dsky

Engine starts fine – strong. then rough idle.
When in drive, I get rough idle, almost stalling to 1200rpm.
(remove IAC valve connector, runs smooth,
but high rpm's)
reconnect IAC valve,then when driving,
Acceleration I get pinging/knocking, greater with more throttle.
When running steady 50mph, I get stumbling, hesitation
When coasting, it’s almost bucking!

Replaced opti-spark, plugs, wires and water pump.
Tested fuel pressure 42lbs before engine start, 39 when running at idle.
all injectors drop engine speed about 50rpm when disconnected.

cleared fault codes, and nothiing new shows up.

Question: WHAT HAVE I MISSED??????

nobodyunknown

Sounds like a O2 sensor.Check your book you may have more than 1.
Also could be dirty fuel & or air filters.

Goody

You need to hook up a code scanner to see what the engine is doing. Also take another look at your spark plugs. See if the engine is running lean (possible clogged fuel filter). What is the reading from the throttle position sensor?

Nathan Plemons

You say it idles high but smooth without the IAC. Can you drive a car without the IAC connected??? If you can, try unhooking it and see if your problems go away.

YLO Z06

Some possible culprits:

- Faulty IAC valve
- Bad EGR valve
- Spark plugs not fully seated in head
- Plug wires not on Opti in correct order
- Carbon-coated 02 sensors (Mine has 3)
- Clogged fuel filter

LouisBarton

How about squirting some Throttle body cleaner in the IAC's air passage hole with the engine off and the IAC removed? If it got better with the max air bypass (IAC removed),maybe you've got a varnish problem in the air bypass circuit of the throttle body. If you remove the IAC's electrical connection with the motor running you have to reset the ECM after reconnection via a procedure described in the official service manual.

95BLKVette

Highly Recommend Check for codes first, as other Forum Member Suggested, then proceed with troubleshooting.

Lloyd Smale

sounds like your lean change your filter if you havent allready! then I would olm check your injectors and check your fuel pressure