i have a 2000 monte with a 3.4. my cars motor was surging at about 2500 rpm for a couple weeks then it started to get a rough idle. the fuel pressure is 55 psi i replaced the egr, throttle position sensor, new plugs and wires, and the mass airflow the converter is good, there are no codes can any one help with what could be the problem. could it be the ecm if so can i replace it with one from a salvage yard from the same type of car with the same motor.

 

Upper plenum gasket. Possibly coil or icm

 

- Check for vacuum leaks
- Take the ICM to the parts store, have them test it (do it about 3-5 times to ensure it passes multiple tests, as sometimes they need to warm up)
- Clean the EGR passage between the EGR and the UIM (on the 3100/3400, they sometimes carbon up and get screwy).

That's the free list.....

After that, you could have something like a bad injector (granted, on a modern car that is ODB-II, it should pop a code specific for that cylinder).

 

the egr is new i did unplug it and ran it the car seemed to run fine i was told that a ecm could be the problem does anyone think this could be also the upper plenum gasket seems to have no leak i spray wd40 around it while it was running and it didn't change the idle at all

 

A new EGR does not "unclog" the passage between the EGR and UIM. Pull the EGR off (and pulling the throttle body can sometimes help depending on the gunk in there) and use carb clean and a form of a stiff wire or think bottle brush down into the passage on the pedestal the EGR sits on that leads to the main part of the UIM.

That is one of the last things to consider.

WD-40 typically "streams" out when sprayed (does not produce a fine mist). If you are looking for a vac leak, get a can of something like starting fluid, that will make a fine mist when sprayed. Spray it around gaskets and vac lines, listen to the engine, you want to hear if the idle changes in response to misting the starting fluid around. If it does, try to focus on the location and get a fix on what might be suching in the mist you are spraying. That is the source of a vac leak.

 

Could it also be a dirty/clogged/damaged IAC? (idle air control)
BadGas, Dirty Injectors?