Well, I'm back again. Things have been running well and reliably up until a few weeks back when my daughter limped it home barely running.

This is a custom build from 350 ZZ-3 Medium Duty GM short block, roller RV cam a bit hotter than most but with strongish vacuum (15 in hg and steady), cleaned up SR Torquer iron heads (built by Gruelick Speed Shop in Phoenix, focus low rpm with good upper rpm support), and orange injectors in a stock 350 size TB. Custom ECM built with Moats stuff and tuned with Tuner Pro and Auto Prom. Ran very strong and clean. It constantly get comments about it being one of the cleanest they have seen when it runs through emissions check (they always expect it to fail because it's a beat up rough POS looking off-road K5 without 1 straight panel). Anyway, I say all that to support that the basic config is known good and works well in general.

Now I could write a book about all I've been through. First thing I noticed is it's low on fuel pressure. Initial tune was at 19-20 PSI, when it came home it was at 16. But sure seemed to be running rich. Oh well, as always fix the pressure first, see what happens. First change filter (easy, cheap, and due), no improvement. Put in spare pump (keep one just in case needed 30 miles from pavement) through "custom" access panel in rear deck, no improvement. Pulled out the adjustable regulator and rebuilt with new spring (apparently old was weak, but not broken?), readjust back to 19 psi. Still acting up, seems rich, and ECM confirms "02 rich error". Dropped the pipe (have to in order to get to 02) and see that the 02 cage is 1/3 burnt away with some sort of barnacle looking deposits, but heavily carboned. Lean AND rich signs, WTF? At this point I'm thinking the pressure went low and made it lean to cook the O2 sensor, then something else took it way rich? Ok, replace obviously bad 02 sensor and pull plugs which are also heavily carboned but otherwise look fine. Seem fine other than the moderately heavy black carbon buildup, so clean, check gap, reinstall. Reset everything. Seems to be running better, but falls on it's face at tip-in off idle. If you can get it past tip-in, it runs good and strong. Check EGR by manual actuation and bypassing. Also note a "burble semi-want-to-back-fire note in the exhaust, actual back fire once and a while, and black smoke. Still rich. Rechecked timing again; included check at static +5 and +10, no real difference. Swap in stock ignition ECM (which should be WAY lean on this motor) and note still rich. Swap in stock pressure regulator at 12 PSI along with clean and check both IAC and TPS; and still way rich (at 12 PSI with stock ECM!!!). Decide that maybe primary ignition weak, so replace coil and ignition module. Still won't pick up off idle, often stumbles to the point of dying. Note that injector pods have very distinct pulses with good cones (especially at 20 psi) and do not leak at all, but I have no spares to try. Also MAP seems ok, may swap on a spare just for grins tonight. I have no spare ESC, not sure how to test. But timing moves around when disconnect is connected, seems to work ok?

Friend who is much better at this than I am comes by last night, brings his Snap-On Scanner. Spends almost 3 hours as he reviews and retests what I've done, draws a blank, but suggest maybe pick-up coil? Fine, I bought a new delco unit and installed, no improvement. The only thing I've not replaced on the primary ignition is plugs and wires (look good, no light show, and no "dead cylinder" signs noted).

The one thing I did note is that if the diverter valve is disconnected (plumbed correctly verified by core support tag and manual) from vacuum, the "burble and back-fire" gets worse, but it stops the stumble off-idle and it becomes fairly driveable. No idea what that means, but I think it's likely just masking the problem? Frankly, I can't see how the diverter could influence this at all, just noting what I observed.

I'm at a loss. ECM continues to note O2 rich error, and I've also noted an ESC error, though I think that was from setting the timing and didn't clear it. I've got new plugs, and will probably try them tonight, but this don't act like plugs, so no real hope there. Went to get some good Delco wires, but they were out and too late for alternatives. I got some Parts Master (supposed) "Premium" 5 year plug wires, and may try them in desperation, but would rather return on Monday for better wires unless someone convinces me that it is a good next step (never seen wire problems like this). I just need to get past this...

