Pickup idle problems

I changed the heater core on my 92 Isuzu pickup and now it revs repetedly. The rev is like (vroom, stop,vroom, stop, vroom, stop). Also, the temp stays cold when going down the highway and warms up when stopped. Any help is greatly appriciated

as far as the rev, checkto makesure all wiring and vac hoses are connected(you might have knocked one loose during youre change out.
and with the heating issue you might have air traped in the system somewhere. just flush and refill. if that dont work change out thermastat, and rad. cap.

Thank you Jackass#1

I just changed the thermostat and I think you are right about the trapped air. I have checked all the wireing but not the vac lines.

I wil be giving the system a flush.

Thanks Again

how did it go?

It went great. The coolant system did have trapped air in it and that is what was causing the rev problem. As soon as I "Burped" the coolant system the rev problem stopped. The old truck is running like a champ now.

Thank you for all your help

glad to help