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:
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:
plugs (NGK non resistor)
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:
Under load 1500rpm:
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?