Idle fluctuates when at operating temp
 

Finally got the new lift pump sorted out.

No more stalling. Regained power and all works as it should.

But now when engine is warmed up the idle ever so slightly fluctuates.

Can feel extra vibration at a stop. Goes away if I put it in park or neutral.

Can see it on the tach and oil pressure gauge.

Maybe 700-750rpm for example.

AC on. Recently fixed. I adjusted the rack damper when I didn't have ac.

Could that be the culprit?

Recently installed a new r4 and new belt.

I don't think there is any spec for adjusting the rack damper with the AC on.

If the AC is on at idle I would expect the idle to not be as good.

What about the Motor and Transmission Mounts?

If the Pop pressures on the Injectors are not even it will effect the idle. The same if the Injector Nozzles are worn out.

Motor/transmission mounts are new

Balanced injectors from greazzer

In neutral, it's smooth as silk

The vibration is probably from dead motor mounts.

I'd try adjusting the rack damper again to solve the idle fluctuation.

When in neutral it is not under any load at all.

Compression test would be the next step (assuming the valveas had been adjusted recently).

Ever check the valve clearances?

did a valve adjustment last oil change. maybe 2-3 months ago.

only recent work was changing out the lift pump and fixing the ac.

could an air leak present this way?

the rubber fuel lines at the tank are original. and the clamps are all originals too.

New from MB, or new aftermarket? When installed?

motor mounts lemforder: approx. 4-5 months ago

transmission mount lemforder: approx. 2 years, along with flex discs and center bearing

Air in the system can make the engine run a little rough.
I once cracked my lines a few weeks after having my injectors rebuilt by greazzer. I cracked open the nut on the metal fuel line and then floored it to get any air out of there. Then did it for the other 5 injector lines. That helped the engine stop a lot of it's shaking.

My idle "fluctuates" at operating temperature too. I mean, I can come FRESH off the highway, 70mph right off the exit and hit a red light and the engine will still have that "behavior" every now and then. I believe you will hear a very slight "hum" at idle correct? The "hum" is like a VERY quick change in idle...aka the "fluctuation". For me this "fluctuation" is synchronized with a rather pronounced vibration from the engine that I can feel. Basically I can hear and feel the fluctuation at the same time, thanks to my shot motor mounts.

I'm in the same situation as you, I'm not sure what is causing it. I will be getting a leak down tester (one day) and get to the bottom of it.

I wonder if it has anything to do with the linkage that runs down the side of the ip.

I adjusted those to "correct" length according to fsm but that may be pointless on an old motor

When I had the symptom of the Engine running very slightly rough after the other stuff was fixed I found that the Fuel Inlet Hose was extremely hard and inelastic. The clamp was turned down tight but I am guessing that when the plastic on the Filter got warm it deformed and allowed air to leak in.
Anyway replaceing the Hose fixed that.

So after further investigation, I found that this only happens when the ac is running and car is idling in gear. If I turn off ac, idle stabilizes.

Not sure if there is a fix aside from bumping up the idle

I'm just curious here, but how would air leak into a pressurized fuel line? :confused: