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Mercury Sable 3.8 V6 - Everyday Diagnostics


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Diagnostic trouble codes (DTCs)

326 - EGR circuit voltage lower than expected

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By Merrick Endres

  • Vehicle: Mercury Sable 3.8 V6
  • Year: 1991
  • Symptom: Surging 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.

Diagnostic trouble codes (DTCs)

326 - EGR circuit voltage lower than expected

Possible causes

  • Vacuum leaks: Vacuum steady at 18 – 19 inches
  • Fuel pressure: Disconnecting pressure regulator vac source had no effect
  • MAF sensor: Cleaned sensor wires with magic cleaner- no change. Sensor steady at .6 V,
    sometimes at .9 V DC, 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 regulator 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
  • ECM


MAF sensor seemed to be in range based on DVM measurement, .6 to .9 V DC. However, unplugging
MAF sensor connector allowed a stable idle and the vehicle was drivable with the occasional stumble but
no stall at idle.

Engine generally idled okay until it was shifted out of Park, and then it started surging. Going back into
Park did not return to a better idle and continued to surge.

Time for the scope!

Here we have the MAF signal waveform, showing steady and heavy spikes. Average DC around .6 V

Here we have the MAF waveform after shifting out of Park- 1 V DC, saw tooth from MAF?


Here we see the alternator output and MAF sensor signal, where oscillations and switching spikes are
present. Could this be caused by the MAF or something else? Average DC is about .6 V DC


The oscillations would continue even after the engine had stalled. If the key was turned off and then
restarted, the oscillations were gone until triggered again by shifting. All accessible power and ground
circuits were checked and cleaned. I unplugged any accessible connectors; this improved the idle 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.


EPC and MAF power after shifting. Instability oscillations


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. It seems that the ECM is
the primary candidate. Opening up the ECM revealed 3 bad electrolytic capacitors.


After changing the 3 electrolytic capacitors, the car is working correctly and as expected.


EPS and MAF signal, .68 V DC



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. That may be the cause of the 326 code.
Without the use of the PicoScope, the car may have ended up with a new MAF sensor, TPS, EVR, EGR
position sensor, EGR valve, etc.

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