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Discussion Starter · #1 ·
I've had the problem ever since I bought the truck back in August, and its just been getting slightly worse and worse as time goes on.

Truck is a 1998 5.9L 4x4 with just under 170,000 miles

Okay, so the problem: about 2-3 minutes after I start it, the RPM's will drop down to around 500 or so, and it just doesn't want to run at all. when the RPM's drop, a loud hissing noise starts under the hood that is coming from the air intake.
If you are driving and the problem starts, it still doesn't want to rev at all. When you hit the gas, the RPM's will go up slightly, but then go straight back down. And if you keep your foot on the gas, it will start to pop and backfire. If you put your foot all the way down, it will downshift and the RPM's will jump up and the problem will go away for the rest of the trip.

If the truck sits for about a half hour or longer without running, the problem will occur every time you start it. It does it worse when it is hot outside or when the engine is already warm. Over the winter when it was really cold outside, the problem didn't really happen.

I tried a new idle air controller with no change. Any help would be great. Thanks in advance!
 

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Discussion Starter · #2 ·
Also I've noticed when driving with the A/C on and I go up a hill, the AC will cut out until I get to the top of the hill
 

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It may be a defective IAC that you replaced it with.

I'd take it back and try another. If not, then I'd lean towards an upstream 02 sensor.
 

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No CEL?

if you have the opportunity, run a scope on the TPS, could be off, but not enough to trigger a CEL.
 

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Discussion Starter · #12 ·
Yes sir. Pulled the upstream, and it was almost completely black. Put the new one on and all is good. Now to find out how to pop the back one off without breaking any more tools...
 

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Try using a tube wrench or a socket wrench. That'll give you more bite on the nut.
 

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Rough idle resolved

I gave you some rep points for posting up, but I wanted to get back to this thread and post a resolved title to help other members.

Just a side note: The black hydrocarbons on the 02 sensor may have been caused by the defective (original) IAC causing it to go bad (prematurely?). In any event, nice to see you got her running smooth.
 

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Discussion Starter · #16 ·
I wanna thank everyone again. That problem has been eating at me for the better part of 10 months. How would I post it as resolved? Or did you beat me to it?
 

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As long as you post your resolution anyone doing a search for "rough Idle" it will locate your resolution because your thread will come up.

I posted a title "rough idle resolved" that should likewise come up under a search. And now that I typed it 2 more times it should come up.

It is an example of why the issue should be used when posting a problem as opposed to "WTF" :LOL:
 

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interesting..i have a rough idle..but not as bad as yours...its rough at start then smooths out mostly..i just passed smog..but ill try changing the 02 out..is the upstream one the furthest? does your ac still cut out heavy throttle? mine goes to the upper vents..was told vacuum issue
 

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does your ac still cut out heavy throttle? mine goes to the upper vents..was told vacuum issue
When you say heavy throttle, do you mean WOT? (Wide Open Throttle) It's designed that way. When you punch it, you're asking for all the power the engine has. So the computer shuts off the a/c to give you as much power as possible.
 

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Discussion Starter · #20 ·
Upstream is the one before the Cat. Thats the one I changed and the problem is no more. I originally thought vacuum leak, and thats what numerous friends said that looked at it. But the O2 was the trick
 
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