Signature 600 idle speed

[TerryB]

Got a 2000 International 9900ix with a Signature 600 that has a low idle of 500 rpm's. Programming is all factory specs. Took it to a local shop that has Cummins Insite. They tried to set the low idle to 725 and they say ECM would not accept the change. They are blaming the timing and metering valves as being the problem. Is that at all possible? No codes. Engine does idle like crap also but it runs and pulls great. I ran the overhead a few months ago thinking I could help the idle didn't help the idle nor did I really find anything out of adjustment on the overhead. Could timing and metering valves be overall problem?