ECM fault causes loping idle in 91 Sable 3.8

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ECM fault causes loping idle in 91 Sable 3.8

Postby merricke » Fri Nov 09, 2012 6:20 pm

Case Study
Surging, loping idle in 91 Sable 3.8 V6. Cause: Engine Control Module

Loping idle in gear, got worse as engine warmed up. Usually okay from cold start.
Shifting could tend to be a little harsh at times.
Engine would usually stall after WOT test.
DTC: 326, EGR ckt voltage lower than expected

Possible causes based on input from various sources
Vacuum leaks Vacuum steady at 18 – 19 inches
Fuel pressure Disconnecting pres reg vac source had no effect
MAF sensor Cleaned sensor wires with magic cleaner- no change. Sensor steady at .6 volts, sometimes at .9 vdc, confusing but in specified range
TPS Resistance measurements smooth and steady. Powered output smooth and steady
EGR system Unplugged vac source- no change
EGR valve operated with vac tester (Mityvac)
Idle deteriorated as expected when EGR valve operated manually
EGR reg valve coil measured 33 ohms; cleaned filter- no change
EGR valve position sensor output in spec with manually applied vac
PCV Not checked
Air Filter Fairly new
AC load Not a factor
Idle Air Controller Cleaned and coil checked- no change
Injectors Not checked, power at speed okay
Ignition Not checked
Cam Not checked
Timing Chain Not checked
Head gasket Not checked

Testing (my inexperienced dead ends are not included)
MAF sensor seemed to be in range based on DVM measurement, .6 to .9 vdc. However, unplugging MAF sensor connector allowed a stable idle, was drivable with occasional stumble but no stall at idle.
Engine generally idled okay until it was shifted out of Park, then it started loping. Going back into Park did not return to a better idle but continued loping.
Time for the scope

MAF signal- unsteady and heavy spikes. Average DC about .6 vdc.
1 maf.psdata
(36.39 KiB) Downloaded 406 times

MAF after shifting out of Park- 1 vdc, sawtooth from MAF or elsewhere?
2 maf after shifting.psdata
(39.33 KiB) Downloaded 343 times

Alternator output and MAF sensor signal- oscillations and switching spikes present. Outside cause or by MAF? Average DC is about .6 vdc.
3 alt out and maf.psdata
(59.94 KiB) Downloaded 321 times

The oscillations would continue even after the engine had stalled. If key was turned off and then restarted, the oscillations were gone until triggered again by shifting.
All accessible power and ground circuits were checked, cleaned, etc.
I unplugged any accessible connectors. Idle improved and heavy spikes disappeared when the Axode2 connector to the transmission was disconnected. The transmission pressure control solenoid (EPC) was included in that connector.

On cold start, MAF and EPC solenoid, idle okay, EPC switching spikes showing up on MAF signal.
4 maf and eps, idle okay.psdata
(45.46 KiB) Downloaded 314 times

EPC and MAF power after shifting. Instability oscillations
5 eps and maf pwr after shifting.psdata
(47.59 KiB) Downloaded 320 times

I wondered about the EPC solenoid chattering or having a failed snubber, but the instability was too uniform. Applying an external snubber diode only made the oscillations worse. Seemed that the ECM was the primary candidate.
Opened up the ECM and found 3 bad electrolytic capacitors. After changing those, the car is back to its old self.

EPS and MAF signal, .68 vdc, small spikes probably normal?
6 eps and maf.psdata
(31.74 KiB) Downloaded 313 times

One of the replaced ECM caps was part of the reference voltage supply circuit for the TPS and EGR position sensor, and probably for the MAF sensor (but I can’t confirm). That may be the cause of the 326 code.

Without the use of the Pico scope, the car may have ended up with a new MAF sensor, TPS, EVR, EGR position sensor, EGR valve (?), etc. As it turned out the repair costs for components was nil.
Posts: 9
Joined: Fri Oct 19, 2012 3:08 pm

Re: ECM fault causes loping idle in 91 Sable 3.8

Postby AuthorMike » Tue Nov 13, 2012 11:04 am

Nice work, great result :)

Thanks for the submission!
Mike Valentine
Technical Author @ Pico Technology UK
Pico Staff Member
Pico Staff Member
Posts: 294
Joined: Mon Jan 31, 2011 11:22 am
Location: UK

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