2002 Chevrolet Cavalier CNG LPR issue?

[wheelhorse1962]

I am the proud owner of a 2002 Chevrolet Cavalier CNG. I have had this vehicle for approx. 1 year. The vehicle has approx. 28,000 miles. This vehicle on occasion experiences the 6-8 second start time while starting in the CNG mode as documented in the GM service bulletins. I am having issues on occasion that the engine power (RPM) failed to respond to acceleration while driving. I have been fortunate enough to restart the vehicle in neutral while still coasting to a stop. I took this vehicle to an authorized Chevrolet Dealer who has a CNG certified mechanic. In my opinion, the certified mechanic knew less about the CNG vehicle then I do. He was “fishing” and after I spent $450.00 for the dealer to read/detect any error codes, (which he indicated he did not find), updated the vehicles computer, changed 4 spark plugs, one air filter and cleaned the throttle plate bore?, the vehicle still possessed the same characteristic of no RPM’s on limited occasion while driving in the CNG mode. Now the dealer wants to remove and replace the gasoline, (I am not quite sure how the gasoline filter effects the operation of the vehicle in the CNG mode), and CNG filters for an additional $271.00. I have declined any additional work.
I have read numerous postings from other Chevrolet Cavalier CNG owners. This past weekend I disassembled the Low Pressure Regulator (LPR). I did not notice any voids or tears in the gasket element. There were some minor oils/liquids in the bowl of the regulator. I cleaned out the LPR and reinstalled the original gasket; fore I did not have a repair kit. Currently the vehicle starts in gasoline mode (light glowing). I turn the vehicle off then on again and it starts in the CNG mode. As of today, I have not experienced the 6-8 second start up or lose of RPM’s while driving. But I do see that the engine service light is glowing, when the vehicle first starts on gasoline then on CNG. I now believe the vehicle issue maybe with the low pressure regulator. From previous CNG Chat posting, it is my understanding that there is the availability of LPR gasket repair kits. Has anyone repaired their LPR and has the repair corrected the issue concerning the lack of CNG flow to the engine? Also the authorized Chevrolet mechanic stated to me that it is normal for the vehicle to start in gasoline mode when the vehicle has sat overnight or after several hours, is this correct?
Thank you very much for your time and assistance on this issue.

[Highmarker]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

Wheelhorse,

The LPR you are looking for has part number RK-PEV-2. It is $112 from Alternative Fuel Systems, Inc. They are located in Illionis (www.altfuel.com).

[tallan]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

The IMPCO CNG Low Pressure Regulator is classified as a two stage, positive pressure regulator because they reduce fuel pressure in two stages, and supply fuel to the air fuel mixer under a slight positive pressure.
The NG from the HPR enters the regulator through the fuel inlet port at about 150 PSI +/- 20 PSI, it then flows through the primary valve, which is held normally open by the primary spring and then into the primary chamber.
A small port connects the primary chamber and the primary diaphragm chamber.
NG then flows through the secondary valve, which is held normally open by the secondary spring, and into the secondary diaphragm chamber. NG then flows out of the regulator and on to the air fuel mixer through the GMS/MCV ( Gas Mass Sensor - Mixer Control Valve ).
Since the Mixer Control Valve is normally closed pressure starts to back up from the air fuel mixer. When the pressure in the secondary chamber reaches positive 4.5 "wc starts to move upward against the secondary spring and closes the secondary valve. The pressure in the primary continues to build until it reaches about 1.5 PSI causing the primary diaphragm to pivot against the primary spring and close the primary valve. It will stay this way untill the engine begins to turn over causing a negative pressure to develope and then the NG will flow from the secondary side into the intake air stream. IMPCO has a diagram of the PEV regulator on their web site so you can see the process.

Just thought you would be interested in the theory of operation.

[wheelhorse1962]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

Tallan;

Thank you very much for the descriptive information concerning the low Pressure Regulator (LPR). This information will come in handy when beginning the rebuilt process for the LPR that I will purchase from Alternative Fuel Systems, Inc.
In another thread you have written a concern about the Gas Mass Sensor (GMS) and that you have rebuilt/repaired this assembly. Can you please elaborate on what you found concerning the PC board that was defective, i.e. corrosion, bad solder, defective component? After I rebuild the LPR, if this rebuild does not correct the issue, my next challenge will be to investigate the GMS. Within the GMS unit are there tubes of sensors that may become dirty or restricted?
Any and all information will be most appreciated.
Thank you again for your input.

