Need some help with idle stumble

[azhobo]

I've been through the threads here, and find nothing exactly like my problem, so I'm asking for some help.
99 old body style, ECSB, 454, NV 4500. Truck has 219K on it, motor has about 88K on a local machine shop complete reman. I have no reason to suspect any internal problems, it runs great on the road, although fuel economy isn't the greatest. One other thing..... in my area yearly emissions checks are required for registration, and the NOX/HC levels have crept up until it failed last year. No black smoke, ever, and no unusual smell at the tailpipe.
Here's what I know.....
runs fine above idle, but has a rythmic stumble at idle. It idles good for 7 seconds, then stumbles for 7 seconds. This doesn't change, always the same pattern, 7 good, 7 bad. It will, or won't bring on the SES light, but never locks it in. It does see a code, P0405, EGR valve sensor low voltage.
Obviously I've done all the routine tune stuff, plugs, wires, cap, rotor, fuel filter, etc, even had it to a shop that had a computer. They put in a new MAF sensor, which I had replaced not long before, with no change, and theirs made no difference either. Idle vacuum is generally at 19-21, but of course drops during the stumble.
I'm probably missing some needed info here, and if so please let me know.
Appreciate all inputs, thanks guys.
Bob

 

[Griffin]

Any luck with getting this figured out?

 

[azhobo]

Any luck with getting this figured out?

No help thus far, looks like either I stumped the entire crew, or nobody loves me. Story of my life!

 

[Griffin]

Haha! Hopefully someone will be able to help you out...

 

[RichLo]

How does it run under a load like pulling a trailer? Any missing?

The OEM style fuel injectors aren't the greatest in these engines.

 

[obs-logan]

i was watching a video about these issues cause i too have some of them p0154,p0151,p0405 is what i have, i checked all my wiring but nothing looked bad but in the video they started leaning towards the alternator and they also had a part 2 confirming it but it sounds almost exactly like what your symptoms and mine are i’m going to get a brand-used alternator after the holidays i’ll keep you guys updated

 

[Carlaisle]

When was the last time you replaced the distributor cap? The L29s are well known for those shorting out internally. You might also check under the hood at night in a pitch black place. If you see a lightning show under the hood you'll have some troubleshooting to do.

 

[obs-logan]

last replaced a month or two ago i have a similar thread out there cause i had it shut off in the drive through and it took me about a month to figure out it was in my distributor then after retuning the distributor to be in position then it started and i got a new cap for it and i’ve put probably 2-3k miles on it since, now im just fighting low voltage codes and less engine power, but after swapping the oxygen sensor for b2s1 it stopped sputtering and missing and now feels a lil sluggish

 

[obs-logan]

also sorry forgot to include the video but here is pt 2 of why they thaught and confirmed alternator

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[Schurkey]

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. TBI systems don't hold pressure when the pump stops running, but the other fuel injection styles should. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Use quality parts sourced from an authorized seller (NO COUNTERFEIT PARTS FROM AMAZON, EBAY, OR OTHER SKETCHY SELLERS!) Be sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.