Here's my problem: I put a new built motor in my car at the end of august and it has been running great until a week ago. Out of nowhere my idle rpm would drop to around 400rpm and then jump up to almost 2000 rpm and then back to 800 rpm. It would repeat this until i started moving again. It only does this after the engine has warmed up to normal operating temperature. My vacuum gauge during this cycle goes from 17in hg to like 5 in hg, jumps to like 20 in hg and then back to 17 in hg. All other gauges are reading normal. I've checked all my vacuum lines, but i really don't know where to even start with this. Any help would be greatly appreciated. TIA

Is the whole motor new? Was it used and you rebuilt it or brand new from somewhere with all new sensors? Or old sensors?

Everything on the motor is brand new. I had new seals put in the head and also had the shop vacuum test the valves, and pressure checked the head. The bottom end was bored out 0.020 and has j.e. pistons, eagle rods, all new bearings, new oil & water pump, new timing belt and mechanical tensioner, all new gaskets, and all other small things are brand new. I reused all of my sensors except for my Coolant Temperature Sensor. Any ideas on what might cause this type of idle problem????

Hmm..... Mine has a funny idle too but nothing like yours. Mine just lopes like it has cams. I think its the idle air control motor (the bottom sensor on the throttlebody under the throttle position sensor(top)) I dont think that the idle motor could do something like what you're describing. Something of this magnitude i would think would throw some sort of trouble code. It's gotta be something mechanical. Do you have a check engine light?

Make sure you have the no leaks in your egr system. Even if you have the egr system blocked off check to see it is sealed tight.

Most early year ecu's were problematic and you are describing a symptom that a bad ecu can cause. What year is your car? Better, what's the build date on your car?

Not a god damn ECU. I hate it when people jump to conclusions.

Murf,

1st, any CEL?

2nd, pinch off the PCV hose while it idles......this will give you an idea if the PCV is stuck open. Open wont affect cold idle.

3rd, Test out the IAC.

4th, with a code scanner or datalogger, see what the TPS is doing. If the TPS has a short, you will see the % going up and down. This will cause the same problem your having.

5th, Start pinching off Vacuum hoses at the Intake with a pair of pliers and see if the problem goes away when you pinch one of them off. Dont forget the Brake booster line. Problem you describe is usually a big leak.

thanks Star, I'll give those a shot. And if it matters at all, i have a 97...

Well, i deserve the dumbass of the year award. As I was checking the pcv valve and vacuum lines, I rest my hand against my intake manifold. And guess what, my freakin intake manifold was loose. 2 of the bolts were completely missing and the other 6 were loose. I have no idea how that happened but i tightened it down, and she runs perfect again. Thanks for everyones help though!