Wheelhorse1962

[tallan]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

The GMS/MCV assembly has two circuit cards in it. One is the AC7-20419 Digital Assembly, I have not found any problems with this PC board yet. The other one is the AC7-17967 Analog Assembly, on this card i have found some bad solder joints. If you take the black cover off you will see the Digital PC board, remove all of the screws then lift the one side of the PC board, it is tied to the Analog card with a flat flex cable soldered on both cards. There are several components that are stuck to the aluminum housing for a heat sink, be careful and work them loose, then the whole assembly will come out. The latest unit I worked on looked like there had been water about ¼” deep sitting in bottom where the flow element sits; it had what looked like hard water deposits and it corroded the hot wire element and the RTD sensor and some of the PC board.
Un-solder the connections and clean up the corrosion and re-solder the components back in place. Use a Ohm meter to verify connections are ok.

On a different unit the GMS/MCV was ok but the butterfly valve was missing the 2 4-40 bolts and it was binding against the housing. If that is the case then clean up the area where it was binding and replace the bolts and use a small amount of lock-tight.

I hope this helps.

[kgjuice]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

Where did you find such a low miles Cav? I'm looking for one...

[wheelhorse1962]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

Hello;

Thank you for your inquiry concerning the 2002 Chevrolet Cavalier. I purchased this vehicle last year from GSA (US Government). The web site is http://www.autoauctions.gsa.gov/
Good Luck on your CNG search.

[azllama]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

Hello,
I have a 2001 chevy Cavalier,when on CNG, it will stall out while driving.The GM dealer replaced one part,now says it the GMS/MCV needs to be replaced at 2900 dollars!!! I can find the cover that you are talking about.Do you remove all the screws that go thru the top of the circuit baord? Also,do you disconect the large cable that goes thru the side of the box and connects to the boards? I tried to remove this cable,did not want to pull to hard,but it would not disconnect.

Also wanted to look for missing/loose screws on the valve,what needs to be removed so I can check out the valve.

Thanks for your help Azllama Phoenix az

[bufordthelittle]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

I bought a 2002 CNG Cavalier last October only to have the GMS go out on it after a month of use. The car had the starting problem described earlier which was no big deal but then it began back-firing and even broke the air filter cover. I have been running the car on gasoline since the dealer part costs $2400-$2700 to fix. I am an Electronics Engineer so with the posts here in this thread, I am thinking about opening it up to fix it. Does anyone know where I can my hands on the block diagrams or schematics or technician manual?

[wheelhorse1962]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

I replaced the gaskets on the low pressure regulator which was not too difficult of a task. I don’t believe this was the issue as my 2002 Chevrolet Cavalier engine still dies while in CNG mode on occasion usually when coming to a stop or slowing down and sometimes while at cruise speed (55 MPH). While still coasting, I can normally restart the vehicle in neutral and continue my driving in CNG mode, but this is becoming very annoying and may be dangerous. Reading the various listing in this forum; I believe the issue is with the mass gas sensor. I am going to investigate the circuit board and connections. As stated in a prior posting, is there a special trick to disconnect the connector from the outside, or is this connector hardwired? Has anyone used CRC mass Air Flow sensor cleaner (UPC 0 78254 05110 9) to assist with correcting a mass gas issue? When I remove the circuit board, I’ll take a few digital pictures and post them for future reference.
Thanks for your help and assistance.

Wheelhorse1962 (one of a few CNG owner’s in Indiana)

[Schase52]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

I cannot be certain that this was related, but I found it odd that since this repair I have not had the car backfire through the intake or stall out on sudden acceleration. I found the my coil packs were not working in the proper power range of 8 - 15 ohms. I pulled these off in the parking lot at AutoZone and had them test the coil packs. Both were operating outside the normal range. $52 and a few minutes later I was on the road and have yet (2 months later) to have an intake backfire or stalling problem.

Again, I cannot be certain they're related, but it is a quick removal of 4 bolts to get the coil packs off and have them tested.

[Schase52]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

About 6 months after replacing the coil packs (6 months with no backfiring through the intake) it started up again. This time I found that I had a very nasty looking spark plug. I replaced my plugs and wires and I'm back to hestitation and backfire free driving!! It seems that the cng system needs to have a quality spark in order to fully consume the fuel that is being pushed into the cylinders. Before spending tons of money on CNG components, be sure to check the coils and plugs. That is all I've done and I've fixed the symptoms that make it appear as a low pressure regulator issue.

[pearceri]

Re: 2002 Chevrolet Cavalier CNG LPR issue?

I bought a 2002 CNG Cavalier last October only to have the GMS go out on it after a month of use. The car had the starting problem described earlier which was no big deal but then it began back-firing and even broke the air filter cover. I have been running the car on gasoline since the dealer part costs $2400-$2700 to fix. I am an Electronics Engineer so with the posts here in this thread, I am thinking about opening it up to fix it. Does anyone know where I can my hands on the block diagrams or schematics or technician manual?

If all else fails, cng Utah will do an exchange on a rebuilt gms for $500, or at least that was the price the last I checked.

Good Luck,

Bob