I read on this site that not to let the engine idle alot. Why is this? When I picked up the truck at the dealer we let it run awhile outside because I still had an idle problem. He never told me when I let with it it wasn't good for it. I had it idling alot the next weekend trying to work on diagnosising the engine idle problem which I hope I have figured out. I only have 300 miles on the new engine and the cat is being replaced in a few days because it is not working and I can't hardly drive it until that problem is solved.

 

Drive it like you stole it.

http://www.mototuneusa.com/break_in_secrets.htm

 

haha
seems like the dealership should fix it fi it's a brand new car... dunno your situation. but if you have access to some tuning software log the LTIT/STIT and see what your idle is set to. other than that, do you have DTC codes popping up (SES light). if so, what codes?
if it's a new motor install check the MAF/air temp sensor. on the newer motors the MAF/temp sensor are in one unit. and btw, the cat isn't going to fix your idle problem, unless somehow it is clogged with something...
another thing to check maybe is your fuel trims, spark advance, and knock retard. if there's anyway you can get someone to log this info it'd be a lot easier to figure out what's going on.

 

The cat is plugged or something is broke loose in there. I have been trying to get a crazy weird idle problem for awhile. The dealer put in another new engine because of a coolant issue. It is a 91 Silverado with the obd 1 so there isn't much you can do to it. It doesn't have an codes. Never has had any. The exhaust is doing so weird things when you drive it. At times you can't hardly hear it and then it will change like something is loose inside. Anyway I have had to basically figure all this out because the mechanics here don't have a clue what is wrong. You know they don't get paid when they are just trying to figure out what is wrong. I just want to make sure I break the engine in right. I am not a lead foot normally.

Everyone on this web site has been very helpful with this idle problem.

 

The Cat Is Probably So Clogegd That The Catalyst Material Is Broken Up Inside, And Occasionally The Piece Will Lodge Itself In The Pipe, And The Exhaust Tone Will All But Dissappear.... And Itll Surge And Buck, And Just Plain Run Like Crap

 

As For Break In... The Minute We Start A Vehicle In Our Shop We Drive The ***** Off Of It.... You Should Do Taht Because Otherwise You Will Not Get The Proper Wear Patetrn On Your Rings Before The Crosshatch Loses Its Edge, And If You Are Easy On It, Or Let It Idle.... It Will Start Life With Already High Amounts Of Blowby, Which Means It Will Last Signifigantly Less Time

 

just unbolt your exhaust and run it pretty much open headers and see if that makes a difference... at idle lol. what kind of exhaust setup do you have? is it stock manifolds and stock exhaust setup? just curious. an investment in longtube headers and a complete exhaust setup would be wise... unless it's a DD beater truck, just get the exhaust redone from the headers back.

 

I have a stainless steel Gibson system I love. It is the only thing I am changing on the truck because it is a show truck that is original and I can only have 3 modifications. I just want it to run like a new engine should. It has been a great truck in the past. At the dealer they told me the engines were already broke in and I didn't have to do anything special to break it in. As soon as I get the cat put on I will drive it alot harder with little idling. I didn't know about the idle thing being a bad thing on the new engine. The mechanic said he drove it alot to break it in so he might have drivin it hard. How long do I have to drive it like that to break it in and to seat the rings? It has 300 miles on it. I drove it like up to 65 and then I would get off the throttle and let it coast down to 35 and start over and over again trying to break it in. I read that on the gm website.

 

I bet it runs ok with the new cat installed. It's probably pretty much broken-in by now. IDK what the best procedure is but have heard to not drive it too hard and vary the throttle settings a lot for about the first 500 to 1,000 miles.

 

Went ballz-out in my Yukon when I bought it (7 miles on odo). After 100K+ miles, it still runs like new... throttle response is still great. Of course, HPTuners makes it better.