The economy gauge in drive at idle is just under a half in the black and in idle neutral the gauge is about 1/8th . Also checked the fan blades all are good all belt driven accesories are good. On drive home from work this afternoon the bad miss is worse when hot and not as bad when it was first started this is a new development. It is very bad when accelerating and a very rough miss/idle when hot

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

hooked up vac gauge and it is fluctuating between 11" and 14" it fluctuates in perfect timing with the idle miss. this seems pretty low to me.
I checked the opperation of the IAC it ops good cleaned it anyway.
Checked all vac lines and hoses including IAC hoses all look good.
Tested OVP it works as advertised.
Checked ign timing looks like about 8deg before TDC.
Sprayed brake clean around injectors and intake no change in idle.
Still runs excellent above idle very quick and smooth in highway!

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

Possibly a stuck intake valve.....pull the injectors and use about 3 bottles of carb cleaner to spray into the ports that lead to the valves. Make sure you crank the engine to make sure you hit each valve at the open and close positions. Also have the throttle body off so you can clean up the excess cleaner draining into the intake manifold. Then get a bottle of Lucas injector cleaner and use a 10 mL syringe and squirt (with a lot of force) the injectors cleaner into each port and crank the engine as well. This will lubricate the valves so that they move more freely after using the carb cleaner. Clean out the excess in the intake manifold and button it back up. Next, go pull your spark plugs and wipe them dry as they will possibly be wet. Next you will need to crank the engine a few times before it fires up. You will have some hesitation and stumbling while the cleaner burns off, also expect a smoke screen to come out if your exhaust as the Lucas cleaner is being burned off so I recommend doing this in a wide open area where you won't disrupt anyone with the cloud of smoke. It may not be completely smooth initially, but go take it on the freeway or open road. Hope this helps. I had the same problem and now mine is cleared up. I need to actually revisit my post to let everyone know the problem has been resolved and to post my write up solution. I took pictures as well. Good luck.

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1990 190E 3.0L

If you do the carbon removal, be sure to disable the ignition system, pull all plugs and turn the engine to expel the liquid otherwise you might hydra lock the engine and bend a rod.

Be sure to cover the plug holes with a rag to prevent paint damage.

You can do a leak down test to see if compression is being lost. This involves pumping air into each cylinder and seeing where it leaks out. This is far better than guessing what is wrong. It is also a way to test the head gasket / head for combustion leaks to coolant by looking for bubbles in the rad.

Today I did smoke tests on the manifold no vac leaks were detected, also checked the off idle micro switch and it also tests good.
Checked the brake booster and it holds vac very well.
I will do the cyl leak down tests tomorrow as I have the equipment for that from when I did the valve stem seals. I will also borrow my friends boroscope camera to take a look in the cylinders. The compression tests were good hot and cold ,185 hot 140 cold, all cyl equal.
I think you guys are right about the intake valve hanging-up a little there is not much else left to cause low vac

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

I did the cyl leak down tests and had no leakage out int or ex valves, no bubbles in coolant, no cross cyl leakage, just air going past the rings. All cyl are around 30 to 35% .I did this test cold ,from what I understand results are usually better hot??

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

You might get less when hot, 30% is pretty high but if the plugs are burning clean I would not be concerned.

Shut off one cylinder at a time to test for where the miss is. If you are careful, use some stiff copper wire / black rubber vacuum hose to make an extension between the distributor and spark plug wire, take a ground wire then touch it to each extension and listen for the same RPM drop. Black vacuum hose has enough carbon in it to conduct electricity. Don't stick your tongue on the wires.

You can also unplug injectors one at a time for the same effect if you have electronic FI.

No electronic FI on this model,but I replaced the plug wires one at a time and also the coil ( I had a spare set) No help same thing a bad miss, vacuum slowly fluctuating between 11" and 14" almost in time with the miss.It must be FI problem I think I have a bad injector at idle and at higher rpm and pressures it smooths out.

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

Yep, but I'd still short out one wire at a time to narrow down to the bad cylinder.

If all cylinders drop the same amount, you have a global problem.

Injectors with poor spray don't idle well.

Just got done testing injectors. The new units I put in position 3 & 4 were opening later and dripping and shooting pencil streams. I put my old silver Mercedes original injectors back in and I now have a beautiful spray pattern on all 6 injectors! What a waste of money. Put it back together and now it runs great vacuum gauge is steady. No miss nice idle

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg