all the vacuum lines are new not sure where this 4 port vacuum control is.

but i have officially stumped 3 mechanic's with this rough idle.


yesterday it did throw a misfire in cylinder #4 but that was a plug we pulled to inspect. but now since they reset the computer for the millionth time the light is off but idles rough /misfire at an idle but when im driving it drives fine. the third mechanic says to drive it til it dies lol...... he's a good ole boy.


but we tested the fuel pressure. its at 55 according to the book its right,.

you can clearly hear a misfire when idling. but when im driving it's fine and i live in the mountains no reduction in performance when climbing only at an IDLE and it's making me crazy.

someone else told me to change the thermostat because if it's running too hot it will tell the computer that it's too hot and cause something i dunno im not technical when it comes to computers on trucks. which in turn will throw the egr code?


please im ready to kick the darn thing.

Chas

But thank you all for your input im trying everything your telling me to.

 

4 port control is the **** you use to select A/C, defrost, heat, etc. You say you have new vacuum lines, do you still have the leak? (wacky heater when you give it the gas) Unless you have something sticking the EGR valve open it shouldn't affect the idle, EGR is closed at idle and wide open throttle - anywhere in between it's open. If you still have the leak I'd say find the vacuum supply line on the motor & plug it off. You'll lose heater controls and 4WD but if the motor runs better reconnect things until you find the leaker.

 

Im attaching a pic of my controls for my A/C is that the same one your talking about?
I also plugged the vacuum line that goes into the firewall for the A/C No change in the idle.




im really on my wits end with this truck. It has a BIG misfire at idle no code just the same p0401 egr p0172 running rich.

at IDLE it about bounces me out of it but runs fine once I hit the gas.

the light is still on. Mechanic doesnt even want to see me anymore.



soon there is going to be a FREE sign on this truck.

 

I realize this is an old thread, however I ran across a 4.3 same year as yours with "fuel rich" and O2 codes and it turned out to be low timing.

The EST wire is under the passenger floor mat high up next to the heater box
[on most models] and is brown w/black stripe and has a black connector that must be disconnected prior to setting the timing.

However, bad wires usually don't show up till slight load on the engine.
Lopey idle even if from a huge vacuum leak is usually calmed down by the computer.

I'm betting on a bad fuel line [CPI Nut Kit] or a bad Spider Unit.
A leaky CPI nut kit aka fuel line is inside the intake manifold and will wash away a clean spot inside the manifold.


I'm hopeful this situation has already been resolved.

As a side note, poor quality plug wires can cause fits, GM factory manual calls for a ohm reading of 12,000-15,000 ohms per ft, with a max reading of 35,000 ohms.

I've given up on aftermarket plug wires as factory units gleaned from salvage yards are better and cheaper. The GM service manual calls for cleaning them with a strong solvent[acetone works well], then wiping them down with silicone grease.

The same procedure goes for door gaskets and wiper blades.