Hello I'm new to posting on here benn reading archives for a while just can't seam to find help with my problem .
I have a question on my 2001 gmc 2500hd 8.1 Allison trans.. I'm having a high idle issue with truck while in gear or moving but does not happen all the time. When in traffic going slow the idle picks up to around 2000 rpm's if I put truck in neutral idle stay high until truck comes to a stop ? Put it in gear the idle goes high again ? Never happens anything above 45 mph ..I've replace throttle body with new tps and throttle control motor .and MAF sensor .i sprayed starting fluid all around intake ect looking for vacuum leaks .i discounted battery over night let it idle for 15 min to relearn new parts ..I have a code scanner not showing any codes check engine lights.. Is there a speed sensor or something that controls idle when truck is in gear ? I've been messing with it for few weeks now been on different discussion forum can't seam to find any answers .i hoping someone reads this has had simular problem can help ..thanks for reading my post

 

I have the same issue. 02 Silverado 2500 8.1 gas

 

I should have updated my post when I fixed the problem . Problem turn out to be a bad wire on the temp sensor of the passenger side cylinder head .conenection was loose bounce around made idle fluctuate .good luck hope it fixes your issue a lot quicker the. Took me to figure mine out