W108 280SE Stalling Issue - PeachParts Mercedes-Benz Forum

w1083 · #1 08-08-2018, 05:48 PM

Car is a W108 280SE 3.5.

So, I was driving and came to a stop when the car suddenly turned off. It turned on right away but when I pressed the brake pedal to go from Park to Drive, it turned off again so I drove it back home using both feet to keep it from shutting off. When I got home, I turned the idle up a bit for a temporary fix and the car doesn't shut off anymore when applying the brakes. My question is, could it be the brake booster diaphragm that went out or possibly the check valve in the brake booster vacuum line failing? The vacuum line itself looks original and ok with no cracking along the line or ends near the fittings.

w1083 · #2 08-13-2018, 11:15 AM

Anyone?

Tomguy · #3 08-13-2018, 12:32 PM

If it were the check valve, it'd have no impact on your issue. The check valve is really there to ensure you can brake at least once in an emergency if you lose engine power. If your booster diaphragm is going bad, it will allow the engine to suck an infinite amount of air - check valve working properly or completely missing.

The only way to really know is to hook up a vacuum gauge to your manifold and have someone apply the brakes. It should show a small pressure increase initially as the booster demands a stronger vacuum, and then level off. If it doesn't level off, or the pressure keeps increasing in your manifold, the booster's shot.

Tomguy · #4 08-13-2018, 12:48 PM

Hate to make a second post but I know if you have email notification, edits don't show up.

One other thing that I failed to mention, is that this is *ASSUMING* the rest of your system is fine too. It could be a bad booster, or it could also be the combination of small leaks anywhere/everywhere else in your DJet system are causing enough vacuum loss in the manifold that braking is just the final straw that broke the proverbial camel's back.

The intake manifold is two halves, which have 8 seals that look a bit like a donut. Then there's the hoses for the MAP, booster, and central locking system. 8 injector seals. The aux air valve circuit. The door locks themselves. Any (or all) of these may be leaking, and contributing to your issue.

karshack · #5 09-03-2018, 06:53 PM

W1083,

Were you able to get this solved?

Just a note about the in-line valve in the manifold brake vacuum hose. You can test that yourself without any diagnostic tools. Remove the hose from the pipe from the manifold, then loosen and remove the vacuum line after the in-line valve. So what you should have in your hand is a length of hose with the in-line valve still attached. Now look a the in-line valve carefully. It should have a directional arrow molded into the plastic. That arrow should aimed toward the brake booster. The direction of the flow of air from the manifold to the brake booster. If you want to make certain of the direction of the valve flow, just blow through the hose from the manifold end. No special tools needed.

On my "67 250S the valve was the wrong way around. The previous owner and my mechanic complained that they could not get the car to factory idle (750-800rpm). This had nothing to do with synching the carbs. Idle below 900-1000 and it would die at a moment's notice. So, I had to live with a high idle just to drive it without fear of stalling. After a trip to the mechanic for something unrelated, he stated that he had checked the booster diaphragm for leaks - but that there was no problem. He thought that the carbs might need a thorough cleaning/rebuilding and re-synching. That weekend, I tried to puzzle my way through the problem, now knowing that there was nothing wrong with the booster (which I had feared). I removed the hose and valve, as stated above, and when I blew, did a Dizzy Gillespie impersonation. The valve was installed the wrong way - the arrow on the valve was facing the manifold. Took the valve off, turned it right way around, re-attached the hose, and the idle rpm was immediately lower and smoother. I was able to get it down to about 770, and then used my airflow gauge to synchronize the carbs. The difference was like night and day. So much quieter and enjoyable.

Of course, there could be some vacuum issue somewhere else in your system, just as Tomguy mentioned. I'd just recommend that you try this easy and no-frills basic test before you get deep into the other vacuum systems.

Best of luck.