Bad gas mass sensor on Cavalier?

[kgmc]

I've been told my 2001 Cavalier with 100,800 miles needs a new gas mass sensor ($2,600). Though I have no reason to doubt the mechanic, I hope he's wrong. Can anyone advise me whether this diagnosis is consistent with the following: I drove the car a couple times about a mile before it began losing power and barely chugging along. It would die and maybe eventually re-start. I pulled the CNG fuse, and it worked great. I know nothing about the mechanics, but these codes, etc., are from my receipt: Gas ECU P0300 and AF ECU P0134 and PI202. The gas mass sensor circuit test found 192 hz on the actual sensor circuit and it should be over 250 hz. The signal dropped at higher rpms, so conclusion was that it's a fault sensor. Picked up the car last night and it seemed to run fine today, about 150 miles on cng. Thanks!

[Lakewood90712]

Re: Bad gas mass sensor on Cavalier?

Siai47 , the CNG-GM vehicle guru on this site will have an answer for you for sure. He may be on vacation , so keep an eye on this thread for a few days. As to the part cost , it is a very expensive indeed.

[siai47]

Re: Bad gas mass sensor on Cavalier?

Wouldn't say that it is the GMS, but it could be. The codes you are seeing could also come from low fuel pressure caused by a number of things from the little filter in the high pressure regulator to issues with the low pressure regulator. The frequency (HZ) thing is a little confusing---all it is saying is that the GMS isn't seeing the amount of requested fuel (requested by the AF ECU) passing through the sensor. If your mechanic can scan the AF ECU, then the thing you want to look at is the requested gas flow v.s. the flow reported by the GMS. These two numbers should track each other. Sometimes the car sets a low frequency code when first trying to start as there just isn't enough NG arriving at the GMS during cranking. In any event, two things. Usually the GMS doesn't cure itself--so if it is running OK one day and not the next--it may be a fuel pressure issue. The other thing is that a GMS/MCV just showed up on E-bay---and if it could be bought right, it wouldn't hurt to have one on the shelf just in case. BTW--I am out of town and just got on a laptop tonight. If you have, or can get a copy of the service manual supplement for any CNG Cavalier from 1999 through 2002, the procedures for checking fuel presures can be used for your car. It is important to check the intermediate fuel pressures (at the entrance to the low pressure regulator) with the car under laod. Also, follow the flow charts in the manual for diagonosis of the codes you recieved.

[kgmc]

Re: Bad gas mass sensor on Cavalier?

Thank you so much for the information. I plan to print this out and take it to another mechanic..... But I might wait to see if it's a problem again. So far, it's running fine on CNG and we've driven probably 400 miles since the mechanic said the GMS was bad.

[01cavalier]

Re: Bad gas mass sensor on Cavalier?

I also have a 2001 cavalier that came with the CNG option. But since there was no fueling stations nearby I took the tank out and that was fine for several months and now I'm getting the p1202 code and my Check Engine lights on. Any idea how to clear this code. Is there a sensor I can just unplug or bypass somthing that will make the CE light go off? Thanks

[siai47]

Re: Bad gas mass sensor on Cavalier?

Removing the fuse in the underhood fuse box marked "CNG" will turn off the power to the AF ECU and prevent any codes from being sent by the CNG fuel system. After you remove the fuse you can disconnect the battery for a couple of minutes, which should clear the code, or go to a auto supply "box" store and have them clear it with a scan tool.

[rhiatt]

Re: Bad gas mass sensor on Cavalier?

I had a really similar issue,
I was running fine on cng and it slowly puttered along, the check engine light came on and then it died. I pulled the fuse and it ran fine. The code that I got from a scanner was p0134. Ive been on a wild goose chase ever since, I replaced the O2 sensor, plugs and wires(which it definatly needed), I even borrowed a gass mass sensor from a guy and tried to see if that changed anything, which it didnt. I've torn the bumper cover off, and all plugs, the AFECM, or any mounts have any corrosion. I have no clue where to go from here, any suggestions?

[siai47]

Re: Bad gas mass sensor on Cavalier?

