Rock in idle rpm sometimes-?

[captsb890]

The rpm at idle--sometimes--when you come to a stop will lower 100- 200 rpm----but then goes back up to a normal idle--700 rpm----------no **** or stumbling , no rpm change during driving-- and then at coming to a stop, the idle sometimes will be at 700 and no 100-200 rpm drop with recover---What is causing that as it just started doing that. The timing is good, new wires and plugs-----could it be either the in-tank fuel pump going bad or maybe rail pump- acting up? It is the same drop 100- 200 with recover in a second or two, doesn't faulter or stumble ---just that 100- 200 and back up---that's it? I don't think it is ignition as it probably would do it all the time, or have driving issues. maybe battery going--bad as it is sometimes only at stopping idle-? any suggestions would be helpful---by the way, the temps here in AZ jumped to 105 and that's when it started happening , that's why I was thinking maybe the battery is causing it-??

 

[Tiha]

First thing to look at is the IAC. probably pull it off and clean it.

 

[L\Bronco]

The rpm at idle--sometimes--when you come to a stop will lower 100- 200 rpm----but then goes back up to a normal idle--700 rpm----------no **** or stumbling , no rpm change during driving-- and then at coming to a stop, the idle sometimes will be at 700 and no 100-200 rpm drop with recover---What is causing that as it just started doing that. The timing is good, new wires and plugs-----could it be either the in-tank fuel pump going bad or maybe rail pump- acting up? It is the same drop 100- 200 with recover in a second or two, doesn't faulter or stumble ---just that 100- 200 and back up---that's it? I don't think it is ignition as it probably would do it all the time, or have driving issues. maybe battery going--bad as it is sometimes only at stopping idle-? any suggestions would be helpful---by the way, the temps here in AZ jumped to 105 and that's when it started happening , that's why I was thinking maybe the battery is causing it-??

Those old OBD1's rely heavily on the base idle setting being correct for proper idle control. (It's also called the hard stop setting)
Can be affected by a dirty IAC valve, a sludged up throttle body, or an incorrect PCV valve.
A quick check is to unplug the IAC valve at idle, the RPM should drop to around 550 (not an exact science) if it stalls, the base idle is too low. Start with cleaning, (Be careful, throttle body spray can harm the TPS) also verify the PCV valve is correct. If all else fails, there is an adjusting S C R E W in the throttle body. (It will be locktight city though) good luck. Hope that helps.
Cheers

 

[Motech]

any suggestions would be helpful---by the way, the temps here in AZ jumped to 105 and that's when it started happening

Running your AC? That'll do it.
@L\Bronco suggestions should help that some too.

 

[captsb890]

The rpm at idle--sometimes--when you come to a stop will lower 100- 200 rpm----but then goes back up to a normal idle--700 rpm----------no **** or stumbling , no rpm change during driving-- and then at coming to a stop, the idle sometimes will be at 700 and no 100-200 rpm drop with recover---What is causing that as it just started doing that. The timing is good, new wires and plugs-----could it be either the in-tank fuel pump going bad or maybe rail pump- acting up? It is the same drop 100- 200 with recover in a second or two, doesn't faulter or stumble ---just that 100- 200 and back up---that's it? I don't think it is ignition as it probably would do it all the time, or have driving issues. maybe battery going--bad as it is sometimes only at stopping idle-? any suggestions would be helpful---by the way, the temps here in AZ jumped to 105 and that's when it started happening , that's why I was thinking maybe the battery is causing it-??

Found the problem--cheap ICM installed---when I changed it----works fine now