89 Jimmy will idle smooth at stop light for the first few seconds then runs rough as engine speed reduces. The longer it sits the rougher the idle. If I increase idle speed it will smooth out.
It has a fresh tune up, doesn't appear to be any vacuum leaks or gasket damage at throttle body. Throttle body still has a small amount of gunk on the very top of it. I have replaced the fuel pump. The IAC valve is not very old. I checked around and two possibles come up. Idle control valve or fuel pressure regulator.
I think it is worth pointing out, the truck has 300,000 miles on it with no major engine work. However it was running like a truck with 30.000 a few weeks ago.

 

what parts did you use for the tune up, and fuel pump?

 

EGR valve could be leaking by at idle. The EGR should be closed while idleing. If it is not you will have a rough idle.

 

I was watching the fuel from the injector nozzles closely and noticed a disruption in the fuel during a stutter. That combined with the smooth to rough idle at stop lights, and low R.P.M has me thinking it's some kind of fuel issues. I think the pressure to the throttle body is good.
So with all that in mind what is the most likely problem?

P.S Tune up / fuel pump was done with mid priced items. I think it all good. It runs very smooth in park.
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