Hey guys, got a good one. 89 GT, stock except for one crank pulley, and some dynomax mufflers. Went to get smog, (passed last time) and failed. High Nox and Hc2 or something. I was told by the tech that there was a random misfire and a bad egr valve.Went ahead and replaced the plugs, wires, cap & rotor, new EGR valve, put in some 91 octane, and Went ahead and free retested it. It passed the first part, where it idles, then he revved it up to 1500, held for 30 seconds, it passed, then took it up to 2500 RPM, but couldn't hold it there, and could not complete the test.

What could it be? I am thinking throttle position sensor, or mass air sensor? I am running out of time, cash and ideas.

Thanks...!

 

I forgot to add, I found a vacuum cable coming from under the intake manifold with a small crack in it. I cut off the crack and replaced it. There are no other apparent cracks, but I'll double check tomorrow.

 

What do you mean it wouldn't stay at 2.5k? that it would surge up and down? or backfire and miss?