Hard stumble under load

[Rittercon]

I have an intermittent "hard stumble" in my '94 Chevy 350 pickup. It occurs when the truck is in the 1,700-ish rpm range and MAP is at 75 KPA-ish. It's a quick event; the motor takes a hard stumble and then recovers. It doesn't happen all the time at that rpm/map range; just sometimes.

I can see the event in my TunerPro log files, but don't know what's causing it. Here is a table that shows the event, plus a few before/after consecutive events. I've highlighted the event in bold italic and used red font for categories where obvious differences occurred, some of which may not be relevant.

TunerPro Sample #	Battery Voltage	Block Learn Add	Block Learn Cell	BLM	Fuel Pump Voltage	Fuel Usage	IAC Desired	IAC Preset Motor Position	Inj DC%	Inj BPW	MAP	Spark Advance	ECU Loop Ref Period	Tach RPM	Knock Counts
6765	13.4	FALSE	10	125	13.7	14.7	68	68	10.48	3.75	74.2	29.9	1666.17	1675	303
6766	13.5	FALSE	10	124	13.7	14.9	68	68	10.35	3.71	73.8	29.9	1666.17	1675	303
6767	13.5	FALSE	10	124	13.7	15.1	68	68	10.27	3.68	74.2	29.5	1666.17	1675	303
6768	13.4	FALSE	11	124	13.8	3.9	68	68	40.38	3.85	74.2	19.7	6301.54	6300	303
6769	13.4	TRUE	10	124	13.8	14.4	105	77	10.73	3.85	74.6	23.9	1683.29	1675	303
6770	13.4	FALSE	10	124	13.8	14.6	105	91	10.73	3.85	76.0	26.0	1666.17	1675	303
6771	13.4	FALSE	10	124	13.7	14.5	105	105	10.82	3.88	76.4	27.1	1666.17	1675	303

The knock counts start at 303 and don't change the entire log and there's no knock retard recorded.
I'm running a BHMX 16196395 $0D chip that I've modified to improve the VE and Spark curves. The truck has a mild cam, Summit aluminum heads, shorty headers, stock TBI & manifold, all CA emission approved.

Greatly appreciate any thoughts!

[dave w]

I wonder if the AE is Active with the stumble? I wonder if PE is active with the stumble?

I'm also curious, why some of the data at sample 6768 become odd? Seemingly the Inj BPW and MAP at sample 6768 are as expected.

dave w

[Rittercon]

Both AE and PE are inactive throughout the event.

The things that jump out are the Inj DC%, Spark and RPMs and I wonder how they're related, cause and effect-wise.

For some reason the ECU Loop Ref and Tach values jump to 6300rpms. In my TP Main Spark Table, the corresponding spark advance at that map/ rpm is 31* vs. 19.7* in the log, so there's a temporary disassociation between spark and MAP/RPM during the event.

All of the values except spark and IAC stabilize immediately after the event. Spark takes about 5 subsequent events to stabilize and Desired IAC takes about 47 events to get back to 68 from 105. IAC Preset catches back up to Desired IAC in 3 events.
FYI- each event lasts about 9/100's of a second, so the entire 7 events in the Table above represent about 6/10ths of a second and the 47 events for the Desired IAC is a little over 4 seconds...

Really have no idea how Inj DC% relates to spark, map/rpm, IAC, etc.

In general, the problem occurs while driving on the highway at 55-60 mph. My motor is running 1675-ish RPM & 45-ish MAP at that speed. When I encounter a hill or long grade, the MAP starts to increase as the motor begins to work a bit. Generally the stumble occurs when MAP is in the 75 kpa range. If it's a steep grade, the MAP keeps increasing until the truck downshifts and there's no stumble. It's when the MAP is relatively stable in the 75 kpa range that it does the stumble.

[dave w]

AE is similar to carburetor Pump Shot. PE is similar to the back barrels of a 4 barrel carburetor opening up. One possible cause of a stumble is no "PUMP SHOT". It would be challenging to know what the PCM will do, when the PCM attempts to compensates for No Pump Shot?

Ultimately, a Wide Band Oxygen Sensor (WBO2) is required for "tuning AE & PE". Link to WBO2 tuning spreadsheet:http://www.gearhead-efi.com/Fuel-Inj...g)-Spreadsheet

AE - AFR should be in the 12.5 to 13.5 range.
PE - AFR should be in the 12.0 to 13.0 range.

Without AFR information, troubleshooting a "Hard Stumble" is CHALLENGING!

dave w

[Rittercon]

Thank-you Dave; that is helpful.

[steveo]

i'd maybe look at an intermittent fault with your distributor base or whatever provides timing reference to the ECM. you didn't jump to 6000+ rpm in one sample, however the rest of the data in that sample remains valid so one would assume datastream isn't corrupt.

the condition that it occurs in...? sometimes under increasing load, a spark leakage issue will be exasperated, maybe that arcing is causing interference with something.

in short something falsely convinced the ecm that you were at redline, and it tried to fuel accordingly

just something to consider