2004 Cavi - Hesitates when on CNG

[Ben Lurkin]

As the title indicates the car hesitates off idle and when accelerating after being completely off the throttle while driving. The problem is intermittent, but more pronounced when the engine is cold. It acts as if it were running too lean and only does it when running on CNG.

Any ideas on this one?

-Lurkin

[Lancer Automotive Group]

Re: 2004 Cavi - Hesitates when on CNG

We had one in here last week with a similar problem, to turned out to be a small hole in the diaphragm, we rebuilt it and now runs wonderful.

[karlhafen]

Re: 2004 Cavi - Hesitates when on CNG

An '04 is port injected. Probably an ignition system (plugs or wires) problem.

[siai47]

Re: 2004 Cavi - Hesitates when on CNG

You need to get to some one who has a Tech 2 scan tool (dealer). The 2004 uses the same ECU for both gas and gasoline fuel. If it is running OK on gasoline and runs bad on CNG, it's most likely in the gas injection system as most of the sensors are shared between the two systems. A couple of things you can check with the Tech 2 are the gas fuel rail pressure and temperature and the injector flow and balance. Make sure the sensors are providing proper inputs to the ECU. If I remember correctly, the fuel rail pressure should be about 40 PSI and the temperature should be close to the air temperature. If something is wrong at the fuel rail a lean or rich condition could occur. You could also have an out of balance condition with the injectors which the Tech 2 can diagnose and might be able to trim. The '04 also has a fuel injector driver module and four cube relays to switch to CNG. A problem with any of these items would be detected on the Tech 2 but you should also have a "check engine" light and no CNG operation if there was a fault. If you live near a dealer that is qualified to work on the CNG Cavalier there should be no problem getting this resolved. If your dealer doesn't know about these cars, order the CNG supplement from his parts department and then he has a path that any mechanic can follow with the scan tool to fix this problem.

[siai47]

Re: 2004 Cavi - Hesitates when on CNG

One more thing---As with any CNG car, good spark plugs are important. The '04 uses coil packs so there are no wires. Change the plugs first and then go to the items I described above if the problem isn't resolved. The only problem with changing the plugs is a bracket that holds the throttle cable needs to be loosened to remove the coil packs. It is really dumb, but is has a nut holding a couple of fuel lines to a stud that holds the bracket. Remove the nut and then loosen the stud that holds the bracket so that it can be rotated forward to clear the coil pack (the stud doesn't need to be removed) There are four bolts that hold the coil pack down to the head cover. Lift the coil pack straight up. If you need to remove the electrical connector, there is a red lock on the connector that needs to be pulled out first before the lock tab on the connector can be pushed in and the connector removed. Use anti-seize compound on the new spark plug threads and some silicone lube on the spark plug boots. Make sure the spring is in place in each of the spark plug boots (it's your plug wire). Tighten the plugs to 13 foot pounds of torque.

[Ben Lurkin]

Re: 2004 Cavi - Hesitates when on CNG

We had one in here last week with a similar problem, to turned out to be a small hole in the diaphragm, we rebuilt it and now runs wonderful.

What did that run? I may have to wait a bit due to Christmas time.

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I changed the plugs and boots when I first got the car; about 5k ago. It was eating cat converters according to GM's service records (#3 in 48,000 miles) and it had a rolling miss. They covered the cat under warranty & said the plugs were good to 100k miles & not to worry. Well, after I changed the plugs/boots; it now runs like it should. It was a couple of months later that the CNG hesitation started showing up.

I'll have to look into the Tech II scanning tool. I've access to the normal OBD II scanner; but there are no current or logged codes.

[Lancer Automotive Group]

Re: 2004 Cavi - Hesitates when on CNG

It would run around $385 to rebuild it that includes flushing the system and cleaning the mass gas sensor as well. We would want to take a quick look at it to is if that is actually the problem, don’t want to pay that much for something that will not fix your problem.

[Highmarker]

Re: 2004 Cavi - Hesitates when on CNG

It would run around $385 to rebuild it that includes flushing the system and cleaning the mass gas sensor as well. We would want to take a quick look at it to is if that is actually the problem, don’t want to pay that much for something that will not fix your problem.

Is there a gas mass sensor on a 2004 Cavalier?

[siai47]

Re: 2004 Cavi - Hesitates when on CNG

There is no gas mass sensor on the 2004. Go to a dealer with a Tech 2. They can check for a misfire while it is running. Failing cats (not the four legged variety ) are a sign of a misfire. You might need a new coil pack or there could be other issues. Check the fuel rail pressure while driving. Also make sure the car has the latest ECU update as there were a couple of SB's out there relating to stalling on deceleration -- might be related.