Find the vacuum leak - PeachParts Mercedes-Benz Forum

A264172 · #1 12-10-2004, 03:49 PM

The 86 300E is acting funny.
Symptoms
-Had been shifting a little rough (I thought it was a bad front flex disk or differential/transmission mounts or rear suspension).
-Have had some loss of gas mileage (thought it was from running on straight cat until I replaced both mufflers, but it did not get all the way back to 19 or so mpg).
-Today came to a stop after 30min +/- highway run and died. Started back up easy but surged a bit and died a few more times over a few miles.
-I noticed that when it surged and lost power, the manifold vacuum pegged on the economy gage (meaning aprox. none?). After it cooled down a bit it seemed a little better and the stuttering/dying disappeared (over another 2 miles)
Haven’t driven since.

Do I have a vacuum leak?
Where should I start looking?

#2 12-10-2004, 05:02 PM

I had a vacuum leak in my 87 300D, I would imagine your vacuum reservoir is the same and also located backside of the right front wheel inner fender, forward from the right front door jam.

In my case, mice had chewed through the plastic tube running from the reservoir to the vacuum source. Put a vaccum hose tee in that tube and a vac guage and see if it holds vacuum!

BB

A264172 · #3 12-10-2004, 05:52 PM

Quote:

Originally Posted by BusyBenz

I had a vacuum leak in my 87 300D, I would imagine your vacuum reservoir is the same and also located backside of the right front wheel inner fender, forward from the right front door jam.

In my case, mice had chewed through the plastic tube running from the reservoir to the vacuum source. Put a vaccum hose tee in that tube and a vac guage and see if it holds vacuum!

BB

It could be Busy, although diesel's vacuums are different also.
I have yet to really crack the CD. I'm sort of looking for direction as to where to start, or maybe more hoping that someone knows exactly what it should be from the symptoms.

I like the mice notion though, I have cats and pigeons at work and any number of critters at home to suspect. As yet I have not done a thorough inspection. I suppose I'll just take off the air cleaner, look around, and then check the CD.

#4 12-10-2004, 06:33 PM

It does sound like a vacuum problem and although diesels use vacuum for some different reasons other than gassers, and visa-versa, your CD manual will list what inches of vacuum is neccessary. If it leaking vacuum, it'll show up on a guage relative to specs, of course.

Just thought I'd mention one cause for reasons for lost vacuum that's not listed in any manual!

My 190E also had a vac leak and produced similar issues as your 124. Seems vacuum leaks are very common and a good place to start looking. Beyond that, it's another ball game!

BB

A264172 · #5 12-11-2004, 11:42 AM

Does anyone know whether an airflow sensor would cause these symptoms? This has only happened when the engine is warm so far.

I am also considering the fuel pump relay.

On the way home last night it cut out on the freeway about 50mph, the plot thickens.

Still pondering.

A264172 · #6 12-11-2004, 09:36 PM

Well Busy Benz it looks like it’s just you and me here so I thought I'd let you know how its going.

I took out the fuel pump relay and the OVP and cleaned them with JVC lectra-motive cleaner.

Then I took off the air cleaner to see what I could see. The big connector from the top of the air cleaner to the valve cover was split at the bottom of the connection to the valve cover, so I electrical taped it up real well. The hard vac line that came down out of that same connector and ran under the air cleaner seemed loose. I pulled it out to clean it and tried to see if it had any flex in it. It did not, and snapped in half. I slapped it all back together and drove down to local auto parts store for some vac line, which I installed in its place (much snugger fit).

Then I test drove it out the highway down through the strip malls. Bought a Christmas tree at Home Depot, Merry Christmas by the way, and tied it on the roof. Back out past the strip malls and back down the highway with no return of my stalling problem. I installed my Christmas tree complete with lights and then repeated my test drive only longer and farther than the first one and still my stall did not return. I’ve never been happier with my usual flex-disk clunk of the past few months than I was today.

I suppose I should really get in there at some point in the future and rehab all the vac lines, as they are probably all brittle and or loose.
The biggest to-do list in the world just got bigger.

Thanks for your help Busy Benz,
And as for the rest of you- ah who needs you.
Marty
:

Mike Murrell · #7 12-12-2004, 01:24 AM

If you suspect a vacuum problem, pop the hood and start eye-balling hoses.

It can be hard to see slits in the smaller hoses.

Starting fluid(ether) can be useful in locating vacuum leaks.

A264172 · #8 12-12-2004, 09:11 PM

Mike,
I'm guessing you would run the engine and spray the starting fluid around the vacuume connectors and look for it being blown about by the leaks?

joel · #9 12-13-2004, 01:02 AM

I use a propane torch, without lighting it of course!

jbaj007 · #10 12-13-2004, 01:24 AM

By sraying ether(or propane) at a suspect vacuum leak, the engine will run a bit faster if it is there. Obviously, only applies to gas engines(because it's manifold vacuum), not diesels.

A264172 · #11 12-13-2004, 11:49 AM

for the clarification.