rpms when idling

[jpac1]

I have a 1991 gmc 4x4 with the 350 and 400,000kms. When I am in drive it idles at 200 - 300 rpm. Is that low? Now my question is that when in drive and sitting at a light or in park when the engine is hot my rpms drop to 150 and goes to 350 up and down as if it is surging( for a lack of a better word). When i am driving the rpms are fine, about 1100 - 1200. Any ideas as to what my problem could be? I read another thread about someones throttle body sticking, could this be my problem? If it is are theyhard to clean yourself? Thanks in advance!

[formulaross]

I suggest getting a second tachometer and checking the accuracy of your tach. 200-300 is VERY low, and I wonder if what you are getting is an accurate reading. Your engine is computer controlled, if I recall correctly, and idle speed is controlled by the computer via the air bypass solenoid. It and the throttle body can be cleaned with Gumout, etc. Be careful, there is a mass airflow (MAF) sensor among the hardware, and possible other sensors, that you need to be gentle with.

[fast6]

I'm having the exact same problems with my 1990 305. It idles very low and surges. I am going today to check the egr valve.
I would suggest to you to look for vacuum leaks, run some injector cleaner through, check all ignition parts, change fuel filter?, check timing, may need valve adj. or have a bad cam lobe...just some ideas. Good luck.

[TweakedGM]

the 91 gmc it surged every once and a while. I changed the plugs to a racing plugs worked good,6 months later it happened again. I figured it worked once try again checked the plugs two where misfiring the I when the bosch plugs worked fine.Then about a 6 to7 months later did it again, same problem as the first one. Then I went and got factory plugs not a problem.

[fast6]

Well I changed the EGR valve and WOW!!! what a differance. The truck now idles at the proper rpm, it runs smoother and is all in all much nicer to drive(smooth).