On going problem your input please

[ONEQUIKSS]

I have a 1987 k5 350 motor is brand new crate motor with no mods. My problem is the motor hunts when started idle goes from say 600rpm to 1500rpm then to almost 200rpm almost like its gonna stall?
once the truck is placed in gear it stops. Sometimes when its started it will not do it its totally random doesnt matter what the weather is etc. I so far have changed the computer, tbi,idle air,temp sensor,egr valve and a new cat..the problem was present when the old motor was in it.

 

[1985_K5_Silverado]

Was the previous engine similarly cammed? What profile is the cam you're currently running? When you take it out of gear, does it start loping/hunting again? Does the ecm toss out any trouble codes?

 

[ou8mud2]

check for vacuum leaks and also your map sensor.

 

[ONEQUIKSS]

yea the new motor is the direct replacment cam is unmodded and there is no trouble codes while hooked up to the computer all the computers signals are within the parameters

Was the previous engine similarly cammed? What profile is the cam you're currently running? When you take it out of gear, does it start loping/hunting again? Does the ecm toss out any trouble codes?

 

[1985_K5_Silverado]

The vacuum leak/map sensor thing does have some possibilities, especially the vacuum line to the map sensor. If you have another piece of vacuum hose handy, replace that map sensor vacuum hose to see if it makes a difference. If you have a timing light, you could hook it up and see if the timing is varying widely as it hunts/lopes around that rpm range, which it might do if the map sensor was getting an erratic vacuum input.

Other possibilities could include a sticky EGR valve, vacuum leaks elsewhere, or a leaky PCV hose.

Have you noticed the CE light flickering or coming on intermittently as the rpms hunt around?

 

[ONEQUIKSS]

No CE light at all changed egr and I dont hear a vacuum leak and I also sprayed the vacuum lines with wd-40 which will change the tone of the engine when sucked into the break if there was one which I didnt find I will check again and also look more into the map sensor.

The vacuum leak/map sensor thing does have some possibilities, especially the vacuum line to the map sensor. If you have another piece of vacuum hose handy, replace that map sensor vacuum hose to see if it makes a difference. If you have a timing light, you could hook it up and see if the timing is varying widely as it hunts/lopes around that rpm range, which it might do if the map sensor was getting an erratic vacuum input.

Other possibilities could include a sticky EGR valve, vacuum leaks elsewhere, or a leaky PCV hose.

Have you noticed the CE light flickering or coming on intermittently as the rpms hunt around?