Surging idle, 91 Sable 3.8

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Surging idle, 91 Sable 3.8

Postby merricke » Fri Oct 19, 2012 10:57 pm

Hi
Patient is a 1991 Mercury Sable, 3.8 liter V6.
Over last couple of years it will stall at idle, then run fine for weeks.
Recently it has developed a surging idle and stall after shifting into gear or after warming up. Engine runs better with MAF sensor disconnected but still has occassional idle stumble w/o stalling.
Scope shows periodic hash on various wires, including alternator output.
I'm not sure what proper waveforms should look like or if the hash is related to the problem.
Idle vacuum is steady at about 19 inches, no leaks that I can find.
The MAF sensor wires are clean.
Check Engine light not on except with MAF disconnected.
Any help would be appreciated.
Waveforms:
MAF signal ground to battery ground, engine off
MAF sig gnd to bat gnd, eng off.psdata
MAF signal ground to battery ground, engine off
(26.34 KiB) Downloaded 180 times


MAF signal ground to battery ground, engine on
MAF sig gnd to bat gnd, eng on.psdata
MAF signal ground to battery ground, engine on
(30.57 KiB) Downloaded 179 times


MAF to battery ground, idle smooth
MAF sig to bat gnd idle.psdata
MAF to battery ground, idle smooth
(36.29 KiB) Downloaded 160 times


MAF, wot test
MAF wot tst bat gnd.psdata
MAF, wot test
(50.74 KiB) Downloaded 174 times


MAF after shifting into gear (and back to park), idle poor
MAF after gear shift.psdata
MAF after shifting into gear (and back to park), idle poor
(39.11 KiB) Downloaded 170 times


MAF, wot test after shifting, signal clearer at higher RPM
MAF wot test after gear shift.psdata
MAF, wot test after shifting, signal clearer at higher RPM
(61.23 KiB) Downloaded 169 times


Injector, idle surging (x10)
INJECTOR engine surging.psdata
Injector, idle surging
(63.6 KiB) Downloaded 180 times
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Re: Surging idle, 91 Sable 3.8

Postby Robski » Sat Oct 20, 2012 8:18 am

Do an alternator ripple test.

Load test the grounds from batt ve- to engine block to PCM - MAF etc.

Any DTC's in any modules ?
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Thu Oct 25, 2012 7:37 pm

If I remember right. It's been a while since I've worked on one of these. Going along with what Robski asked you. Grounding problems were notorious on these cars. There is black metal box, "the Integrated Control Relay module" that sits right above and behind the radiator that holds the relays for fuel pump, electric fans, etc,.. I believe there are two ground leads that comes from this module and connect at the negative battery terminal. Looks for loose or in an way poor connections. If this problem seems to happen when you come off the freeway look to a sticking overdrive solinoid in the transmission. EEC_IV Ford cars would not set codes easily. You could have something that was slightly out of range, just enough to cause a problem, but no code would be set. An example of this is a tps reading of something like 1.2 volts at idle. Won't set a code, but the computer will think that you are at part throttle, and not closed throttle. And thinking that will apply vacuum to the egr valve when you don't want it. This can cause the car to stall. I will get you a cheat sheet at the end of my work day to help you figure it out.

Mab
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Thu Oct 25, 2012 8:12 pm

These are for a 1990, but almost identical. I already had them scanned in. This is from a book that really taught me how to look at Ford EEC-IV cars that were a problem. Usually you would hook a break-out box up between the computer and the wire harness to do your diagnosis, but you can do without.

Page372.jpg


Page373.jpg


Page374.jpg


Can you get a hold of a scan tool for this car? You will want to look at fuel trim data. In '91 it should be there. Surging idle would have me looking immediately at long term fuel trim, and at the egr valve. Is the computer engaging it when it should be? You also have two oxygen sensors, one for each bank of the engine. Are they both working?

Good luck,

Mab

I just switched from tif to jpeg. Hopefully this will help you. Tif is set up nicely for printing but this should work too.
Last edited by hexibot43 on Mon Oct 29, 2012 4:10 pm, edited 2 times in total.
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Re: Surging idle, 91 Sable 3.8

Postby merricke » Fri Oct 26, 2012 9:44 pm

Hi again, thanks for the tickler, it got me going on a long learning process on how this fuel injection system works.
And Mab, but I can't open the links (red X showing)

Here’s what I think I know:

Code 326 (Pressure Feedback EGR Circuit Voltage Low) keeps getting set. I think the EVR (EGR regulator valve) may be at fault because it doesn’t seal in any direction, or does it have a built in bleed function? But I don’t think this is the main problem.

