Rebuilt Gas Mass Sensors for Cavaliers

[CNG Utah]

Edit: Due to the decline in demand for gas mass sensors rebuilds are now $150. Contact us at 801-802-6080 if you need one and we will put you in contact with our technician.

Original post:
We've recently found someone who is able to rebuild GM Gas Mass Sensors for us. We've been testing several of them and they've been running flawlessly for well over a year.

We decided this was a good resource that we would like to share with the CNG community, so if you need a Gas Mass Sensor (GMS) for your 1998-2002 Cavalier we should be able to help you out.

The cost is $500 to rebuild your existing GMS (or at least trade us the old one), or $700 for a rebuilt GMS with out a core to trade in.

Order information is here:
http://www.cngutah.com/forsale/gms/main.html

We've also made a short video showing how to remove and replace the gas mass sensor.
http://youtu.be/jAjxwR_CFDo

-Derek

[Highmarker]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Derek,

With an 80% success rate, is there some type of warranty?

[CNG Utah]

Re: Rebuilt Gas Mass Sensors for Cavaliers

If our guy can't get it working we won't charge you, but we do have a 30 day warranty on fixed units.
-Derek

[CNG Utah]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Update:
It's been well over a year now, and our tech has pretty much got the GMS figured out. He has located a few trouble spots inside the unit and replaces those components whether they have failed or not. He also has a known good Cavalier and he road tests every GMS after the rebuild to make sure it is functioning properly. This has pushed the success rate to nearly 100%.
We've had a few customers whose car still wasn't functioning correctly after replacing the GMS, but it always turned out to be another issue somewhere in the system. Now we usually prefer to have our tech put the GMS on the car and test it to make sure it takes care of the problem. We still ship them out of state as well.
-Derek

[smirch9]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Do you offer them for GM trucks? It has the factory converted Impco system. Thanks

[aggiea]

Re: Rebuilt Gas Mass Sensors for Cavaliers

I highly recommend CNGUtah's rebuilt Gas Mass Sensors. If you don't know if your Gas Mass Sensor is bad CNGUtah has a tech that can test it for you. I also wish to add the following information that I have learned on my own and later from information from an authentic repair manual for a '99 Cavalier. First, if you ever disconnect the battery, the PCM and AF ECU both reset themselves making the car only drivable on gasoline. I have found that the AF ECU cannot run its self diagnostics using the ECT sensor without the CNG fuse in place. In order for the AF ECU to run its diagnostics, leave the fuse in place and use an allen wrench the shut off the fuel flow on the CNG tank. This will allow you todrive using gasoline while the AF ECU runs its diagnostics. There is a way to test the ECT sensor using a tech2 tool to ensure the sensor is good. The engine must be at ambient temperature and the ECT must be within 5 degrees f. (3 degrees c.) of the IAT sensor. Next, The o2 sensor must be OEM and must be working 100 percent. The Knock sensor must also be working 100 percent. If either of these sensors is not running 100 percent, the gasoline system may stil run, but the CNG system will not. The knock sensor must be able to advance ignition timing to drive on CNG, or the engine will not run. If it does run, you will get a system too rich code provided the o2 sensor is working 100 percent. The MAP sensor must also be working 100 percent in CNG mode because is must respond quickly due to rapid changes in intake manifold pressure. Next, CNG requires that the spark plugs provide the strongest spark possible. Iridium spark plugs are the best to achieve this; however, the ignition system must be running 100 percent, or the plugs will not provide the strongest spark possible. The coils must be in good condition, or the electronic timing control module will go bad. The electronic control module is also a secondary knock sensor. If the ignition system does not provide adequate power to the electronic timing control module, the vehicle may not run on either gasoline or CNG (it may run on gasoline, but poorly). In addition, a leaking valve cover gasket may allow engine oil to foul spark plugs. The valve cover also functions as a PCV valve functions, as it is a calibrated valve cover. The is a hose on the valve cover that is connected to the intake hose. This hose pulls air from the intake hose, through the valve cover where it mixes with eccess crank cas pressure, then the mixture is runs into the intake manifold through a valve. If this valve fails or functions poorly, the engine will run poorly on CNG. There is also a valve in the intake plastic part just before it reaches the throttle valve. This valve must also rremain unclogged, or the engine will not pull in enough air. On the intake manifold, there is an idle air control valve that also must remain unclogged and working properly, or idle problems will result. Next, the low pressure solenoid lock valve receives power from the AF ECU and grongs on the engine to open the valve. The green wire will only have power during gasoline operation to re-route power from the valve causing it to close. Next, the high presser regulator must not have an internal leak, or too much CNG will make it to the low pressure regulator and flood the engine. If the HPR has an external leak, the engine will run too lean. Do not service this part while the engine is hot beccause it is heated by radiator fluid. The tank has a high pressure lock valve that closes when not in CNG operation. If it does not open, CNG will not make it to the HPR. Before the HPL is the tank pressure sensor, and before that is the tank manual shut off valve. The tank also has a temperature sensor to adjust cng flow according to ambient temperature. This covers most of the system without diagnostic procedures. I hope this helps .

