Idle problems

[Burbdood 74]

Ok, 1988 Suburban, 350 TBI. I noticed today that if I just blip the throttle when it's in park, the idle goes down about 100-200 rpm's and it starts to surge. If I give it a little gas and let off of it slowly, the idle comes back up and levels off. I was able to do this a couple of times in my driveway.

I don't have any codes and this is new to me. I rebuilt the TBI last summer but are my throttle shafts worn out causing my weird idle?

Please help me out here.

 

[Fordum]

Man, if this was a 89 to about 93 Ford, I know exactly what it is.......

I just advised someone else about the base idle setting, but that was not his problem, so I'll try it out on you..........

There is a mechanical stop that you set that limits how much closure of the butterflys you get when let off on the throttle, and yours might be right on the edge.
There is a procedure here for setting it.

If not that, then I would suspect the EGR valve, or the distributor advance if its vacuum controlled on this engine.

Of course worn throttle shafts might cause that. But I would suspect it would do it all the time.

When you suddenly blip it like that, you punch the vacuum in the head suddenly. So, anything that is operated by vacuum is suddenly not seeing any, and will move if its being held by it, but then just as suddenly sees strong vacuum.
So, whatever that is, might stick. That is why I suggested EGR and advance.

 

[Burbdood 74]

Thanks Fordum.

The distributor is electronically advanced and I put in a new EGR and my used one for comparison and it does it to both.

At this point I am really suspecting that the TBI unit is just junk. Ever since I rebuilt it, it just doesn't run as sweet as it used to. So, when tax money gets here, I'm thinking of replacing the TBI unit with a rebuilt one or just try to get this one rebuilt.