CNG Cavalier Stalls and Run Rough

[sanderson]

I have a 2000 bi-fuel Cavalier that has been running great both on natural gas and regular gas until this past Friday. The check engine light came on, but the car still ran fine. It was running on regular gas since it was out of NG. I took it to a parts store that was nearby and they read the code on it which was P1215. They told me I needed to replace the computer, but the guy there didn't sound that confident. Someone else there told me to just unplug the battery and it would reboot the computer. After doing this the light went off for about two days and it seemed to run fine. Then my wife was driving the car and it started running rough and died on the side of the road, with the check engine light on again. I disconnected the battery for awhile, connected it again and it started up and I drove it home. Now the light comes right on and stays on after clearing the codes. I tried pulling the 20 AMP fuse to disable the NG to see if it might be trying to run on NG and it still runs the same and I still het the P1215 code. Any ideas of what to check or what this problem might be would be appreciated. The reader I am using says this code is Manufacturer Control Fuel Air Monitoring, but the Chevy site says it is a Ground Fault, but no more details.

Thanks.

[Highmarker]

Re: CNG Cavalier Stalls and Run Rough

The Cavaliers have two computers. One is the main ECU and the other is the alternative fuel ECU (AFECU). It sounds like it might be the main ECU because you have the same problem when running on gasoline.

[siai47]

Re: CNG Cavalier Stalls and Run Rough

P1215 only relates to the AF ECU. This DTC sets when there is a problem with the control module output circuits within the AF ECU. The computer only monitors the ground side of four circuits. The first is MIL control (malfunction indicator lamp). This is working correctly as you say the MIL (check engine) lamp is on. The other three are NGO (natural gas operation) enable, high pressure lock off enable, and low pressure lockoff enable. The DTC sets when the computer comands a ground or open to one of the circuits and does not get the desired result. I am suprised that this DTC is affecting gasoline operation---unless there is a problem with the NGO enable circuit. Failures of the HPL or LPL would just prohibit natural gas operation. The NGO enable is a single wire running from the AF ECU to the PCM (powertrain control module--the "other" ECU). If this wire was grounded somewhere, it could set P1215 and really mess up gasoline operation--even with the 20 amp fuse removed, the ground would make the PCM think natural gas operation was being requested. This wire is pink in color and comes from connector C001 pin #3 in the AF ECU and goes to connector C1 pin #71 in the PCM. This wire should show a ground state when CNG operation is requested and an open (voltage showing) when requesting gasoline operation. Check it first at the PCM. It would be a good place to start. BTW the PCM on the 2000 Cavalier is a bit hard to find--it is in the passenger side front wheelwell area by the frame rail--couldn't be in a worse place. When you are in the area, check the harness for any damage as it runs down behind the radiator and is subject to damage. As a last resort, remove pin #71 from the PCM and see if you can restore gasoline operation. If the AF ECU is showing ground on pin #3 when gasoline operation is occurring, then the AF ECU is faulty.

[siai47]

Re: CNG Cavalier Stalls and Run Rough

Hate to double post, but there is a better way to get to this pink NGO wire. There is a Black 10 way connector located on the top left side of the engine. The NGO wire passes through this connector. It is located in cavity "J". There are two pink wires going to the connector. The other wire is ignition power. They are located on the top row (toward the plastic retainers that hold the connectors together) and are right next to each other. The ignition power is in the center of the row---the wire you are looking to test is the other pink wire either to the left or right of the center of the row depending on which side of the connector you are looking at. With the ignition off and the connector seperated, using a ohmmeter, you should not have a ground at either side of the connector. Another point---If this wire is the problem, if there is CNG in the tank, the car should start and run on CNG but not run correctly on gasoline. You can not run the engine with the connector seperated. If you find a grounded wire on either side of the connector, follow the harness to whichever ECU it goes to. Have someone watch the ohmmeter while you move the harness around to see if there is a wire shorting on something. Hopefully this is somewhat the right track as it is the only track I can think of with the information I have.