1988 2.9 At a loss with this cold idle issue

[88ranger91]

Watch the video I took on YouTube and tell me what you think. Only does it when cold and used to only do it for a few seconds on first cold start up of the day but I’ve found this winter even after a few minutes of driving when I come to a stop with the lights on and fan blowing hot air it will jump just like this. Battery is strong at 12.6 and alternator is at 14+. Any ideas??? Thanks guys!

https://youtu.be/UDqZo6LXjNs

 

[Bird76Mojo]

Is the engine changing RPM when the tachometer does, or is it just the tach that's jumping? I couldn't tell by the video. If so, it may be an electrical issue as the tach itself is electric.

Also, are you pushing the A/C button? (I couldn't tell for certain) If so, the compressor may be pulling quite a few amps to engage the clutch. Then the higher speed you turn the fan too, it pulls even more amps. The heater/AC fan motor pulls a ton of amperage on these trucks.

How old is the battery and have you checked the fluid in it or is it a permanently sealed battery?

You could start removing the various grounds on the truck, cleaning the connection surfaces, and re-attaching them. Just as a precautionary/preventative maintenance thing.. It would help to eliminate a potential ground issue and it never hurts to assure you have good grounds on the truck.



GB

 

[Locotomb]

I would also check the connectors by the coil. I just had this issue, the harness chaffed on the corner of the coil causing a short in one of the connectors. I would also make sure the distributor wire is as isolated as possible from the main harness. I believe the tach+ to instr. cluster is Dkgr/Y and tach- is Bk/Y.