WHAT do you call a car that stalls four times over a weekend? How about Chevrolet Optra Magnum CNG?

The Thai-made Chevy's engine simply goes off like a candle - once while coasting down a slope, once while climbing a carpark ramp and twice at the lights. Each time, you shift to Park, pull the handbrake and crank the engine, which, I am happy to report, fires up instantaneously.

The test car may well be a lemon (now that's a word I rarely use in a review), and it may not be representative of other Chevys. Then again, that can also be said for cars which get rave reviews.

The Chevrolet Optra had such a review when it was first launched in 2004.

And in some ways, the Optra Magnum CNG is still the same car. It is essentially an Optra sedan (or estate) that has been fitted with a conversion kit that allows it to run on compressed natural gas (CNG) as well as petrol.

CNG cars are gradually becoming as hot as Justin Timberlake, thanks to the fact that it now costs $100 or more to tank up a compact with petrol. That and a generous green vehicle tax rebate accorded to CNG cars here.

While most CNG conversions are done here in Singapore, the Optra Magnums are converted in Chevrolet's plant in Thailand.

Because of that, the cars come with a manufacturer's three-year/100,000km warranty - just like any other Chevy. If other Optra Magnums are anything like the test car, you're going to need it.

General Motors says the test car is actually a version meant for Thailand (Thais reading this, take note). So, perhaps the Singapore cars, due here soon, will have better quality control.

Then again, General Motors South-east Asia operations president Steve Carlisle says there has not been any major technical glitches since the bi-fuel Optras were introduced in Thailand two years ago. (In Bangkok traffic, having to restart the engine could well be considered a minor inconvenience compared to, say, having to answer the call of nature.)

Sorry if I belabour the point, but I've never had a car stall on me so many times.

If you accept the irregularity as nothing more than an irregularity, let me say there are worse CNG cars to buy than the Optra Magnum.

GM has done more to the car than merely strapping on a big gas tank and the required piping. It has fine-tuned the engine and shored up the car's rear dampers (to offset the extra 70 to 80kg in the boot).

This has given the Optra rather unique ride and handling characteristics. Despite the stiffer damping and higher laden weight, the car is clearly set up for comfort. And for the record, it gets full marks on this front by going over speed humps like they don't exist and by keeping road noise to a whisper even at high speeds.

But it is wishy-washy when it comes to quick lane changing or simple cornering. You turn the steering in one direction and you feel the car's body literally shifting the opposite way.

Unlike in some CNG-converted cars here, there is no detectable power loss when the Optra is running on gas. However, it does take longer than usual to switch over from petrol mode, which is employed when starting up.

Hence, despite consciously choosing to run on gas over the entire weekend, I had used up one quarter tank of the precious fluid by Monday.

One final complaint: The air-con is weak. For a car originating from tropical Thailand, that is puzzling.

But if you are really bent on cutting your fuel bills or saving the Earth, you really shouldn't be using the air-con in the first place.

This article was first published in The Straits Times on Apr 12, 2008.