McLaren's Kimi Raikkonen led the field in the rain-affected opening practice sessions for Sunday's Belgian Grand Prix at Spa-Francorchamps.
Raikkonen's McLaren has struggled for reliability this season
The Finn set the morning's fastest time of one minute 48.206 seconds, with the team's test driver Alex Wurz second and Renault's Giancarlo Fisichella third.
Renault star Fernando Alonso, who could win the title this weekend, sat out the session to protect his engine.
The afternoon session was called off after torrential rain failed to relent.
McLaren changed the engine in Juan Pablo Montoya's car so he did not run.
Normally engines must last for two races and any unscheduled changes incur a 10-place penalty.
But the Colombian, who won in Italy last weekend, was not punished because the problem was detected before he had gone out on to the track.
Renault, however, may have to replace the engine in Italian Giancarlo Fisichella's car before qualifying, denting the team's hopes of staying ahead of McLaren.
Renault are eight points clear of their Mercedes-powered rivals in the constructors' championship, with four races remaining.
If Fisichella needs a new engine he will lose 10 places on the starting grid.
A spokesman said a problem was detected after Monza but the team wanted to test the engine before making a decision on whether it needed replacing. That plan was scuppered by Friday's rain.
Toyota tester Ricardo Zonta took fourth place in the session, just ahead of Williams' Mark Webber and BAR's Jenson Button.
Michael Schumacher was two seconds off the pace in eighth for Ferrari while David Coulthard did not set a time in his Red Bull.
Overnight rain that continued into Friday morning left the circuit wet by the time the session started.
Many drivers stayed in their garage for much of the session and with 10 minutes left to go only seven of the 25 cars had set a lap time but a flurry of activity came in the closing minutes.
Minardi's test driver Enrico Toccacelo lost the back of his car under braking for Blanchimont and sliding through the gravel before hitting the tyre barrier.
As the track dried out, more cars ventured out of the pits and Christijan Albers spun his Minardi in the same fashion as Toccacelo while Vitantonio Liuzzi misjudged his braking and missed the chicane at Les Combes.
Friday free practice times
1. Kimi Raikkonen (Fin) McLaren-Mercedes 1:48.206
2. Alexander Wurz (Aut) McLaren-Mercedes 1:48.216
3. Giancarlo Fisichella (Ita) Renault 1:48.619
4. Ricardo Zonta (Bra) Toyota 1:49.445
5. Mark Webber (Aus) Williams-BMW 1:49.692
6. Jenson Button (GB) BAR-Honda 1:49.890
7. Jarno Trulli (Ita) Toyota 1:50.542
8. Michael Schumacher (Ger) Ferrari 1:50.564
9. Ralf Schumacher (Ger) Toyota 1:50.820
10. Vitantonio Liuzzi (Ita) Red Bull 1:50.951
11. Takuma Sato (Jpn) BAR-Honda 1:51.003
12. Rubens Barrichello (Bra) Ferrari 1:51.177
13. Jacques Villeneuve (Can) Sauber-Petronas 1:52.173
14. Felipe Massa (Bra) Sauber-Petronas 1:52.236
15. Narain Karthikeyan (Ind) Jordan 1:53.148
16. Antonio Pizzonia (Bra) Williams-BMW 1:53.535
17. Christijan Albers (Ned) Minardi 1:53.807
18. Nicolas Kiesa (Den) Jordan 1:54.437
19. Robert Doornbos (Ned) Minardi 1:54.973
20. Tiago Monteiro (Por) Jordan 1:55.174
21. Enrico Toccacelo (Ita) Minardi 2:02.502
22. David Coulthard (Sco) Red Bull no time
23. Christian Klien (Aut) Red Bull n.t
24. Fernando Alonso (Esp) Renault n.t
25. Juan Pablo Montoya (Col) McLaren-Mercedes n.t