2002 Suburban 125,000 miles

When slowing the vehicle, foot off throttle, transmission automatically downshifting, motor revs 100-200 RPM at each shift.

Occasional rough idle but no other signs of Transmission issues or motor issues.

I was thinking possibly the IAC but can anyone confirm what might be causing this revving.
This started about 6 months ago. Has not changed since it began.

Thanks

 

IAC should only control the engine at idle stop or neutral (as I understand it)
I would consider cleaning the throttle body and plate

 

Slight revving while down shifting is normal, this means the transmission is going through the clutches and they aren't slipping. But the rough idle is from a dirty throttle body, the LS base motors don't have a separate IAC. The idle is controlled by the stupid fly by wire throttle body. Be careful when cleaning. Don't just spray into the intake! Have someone press the gas peda while you are cleaning the throttle-body.DO NOT UNPLUG THROTTLE BODY! When you unplug it it has to "relearn" the throttle body and likes to idle from 2000 to 3000 rpm . It high idles ALL THE TIME until you have driven MORE than 150miles. I learned that the hard way. Mileage went from 14.5 to 10.8!!!!!! Then back up to 14.5. But stayed at 10.8 way too long!

Don't know what year this started but the newer LS motors have self cleaning injectors so injector cleaner is kinda useless. Do Not use seafoam! seafoam is very bad for motors period. The short term may seem great, but long term large amounts of damage is done...

A good injector cleaner such as Chevron or Lucas will help with the rest of the fuel system and new AC Delco spark plugs help with rough idle. I went from 500~400rpm rough idle in gear with ac on to 600 rpm smooth idle in gear with ac on, with new spark-plugs and throttle-body cleaning, and treatment of Lucas last summer, and treatment of chevron 2 weeks ago...

 

Thank you for the information.

I will give your suggestions a try.

Never knew that about Seafoam though. Good Info.

 

Think of it like this, let the idle speed in 1st gear, then shift it to 2nd. You will have a decrease in rpm. The overdrive is a function of an even higher gear when cruising, drops the rpm to save gas.

 

Could you go into detail on the seafoam? I would never use it on the throttle body or vacuum tubes but I was under the impression putting it in your fuel had been deemed safe and putting it in your oil was great for cleaning out carbon deposits and such but only if you change the oil literally 30 seconds of idling later.

Just curious because I definitely used it quite a bit on my boat motor and a little in the fuel of the F like once

 

if you tear apart a motor that has had treatments from seafoam you will see the damage done. esecially on Carb motors. it takes any build up in the intake and fuel system and deposits it in the cylinders of the motor, it then dries out the piston rings rings, often causing them score. and when you add to your oil.... well. if there is a lot of build up and neglect best not do it. if you do it quick your oil passages wont get blocked by debree...

if you ask a expert mechanic who is Australia, by the youtube name of V8Jagnut he will tell all about why it is bad, he done several vids on a sad sad motor that was treated with seafoam. boy it was not good...