Vortec 5.7 low idle

[shovenose]

This is my 4th gmt400 truck, so I know it's not normal. 99 burb k1500 with 123k on it. Low idle to the point that it isn't smooth. Just bought it so no maintenance history. Dealsership I bought it from supposedly just replaced cats. No service engine/check engine light. Any ideas? Thanks...

 

[littleblazer]

I'm not familiar with the 5.7 setup since well... never had an issue with the motor for the 210k we had it. Iirc the tb is controlled by both a servo and a stop for the cable like a carb I think. I would check that first and then confirm that the plug wires are in the correct firing order. I've heard of that being messed up. If that doesn't fix it I'd look into timing and maybe a tune up. (Plugs cap rotor.) Especially if it was never done. Does it go away slightly higher in the band if you push the pedal or does it sound like it misses?

 

[shovenose]

hmm so real rpm according to scantool is about 550rpm. seems ok to me, though the dash gauge shows like 300rpm which was worrying me. is about 550 ok? in that case, what could cause slight roughness at idle? i'm thinking i'm going to just run some seafoam through it and then do a good tune up cap rotor wires plugs air filter and then see how it goes. good plan?

 

[littleblazer]

Sounds about right. The 5.7 I always thought should be 600-650 in park. Is that with ac on?

 

[Mooseman]

Could be a vacuum leak but you would think that would throw a code. Maybe just needs a good tuneup. Idle issues can also be related to the idle air control solenoid on the throttle body which could be sticky, dirty or the passages in the TB are dirty.

What do the fuel trims look like?

 

[littleblazer]

Could be a vacuum leak but you would think that would throw a code. Maybe just needs a good tuneup. Idle issues can also be related to the idle air control solenoid on the throttle body which could be sticky, dirty or the passages in the TB are dirty.

What do the fuel trims look like?

That's what that is. I was thinking the older tpi.

 

[shovenose]

oh, yes, i will clean throttle body and maf. that's a great idea, as i have specific cleaners on hand for both. fuel trims were very good. the 550rpm i quoted was in drive at a stop light at 194f coolant temp. ac works but i have yet to use it.