2002 Cavalier Problem

[rguzek]

I have a problem with my 2002 cav. The car has around 30K miles. The car only runs on CNG when it's cold, first thing in the morning. It will stay on CNG for ten or fifteen minutes then you feel a momentary power drop and it switches over to gasoline. It only starts on gasoline after that, until the next morning. Any Ideas?

Additionally, I have not been able to get a the CNG maintenance manual for this car. The fleet dealers in my region never return my calls, or follow up. Does anyone know where I can buy one?

[GreenCav]

Re: 2002 Cavalier Problem

According to the manual, some of the situations that will cause an unwanted switch to gasoline are:

1. Fuel pressure sensor indicates fuel pressure drops below 0.85 volts. CNG will not be enabled until FPS indicates fuel pressure is greater than 1.5 volts. (I will assume you have more than enough fuel in the tank)

2. An AF ECU DTC is set.

3. A wide open mixture control valve is detected. I've had this happen going up extremely steep grades at speeds >50 mph. (That wouldn't explain why it wouldn't start back up though until it cools)

I would definitely look at and possibly test the voltage coming off the FPS, it could be faulty. I'd also ensure your 02 sensor is good too, especially since this problem seems to be directly related to the operating temperture of the engine. YOu can test both of these with a simple voltmeter. Any other diagnostic test will probably require one of the special tools that were designed for the cars.

[freedml]

Re: 2002 Cavalier Problem

Helm has it http://www.helminc.com/helm/Result.a...KT19CFSJWL4DFC

[rguzek]

Re: 2002 Cavalier Problem

I took my cavalier into a local shop (not CNG experienced or certified). They said they were confident they could troubleshoot the car. They said their diagnostic computer identified the problem as a "bad low pressure valve assemply". They also said there is a "bulletin" on this problem. They have to order the part and they said it will cost $400-500 dollars to repair it. The diagnosis cost me $68. I'm slightly skeptical, but I'm going to give it a shot and see if they can fix it.

One thing I forgot to mention in my original posting was that the check engine light had come on steady. I didn't associate this with the problem originally because it had been coming on randomly in the past with no apparent malfunctions. The operations manual I have said this indicated an issue with the pollution monitoring or something, but according to the Chevy repair shop this is also tied to the CNG system. I don't have the CNG manual myself though.

[rguzek]

Re: 2002 Cavalier Problem

The local Chevy shop identified and replaced the low pressure lock out valve and this appears to have corrected my problems.