2000 Cavalier runs badly on CNG

[PaulReeder]

Hello All-
My 2000 Cavalier runs badly on CNG.
With the engine cold, it starts OK on CNG and drives somewhat OK. But when the engine reaches operating temperature, it stalls and can sometimes be restarted. If I can restart it, the car runs badly, and quickly stalls again, acting like the fuel was shut off completely. I pull the CNG fuse at that point. The car will then run very well on gasoline.
Other info: The pressure before the LPR stays around 125 PSI, so I believe the HPR is OK. I rebuilt the LPR, but the symptoms are still the same. When I bought the car last November, it ran fine on CNG. I was unable to refuel the CNG until April, at which time the bad performance started. I have no engine codes.
Any advice is appreciated.

-Paul Reeder

[Insight]

Re: 2000 Cavalier runs badly on CNG

I had a problem that sounds similar when I rebuilt the LPR on my 02 Cav. I recieved the repair kit without instruction sheets and did not think about the way the LPR functions. It's a man thing I guess to dive in. The seal came to me as a straight piece of metal. You are required to bend the seal arm to maintain the proper spacing between the seal and the bottom of the regulator. Do you have these instructions?

When I had left the seal straight the car started, but then chugged, sputtered, and lurched when running on CNG. The seal was closing and staying closed until the pressure built back to full and then would pop open supplying gas flow agian. Seal up as flow started and repeat the cycle. Ran great after I got a copy of the instructions and bent the seal arm to be similar to the arm I removed. Let me know if you need the instructions. I have them here somewhere.

[PaulReeder]

Re: 2000 Cavalier runs badly on CNG

Yea that man thing has got me in trouble more than once.

I did have the instructions that came with the kit and I bent the arm as shown. The picture was not very clear but I think I understood what they wanted. "Lever pin 3/32" below the top of the casting".

That said, when I compared the newly bent arm with the old one, there was a difference of about 1/4". A lot, I thought, but I went with the instructions.

Maybe I'll take a second look at that and bend it similar to the original. Not much to lose at this point.

Also, I found the insides of the LPR to be lightly coated with some type of oily film. I say oil but I'm not really sure what it was. It did not smell much like anything I could name. Was yours dry?

Thanks for the reply. I got the car. I got the Phil. Now I need to get this thing working!

[Insight]

Re: 2000 Cavalier runs badly on CNG

Oil on the diaphragm is from you CNG source. This is compresser oil from the well head and your local compression or the previous owners. There is a filter in the high pressure system to capture some of the contaminant. I would not worry about a minor film in the LPR.

3/32" sounds right. However, I did not measure for the 3/32", I bent it to be identical to the removed part since it was running well previously. I had a small hole in the diaphragm that initiated the repair. I did find the email with the instructions from AltFuel if anyone needs the instructions. Let us know how it works out for you.

[siai47]

Re: 2000 Cavalier runs badly on CNG

I looked at this thread when you first posted it. I still don't have a good answer or even a good place to start. You say it starts OK when cold and drives "somewhat" OK on CNG. When it first starts, does it idle OK? If you don't drive it and let it get to operating temperture does it still idle OK? You state you have 125 psi at the LPR inlet. Is this static pressure or are you able to monitor the pressure when driving? Do you have the service supplement for the car? There are two other pressure checks that can be made to verify the operation of the LPR. When you are having trouble re-starting it, does the fuel changover to gasoline occur after eight seconds of continious cranking? Although it sounds like a fuel, not ignition, problem it might still be wise to check the plugs and wires. The problems you are stating should set some codes unless the AF ECU is acting up or a sensor is feeding the ECU wrong information. What type of scan tool did you use to retrieve the codes? Is there any way you can beg, borrow or (well you shouldn't) steal a Tech2. With that you could look at the data from the ECU and see if it makes any sense. I hope that over the Winter what happened to the AF ECU in my 2000 Cavalier didn't happen to your vehicle. (see Cavalier AF ECU failure) .

[PaulReeder]

Re: 2000 Cavalier runs badly on CNG

Hello saia47-

When it first starts, does it idle OK? If you don't drive it and let it get to operating temperture does it still idle OK? No, it will die. It does restart, but will idle (& some revving) for about a minute, then dies. Lather, rinse, repeat.

You state you have 125 psi at the LPR inlet. Is this static pressure or are you able to monitor the pressure when driving? I did not have the gauge connected while driving, but the 125 psi stayed constant (+- 10 psi) during the above scenerio. And the pressure drops off after the car quits and the solenoid valve closes.


Do you have the service supplement for the car? I bought a used Helms off EBAY and expect it any day.

When you are having trouble re-starting it, does the fuel changover to gasoline occur after eight seconds of continious cranking? I don't crank it that long. I do monitor the LED on the rocker switch.

