CNG alternate fuel engine control module (AFECM) might still be in effect.

Initially after I bought the car, it ran well on CNG, but suddenly the car began running only on gasoline.

I can see by holding down the fuel toggle switch (on the dash) that the CNG gauge is reading empty, when in fact I just filled it (topped it off) from a 3,600 psi CNG pump -- so it is full.

Is it possible that some sort of CNG tank pressure sensor or a relay is not working properly and so the electronic engine control module thinks the CNG is empty and so is running the car exclusively on gasoline?

One mechanic looked at the car, although he didn't have GM/Chevy diagnostic equipment. He was able to tell me he observed 0.49 Volts on the pressure sensor and temp sensor read 73 degrees. The 5-volt reference checked out OK. The ground circuit was OK. He jumped the ground and signal circuits and measured 0.49 V on the AFECM. He checked the ground and signal circuits for shorts -- none were found.

What are your thoughts on what might be wrong and how to trouble shoot the problem?

The car is in Provo, UT, near BYU. I checked with the two local Chevy dealers in Provo and they don't have a CNG/Alt fuel certified mechanic.

The GM/Chevy web site lists the following Salt Lake City GM/Chevy dealers as having CNG mechanics: Jerry Seiner Chevrolet, Salt Lake Valley GMC, Young Chevrolet. Any advice on which one to send the car to for service? Do they all have certified CNG/alt fuel mechanics and specialized diagnostic equipment?

[UtahGasBill]

Re: 2000 Cavalier CNG shows empty when full -- won't run on CNG

Is the check engine light on? Gus Paulos has a very good tech when it comes to these vehicles.

[siai47]

Re: 2000 Cavalier CNG shows empty when full -- won't run on CNG

If there is no "check engine" lamp on when running on gasoline, is the little red light illuminated in the "fuel" switch? If it is not, does it illuminate when you first turn the key on or during engine cranking? If it never lights, check the CNG fuse under the hood. If the fuse is good, check the fuel pump relay. If those two items are good, then the next thing may be the expensive AFECU.

[jimcive]

Re: 2000 Cavalier CNG shows empty when full -- won't run on CNG

To UtahGasBill: The check engine light is off when the car is running.

To siai47: When running on gasoline, the little red light is illuminated in the "fuel" switch. I infer from your response that since rocker-switch light is illuminated, as is normal when running on gasoline, that the CNG fuse must be OK and so does not have to be checked. Further, you deduced that the information presented is pointing to the need to replace the AFECU (Alt. Fuel Engine Control Unit?)?

It looks like I should get the car into a dealer ASAP in case the 8/80,000 warrantee still covers this part.

[CNG MOTORS]

Re: 2000 Cavalier CNG shows empty when full -- won't run on CNG

not sur eif this year has the fuel lock off fuse if it does it's under the hood on the drivers side next to the CNG fuse. Check both fuses, take 30 seconds to check before you spend several hundred on a diag. I put my money on the toasted lock off fuse.