Rough Idle in park when warmed up

Joined
May 13, 2008
Messages
350
Location
Boulder, CO
I searched for this and found tons of threads on rough idle but mine seems different than most.

My truck idles at 500 rpm when in drive, very smoothly. When it's in park or neutral the idle kicks up to about 600 rpm and really starts shuttering and missing, dropping below 600, idling back up and so on. It only does this after it has warmed up and I have driven it for a while. When it is cold it actually idles fine, the idle is higher (which I have read is common), but it runs fine. It drives fine at highway speeds and seems to have plenty of power.

Some history, my cruiser is a 96 with 172K, I just did TB cleaning, plugs, wires, PCV valve and grommet, replaced vacuum lines, replaced air intake hose (ripped old in half during tune-up). Mileage dropped off about 2mpg after the tune up, but I can't isolate that mileage drop to my tune-up because of winter blend fuel being used in CO. I am in the midst of running a tank with BG 44k right now, but I can't tell that it's helping at all and I have almost ran the whole tank. The truck will throw a P0401 code if I am on the highway for a long time after it has been going 65-70 + for a while, but it never throws it around town.

Is there a specific problem that is associated with this? I know the wiring harness that melts is a common suspect-I plan to take a look this weekend. Could I have screwed something up on re-installation? During the tune-up I noticed a very small crack in the hose surrounding the base of the PCV valve but it is very small. I was thinking if I messed something up on re-install it would not run well at higher speeds and acceleration. I run 85 grade fuel would that make it idle poorly in park/neutral? Any usual suspects I am missing?

Thanks.
 
Joined
May 13, 2008
Messages
350
Location
Boulder, CO
I'll add that to the list, from my searches it sounds like that has not helped the idle issue, but I'm sure it couldn't hurt.
 
Joined
Nov 2, 2004
Messages
336
Location
Boulder, CO
I had a discussion with Robbie, aka PowderPig the other day about the correct octane of fuel to run. I typically run 87 but would occasionally run a tank of 85 when fuel prices were pushing $4.00 here in Colorado. Robbie kindly reminded me that the manual specifically states that the truck requires 87. 'Nuff said, that's all I run now.

What kind of parts did you use when you did all the tune up work recently? Were you having the same problems before you did the tune up? I'm local and I'd be happy to lend you a hand if necessary.
 
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