What type of scan tool did you use to retrieve the codes? I have a PC based system, "Car-Code", but I cannot read AF ECU data. I'm looking at this EBAY item #220385133834 if I can't get "Car-Code" to work. I am not getting any codes. Is there a scan tool you might recommend?

I hope that over the Winter what happened to the AF ECU in my 2000 Cavalier didn't happen to your vehicle. (see Cavalier AF ECU failure)
I bought the car (49K miles) last November & it purred on CNG. It was driven all winter here in Ohio, but not any CNG operation. When I finally refuelled the CNG, my problems started. I read your post with great interest, and (Shooting in the dark here) will take a look at that ECU.

Thanks for your help,
-Paul Reeder

[siai47]

Re: 2000 Cavalier runs badly on CNG

OK, lets try to nail this down. The fact that you have 125 psi at the LPR and you rebuilt the LPR (which didn't effect the problem) would make me think that NG is getting through the system. At this point, the only thing between the engine and the gas it needs is a little throttle valve in the MGS/MCV. This is the little retangular box connected to the LPR. This is also the point at which a scan tool (a GM Tech2) is necessary. Code readers are designed to read DTC's (diagnostic trouble codes). Some will also read data streams that are common between vehicles using the OBDII standard. As you go up in cost, scanners will read the OBDII codes plus manufacturer specific data streams. The problem is, even though most high end scan tools could read the GM AF ECU data, nobody has ever programmed one to read it due to the limited number of vehicles out there. The item you are looking at on E-bay will not do the task you are looking for. All GM dealers have the Tech2 and a lot of independent shops also have one. All Tech2's can read the GM alternative fuel data. When you get to someone who has one, go to the data page for the alternative fuel system. There are two pages for the Cavailer (I believe this is on either page). Look for the fuel flow data. This will be natural gas flow expressed in grams. Compare the requested flow (from the ECU) with the acutal flow (as reported by the MGS) These two should match and track one another. Also look at the fuel trim data (short and long term) to be sure it is not something way out of spec, like -20. This will let you know if the MGS/MCV is working correctly. Make sure additional data like engine temperture, air temperture and barometric pressure are correct.

Go over the basics one more time before paying someone to use the Tech2. Make sure all the connections in the induction system are tight---all the way down to the throttle body on the back of the engine. Make sure there are no cracks in the hose that connects the LPR to the MGS/MCV and the hose that connects the MGS/MCV to the gas distribution adapter (intake pipe) is also free from cracks. Finally, change the spark plugs and wires. Make sure the wires are in the clips properly and that there is wire loom around the bundle of wires that pass near the engine lift ring on the drivers side of the engine. Good plugs and wires are very important when running on CNG as the firing voltage is about double the requirement for proper running on gasoline. My misfortune with the AF ECU failure happened quickly--total failure within a couple of days of first having a problem. If you find the problem and it is not a failure of the AF ECU, I would still remove it and seal the edges of the housing to prevent any further damage to the PC board inside from corrosion .

[PaulReeder]

Re: 2000 Cavalier runs badly on CNG

Hey siai47-

Much obliged for the thorough reply. The proper scan tool sounds pretty cool. I'll definitely look into that. BTW, I got the manuals today. Nothing about AF. Let me guess: there's a supplement..
Quick update: I dug into the drivers side headlight in search of the AF ECU.
Not a good find. The ECU mounting bracket is supported by 3 rubber mounts. One was broke, the 2nd had no nut, and the third was doing the best it could. But not enough. The ECU was flopping around, not in its bracket. Apparently the car had some body work (Lucky me) done in that area and whoever worked on it did not secure things properly. And he left a nice 1/4" drive Snap-On ratchet.
The ECU had some NGWCP (Not Good White Corrosion Powder) fall out when the connectors were disconnected. About half of the ECU screws are frozen. They are soaking now with a penetrant and the CNG saga continues tommorrow.

Thanks Again,
-Paul Reeder

[siai47]

Re: 2000 Cavalier runs badly on CNG

Yes there is a supplement. If you can't find it on E-bay it is still available from www.helminc.com for $35.00. It is invaluable when working on these cars. If you are into the AF ECU mount, its time to take the bumper cover off to make things way easier for you. You are doing better then I was if only half the screws are frozen on the ECU. All of mine were frozen and after soaking in "Breakfree" for a couple of days, none came out. I took a Dremmel tool and cut a groove in each screw head and used an impact screwdriver to remove them. The circuit board itself had a good conformal coating on it, however there was a lot of aluminium oxide laying around the bottom of the housing near the connectors. To remove the circuit board, remove all of the screws in the board except the two that have the coating covering them. There is silicone holding a heat sink in the middle of the board to the cover but a slight tug on the board lets it come off. I cleaned up the board, blew out the area between the board and the connectors with air, and reassembled. Go to a Yamaha motorcycle dealer and get a tube of "Yamabond" sealer. It is a grey, thin sealer used in motorcycle crankcases. It is perfect to spead on the flanges of the cover and body of the ECU housing to keep the salt crud from reaching the "O" ring seal. If this had been done by Impco we wouldn't be going through this drill. BTW---now that you know there has been body work in the area of the ECU, check all the wire harnesses in the area (including the wires and connectors to the four relays mounted above the ECU as they are part of the CNG system). Mine hasn't been in an accident, but the harness loom was chaffed through right behind the headlight.

