I had a customer drop his car off, light mods, heads,cam,bolt-on's, etc.

He complained of a high idle and surging, plus the car had two codes.
Right away I'm thinking vacuum leak.

Instead of wasting time and money, the first thing we do in a situation like this is hook up the Elf Smoke Machine, which detects vacuum leaks by emitting smoke from the leak/hole.

It works by pressurizing the system with 5psi and smoke.
Once the system is pressurized, within seconds, the smoke will show where the leak is.
I get asked all the time how to find a leak and what's the cause of the P0171 and P0174 codes.
Here's one solution.

http://www.ls1truck.com/newstorage/v...kedetected.wmv


This code can also be caused by bad front O2 sensors, but in this case it was a faulty intake manifold.