2001 Cavalier problems

[goodforyou]

To make a long story short, our car has problems starting when it's out of cng. It has leaked gas. Check engine light comes on when running on gasoline, but goes off when full of cng. Now the problem is it won't take a fill at all. Not the pump, the other car fuels fine.
We have read many things about solenoid valves, gas mass sensors, etc, but can not find any sites that show diagrams of the cng components or their whereabouts.

Why would the check engine light come on only when it runs on gasoline?

[siai47]

Re: 2001 Cavalier problems

The first thing to do is go somewhere that can put a scan tool on the car and see which DTC's (diagnostic trouble codes) are showing--Autozone, PEP Boys, etc.--can do this, sometimes for free. The ones we are interested in will start with a "P" (powertrain) and have four numbers following. write down the codes and then see if they can be cleared. When the check engine light comes back on, have the codes read again and see if they are the same or different. It is possible that the car will not take a fill because it is already full of CNG, but because of some other problem it will not run on CNG. After getting the codes, post them at this thread, and we will go from there . When you say it "leaked gas" are you refering to CNG or gasoline? As for component locations, we can get to that when we start to nail down what is going on from the DTC's.

[goodforyou]

Re: 2001 Cavalier problems

Well, it's the freaking gas mass sensor (low frequency) P1202.

What now? Is there any way I can get this part somewhere instead of going through the dealer?

[siai47]

Re: 2001 Cavalier problems

The code you have relates to the operation of the rectangular unit mounted on top engine. This unit is a combination of two things, the gas mass sensor and the mixture control valve. It is refered to as the GMS/MCV. The sensor section works much like a mass airflow sensor found in almost all cars except it is measuring the flow of CNG. When the engine's computer requests gas flow, it tells the GMS/MCV how much gas is needed. In response the MCV opens a small butterfly valve to admit the proper amount of gas into the intake system. The GMS reports to the computer how much gas is actually passing through the system and the computer verifies the correct amount is being sent. If the actual amount of gas and the requested amount of gas are different, a code (in this case P1202) sets which disables the cng system. This is usually caused by a failing sensor inside the GMS/MCV. They are not repairable but sometimes they only need cleaning. (Lancer Automotive on this site can do this work). If the unit has failed completely, it needs to be replaced. There have been a number of them on E-bay recently for a couple of hundred bucks. I bought one just for a spare as this is one of the most troublesome components on the '98 through '02 Cavaliers.