Ok I was doing some thinking today...and I read that the vacuum gauge on the instrument cluster attaches to the intake manifold to read the pressure inside.

With that in mind, if my vacuum gauge fluctuates slightly, doesn't that mean that there is not an even, constant amount of air entering the intake manifold at idle?

Does that mean a vacuum leak, or would the airflow plate be moving slightly based on electronic signals from elsewhere? What are your thoughts on this?
I have gone through every single hose on that engine...All the idle air hoses, all the ones that attach to the intake manifold and the one under the KE mixture unit.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

If it is moving around when the engine is idling I'd say there is a leak. Mine is stone steady at idle, especially when in drive.

Fluctuations would point to a leak somewhere....

Does the o-ring on the top of the dipstick seal well? That is an overlooked vac leak source....I found mine to be flat and not sealing well.

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I just don't get where....
Maybe I need to get it smoke tested again.

Yeah it seals well...why would that be a vacuum leak though? It doesn't affect pressure in the intake manifold.

Is there a way to test the air pressure in the intake manifold with a gauge? Is there a spec I can check it against?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

You must have a leak somewhere.

At idle with a warm engine and in neutral the vacuum gauge on my 260e sits at zero - the opposite of the red. With drive engaged it sits slightly off into the main section. Stationary in drive with the AC on, the needle sits about 3/4 to the top - nowhere near the red section.

Get a stand alone vacuum gauge. Go under the bonnet. Disconnect every vacuum line (including the large one for the brake assist) and block the exits off. Connect the external vacuum gauge to one of the outlets. Start eliminating leaks by connecting each vacuum outlet in turn.

Other suspects. The vacuum levels the lights - you may have a leak in one of them or the control switch near the light switch. The heater controls are by vacuum sometimes a controller is suspect and leaks vacuum. Make sure the AFM rubber is leak proof. Make sure the one way valves to the Brake are good. etc etc etc.

A warped valve cover can also cause a problem - even with a new gasket. Does it leak oil? How about the oil filler cap? Tight and the gasket in it good?

The Air fuel mixture must be spot on. The air damper in the AFM set at the correct level.

As mentioned above. The oil level stick has a gasket top and bottom.

With the engine on and car in idle. Can you hear a hissing sound in the interior of the car anywhere? Quite garage and no audio system on for this test.

The US spec cars do not have the vacuum adjustable headlamps.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

I see that Wagon has Euro headlights. Are these not converted to level control?

No hissing sounds. No oil leaks. I have followed the DIY on vacuum detection using the O2 sensor voltage as the meter in the past and didn't find any changes when reconnecting all the vacuum elements.

At idle in drive, it sits just slightly left of the start of red. With A/C on, it sits dead on the very start of red.

The oil filler cap is leaking slightly though...but realistically is that going to really kill my MPG and power...this much? Is there a gasket that goes around the filler hole?

I found a very nice looking valve cover at the junkyard...even better than my one which still looks great. Maybe I'll swap it over to solve the leak?

Hardly ever rains in CA, and my foot wells are fine.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

I was referring specifically to the O2 sensor connectors -- to be sure they are secure and not corroded.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

That doesn't sound right for the economy gauge, in drive mine sits in the black just off being all the way to the left, in Park its all the way to the left, in drive with A/C on it only moves slightly, but still way into the black....nowhere near the red zone.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Have you checked your exhaust for restriction? Bad cat, bad resonator, bad muffler...high back pressure could be causing this...especially as you are seeing lack of power at higher rpm's and bad fuel mileage....Can check short term by removing the O2 sensor to let exhaust escape with less restriction..if performance improves perceptively...you have a pretty good idea.

I only have a lack of power at idle and I have had the cat replaced. I'll unplug the O2 sensor and see what happens.

Thanks.

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

See my remarks earlier today about cleaning the O2 sensor connector:
water in the floor cure for w124's

__________________
2008 E350 4matic / Black/Anthracite

------------------------------------
Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

But you said you have slower performance... how would you even know if you only have a lack of power at idle? At idle you are using no power!

OK, I don't know your model in particular, but I've been a mechanic in a previous life and I'm a mechanical engineer in the engine development business... so I'll stick in my .02.

Idle vacuum should be very high. It sounds like yours is low. The fluctuation I wouldn't worry about just yet... if the engine is idling poorly, the fluctuating idle speed will cause a fluctuating vacuum reading. Low vacuum... it isn't because of a leak (although you might still have one). It's because the engine is idling with more throttle than it should require. It's doing this to take in more air and fuel because it is running inefficiently. Why is it running inefficiently? That will be the key to solving everything. Possibilities:
Incorrect A:F ratio. Probably not, because you've got on O2 sensor, no fault codes, and you've been through the fuel system. Still could be a subtle cause like bad intake valve guides, or a faulty PCV system (that's why someone posted about the dipstick and oil cap). Possibly something with the evaporative emission system, but I have no clue how that works on your car.
Exhaust restriction. You say you have checked the cat, also it would manifest more as running out of power with a slight lag as you accelerate.
Retarded or erratic ignition timing. I haven't seen that you've checked this. Put a timing light on it, see where it is at idle. See if it is stable. See if it advances sharply and cleanly when the engine is revved up. Erratic and retarded timing would cause all your problems.
Poor compression. Your engine looks beautiful. It could still have screwed up cam timing which is essentially like having poor compression. Especially possible because the system has obviously been molested. (No reflection on your mechanical capabilities, but a realization that something that has been touched is more likely to have a problem.) Retrace your steps. Why was the head off? Are any of your valves bent or sticking? Weak or broken valve springs? Tight valves anyplace? Bad hydraulic tappet(s) making a valve stay open?

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1987 W201 190D

Thank you.

I have low power at the bottom end, I didn't mean idle, sorry. Along with very poor MPG. My last fillup was 12.1mpg.

To this post and others, the 1988 model does not have an EGR valve.
Compression is 190psi all cylinders.

I had the entire head rebuilt including new valve guides + seals. Head pressure tested and skimmed. I had the head off because I was losing a lot of oil and coolant and was told by a local indy that the head gasket was leaking. It was a great opportunity to really get to know the engine (my first rebuild) and it didn't cost me a whole lot. Experience was well worth it. Had some help from my dad of course...but he is stumped on the rough idle too. He goes by "Merky" on this forum, but rarely posts. There were a few other items such as the water pump that needed replacing. The valves looked great.
I have a large thread with tons of pictures that I took throughout the whole thing here:
My 300TE Head Gasket Thread (yes pics!)

When I rebuilt the top end, I set the cam timing as it shows in the MB manual to perfection. Should I need to get a light to still check it?

EDIT: Here is a video on youtube
http://www.youtube.com/watch?v=kMAVAjyVSTE
You can see the idle economy gauge and rpm fluctuations. Maybe this will give you some ideas.

I would love to take it to the dealer and have it fixed....but I A) expect a huge bill B) Expect several expensive items to be fixed before the real solution is found

__________________
2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

No guarantee taking it to a dealer will be any good. The local M.B. factory operated stealerships near me have "technicians" instead of mechanics. Many of these technicians are not much older than my M103 engine. They are all geared up to operate on the computer controls of the newer models. - The diagnose and replace system.
When I need advice from them (extremely rarely) they need to call their "old guy" or do a search from their on line manuals. Sometimes they even point me in the direction of an indie specialist - often an old dealership mechanic out on his own.

You would need to find an Indie specialist.

For injection problems I take my cars to a franchise Bosch injection specialist. Even the dealerships send their cars to these guys.