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1987 Mercedes-Benz 560SEC
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Discussion Starter #1
Hi everyone,

I'm on a mission to fix vacuum leaks on my '87 560sec as I have rough idle, what sounds like an exhaust leak coming from the rear of the engine at idle. My economy gauge also doesn't work (pinned left) but there's no white smoke coming from my exhaust. I used a smoke machine and the only smoke I saw was coming from the rear of the upper intake manifold (you can hopefully see it in the picture) where it joins the lower plenum chamber. It's a pretty large leak as my neighbor was only mouth to blow the smoke through and there was quite a bit of smoke coming out. Is the plenum gasket a common failure point and is it a relatively simple DIY job?
2643912
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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I'd say it's a pretty good-sized job but very doable.

Your economy gauge is supposed to be all the way left. Or do you mean it stays left even when driving?
 

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1987 Mercedes-Benz 560SEC
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Discussion Starter #3
I'd say it's a pretty good-sized job but very doable.

Your economy gauge is supposed to be all the way left. Or do you mean it stays left even when driving?
Yes, sorry should've clarified. It stays left even when driving.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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Yes, sorry should've clarified. It stays left even when driving.
Okay.

You might want to check the vac lines going into the car at the secondary firewall. If any of them don't hold vacuum, that will indicate a leak adding to your issues. Perhaps your economy gauge is functional but there's a leak in an elbow or plastic vac line.
 

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1987 Mercedes-Benz 560SEC
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Discussion Starter #5
Okay.

You might want to check the vac lines going into the car at the secondary firewall. If any of them don't hold vacuum, that will indicate a leak adding to your issues. Perhaps your economy gauge is functional but there's a leak in an elbow or plastic vac line.
I replaced the 4 way vacuum connector on the secondary firewall as it was seriously loose. I also replaced the vacuum line going out of the back of the intake manifold to the connector. Haven't checked any of the other connectors but they all seem to be pretty tight.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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I replaced the 4 way vacuum connector on the secondary firewall as it was seriously loose. I also replaced the vacuum line going out of the back of the intake manifold to the connector. Haven't checked any of the other connectors but they all seem to be pretty tight.
That's good but you need to use a Mighty-Vac or similar to test the actual lines going into the car. If they don't hold vacuum, you've got some work to do.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,603 Posts
Also, does the car shift okay? There's a vacuum fitting at the rear of the intake manifold with two nipples. One goes to that four-way fitting you replaced and the other points straight down and goes to the trans. It's hard to see and if the line has fallen off, or if there's a split in the rubber connector, your shifts will be harsh and you'd see a lot of smoke back there during a smoke test.
 

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1987 Mercedes-Benz 560SEC
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11 Posts
Discussion Starter #8
Also, does the car shift okay? There's a vacuum fitting at the rear of the intake manifold with two nipples. One goes to that four-way fitting you replaced and the other points straight down and goes to the trans. It's hard to see and if the line has fallen off, or if there's a split in the rubber connector, your shifts will be harsh and you'd see a lot of smoke back there during a smoke test.
Car shifts smoothly. Hard shifts sometimes, but not very often. I'll check the line tomorrow.
 

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Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,778 Posts
Okay.

You might want to check the vac lines going into the car at the secondary firewall. If any of them don't hold vacuum, that will indicate a leak adding to your issues. Perhaps your economy gauge is functional but there's a leak in an elbow or plastic vac line.
Hey LuckyMike,

In addition to others suggestions & comments, just jumping in late here with an observation on the Economy Gauge.

His stays full left with the engine on or off, meaning there is something wrong with the gauge itself.
Without any vacuum applied, the needle should be all the way to the right.

M.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,603 Posts
Hey LuckyMike,

In addition to others suggestions & comments, just jumping in late here with an observation on the Economy Gauge.

His stays full left with the engine on or off, meaning there is something wrong with the gauge itself.
Without any vacuum applied, the needle should be all the way to the right.

M.
Ha! Never noticed! Thank you for pointing that out.
 

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1991 560sec. 1991 560sel 1969 280SL
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1,333 Posts
If I were you I would pull the instrument cluster and check to see if the vacuum hose to the nipple on the back of cluster is hooked up or not. I would say that is what’s happened.
 

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Premium Member
1999 500SL, 1988 SEC
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1,353 Posts
The intake is doable but if you go that far I would replace the clowns foot as well. It gets brittle. I offer this suggestion to all as well. If I were keeping a car I would replace all the vacuum lines under the hood. I bet total $10.00 and a little time. Then you know you are good. Replace the nylon lines as well.
 

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Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,778 Posts
If I were you I would pull the instrument cluster and check to see if the vacuum hose to the nipple on the back of cluster is hooked up or not. I would say that is what’s happened.
His gauge is permanently indicating FULL vacuum, all the time.. Engine ON or OFF. "My economy gauge also doesn't work (pinned left)"..

M
 

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1991 560sec. 1991 560sel 1969 280SL
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Keep your pants on M. lol. Misread that’s all
 
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