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post #1 of 112 (permalink) Old 09-15-2018, 05:33 PM Thread Starter
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Odd idle issue

First new post and hello to all. I've read thru dozens if not hundreds of idle threads going back more then a decade and have noticed that this seems to be a real problem for the M106 engine series. On to the problem and what's been covered.
1991 W126 M106 Engine.
Idles right at 735rpm in Park and will drop to 600 in Drive. While at idle it has this odd shudder or intermittent miss.

here's the history.
Pulled and rebuilt the engine at 186k miles when the head gasket popped. Had the head rebuilt with new valves, seals, etc.. Liners were near perfect and only required new rings. new bearings gaskets etc.. After getting all together it ran like a dream, purred so quiet at idle it you'd guess it was not running. it's been 5yrs and ~40k miles.. and that idle went poof, bye bye. The engine rebuild also included replacing all rubber bits, vacuum lines and rubber nipples.

Last yr i replace the following:
Fuel pump
Fuel Accumulator
Filter and all hoses
Fuel injectors and 'O' rings
EHA valve as the original sprung a leak
Upped the Alternator to a unit from the newer 500 if I recall correctly, much bigger and more amperage. Headlights actually work only slightly dimmer now

For those wondering the idle was perfect then and continued so after the work. The idle recently as in back in March went sideways.

I've in this last week replaced these items:
Cap
Rotor
wires
plugs (NGK non resistor)
O2 sensor
New Catalytic converter (CA)

I've set the duty cycle to 47~50% on negative reference.
Upper chamber pressures are 80psi lower 76psi
EHA valve at idle is 0mA, -45 on deacceleration and ~+15 when blipped.
Vacuum gauge reading at idle is 17" Hg steady even with the funny miss/shudder.
O2 sensor reading at idle is avg of 0.44V with cross counts from 0.1 to 0.9V Scope trace shows equal distribution of wave form. On the road under load the sensor reading is centered with a range from 0.37~0.4 at part throttle and 0.4~0.5 WOT.
Acceleration is not as it once was, a bit sluggish.

5 gas analyzer readings are as follows post cat:
at idle:
HC: ~250
CO: 0.5%
CO2: 13.8%
O2: 1.5%

Under load 1500rpm:
HC: 105
CO: 0.09%
CO2: 14.6%
O2: 0%

I removed the OV relay and checked the components, solder joints, no problems - didn't expect any as it's also only 3yrs old.
I've double checked the coolant sensors, air temp sensor, fuel pressures etc.. and can not find anything out of spec. Yet it will not idle correctly. Per the gas readings it's rich at idle and under load a touch rich but not what I'd call crazy. Could do with a touch leaner to help the burn be more complete but if I adjust the EHA and plate for a leaner setting the idle issue gets worse and acceleration goes out the window.

Fuel lines are all clean, balance test does not change the idle problem. pulling plug wires one at time also has no effect on the idle problem. So it's not a single cylinder that's causing this. Plugs are all tan and clean, no soot in the exhaust manifolds either.

This is a most baffling and frustrating thing to sort. I bought this car 8yrs ago from the original owner as it was pampered and well taken care of, near perfect condition. To have this issue now ruins the driving experience. Last year I also went thru and replaced all the rubber suspension bits, new shocks, wheel bearings and brakes. That was a lot of work as I was quite surprised at the sheer amount of rubber in the steering and suspension.

Does anyone have any idea as to why it shudders in drive at idle? something iv'e overlooked? I'm not overly familiar with these vehicles or engine family but do know my way around the Bosch CIS system.

here's one last odd bit, using the amp loop to watch the idle RPM with the DMM the shudder/miss corresponds to a blip in the RPM and moves randomly along the cylinders. In other words it's not specific to any single cylinder. So the RPM will read a steady 730 then blip to ~1100, the tachometer does not show this I only noticed when checking the RPM via the DMM and then checking all the cylinders. It could very well be SNR aka noise as the plugs are non-resistor and the meter requires using the AC function to filter but it's awfully coincidental. Transient voltage spike?
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post #2 of 112 (permalink) Old 09-15-2018, 06:10 PM
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So the wires are tight but the resistor in the wire suppressors and the plugs might be issue, old coil too. Just saying double secret resistor is not good. Try to widen the gap to .040.


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post #3 of 112 (permalink) Old 09-15-2018, 06:59 PM
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Check for a vacuum leak, also do a compression test.
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post #4 of 112 (permalink) Old 09-15-2018, 07:13 PM Thread Starter
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There's no vacuum leaks, vacuum gauge is rock steady even though the idle is rough. Once off idle it's smooth as glass again.
Just swapped out for a new coil, no change.
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post #5 of 112 (permalink) Old 09-15-2018, 09:13 PM Thread Starter
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Well I guess I'm down to rebuilding the FD or an ECU is on the fritz... So frustrating.
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post #6 of 112 (permalink) Old 09-16-2018, 05:42 AM
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It sounds ignition related , can you run a scope on the ignition. You might also check the ezl module or crank sensor. These would effect all cylinders.

It is rich at idle which to me says unburnt fuel due to a faulty ignition component.

Don’t be disheartened, old cars need work and it sounds like most of the heavy lifting has been done.

Best of luck.
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post #7 of 112 (permalink) Old 09-16-2018, 11:51 AM Thread Starter
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Changed the oil, went back to conventional shell rotella 15-40, smoothed things out a touch and picked up the pressure. Also checked the trans fluid level and found it 1L low, ouch.. filled it back up and that also helped in ridding the shaking at idle in gear. There is still an odd slight lumpy gallop in the idle so I'll be going thru the EZL system this afternoon.

I've always had good response with Liqui molly oils, in this instance the M106 is not very fond of synthetic oil.
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post #8 of 112 (permalink) Old 09-16-2018, 02:20 PM Thread Starter
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Alright, update:
I also have a '91 190 2.6, I borrowed and swapped over the coil first, no change, then the EZL unit, no change. The 190 runs without issue at idle BTW.
In trying to read the timing the light is erratic hence the quick swap of the EZL unit to check that first. I think the Crank sensor is suspect at this point. So now to wrestle out the old one... bugger of a location.
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post #9 of 112 (permalink) Old 09-16-2018, 03:09 PM Thread Starter
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[email protected]$%%^! Not the CPK.. WTH!
erratic idle for some unknown freaking reason!
It's the strangest thing.. it's like some morse code signaling going on. My scope is analog so I can't trace it over time and I don't have that capability with any of my DMM's, just real time reading.
like this..
RPM
730
735
730
732
730
1100 <--- that's the odd spike
730
etc..
all cylinders do this and it's seemingly random..
Going to try and find a way to trace it over time...
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post #10 of 112 (permalink) Old 09-16-2018, 05:08 PM
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* The electronic control unit supplies a specific voltage to the air valve which determines the respective valve opening and thereby the engine speed.
* The nominal idle speed is controlled depending on temperature, between 1200 RPM at about -23F (-30C) to 900 RPM at 158F (70C).
* With the ignition switched on (engine not running) the idle speed air valve is activated by a specific voltage. The port for the fixed operating mode is closed and the control port is opened.
* If the voltage supply fails, the port for the fixed operating mode is opened automatically.

Just guessing at this point , try unplugging the idle air control valve this will put it in fixed mode maybe the problem lies there or with what controls the valve.


Also try unplugging the eha , not at the same time, when running at idle to see if you still get those rpm spikes.
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