Before you go chasing any more wild geese lets try to nail down the bad goose. P0134 can be set by either the PCM (gasoline) or the AF ECU (CNG). In either event, the problem is the same and relates to activity of the pre-cat O2S located in the exhaust manifold. In this case the AF ECU works in conjunction with the PCM to monitor the O2S operation and if they don't see a change of voltage from the sensor, the MIL will be set. My first question would be can you operate the car on gasoline only (CNG fuse pulled) and drive normally without seeing any codes? What brand of scan tool were you using? Most are blind to codes coming from the AF ECU so I am thinking the stored code you found was coming from the PCM. This failure should cause the AF ECU to go from closed loop operation (normal) to open loop operation. This change should not make the car die or have serious performance issues---it should just have a check engine light and continue running. I think something else is going on in the AF ECU that you cannot detect without a GM Tech2 scantool and the code you are seeing might be a secondary result. If you are using a scan tool (not a code reader) take a look at the O2S signal voltages when on gasoline operation and see if they are changing when you open and close the throttle. Also check the loop status and be sure you are running in closed loop.

[rhiatt]

Re: Bad gas mass sensor on Cavalier?

Car runs great with the cng fuse pulled and has no check engine light, when I put the fuse in the check engine light comes on and the car will lose power quickly and after about two minutes it is all it can do to stay running. I had auto zone scan the car so I'm guessing it was a cheap scanner. I did just replace a valve cover breather hose that was bad and I left the battery unplugged to clear the code. I haven't tried it again yet.

[siai47]

Re: Bad gas mass sensor on Cavalier?

Sounds like a O2S problem when running in CNG mode. Two minutes is the exact amount of time the car runs in open loop (fixed fuel flow maps). After two minutes the AF ECU switches to closed loop and at that point the oxygen sensor is used to control air/fuel ratio. However, a problem in closed loop operation should cause the AF ECU to revert back to open loop so you can continue to drive. When you replaced the oxygen sensor what brand did you use? Bosch sensors (popular at big box auto supplies) just won't work on a GM CNG system. I found that out the hard way--they work fine on gasoline but don't have a quick enough response time to work with CNG. A GM or a Denso oxygen sensor (you can find them cheap on Amazon) work fine. Without a Tech2 that can read the data from the AF ECU you are stabbing in the dark about this problem. The low cost code readers may not be even able to erase fault codes (DTC's) stored in the AF ECU. However, a couple of minutes with the CNG fuse removed should clear them. Try to find a friendly dealer or local shop that has a Tech2 and look at DTC's in the AF ECU and look at the fuel flow, fuel trim, air/fuel ratio and O2S activity as reported by the AF ECU. This should help locate your problem. Until then, make sure you have a GM or Denso oxygen sensor in place--if not replace it.

[rhiatt]

Re: Bad gas mass sensor on Cavalier?

well, bosch is exactly what I put in. Im a rookie to cng so I guess ive learned the hard way too. well, that helps a ton, I will post after I get around to switching it out. Thanks!

[rhiatt]

Re: Bad gas mass sensor on Cavalier?

Just put in a denso o2 sensor, I had to wire it in but the colors were different, I didn't think it was critical which way they went. Anyway I'm dealing with the exact same problem with the new sensor. Does anybody know someone who wants a cavalier? This car is starting to get the best of me.

[reclineaholic]

Re: Bad gas mass sensor on Cavalier?

Just put in a denso o2 sensor, I had to wire it in but the colors were different, I didn't think it was critical which way they went. Anyway I'm dealing with the exact same problem with the new sensor. Does anybody know someone who wants a cavalier? This car is starting to get the best of me.

Yep, that car is probably no good anymore but I would give you a few hundred dollars and take it off your hands, kind of as a favor you know....

[siai47]

Re: Bad gas mass sensor on Cavalier?

You should have been able to get a Denso sensor with the correct plug on it. You could be still chasing the same problem. At this point, you need to get the car to a Tech2 scan tool and look at the O2 sensor response when running on CNG along with the fuel mixture and fuel flows to spot the problem. Never let the car (however evil) get the best of you.

[nttrainer]

Re: Bad gas mass sensor on Cavalier?

Actually, I bought the car from him and put in a stock O2 sensor from a wrecked bi-fuel Cavalier and it's working great.... Thanks rhiatt