QuakerJ

Hey folks, I have a 1996 Maxima GLE 220k miles (auto trans) which has a rather pronounced stumble/rough idle, only when the engine is warm, and the vehicle in gear. I also feel it randomly while driving, but only when the engine is under load (accelerating) at low RPM, under 1500.

I have three codes that are on all the time: P1400 (EGRC valve), P0500 (vehicle speed sensor), and V0600 (serial data something or other).

The stumble feels almost as if you cut the fuel for a split second. Idle will drop to 400 or so, then bounce back up, so it's literally a stumble, not really a "miss"-- it doesn't feel related to the ignition. It even killed the engine once, when the A/C was coming on at the same moment the idle was falling. It does it pretty consistently (once a second or so) at idle while in gear, but stops if you put it in park or neutral. If you're driving under moderate load while the RPM is low (below 1500 or so), it will do it then too, but not as noticeable. Feels just like fuel flow stops for a split second, I don't know how else to describe it. Car runs fantastic otherwise.

In an effort to chase down the problem, I just got done replacing the following: knock sensor & harness (previously was getting a knock sensor code, which is now resolved), EGR guide tube (new tube that goes from EGR manifold to upper intake), rebuilt fuel injectors, fuel filter, all EGR and intake manifold gaskets, PCV valve, air filter, probably a couple other little things I'm forgetting.

Basically I just went through everything down to the lower intake manifold-- checked for bad vacuum lines (replaced a couple), stuff like that. I knew the knock sensor was bad (mouse chewed through harness), so I tried to eliminate as many possibilities as I could, while I was into the engine that far. The knock sensor code is gone, but the darn stumble is still there....

I'm not exactly sure how the idle is controlled on a '96 VQ30. Solely by the IAC valve sticking off the side of the intake? Could this be the problem? I tried taking that apart, scraped some carbon off the inside, but it looked okay so far as I could tell. Fuel trims look good when viewed on a scan gauge. They were a bit erratic before I replaced the injectors, but have definitely smoothed out.

I'm not sure what to make of the EGRC, speed sensor, and serial data code. For grins, I tried a junkyard EGRC in place of my stock one, reset the codes, but the code came back. Speed sensor code is a mystery, my speedometer works fine. Just as clueless on the serial data.

I've been doing a ton of work to the car; replaced most front suspension components, water pump, hood struts, new brakes, etc. to try and keep this fine machine driving like new. Would love to get this last little gremlin chased out! Any help/suggestions/input would be greatly appreciated!