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This is a do over from a past post.  98 ch613 with a E7-427 (remanufactered by Mack in 2008)  has about 150,000 miles on the engine.

Anyway, idles fine when cold, kinda rough when hot, real rough after getting off expressway.  Runs and pulls like a mother%$##^%$^. No trouble codes.  My local guy doesnt know whats wrong ( and hes real good)  He said take it to the Mack dealer.  Mack dealer cant figure it out but told me they see this all the time and run it as is.  

Just all of a sudden idles like sh*&^.  Changed fuel filters a few times same issue.  Had a bad injector and Eup. Changed them but no change in idle.

She does seem like she's trying to smooth out.....sometimes it does and will idle fine for a minute or so but then back to roughness.  Sometimes sounds like a misfire or only running on 4 or 5 cylinders but when i get moving she gets up and goes with no problems.

Any ideas?

Thanks!!

 

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DO NOT overthink this........it is perfectly fine. We always joke that it's a oversized camshaft idle. Sometimes you can get it to smoothen out by putting on a new fuel return check valve. But it will come back. 

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Yup, and if you listen 10 minutes more you’ll be certain a roller is flat-spotting and you need a camshaft replacement. 

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But why would it start all of a sudden?  Have you guys heard of this?

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Yes  and No answer on my part! We have had engines come in idling perfectly fine ! Have a cam replaced and leave with an erratic Idle . leave with this  idle syndrome  not sure of the cause! I  do know one thing for sure Its not  worth chasing! you can spend a Big bag of money chasing it!

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Dumb idea, change your engine speed sensor, maybe it's giving mildly erratic signals. Not enough to throw off your tachometer, but maybe enough that the computer is trying to regulate fuel injections to maintain idle speed.  

Before doing that though, CLEAN YOUR BATTERY CABLES, and make sure they're SNUG. I had a 1999 dodge 24v automatic that would shift in and out of overdrive right at 43mph because of mediocre battery connections. It caused subtle voltage fluctuations that were just enough to throw that one thing off at that one speed.

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Clean battery terminals are a necessity on this new computerized crap, and it's one if the things mechanics are supposed to disconnect before doing work on an engine.  And I'll bet 95 times out of 100 that the mechanic doesn't polish terminals before hooking back up. 

Old mechanical trucks never cared about this sort of thing but the newer stuff can be thrown off by it.

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