Surging idle, 91 Sable 3.8

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Re: Surging idle, 91 Sable 3.8

Postby merricke » Wed Oct 31, 2012 5:30 pm

Happy Halloween

Hey, you guys are a great help keeping me thinking. I’m finally learning about fuel injection and also getting some time with the scope.

Latest:
From a cold start the EPC is working fine, shifts are smooth and idle is good. Over 20 minutes or so things start to fall apart. Idle is stumbling and shifting is getting clunky.

I’ve rechecked power and ground circuits and can’t find any smoking bullets.

What I think is happening is that the ECM is going unstable and oscillating at the EPC output. Even when okay, the edge spikes from the EPC switching are showing up on the power bus. When the oscillations start, the EPC function starts going away and the MAF sensor is affected, leaving a lack of tranny pressure control and ECM problems.

Waveform of the EPC solenoid and the power bus taken at the MAF sensor. I’ll assume this is the proper EPC control at idle.
sol and pwr, idle.psdata
(27.29 KiB) Downloaded 169 times


After shifting or warming up, the oscillations start.
sol and pwr, after shifting.psdata
(43.29 KiB) Downloaded 176 times


To me, the evidence is pointing to problems in the ECM. BUT…. I’m having trouble getting it out. It’s buried behind the AC drier/accumulator and I haven’t been able to get the connector disconnected. Anything I’m missing about that connector (the center bolt is loose, but the connector won’t budge)?

Thanks again.
Merrick
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Wed Oct 31, 2012 7:34 pm

Merricke,
I just looked at your waveforms. I'm really shocked to see so much movement in the power circuit. Not exactly sure where you are taking this signal from but I'm very skeptical of it. It looks like some of the readings I've done myself.

Can you try this - Just look at the power lead. Remove the probe from your signal line completely. Maybe even just run one channel on the scope. See if you still see this noise on the power line. Somehow I don't think you will. I don't see how that one solenoid could swing the battery voltage so far....how is it going to overcome the battery itself? Double check yourself. If I'm wrong, I'm wrong. But if I'm right you're not looking at a "real" problem.


Marcello
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Re: Surging idle, 91 Sable 3.8

Postby merricke » Mon Nov 05, 2012 3:48 pm

Hi again.

I assume you’re concerned about external noise or scope and/or setup problems. I tried a few setup changes and checked for noise from my fluorescent trouble light, but couldn’t find any problems. The back emf of 2v on the EPC solenoid seemed pretty big, but ?? I had determined earlier that the heavy impulse switching spike peaks increased further I got away from the battery post, but there was no deterioration in the DC level, so I wasn’t suspecting power or ground problems.

The good news is that I was able to get the ECM out and discovered 3 leaking and/or bad electrolytic capacitors in residence (after only 21 years). One was a tank/filter for the TPS and EGR pressure feedback sensor reference voltages. I think it’s reasonable to assume that all three are power supply related (but I couldn’t find a schematic anywhere) so I’ll stay with my instability/oscillation theory. Anyway, after replacing the caps the problem is solved, engine is performing well and the tranny is shifting like an old lady car again.

EPC solenoid and alternator output voltages. Still looks like 2.3 volts of back emf unless this is an overall level shift, but I don’t know how. The little jitter on the power line may be a probe connection issue.
sol and alt output.psdata
(23.16 KiB) Downloaded 169 times

EPC solenoid and power at MAF sensor (far down line from battery post)
sol and maf power.psdata
(24.86 KiB) Downloaded 168 times

EPC solenoid and MAF sensor signal
sol and maf signal.psdata
(31.6 KiB) Downloaded 167 times

EPC solenoid and MAF sensor, wot test. Still some random spikes but I’ll have to wait for them to become more permanent.
sol and maf, wot test.psdata
(54.93 KiB) Downloaded 170 times

I opened up the EVR valve to have a look. The unit does have a calibrated vacuum leak and mine had a failed shorted snubber diode. After replacing the diode the valve is working and all the dtc’s are gone.

Thanks again for the help. I’ll summarize this episode and put it in the case history file so maybe I can win the prize 8 channel, 20 GHz Pico scope kit…..

Merrick
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Re: Surging idle, 91 Sable 3.8

Postby Avdr » Tue Nov 06, 2012 6:47 pm

Pleased to hear you got it sorted!
Have enjoyed following your thread on this car, and especially your last post finding the capacitors to be duff. It's been very informative.

Andy
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Wed Nov 07, 2012 4:56 am

Congrats on getting it figured out. I really wish I was there. I don't think I've ever seen a problem like that one. I really wonder if I would have caught it? I'm still surprised by what I was seeing in the ground circuit waveform. I didn't want to believe it. Glad to see you not only held to your finding, but took it beyond what most mechanics I know could do. You fixed the computer without replacement.

Mab
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Re: Surging idle, 91 Sable 3.8

Postby merricke » Thu Nov 08, 2012 4:07 pm

Thanks guys, I appreciate your support on this project. I really learned a lot about engine/ transmission control.

After too much time staring at the engine wondering what the heck was going on, it's good to know others enjoyed the adventure. My wife's collective comment was, "I don't care, it's just a vehicle" (Maybe she was hoping to upgrade).

Merrick
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