Surging at idle

[Figaros]

My 1999 k2500 7.4L has started surging from 550 to 750 rpm's at idle, in gear it gets better but still slight surge. Any idea's.
Thanks

[taillight]

Lood for a vacuum leak. Clean throttle body and IAC.

[Figaros]

Cleaned TB and IAC. Still surges, sometimes not at all. Seems that when it does do it, it does it from the turn of the key. If it doesn't do it right off the bat, then it does fine. If it surges right away, it keeps doing it,

[taillight]

I would still look for a vacuum leak like in the brake booster hose that does not always seal or an intake gasket. Iac could be sticking.

[Figaros]

Ok, I bought a scanner and the darndist thing happens when the idle starts surging. All 4 o2 sensors drop down to 10-35 mV amd the timing starts fluctuating from 21.5 to 29 deg. Then I idled it up to 2000 rpm for a few seconds, let it idle back down and then it came right down into line with no surging and timing only fluctuates between 24 and 24.5 deg and the o2 sensors start fluctuating between 50 and 900mV as they should. The other wierd thing was while on the freeway the B2S2 o2 sensor for a while stayed down in the 100 range while the B1S2 kept fluctuating, but then at other times they are right in line with each other.
Any advide?

[taillight]

I think the timing is effecting the O2 sensors. Now why is the timing changing ? Engine speed and MAF sensor control timing. Because you have both MAF and MAS sensors, with the engine off unplug the MAF sensor and see how it runs. It will set a code but you can errase that later. If the surge goes away the was the MAF sensor.

[Figaros]

Unpluged MAF sensor, would hardly run, MAP sensor # went way up and ran like it had an RV cam in it. Timing went way up in the 30's. Could it be the distributor gear? Man this is killing me!
Help

[taillight]

Ok, lets go back to the original problem surging. Have you checked your fuel pressure ? Put a fuel pressure guage on and see what happing to the pressure when it is surging. You can rent a guage from some auto parts stores. The pressure should hold steady.

[Figaros]

Ok, the fuel pressure holds steady at 50psi. I haven't gotten it to surg yet though. By the way, I have noticed that when it is surging and I turn the engine on there is no stair stepping of the idle down to the normal range. It just goes up to 1200 or so at start up and then drops straight down to surging. When it doesn't surge at start up it starts at about 1200 then slowly steps down to 900 then pauses ans slowly steps down to 750 and holds there. I will post back the pressure results when it is surging.
Ok.. just surged on me and YES the fuel pressure surges right along with the idle. Not much but does surge bach & forth between 49.5 & 50 psi.

[taillight]

50 is the very lowest the fuel pressure should be . With the pressure guage on and engine idling pull off the vacuum hose to the regulator. The pressure should be 55-62.

[Figaros]

Where is the vacuum hose located? I've looked all over. I assume it is somewhere by the coil because ALLdata says to remove the coil to replace regulator but doesn't show location of the part?

[maxwedge]

If this is a Vortec ,50 is way too low, 60-65 is normal.

[Figaros]

Yes, it is a Vortec 7.4L
So... is it definately the pressure regulator?
What else could it be?
I don't want to remove that upper plenum for nothing.
I also read on another post to pinch of return line to tank and check pressure, no change in my readings with or without line pinched.

[777stickman]

Some things that effect fuel pressure.

1. fuel pump. How many miles and is it original?
2. fuel filter. When was the last time it was changed?
3. pressure regulator. Most of the "top guns" (max, MT, etc) have detailed instructions on how to test the reg, but it probably means you'll have to access the reg to test it.

Most low pressure problems have been fixed with a pump replacement and some have been reg replacement.

In this day and age detailed testing is required before you start throwing parts at a problem in hopes of fixing it.

[Figaros]

145,000 miles.
Dont know if pump is original, I'm not the original owner.
Just replaced the fuel filter.
Just replaced all 8 injectors. (Had the same surging problem before I replaced the injectors and seemed to go away for 1000 or more miles but then returned)
P.S. I have 60psi after pump starts & during startup but then drops to 50 and stays there.