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-Injection/showthread.php?5381-Wide-Band-O2-(WBO2-Tuning)-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

[Rittercon]

I'm going to research the arcing potential. That sounds very plausible.
Thanks-

[Rittercon]

Turns out the culprit was the ignition module. About a year ago I replaced my stock AC Delco distributer with a Pertronix D1020 Flame-Thrower HEI distributer.

Just for grins, I took out the Flame Thrower's ignition module and replaced it with my old stock AC module. Problem solved and no more stumbling!

At this point not sure if I should get a new module or just leave the old one as-is.

[PlayingWithTBI]

I have been having the same issue with the timing jumping at ~2000 - 2400RPM (around 60 - 65MPH w/4,10 gears) in my 88 5.7. You can see it at 0:12:53:852 on this log.

I was wondering if the ICM was the issue. I bought a Spectra Premium GM04 distributor and a PerTronix D3002 coil (only 3K Miles on them). I think I'm going to buy an AC Delco and try it too! Thanks for this input! :rockon:

[PlayingWithTBI]

BTW I read somewhere you need to use the good quality dielectric grease under it, anyone else have any knowledge about this?

[Rittercon]

When I removed the Pertronix ICM, there was a coat of thermal paste between it and the distributer. It looked like the stuff used on computer CPUs. My old AC Delco ICM didn't look like it had anything on it. Just to be safe, I bought a tube of Arctic MX-4 Thermal Compound and coated the bottom of the AC Delco before installing it.
Good luck with your spark advance issue.

[1project2many]

Thermal compound is the preferred solution. Dielectric grease does not aid in heat transfer. If the distributor is corroded and showing white powder where the module attaches it's a good idea to try and clean it.

[PlayingWithTBI]

Thanks guys! I ordered the MX-4 and the AC Delco 19179580 (ADO-1984A Summit Racing PN) ICM. It'll be a couple weeks before I get to it but I'll post the results.

[brian617]

A used GM module is almost always better than any aftermarket I've come across lol.

[PlayingWithTBI]

Unbelievable! I put in the AC Delco ICM with the MX-4 compound, dielectric silicone on the connectors and it runs GREAT!!! I thought I was getting a high quality distributor but, after 3,000 Miles?!?! I'm sold!

[Rittercon]

Always satisfying when you follow a hunch and it turns out to be the solution. Congrats.