in this engine swap. lt1 in a 90 tbi camaro. i have wired everything up. i also have a aftermarket tacho.... when i started the car i reved it alittle. not to much. the aftermarket tach read 2,000 which is about right. but the factory just coasted up to about 5,000 or so and wheni let off the gas it drifts back down to 1500 again. is there a way to fix this?

 

All I can think of is to check your wiring for a bad connection to the factory tach, or bad ground to the factory dash, since either could allow the tach to pick up ignition noise. Seems odd for it to happen at high revs only though; does the factory tach read correctly at idle?