Possible vacuum leak? RPM fluctuation around 1500rpm ONLY WHILE COASTING
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Newbie
Joined: May 2011
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From: Pacific Northwest
1987 560sel
Possible vacuum leak? RPM fluctuation around 1500rpm ONLY WHILE COASTING
First off, Thank you for all the support I have gotten in helping me solve some issues I have had with my vehicle is owning it for the past few months.
Alright,, I love my 560SEL. I have fixed the various problems except one.
I have never been the most excited with auto. trannys so I wouldn't know if this is a torque converter issue or tranny issue, or a vacuum leak Like I am assuming.
Whenever I coast (foot off the gas pedal) usually happens going downhill, My RPM's drop to 1500RPM then goes up and down about 200 RPM from about 1350-1550 rpm until I either slow down to a speed where the RPM's would normally be below 1500RPM and it stops.
IT DOES NOT HAPPEN WHEN I HAVE MY FOOT ON THE BRAKE and off the gas pedal (assuming this has something to do with vacuum with my brakes?) might just be im slowing down to a speed where the RPM's will be low enough anyways to where its not above 1500 and starting to fluctuate
If I start to coast going faster (foot off the pedal at over 2000RPMs) it either takes a longer time to get back to 1500 and start fluctuating, or It does not occur between the time when I let off the gas and put my foot back on it to accelerate again.
It is just annoying during the rare times where my foot is off the gas and it will do this.
Any help, or direction would be awesome! Links or pointers to the right area would help too!
Alright,, I love my 560SEL. I have fixed the various problems except one.
I have never been the most excited with auto. trannys so I wouldn't know if this is a torque converter issue or tranny issue, or a vacuum leak Like I am assuming.
Whenever I coast (foot off the gas pedal) usually happens going downhill, My RPM's drop to 1500RPM then goes up and down about 200 RPM from about 1350-1550 rpm until I either slow down to a speed where the RPM's would normally be below 1500RPM and it stops.
IT DOES NOT HAPPEN WHEN I HAVE MY FOOT ON THE BRAKE and off the gas pedal (assuming this has something to do with vacuum with my brakes?) might just be im slowing down to a speed where the RPM's will be low enough anyways to where its not above 1500 and starting to fluctuate
If I start to coast going faster (foot off the pedal at over 2000RPMs) it either takes a longer time to get back to 1500 and start fluctuating, or It does not occur between the time when I let off the gas and put my foot back on it to accelerate again.
It is just annoying during the rare times where my foot is off the gas and it will do this.
Any help, or direction would be awesome! Links or pointers to the right area would help too!
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G63, C190 AMG GTC, W209 CLK63 BS, W208 CLK430
I think you're correct in looking at vacuum, this doesn't seem like a transmission problem unless the 560SEL has some sort of crazy gadgetry I'm unaware of. A friend had a 560SEL AMG a few years ago... Loved that car. It was lost to fire, and being a gray market car, insurance paid him pennies on the dollar 
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380 SEL
I have a nearly identical problem that came on suddenly with my 380 SEL, if you have a link to an image of this "plastic booster line" I would like to check that out on my car. Not sure where that is, duh, Thanks!
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380se
No need to post a pic, go to the post with (oh Crap! What the hell is this??!!) with the full engine pics where there is gunk on the air intake. Go to the full engine shot. The vacuum brake booster line is the large white plastic line that crosses in front of the red transmission dipstick in that picture. On my car it is reversed, the battery is on the PAX side and the brake and fuze box is on the driver side. No matter, the line I pointed out is the one that had developed a crack in my car. It was intermittent in nature. Also, on my car it is a large black line, much larger than the other small vacuum lines.
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380 SEL
Thanks a million for the reply, I found the picture. Very clear, my car is also reversed from the image in that the brake fluid thingy is on the other side, should be simple to find the line now! Ciao
Last edited by northbeach; Aug 6, 2011 at 08:45 PM.
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380 SEL
okay, i checked the line thanks to your directions. looks brand new and in perfect working order, with tight connections all around. guess i will attack the spark plugs tomorrow, i suspect one of the wires has gone bad, if not a plug. thanks again for directing me to the plastic booster line--- after jiggling the spark plug connections the problem vanished for about an hour, then returned later in the day.
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380se
I have no other great ideas. I am kind of parts replace kind of guy. I just start replacing things until it either works or I run out of $$$.
If you pull the electric connector to the Idle Control Valve your idle speed should jump up to 1500-2k rpm, I forget the exact speed. If this is what happens and then when you return the connector your idle drops back down to normal I think you can rule out the ICV. I ended up replacing my idle control module as well recently when I would get intermittent squirts of high idle. I figure there was a solder joint or part going bad. It has not happened since.
A rebuilt unit was a few hundred dollars from a local import parts shop. My dealership wanted 600+ and it is easy to swap out. Located on the firewall next to the transmission control module (this is the big one right in the center). The idle control module is the smaller box right next to it. Just be careful with the connector, it is easy on/easy off but if the plastic housing comes apart you will be stuck putting all the little connector pins back in the right order. A major PITA that happened to me because I was not as careful as I should have been.
Good luck.
If you pull the electric connector to the Idle Control Valve your idle speed should jump up to 1500-2k rpm, I forget the exact speed. If this is what happens and then when you return the connector your idle drops back down to normal I think you can rule out the ICV. I ended up replacing my idle control module as well recently when I would get intermittent squirts of high idle. I figure there was a solder joint or part going bad. It has not happened since.
A rebuilt unit was a few hundred dollars from a local import parts shop. My dealership wanted 600+ and it is easy to swap out. Located on the firewall next to the transmission control module (this is the big one right in the center). The idle control module is the smaller box right next to it. Just be careful with the connector, it is easy on/easy off but if the plastic housing comes apart you will be stuck putting all the little connector pins back in the right order. A major PITA that happened to me because I was not as careful as I should have been.
Good luck.
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380se
I went back and read your problem again, I still think it some kind of vacuum problem, there are a ton of those plastic lines. They are mostly ok, it is the plastic Y connectors that go bad. Hunt around over by your master cylinder on the engine side of the insulation barrier. There is a plastic Y connector that can be at risk. It is in the same area as the booster line I mentioned. These will be the small lines however.
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380 SEL
I went back and read your problem again, I still think it some kind of vacuum problem, there are a ton of those plastic lines. They are mostly ok, it is the plastic Y connectors that go bad. Hunt around over by your master cylinder on the engine side of the insulation barrier. There is a plastic Y connector that can be at risk. It is in the same area as the booster line I mentioned. These will be the small lines however.
