Hi all. Im new to the forum. i have a 95 f 150 4x4 xlt 5 speed (m5od) ext cab. Truck originally had a 5.0 vin N with 120000 ish miles, that went bad. bought a 95 e250 van with 85000 milesfor $600 with a 5.8 liter vin H. stripped the 5.8 down to the block ( didn't remove crank pistons cam as per inspection) did new oil pump, new timing chain, one new head, rebuilt the other one, new water pump, 4 new injectors. ohm tested and cleaned and rebuilt the other 4, new heater core, pace setter headers, cleaned prepped and painted everything, new y pipe with new cat, new motor mounts new tranny mount and other miscelanous parts along the way over the recent winter months. put the beast in 2 weeks ago. it fired right up. no lights or anything. passed inspection the next day. also running withe the 5.0 computer and harness. now my question When i start my truck it idles around 1500 ""normal"" then drops to around 750 ""normal"" now if im sitting in idle even after driving a little while it will idle up to around 1200 for abouit 10 to 20 seconds. then bac to a normal 750. is this normal

 

Does or did the truck have AC? The computer commands an idle speed increase when the AC compressor clutch engages which is normally just enough to offset the additional load, but if the AC system is no longer charged the AC clutch won't engage and then engine will gain more rpm than it should.

 

The ac does work fine. Didn't have to disconnect it during the motor swap. With the hood open it idles up a little but I don't hear the clutch engage on the compressor

 

Couple questions:

Is this something that just happens once in a while, or does it happen all the time, like it idles at 750 for 10 sec, then jumps to 1200 for 10 sec then drops back down and so forth?

If it is something that happens once in a while, are there any conditions at all that cause it, or does it seem random so far (if it seems random, try to keep track of it to find the common denominator).

 

My first thought, with the engine swap, is would you need to change some of the computer settings? Maybe that's the source of the idle problem.

Other thought would be a vacuum leak, the up and down of the idle may be the computer trying to compensate for the reading it's getting from the HEGO. Just a guess.