Engine surging at stop.
 

97 LT1 A4. Idles great when in park and neutral and typically sits around 750-900 RPM. However if I am at a stoplight holding the brake in gear and the engine is around 180-210 degrees, it will begin idling sort of rough and the RPMs will drop by about 200 and then shoot up to over 1000 and the car will jerk forward. This doesnt happen all the time, but it is common. I have no clue what could be causing it. Any ideas?

Re: Engine surging at stop.
 

Engine stock, or modified? If stock, it may be idling higher than spec. Fully warmed up idle programming is 550 RPM in gear, 650 RPM in neutral. Unfortunately stock tach can be high by 100-200 RPM. Only way to know the exact RPM is to read it from the PCM w/ a scanner.

What brand Opti? I've traced some wild surging to a production run of MSD’s from about 5 or 6 years ago.

An erratic coolant temp reading can cause an intermittent surge, since below ~155°F the PCM elevates idle speed. Can go as high as 1,200 RPM at real low temps. Again, dash gauge sensor is not used by the PCM, It reads the one in the water pump housing. Scanner required.

Another possibility is a throttle blade that is not consistently closing. Or an erratic TPS sensor. When the PCM sees the throttle open, it starts to open up the idle air control motor. That can't be seen by a “regular” scanner. Requires one with GM Enhanced Parameters.

Those mostly could cause the increase. The brief drop is more problematic. The running rough may indicate it's caused by a misfire.

Re: Engine surging at stop.
 

Engine is all bolt ons, everything internal is stock. The opti was replaced by the previous owner so I am unsure of the brand but seems to work fine. I just recently replaced the TPS and IAC and that didn't seem to help the issue. It only surges to high idle when the engine is at normal operating temp and the brake is being held while in gear. It has also acted like it wants to stall and die out a few times in the past when taking off from a complete stop. And I have 3 different codes that may or may not have something to do with it. An AIR code, an EGR code, and a p1361.

Re: Engine surging at stop.
 

Was the PCM tuned?

And did you ever solve the P1361 code?

Re: Engine surging at stop.
 

I know the PCM was replaced by the previous owner but I am unsure if it had a stock tune or not.
And unfortunately I never did solve the p1361. I plan on messing with it more next time I have good access to the ICM.