Fluctuating Idle

[Decolliber]

After installing all new fuel injectors in my 88 Carrera, the engine runs much better. But when starting cold, the idle fluctuates between 900 and 1300rpm, though once warmed up the idle is a steady 950. I can understand why more efficient new injectors would raise the idle, but why would they make it fluctuate? I assume the appropriate solution is to to turn the idle screw clockwise to lower the idle. Is that correct? Since I just had the idle mixture adjusted (before new injectors) I wonder if I should have that redajusted.
In general, what causes an engine's idle speed to continually rise and fall? Why would new injectors make it do this when it was not doing it before?

[Tyson Schmidt]

Leanness is usually the culprit of idle instability. Bad injectors can often dump too much fuel. You may just need to drive the car a while to let the computer relearn the proper parameters with the new injectors.

There are too many things that can cause this. Vacuum leaks etc. Make sure the intake manifold bolts are all snug. The gaskets will shrink after a time and cause minute vacuum leaks.

Just for fun, you might try resetting the computer by disconnecting the battery for 30 seconds or so. Then take it out for a good half hour test drive with lots of different types of driving. But mostly city, and idling, etc.

[jtratza]

It can also be fluctuating if its running rich (like mine was when we set the mass flow sensor to run rich and I reconnected the O2 sensor causing it to run even richer.) Try disconnecting the idle control value when it is fluctuating. The control value may be gummed up or shot which seemed to be another problem I was having with the idle.

[john walker's workshop]

850RPM hot, is about all the idle speed the computer can handle, without trying to correct it repetedly. set it there and see if the warmup period is smoother.