That's actually an abbreviated story. Many of these things I've checked more than once, and in different ways (like EGR). But I'm about tapped out on ideas and resorting to "scatter gun" parts changes. Any suggestions???

 

if the diverter valve is dumping fresh air into the exhaust up stream of the O2 sensor, the motor will run very rich.
i've replace a good number of diverter valves for that very reason.
some of those would show rich on the O2 sensor but the ECM continued to add fuel for some reason.

 

Thanks.

The truck motors run a large air line (maybe 5/8"?) from air filter housing (unfiltered side) down to the valve at the front of the passenger valve cover. There is then a second composite line (steel and rubber) that goes around (for some crazy reason) the back of the motor to a fitting on the #1 cylinder manifold tube. There is also an electrical connection (2 wire?) and vacuum going to the valve.

So, how is that valve tested/repaired/whatever? I assume replacement it the "fix", but is there some way to accurately test? I noted that with the vacuum to the diverter valve plugged, the burble/back-fire got worse, but the tip-in stumble was completely gone. With it connected to vacuum, the burble/back-fire is mostly gone, but it stumbles hard on tip-in. Not sure if that helps, but figured I would toss that out...

 

Ok, borrowed a known good ESC and injector pod assembly from a friend's similarly built Nova. Made no difference at all. Also disconnected the diverter valve and verified the check valves working correctly, then caped the line completely. Also, like everything else, no difference. How is this possible. It's now running 12 psi and a stock ECM on a very free breathing motor and it's STILL running rich enough to make distinct black smoke even at idle, and billows pretty good when you rev it a bit. The entire primary ignition has been tested, and ultimately rebuilt in desperation. The entire fuel system has been tested good, and spite of that, either rebuilt or swapped with known good components. The air intake doesn't offer a lot of choice, but with new filter or with filter off, it makes no difference. Valves have been checked AND adjusted, with compression test showing 150+ psi on every cylinder within the second or 3rd puff. Lean I can see maybe having trouble tracing down due to "false air" or something, but rich should be a piece of cake. It runs too smooth (when not stumbling off idle) to really suspect any of the cylinder specific secondary ignition (plug wires and plugs) since it should be inducing a noticeable miss/vibration, and I don't have that at all. And it's not a matter of confused perspective, I can confirm my expected results by pulling any wire and note the clear miss/vibration that is otherwise not present. Note, I already tried replacing the coil wire since it is common to all cylinders.

Tomorrow I'm going to do a verified TDC test to check is the timing mark may have shifted. And I'm going to replace both wires and plugs. Might as well, everything else is new. I HATE just getting into a parts swap marathon, but obviously that's what I've been reduced to here. I've never seen anything like this. It's got to be something simple I'm overlooking...

 

The large valve from the valvecover to the intake is to burn off the gasses from the motor in the oil. I've got one on my T/A.

Maybe I missed it, but have you ruled out all of your other sensors? MAF, MAP etc? I know if one of those goes bad it can cause some strange stuff to happen.

 

The PCV has been both tested good and eliminated completely at various times. Likewise the EGR control solenoid and EGR was manually tested. MAP, TPS, and ESC have been swapped with known good, O2 sensor is new. Likewise the entire upper throttle body pod (injectors and regulator), and pressures ranging from the original 20 psi down to 12 tried. Entire primary ignition was rebuild with new pick-up coil, module and coil (technically both primary and secondary). Common areas of secondary also replaced including new cap, new rotor, and coil wire. Grounds were tested and reseated. All external vacuum eliminated. Diverter/AIR system eliminated/plugged for testing. And stock ECM was installed for testing.

Today I think I'll be putting on new wires and plugs just to complete the entire ignition rebuild. Not sure where else to look.

 

have you checked to make sure you dont have a pluged cat or perhaps the timing chain jumped a tooth?

 

No plugged cat, it runs great if you can get it off-idle. Likewise for jumped timing.