Ok, so here's the deal. It's a new engine and I'm still working out the bugs.. so to speak. I go out today to attempt to set my minimum idle air so I bump the idle up a bit before I start the car. I start the car cold and it starts to surge back and forth between 1k and 1100 rpms... not a slow surge, pretty quick actually... back and forth. So I press the gas a bit and hold the idle at about 1800-1900 rpms and it idles fine... I let off and it starts idle fine.. well, a little rough but didn't surge. As the engine warms up it starts to idle smoothly at about 1200rpms so I'm satisfied. I put my foot on the brake and put the car in reverse. The car immediately drops to 500rpms and idles like **** for just a few seconds and then immediately jumps to 800rpms and idles great... nice and smooth. I shut the car off for a minute and then restarted it.... it began the surge again.. with the car running about 165 degrees so I got confused. I revved the car toa bout 1900rpms again and held it for about 20-25 secnds and then let off and it started idling smootly at 1200rpms again. Very confusing.

Question one.... what could be causing the surge?? IAC maybe?? when I paper clip the ALDL box and turn the key on, it sounds quite sickly. But at the same time I didn't think the IAC really did anything once the engine got to operating temperature which it was the 2nd time I started it.

Question two... what could be causing the sudden drop to 500rpms for a few seconds before it jumps back to 800rpms and idles fine in gear?? IAC again possibly??

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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.

 

TTT

 

How did you adjust the IAC and tps? Is everything clean and lubed inside the throttle body? How is fuel pressure?

Normally an idle surge is caused by a vaccuum leak, but normally that is pretty obvious. A fuel delivery problem could also cause this, as could an ignition problem. Check and see if your plugs have gotten fouled, they may have before you set the timing. A loose connection from the cap to each plug may be the culprit as well, so could a bad/clogged injector.

 

I haven't adjusted the IAC and TPS just yet. However, I didn't touch the TPS since it was on my 305 and it was find when on the 305. My fuel pressure is at 40psi with 24# injectors and the stock 305 chip. I believe it's running rich.

As far as I can tell there are no vacuum leaks. As I said before, my fuel pressure may be a little high causing the surge. It is possible that my plugs are fouled or a wire isn't propertly connected. The injectors are clean and flowmatched with brand new screens and o-rings. I doubt that's the problem.

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1986 Trans AM
355 TPI
4 Wheel Disc Brakes
9 bolt Borg Warner Rear (2.77's....oh joy) :P
6" rods, approx. 9.7:1 Compression, Stock TPI (soon to be ported), 24#/hr LTI injectors (cleaned and flowmatched by Cruizin Performance), Hedman Shorty Headers, 3" exhaust with 80 series flowmaster, SBC 993 heads completely redone and modified for 1.6 RR, 1.52 RR (for now), Comp Cams XE262-14 TPI Cam, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Ignition, Hypertech 53,000 volt coil.

Track times to come.