Alternator output seems okay.

Grounds seem to be okay.

MLPS (manual lever position sensor, aka transmission range sensor) may be a factor, I’ll get it cleaned up and recheck.

A transmission solenoid circuit (pressure regulator (or lock up) ??) by either the solenoid or the ECM may be the main problem. No codes coming up but when this circuit starts hyper-PWM regulating, the MAF starts a saw tooth oscillation with a loping idle. The hyper regulating starts when the tranny is shifted out of neutral. The solenoid regulation is creating spikes on the power line and is reflected on the MAF signal.

Alternator output and MAF, idle
alt and maf idle.psdata
(59.7 KiB) Downloaded 171 times


Alternator and MAF, stalled
alt and maf stalled.psdata
(58.62 KiB) Downloaded 162 times


Alternator and MAF, MLPS connector disconnected
maf and alt idle mlps discon.psdata
(43.98 KiB) Downloaded 160 times


Alt and MAF, MLPS and AXODE2 connectors disconnected (opens solenoid ckt)
alt and maf mlsd and axode 2 discon.psdata
(35.23 KiB) Downloaded 164 times


MAF and tranny solenoid
maf and axode sol.psdata
(45.5 KiB) Downloaded 157 times


MAF and tranny solenoid, WOT test.
maf and ax sol, mlps open, wot test.psdata
(83.84 KiB) Downloaded 166 times
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Re: Surging idle, 91 Sable 3.8

Postby merricke » Fri Oct 26, 2012 10:04 pm

Hi Mab

No, no access to a scanner. Local parts stores only have newer readers.

With the axode2 connector removed the power line and MAF hash goes away and idle is perfect.

I found a couple of ground connectors that looked suspect but no improvement.

I tried uploading some more waveforms but...?? (problem with being a newbie)

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Re: Surging idle, 91 Sable 3.8

Postby merricke » Fri Oct 26, 2012 10:11 pm

hexibot43 wrote:These are for a 1990, but almost identical..

reply test
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Sun Oct 28, 2012 7:03 pm

I just updated my post with the pics...and changed them jpeg. I can click on each one and get to the image for viewing and downloading. I hope you can? I guess we'll see. If this doesn't work I'll put them somewhere else and send you a link. I had a few moments at the shop but have to get out of here and take my family out for the day. When I get home tonight I'll write more.

Marcello aka Mab
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Re: Surging idle, 91 Sable 3.8

Postby merricke » Mon Oct 29, 2012 4:23 pm

hexibot43 wrote:I just updated my post with the pics


Got 'em- thanks.

I've confirmed that it's the tranny pressure regulator (EPC in the Ford book) that can be disconnected and without its switching spikes, the power lines are almost completely clean, the MAF signal is stable, and the idle is good. The EPC solenoid is in spec at 4 ohms and clicks when manually energized. I haven't checked the pressure, but shifting is harsh while the ckt is open. I thought maybe a snubber diode across the solenoid had opened up, but sticking one on didn't help.

In the meantime, the EVR solenoid has failed shorted so the EGR valve is not operating (can't tell that this is a problem).

Questions:
-Know what the EEC-IV drive ckt for the EPC solenoid is?
-Know what the duty cycle for the EPC should be at idle? (I subbed in a 1K resistor for the solenoid and got a nice square wave but at a longer period duty cycle, 59% over a period of .8 sec w/engine off)

I'll do some more checking of the power and ground circuits.
Thanks again.
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Re: Surging idle, 91 Sable 3.8

Postby hexibot43 » Tue Oct 30, 2012 5:14 am

Merricke,

I honestly don't know how those guys that run help lines do it. I am very hesitant to go along with your call of EPC. Now I am not saying it's wrong but I worry you are being mislead. I will admit I've never had to deal with one. I've never seen one go bad but that doesn't mean it doesn't happen. It is used to adjust the control pressure of the transmission.

http://www.ehow.com/info_7752506_epc-transmission-switch-do.html

By reducing the load on the engine you can disguise the problem. And I know you are seeing noise, that goes away when you do this but sometimes things aren't always clean. I've seen a lot of signals that I thought were wrong because they didn't look like a book, but they were real world. Now I've seen lots of lock up torque converter solenoids go bad. Could you be reducing the extra load that is being applied by the lock-up torque converter? If you hook the EPC solenoid back up how does the vehicle drive off of idle? Shifts good? Shift timing right? If so, please do some more checking.

I am sorry I'm not being more help.
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