[Insight]

Re: 2000 Cavalier runs badly on CNG

Sounds like you are on the right track. I had a similar issue on my 02. Computer worked below 50 F but shorted out above and did not work at all. White aluminum oxide everywhere in board. Milled off all of the bolt heads, but can't see any way to repair the unit. I was not able to revive and have yet to find the right model year AFU. It is a pain to find a dealer in Tulsa to flash the unit I got from a 99 on ebay. I have been getting the big blow off since it is a CNG car. Car runs, but I have the check engine light on when the car is turned off. I am with saia that it is not the LPR at this point and I am curious about the impact of the HPR filter on running poorly if fouled. You did describe oil of some kind on the LPR diaphragm you replaced. I am thinking this is a potential problem for me as well since I am experiencing a chuggle when at high speed and steady foot pedal. The supplement is a must have for diagnosing problems with the CNG system. It has a great deal of good information.

[siai47]

Re: 2000 Cavalier runs badly on CNG

Just about any AF ECU from 99 through 2002 will work. There are some changes throughout the series but they can all be reflashed. The 98 unit is not OBDII compatible and after 2002 there is a whole new system which eliminates the AF ECU. Any GM dealer can flash the ECU. I had one done at a dealer that had never seen a CNG anything. Take the car to the dealer, he will hook up the Tech2 scan tool to the car, connect the scan tool to a internet based computer. Then through the GM service information system, the scan tool will send VIN information to GM which will in turn send the proper calibration for the computers in your car. When they re-flashed my ECU they found an update for the gasoline ECU and applied that also. Total time 30 minutes, total cost $37.00 .

[PaulReeder]

Re: 2000 Cavalier runs badly on CNG

Hi Guys-

I opened up the ECU and found lotsa corrosion, as saia47 has written about lately. I washed the board, reassembled with a new o-ring & RTV. Then I fixed the rubber mounts, installed the ECU & crossed my fingers. I started the car on CNG. It ran, but did start stalling after about 5 minutes. Acting pretty much as before.
But.
The next day I was trying to access the AF ECU with my laptop and had the car idling for about 10 minutes. No stall. So, what the heck, lets drive it. It ran great! I thought it might be on on gasoline, but no, it was purring on CNG! Stops & starts, 60 mph. no stalling at intersections.
But.
After about 10 minutes of road work, it balked after waiting for a traffic light, but did not stall. I drove back to my house, pulled in the driveway and it stalled. Restart, run a few seconds stall. Crap.
So, whatever the issue is, it's intermittent. A mechanics nightmare.
As I said earlier, I was trying to access the AF ECU data. I'm not sure that I did, but my Short Fuel trim was from (-4.68) to (+7.03). averaging 2.36.
The long term trim was from (0.0) to (+17.19), averaging 5.12. I could not see anything about fuel flow, grams or otherwise. Frankly, I don't really understand what it's about, but I've emailed the "Car-Code" guy for help. See OBD-2.com. You're right, saia47, about stealing a Tech 2. I had no idea they were gold!

I need that supplement. Helms does not show it. Anybody know where it can be had?

Thanks,
-Paul Reeder

[siai47]

Re: 2000 Cavalier runs badly on CNG

Hopefully you can get somewhere on this one. I don't know how the data is shown on the PC based scantool you are using, however it appears you might be getting some AF ECU data, possibly not. What you need to view is air and fuel flow data. There are three of these data points visible on the Tech2. All are expressed in g/s (grams per second). First is airflow, next is fuel flow requested and finally actual fuel flow. With these pieces of information you can look at the air/fuel ratio to be sure the engine is running around the 16:1 ratio needed for CNG operation. In addition the requested and actual fuel flow data will let you know about the health of the CNG gas mass sensor. If this information is correct---the car will run properly on CNG.

As to Helm, you are right, I pulled up their website and no mention of CNG supplements. They had them a couple of months ago for $35. You might want to give them a call anyway and ask. 1999 to 2001 supplements will work for your particular car. In a pinch the 1998 supplement will work, but will be lacking much of the OBDII information as the 98's were not OBD compliant.

[PaulReeder]

Re: 2000 Cavalier runs badly on CNG

Thanks siai47 (not saia47).
Do you think a 1999 Supplement would work for my 2000?

[siai47]

Re: 2000 Cavalier runs badly on CNG

Yes it would