1999 C3500 L29 issues

New member here. I have a couple issues with my daily driven 1999 crew cab dually with 454 automatic.

The first issue is an intermittent high idle. It does not seem to be related to temperature, the A/C being on, or anything else I can figure. I tried changing the IAC motor, and it seemed to go away, until a week or so later when the idle went real high and it threw an IAC circuit code, so I put the old motor back in, and started dealing with the intermittent issue again.

The second issue is when cruising on the highway, it will start missing. It doesn't throw any codes, but this seems to be more prevalent when it is warmer, say over 70° F. Today, I noticed on the live data when this was happening, the timing would drop into the mid teens. When it was running good, the timing was up in the high 20s, but never more than 30° when cruising at about 3000 rpm. Are the knock sensors going bad and pulling the timing back? I don't hear it pinging. It just starts breaking up. Today was the worst it's been. It seems to pull fine accelerating at part throttle and at WOT.

The upstream oxygen sensors are only a couple weeks old. I changed them after I got an O2 sensor code a few times, usually coupled with a rich code (P0174 if I recall correctly). The long term feel trims just before that were pretty high negative, but they seem to be back in order now.

Plugs and wires are a few years old, replaced with OEM type wires and AC platinum plugs of the OEM number.

I've had the truck for about 8 years, but didn't drive it daily until about six months ago. It has about 165k miles on it.

Any help or ideas would be greatly appreciated.

Thanks in advance.

Can you watch misfire data while driving? Knowing if the miss is isolated to a single or two cylinders would help with troubleshooting.

Also would be good to know what the commanded idle speed is versus actual, and current IAC steps. That way you could see if it's an issue with the valve or with the control system commanding an idle that is too high.

Unfortunately, my scan tool does not show IAC counts, misfires, or commanded idle speed. Is there a recommended tools that is reasonably priced that would show more information?

This Autel may support it, not sure. I would contact them to find out. I have an old, OLD Matco branded OTC scanner and that does everything and a good many factory tests. Those can be picked up cheap, but you have to keep your eye out.

Hi 2nd79, where you ever able to resolve your issue? I’m having p0171 & p0174 with similar parts being replaced like yours. I have some exhaust leaks on my manifold gaskets and flanges which are upstream of the O2s, I was thinking my leaks could be my culprit??

Verify fuel pressure.

Connect a scan tool, look at the data stream just as was suggested. Verify ALL sensors, and computer outputs. Misfire counts, etc.

BOTH P0171 & P0174 mean BOTH banks are running lean, so suspect a fuel delivery issue. I'll echo what @Schurkey said.

Check fuel pressure against specs.: with key on/engine off, and while idle and under load. Confirm engine is running lean at different RPMs before condemning "stuff."