Smooth idle at 3800 ft... ADA issue?
 

This is an odd one...

Mom lives at 3800ft. Not that high, but enough to make a difference.
Whenever I'm up there, the car idles very smoothly. Enough to make me think all the tinkering I've done at sea level has finally made a difference... then I drive home and find the opposite.

If the ADA (not ALDA) on my injection pump was defective, seized or otherwise restricted, could this be a possible explanation?
I'm imagining some sort of fuel restriction scenario.
I know that when vacuum is applied to the ADA, the engine begins to stutter a bit. If enough vacuum is applied, the engine will shutdown -vacuum simulating reduced atmospheric pressure.

I've found two instances in the archives of people removing their ADAs to great effect. However, I'd like to understand what conditions might cause this effect I'm experiencing.

Seems plausible... No takers?

3800 ft is far from "high altitude". I'd be surprised if there's really any effect from atmospheric pressure. Perhaps the difference is humidity or temperature related. I'd expect an atmospheric issue to affect power more than idle. The turbo cars with the ALDA have virtually zero effect on fueling at idle or low throttle inputs. The ALDA is there to prevent smoking until the turbo spools, then enrich fuel to match the boost pressure. The ADA is to help reduce fueling at higher altitudes for the same reason.

My guess,with out seeing,hearing,feeling,injection pump goes lean when worn.
injectors on one or more cyl. misting (improper spray),at increased alt. ada
increases fuel some, enriching light spraying nozzles,idle smoothes out.

Diseasel -agreed re: altitude, clutching at straws here.

Hercules -very interesting... I didn’t know that worn pumps went lean. If this logic follows, either turning up my pump a little, or removing the Ada might compensate.

If the pump had worn "lean", the ADA would be making it LEANER at altitude, which should exacerbate your problem if that were the case.

Pump does go lean,worn parts. most common symptom hard starting,also low power,rough idle. You are CORRECT ada would cause even leaner running at altitude. My mistake.
If ada taken off will run richer,owner says now idles good ??

Thinking harder,worn internal pump linkage,higher altitude moves linkage out of worn area. Still would need injector problem or other injection pump internal worn linkages.
Ever have this happen:slow take off,engine accelerates smoothly,tad more throttle,engines rpm drops,just a very little more throttle rpm again increases.
(worn rack gears),Seen this a few times. Take off faster never feel it.
Pump re-builders will miss this almost ever time!

I gave the acceleration test a go -didn’t experience what you described.
Is there any situation where Ada may be stuck in an altitude-dilation/contraction?
As in, my pump is running lean for sea-level, and right on at slight elevation?

Ada is not stuck,it works.Worn spot on linkages inside. More interesting,why engine
smooths when running richer. Is engine running faster,hiding imbalance problem?

It's possible...

I've done so many things that I'm out of ideas at this point.

Changed all the fuel hoses, new mounts, new engine dampers, injectors rebuilt, lift pump rebuilt, new OFV spring, multiple diesel purges, compression between 360-390 across cylinders, multiple valve adjustments, 3 degrees chain stretch -advanced pump timing to 25 degrees, multiple new air filters... I think that's the lot of it.

For a 240, I have good power and acceleration but the idle has always been fairly lousy.

How many miles. Possible to have worn valve guide,valve hangs at low speeds,not during
compression test,not at a little faster speed. Had it happen.took a while to find.
When engine is missing test with a strip of note book paper at exhaust outlet,paper should
have a steady pulse out wards,not any sucked inwards.

197,979 -but it’s been like this since at least 170k. I bought the car at 156k but really only began noticing things as I started learning and paying attention.

Tried the muffler test. It’s a solid puff puff, pulse outward.
I wouldn’t say it’s missing, it’s just rough.

If the idle can be raised a small amount and engine idles good,than the fuel rack
inside pump has a wear spot,can be on just one cylinder. Just drive it.

Isn't it the other way around, thinner air increase in fuel to air ratio, at least that's how I figured when messing with mine.

From memory (the details are possibly on 'wobble thread) unscrewing the dome screw allows the spring to push plunger upwards, this increased mixture noticeably spinning the wheels on odd occasion.

Sherm if yours has the same model check the diaphragm is not leaking, use mityvac to hold vacuum.