View Full Version : Stumble when throttle blipped

28th December 2000, 11:44
Just wondering if anyone else has had a problem with a stumble when the throttle is blipped after changing the engine specs due to a rebuild or cam change etc.

In my case I have had the engine rebuilt and the only significant changes were head work and cam changes. The cams were based on Sti ver 3 specs, but upped a bit in lift and a smaller bit in duration. The engine idles happily at the stock rpm of around 800 - 850, on the same injector open times as pre-rebuild. This surprised me, and shows the cams can't be THAT different. I have tried playing around with the PUMP and OFFIDLE settings on my aftermarket ecu, with no success. The PUMP settings are perfect in every other aspect, i.e. smooth crisp acceleration with no stumbles as long as the rpm is above idle. If I open the throttle slowly when idling, it doesn't stumble.

Has anyone else experienced this? Do you think increasing the idle speed might cure it? Before the rebuild I never had this problem. Is it something I'll just have to put up with for the sake of more power? Some people have suggested that problems like this stem from crappy injector spray patterns and the like. What about fuel pressures? Any ideas?

Bob Rawle
28th December 2000, 18:51
Sounds like you don't have enough fuel in the zones bordering the idle area, the accel (or pump) should not be used to compensate for a lean mixture.