Cold start... the idle pulsates between 1800 and 1000 rpm (consistent pulse almost like someone tapping the gas pedal)... then settles down to 800 or so... thoughts?

 

try this??

https://www.yotatech.com/search.php?searchid=6753756

 

@Team420: I tried it and no search results. This is an issue I've mentioned in the site forum before where search results are tied to a user. You're better off just posting the search keywords. Try logging out then try the link.

 

I'd guess that either the coolant temp sensor (on front of intake manifold) might be malfunctioning or the idle air control valve (below the throttle body) might not be operating correctly.

 

Just changed the coolant... and I know it is not bleed completely... could that cause it?

 

Yes it could cause it or even low on coolant. I would recheck the coolant . Let it run with the cap off until it warms up. Wait for the bubbles to surface and the coolant to start flowing.

 

I would guess idle air control valve bellow the throttle body or the air flow meter..... any codes?

 

no.. no codes. This just started after replacing the t-stat and 1/3 of the coolant... the t-stat is good.. was tested and is OEM. I couldn't get the air out no matter what I did... but I am getting that air out this weekend if I have to stand the truck on the rear bumper to bleed it... will post back >

 

Yes... the system needs to be free of air to operate properly...

park on a hill, with the front end facing uphill, turn on the heat to full blast( if u got a runner w rear heat, dont forget to turn that one on also), start the truck with rad cap off, let run for at least 20 min, making sure to top of as needed, it might even help to "massage" the upper hose.

I usually take the upper hose off the rad, twist it upward, and fill coolant thru there, then reconnect, and top off rad, and go from there, with the same proceedure, as listed above... hope that helps.

 

Might not be the coolant level because the surging starts only once the thermostat opens. The coolant related surges are due to the water pump loosing suction. Basically, the pump can throw the coolant out of the engine but can't draw enough in. Parking facing forward up a steep hill can help, open the radiator cap as to not let the surges put too much force on the pump seal/gasket.

If your problem stops once the truck is warm (operating temp with the Tstat open) I'd say it's not air in the coolant.

Just remember the only two laws of engineering: 1.) Always check the easy stuff first. 2.) Don't F it up worse than it already was.