All -

In the course of putting my new(er) 4A1 on the '73 280C, I discovered a major vacuum leak in the intake manifold gasket where the manifold meets the intake ports. I can actually see it with the carb off. The gasket protrudes a bit beyond the manifold, and there is a 1/8" gap where a piece appears to be missing. I can put my finger near it and feel the leak.

Is there a sealer that I can use to close this off until I can get to the gasket replacement - like put it over the leak and let it cure? Both surfaces are aluminum. I was thinking JBWeld, but I really don't want something I need to grind off. Obviously, these surfaces can get very hot.

TIA

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Don't know -- I'd probably wait for a new gasket. You're right, it gets pretty warm there, so "cheap" fixes may be more trouble than not.

If you can find an old intake manifold gasket off something else, you might be able to cut a bit to fit in the hole, but leave it large enough on the outside so it won't get sucked through!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Work some RTV black or grey silicone sealant into the gap. It's pretty resistant to high temperatures. I've even used it to seal where a badly corroded exhaust manifold was leaking at the cylinder head mounting.

Happy Motoring, Mark

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DrDKW

Yeah.. I have been thinking this could be the problem I have been having with my rear zenith. It's really hard to see bad there witht he carb on and so far my attempts to detect a leak in the manifold had not produced anything.

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MB 72 250 M130.923 114.011 170k The Beauty

I took Mark's advice and used RTV gray. Applied it to a cold engine and let it sit overnight. Started the car yesterday, and it seems to have worked. For the first time I can remember, the car started and warmed up without stalling, and without my having to hit the gas to keep it going. Idle is better as well.

Now I probably need to go back and undo what I did to get it to run semi-right with the vacuum leak.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

So I finally got a chance to drive the car yesterday. What a difference!

Much smoother, more power. Much better warm-up.

But some interesting other changes:

Now pinging a bit under load. - obviously getting more advance out of the distributor. Need to back off the advance needed before to get any sort of a smooth idle.

Will not start warm without a pump of the pedal. Before, it did not need this, but now would not even fire without some gas.

Hard cranking when warm gone. Improved vacuum holding timing more retarded when engine shut off?

Really have to back off the gas quickly to get a shift to 4th. Obviously need to tweak the modular for the improved vacuum.

Brake pedal harder. DOH!

I am wondering how many 4A1's have been maligned when the real issue wass a vacuum leak like this one?

And I did order the gasket today.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Since the advent of Lambda control fuel injection, vacuum leak problems tend to get ignored, I think. They cause serious drivability problems with carbs, but with feedback control EFI they just run bad, usually start OK and just make you crazy.

Even carbs can work OK with a serious vac leak -- my Mom's old Ford Fairmont with a 1-barrel carb had a split diaphram on the charcoal canister and a bad advance temp control sensor (stuck at full advance all the time), and ran OK most of the time, only required premium on the highway to prevent knocking. Ran MUCH better when I finally found the leak, just before I got rid of it.

My advice to anyone with "carburetor" problems or drivability problems of any kind is to first fix all the vac leaks. Nothing is gonna work right until you do, no matter what you replace! Once you get all the air going through the proper metering device, be it a carb or a mass flow sensor, you can adjust it perfectly. So long as you have a vac leak, you will only chase your tail.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!