[CNG Utah]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Thanks for the praise and information aggiea!

Smirch9, we can do the GMS for the Impco GM trucks with the aspirated system as well. Just give us a call if you have any questions.
-Derek

[skinney]

Re: Rebuilt Gas Mass Sensors for Cavaliers

the 2003 does not have the GMS?

[siai47]

Re: Rebuilt Gas Mass Sensors for Cavaliers

No, the 2003 is fuel injected and therefore no GMS/MCV is required

[skinney]

Re: Rebuilt Gas Mass Sensors for Cavaliers

what is MCV?

[aggiea]

Re: Rebuilt Gas Mass Sensors for Cavaliers

what is MCV?

Mass Control Valve

[taigacarol]

Re: Rebuilt Gas Mass Sensors for Cavaliers

I am looking to get my son a 2002, is there a certain mileage that these tend to go out?

[CNG Utah]

Re: Rebuilt Gas Mass Sensors for Cavaliers

There isn't a certain mileage when they tend to go out. The main contributing factor that I have seen is how much the car has been run on CNG. I've seen some fleets run their cars almost entirely on gasoline, and those seem to be the ones that have the most issues with the GMS.
-Derek

[mtrimble]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Well, it is good to know about CNGUtah. I just sent them a note. But all the rest you wrote seems like you are trying to scare us off from locating the problem. Too many possible problems! I need to review what is AF and HPR and ECU and HPL! Too many abrievations!

[siai47]

Re: Rebuilt Gas Mass Sensors for Cavaliers

Like any modern car, there are a lot of things that can go wrong but nothing to scare you. AF and ECU are used together as AF-ECU=alternative fuel engine control unit. If you are driving a pre 2003 Cavalier, or any of the older fumigation system vans or pickups, you have one of these to control CNG operation. HPR=high pressure regulator. This reduces the high pressure in the tank (3600 PSI) to a more managable 150 or so PSI. It is used on all the Impco style systems used on GM vehicles. HPL and LPL are lockoff valves for the CNG system. The HPL (high pressure lockoff) valve is inside the fuel tank and is electrically operated to provide fuel to the HPR. The LPL (low pressure lockoff) valve is on the other end of the system near or on the engine to prevent fuel from the other side of the HPR reaching the final components in the sytem, either the LPR (low pressure regulator on a fumigation system) or the LPR and fuel rail (on a fuel injected system). On a fumigation system the LPR reduces the pressure further to about a 7" water column of water, about the same as pressure on your home natural gas system. After the LPR you reach the GMS/MCV (gas mass sensor/mixture control valve). This is the final stop before gas goes into the intake system. It is the rectangular black box on top of the engine in a Cavalier (and on the side on some trucks) that has a large hose going in and a small one coming out connected to the intake pipe. There are two components in this box. first, the gas passes through the GMS section which determines the amount of flow of CNG passing through. This is very much like a mass airflow sensor found on most cars except natural gas, not air is being measured. The MCV component is a small servo driven throttle plate that controls how much gas can go into the engine based on the engine's load and fuel requirements. The AF-ECU controls this throttle plate based on information from the GMS and various engine sensors. In a fuel injected car, the GMS/MCV is eliminated and fuel at a preset pressure is delivered to a fuel rail which supplies individual fuel injectors with natural gas from it's LPR at about 35 PSI. The fuel system uses the same ECM (engine control module) that the gasoline side does with a different fuel map for CNG operation. That it in a nutshell. I'll have to remember to UNA (use no abbreviations)