TBI Rough Idle when warm

[studigggs]

So I noticed my '90 K1500 5.7L was idling rough in closed loop a couple weeks ago. Hooked up the scanner today and see that at warm idle, my IAC was dropping to 7-13 whereas I recall it generally stays around 30-40 at warm idle (been a little while since I've scanned this truck). In closed loop my Integrator was also down to 118 when it was rough running, so I assumed I had a fuel issue. Fuel trims pop back to 128 +/-2 above 700 RPM. I checked out the injectors and see decent spray patterns with no detectable drips. I do have some seepage around the injector hold-down (see pic below), so could this small leak be enough to impact the A/F ratio at idle, but at cruise its just in the noise? Its been 35k since a tune-up, so I'll add a TBI rebuild kit to the list. Unfortunately I wont have time to work on this thing until 2020, but I do have an hour or so a day to check sensors/scan data. Open to corrections/suggestions/thoughts in the meantime.

Things I've checked:

Checked base timing and its right at 0

Secondary ignition: 2-4-6-8 are running consistently 2 kV higher than the driver bank on Spark and Power KV which, I think, reinforces my leak assumption since The O2 sensor only reads off 1-3-5-7. Could more fuel be getting to the passenger bank and the ECM cant detect/correct for it leading to higher KV?

I do see the IAC indexing in/out with throttle snaps, and there is some crud down in the corner across from the pintle (another sign its time for a rebuild)

Fuel Pressure: Havent checked, but I now have an excuse to install that schrader valve at the TBI which I purchased a few months ago.

https://imgur.com/vFCOLC6

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[Supercharged111]

Is it down on power too or just the idle? I had a plugged fuel filter give me those fits on my TBI truck.

 

[studigggs]

Is it down on power too or just the idle? I had a plugged fuel filter give me those fits on my TBI truck.

No power loss since I've noticed this stumbling. Fuel filter was replaced with a delco a few thousand miles ago.

 

[PlayingWithTBI]

What distributor and ICM do you have? I've seen a lot of issues with the ICM going out within a couple thousand miles. You can spray water on the coil and see if it stumbles too, a Q&D test.

 

[studigggs]

Distributor is factory. ICM is ACDelco D1943A which i replaced in 2015, coil is a Delco D503A which was installed in 2017. I'll try the water spray tonight. Thanks.

 

[MIHELA]

Also check the pickup coil. They can cause random idle misfires which the computer cannot detect but would show as a rich condition.

 

[Schurkey]

Checked base timing and its right at 0

With the wire disconnected?

Secondary ignition: 2-4-6-8 are running consistently 2 kV higher than the driver bank on Spark and Power KV which, I think, reinforces my leak assumption since The O2 sensor only reads off 1-3-5-7. Could more fuel be getting to the passenger bank and the ECM cant detect/correct for it leading to higher KV?

A lean mixture should have higher firing voltage than a rich mixture, if both are reasonably close to correct.

random idle misfires which the computer cannot detect but would show as a rich condition.

Misfire results in a false-Lean indication from the O2 sensor. All the oxygen that DIDN'T combine with the fuel drives the O2 sensor crazy.

 

[studigggs]

I am an extreme novice on secondary ignition, but if there was a primary ignition issue, wouldn't I see that in the KV data? Power/Spark KV on driver side was at 10/2 kV respectively and the passenger bank was at 12/4 kV. When it stumbled at idle, I saw the passenger bank (I caught cyl 6 and 8 on the scope) jump up to 15-16kV on Power kV and 5-6 on Spark. I didnt see any variations in Power or Spark on the driver side during the stumble. I cant see an actual waveform with my ancient MT2400, but If it was an ignition miss, wouldn't I see lower or no value on Power kV during the stumble? And since the entire passenger bank had higher KV than the left, and the only thing 2-4-6-8 have in common is air and fuel, wouldn't it point to one of those as the culprit? Not doubting the suggestions I should check my primary side, but just asking for my own diagnosing knowledge.

 

[studigggs]

With the wire disconnected?


A lean mixture should have higher firing voltage than a rich mixture, if both are reasonably close to correct.


Misfire results in a false-Lean indication from the O2 sensor. All the oxygen that DIDN'T combine with the fuel drives the O2 sensor crazy.

I did disconnect the wire when setting base timing. Good to know on the firing KV relative values when Rich/Lean as well. I had that backwards.

 

[Schurkey]

I am an extreme novice on secondary ignition, but if there was a primary ignition issue, wouldn't I see that in the KV data? Power/Spark KV on driver side was at 10/2 kV respectively and the passenger bank was at 12/4 kV.

I've never used a Snap-On MT2400. What is "Power/Spark" referring to? I understand voltage required to fire the plug would be in the KV range, but I don't understand the terms "power" and "spark" in this application. Is "spark" the millisecond duration of the spark? In general, as voltage goes up, duration goes DOWN, which is the opposite of what your results are showing.

"Primary" and "Secondary" ignition are of course very closely related. A problem on the primary side can easily show up as a problem on the secondary side, too. However, some primary-side problems won't cause low output on the secondary side; in part because most folks look at voltage and largely ignore duration; and in part because some primary problems are more related to timing than to output power.

When it stumbled at idle, I saw the passenger bank (I caught cyl 6 and 8 on the scope) jump up to 15-16kV on Power kV and 5-6 on Spark. I didnt see any variations in Power or Spark on the driver side during the stumble. I cant see an actual waveform with my ancient MT2400, but If it was an ignition miss, wouldn't I see lower or no value on Power kV during the stumble?

Depends on the reason for the misfire. A mixture too lean to burn would drive the voltage higher because air is less conductive than a burnable air-fuel mixture. OTOH, if the spark were grounding or the coil was weak, the KV would be low.

And since the entire passenger bank had higher KV than the left, and the only thing 2-4-6-8 have in common is air and fuel, wouldn't it point to one of those as the culprit?

2-4-6-8 don't share "fuel". Like most V-8 vehicles with a "wet" intake manifold, the TBI uses a dual-plane intake system, where the outer two cylinders of one bank, and the inner two cylinders of the other bank share fuel.

So 2-3-5-8 share one injector, and 1-4-6-7 share the other injector. If you look at how the intake manifold runners are laid-out, you'll see how the injectors are shared among the cylinders.

A 2KV difference in firing voltage could EASILY be spark plug gap or condition. "I" would be looking at normal tune-up stuff: Plug condition, plug wire condition, cap/rotor condition, etc.

Yes, pickup coils can be a problem. I had a high-speed misfire on my '88, that got worse and worse (lower-speed misfires) until the truck stalled at a stop sign and wouldn't re-start. Pickup coil wires can break, or the magnets get weak; either one reduces or ends the signal to the ignition module.