Short supply of diesels has contributed to an 18.8 percent decline in Opel's western European sales in the first nine months of 2000 compared with the same period a year ago.
Astra and Vectra suffered steep declines, with Astra off 21 percent and Vectra 27 percent.
Opel has been hurt by an overall sales drop in its main German market. But lack of diesel engines has affected Astra and Vectra sales in other markets.
'Our capacity at the Kaiserslautern [Germany] engine plant is 300,000 diesel engines,' said a GM spokesman.
But Kaiserslautern only produces direct-injection 2.0- and 2.2-liter diesels. Opel needs more smaller-capacity diesels.
'That need could be fulfilled within the Fiat-GM alliance,' said the spokesman. 'We are considering the application of Fiat diesels for use in Opel models.'
Industry sources have already confirmed that Fiat's new small 1.25-liter multijet diesel will become available to Opel from 2002.
The lower-medium Astra and upper-medium Vectra were also hit by declining sales in their respective segments. The lower-medium segment made up 26.8 percent of the overall new-car market in 1998. That has fallen to 25 percent this year.
The upper-medium segment's share has dropped from 19 percent in 1998 to 15.5 percent this year.
In the UK, where Opels are sold under the Vauxhall name, sales have remained relatively stable. But in September GM's UK brand saw its sales slip from 49,150 to 43,861 units.
Total GM sales in Europe were down by 7.0 percent in the January-September period.Vauxhalls accounted for about 18.7 percent of GM's overall European volume of 1.3 million during the period.
Opel's sales performance is worst in Germany, where the overall car market has fallen by more than 12 percent.
'It is our home market, so in terms of volume it affects the company's overall performance rather seriously,' said the GM spokesman.
Various factors have affected German sales, including high fuel prices and worries about the state of the economy.
'People driving relatively new cars, those with lower and middle incomes with one- to four-year-old vehicles, can easily postpone a [new-car] purchase if the [German] economy starts to hiccup,' said Jeff Hurlbert, vice president sales, marketing and aftersales of GM Europe.
Hurlbert said demand in the UK was weak in September, and that France and Holland were down, too.
Zafira, Opel/Vauxhall's compact minivan, is doing well, with 156,800 units sold in the first nine months. About 60 percent of this volume came from customers who owned rival brands. But Zafira also cannibalized other Opel and Vauxhall models - 10 percent each from Astra and Vectra, and another 10 percent from other models. A further 10 percent of Zafira sales went to people buying it as a second car.
Despite its strength, Zafira dould not overtake the Renault Scenic, which sold more than 260,000 units in the first nine months.
Bradford Wernle contributed