Me and my Dad just swapped an engine from a 98 Blazer into a 99 bravada. Everything is fine and it runs well but after driving for a bit and letting the truck sit at idle there's a deep sounding vibration. It's idling at about 550rpm and I'm just wondering whereI should start looking. He thinks we could just raise the idle time and it'll be fine but maybe we should be looking somewhere else.

 

did you replace the motor and trans mounts? You cant "raise" the idle.

 

Yes all mounts are new. And no? Idk he's said he's done it to other GM vehicles. Meaning having it idle at 600 rpm rather than 550.

Also there is no vibrations when driving at any speed. Only when it's parked and sitting idle after driving for a little bit.

 

So we raised the idle to about 650rpm and the hum went away but it seems the computer corrects what we change it to and it just goes back to 550.

Any other things I could check for? I guess at this point I wouldn't call it a vibration but more of a deep hum that's very noticeable behind the wheel.

 

Bump.

 

I got a replacement parts store trans mount for my 97 that was made incorrectly and contacted the t-case when bolted down...it caused a nasty vibration at idle.

FWIW the only way to change the idle on something with EFI is with tuning software...