I need help eliminating what I think is a vacuum leak.
Car: 1995 E320
History: about 4 months ago I replaced head gasket and intake manifold gasket/seals. The car ran great for a month then started idling rough
Symptoms: When cold the idle is both rough and surges. After 2 minutes the surging is gone but the roughness is still there. After 10-15 minutes or when the engine is fully hot the rough idle is much less severe, sometimes not noticeable.
Under acceleration the engine is smooth whether hot or cold.
My diagnosis: I can hear a distinct hiss from under the intake manifold, seems to come near cylinder 3-4. I sprayed carb cleaner in that area where the manifold meets the head and the engine surges.
I replaced the vacuum hose that is in that area (both the small tubing vacuum hose and the shaped rubber hose that goes from under the manifold to the Throttle body). Therefor I am pretty sure the leak is coming from where the intake manifold meets the head.
My question is how do I tighten the manifold bolts (Allen head). First, it seems there is only one bolt between every other cylinder (spaced quite far apart - how does it manage to keep a tight seal). Second, it seems to be impossible to get to. I couldn't manage with any of the flex attachements I have.
If I manage to tighten the manifold bolts and it doesn't cure the leak, Any advice would be welcome.

The hands down best way to find a vacuum leak is with a smoke machine. See if your local MB indy mechanic has one.

You must replace ALL ALL ALL the rubber vacuum parts in the engine compartment. There are a bunch of tees, Y's, and short pieces of tubing all over the motor. Track them down and replace them all. The vacuum systems diagram on EPC shows where the vast majority of them are. All the vacuum parts can be ordered new from MB.

Huh. This is a tough one, sho 'nuff. I did mine on the bench when I had the head off the motor and even then it was gnarly. Don't do this job around women and children - causes lots of blue air in the garage. You can use a ball-end Allen wrench or ball-end Allen socket bit on those bolts. What I did was got disgusted and threw out those dang Allen screws and replaced them with regular Murrican 8.8 hex head bolts because with those you can use a socket and universal and no more Allen bolt foolishness. A shop-originated engineering change order, if you will. Another trick is to take a ball-end Allen wrench of the right size, cut the ball end off with about 1" of hex using a Dremel tool, then use a 1/4" drive 6-pt socket on the cut-off Allen wrench and then drive that using universals, extensions, and your torque wrench. So visualize a 1" long straight Allen wrench without the hook end and with a ball end. Make sure you buy the best quality Allen wrench you can. This is no time for a Harbor Freight 99-cent special.

Make sure you lube the bolts slightly when assembling and use a torque wrench that is within the correct range, i.e. within 20% to 80% of the wrench's scale for a clicker-type. (This isn't an issue for the beam pointer type of torque wrench.) Hope this helps - let us know what you find.

When I did the head gasket at the same time I replaced ALL the vacuum tubing including all the "T" & "Y" connectors. So I am fairly sure the leak is not a vacuum tube.
While the head was off I detached the intake manifold and replaced the gaskets/seals between the head and intake manifold. THEN it was relatively easy.
Now with the whole thing reassembled I couldn't get in there.
Your suggestion to cut of a short pc of allen head is good - that may work. I'll give it a try. I had to do the same thing for my R107 motor mounts (& also some other tough to reach bolts).
Regarding torquing, I'll tighten while the engine is running to see if I get any change - if I do then I'll keep tightening till the vacuum stops leaking. If I don't get any change then I'll stop tightening and take it to my indie (we'll find out if he has a smoke machine)

Right on. Be careful though because that manifold is plastic and it's not metal-reinforced. It's stout, but still...

Please let us know what you find. Pete

OK I just thought of something else. When I had my intake off, I accidentally broke off the plastic hose fitting on the botttom of the intake manifold where those two hoses connect (the hose to the throttle body and the hose to the PCV system housing down by the power steering pump). Sucker just snapped right off due to being brittle. Friend of mine who was an indy shop owner said this happens with some frequency on these cars due to the heat and age making the plastic brittle. He said to clean it real good and glue it back on with JB Weld because the alternative is a new (or salvage) intake manifold. So I did. I have not run it yet so I don't know if that fixed it or not. SOOO I am thinking when you did your hose replacement you nay have cracked that hose fitting because that hose fitting is located right in the area you are talking about, i.e. between cyl #3 and #4. There may be a crack there that is causing the vacuum leak. Try your carb cleaner spray trick again, only try it in that area and see if that makes the motor smooth out.

Also spray the carb cleaner around the injector nozzles - maybe one of the injector o-rings is leaking. Did you replace the injector o-rings when you had the intake all apart for the head gasket job?

Intake manifold is plastic...Good reminder. I would really kick myself if I heard it crack.
The two hoses at bottom of intake manifold...I had to replace both of them because they were hard as a rock (one broke on removal, the other broke on reassembly) BUT I never thought of checking whether I broke the plastic neck they fit on to. That is very much the area where the carb cleaner spray gives a reaction. Early on in tracing the leak I thought I didn't have a good seal on one of those hoses so I replaced the spring clip hose clamp with a screw tightening type - now I'll check the plastic neck to see if it is cracked.

I didn't replace the injector O-ring seals so I'm not going there.

When i did my head-gasket, that area broke because it was so brittle...JBWeld took care of it...

Most of intakes break around that area because of the damn original hose clamps...should definitely inspect it and see what you find. buy a cheapo telescopic mirror and flash light if you don't have one

good luck

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1993 300E, 2.8 M104 .....

IF i remember correctly, just yank it out..but make sure the clamp is very loose and not even touching the tube....

Good luck !

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1993 300E, 2.8 M104 .....

I am happy to report that I completed the breather hose replacement and the car runs smooth, no CEL (yet...) and all is good once again. The upright tube did come off with a good yank, the help and advice from people who replied kept me going and I am sure if I had brought this to MB dealer or indy it would have cost a fortune and probably replaced things that didn't need to be.
Much obliged to all