Let's play the 'what could cause all three of these' game :) - PeachParts Mercedes-Benz Forum

ck42 · #1 07-25-2007, 09:27 AM

1) Last couple of weeks, wife has said the engine wouldn't shutoff a couple of times (very intermittent). Shuts off manually using the lever.

2) Complained of engine power actually decreasing while acclerating onto an on-ramp....says it actually started losing speed.

3) Just mentioned that intermittently the brake pedal would be completely STIFF "felt like I was pressing on a brick". Car would slow but had to press hard.

So....I've got my idea of what this is, but I'm not 100% about how it 'works' with #3.

My answer: vacuum. Easily explains 1,2. But for #3, I'm not sure where/how vacuum ties into the brake system.

I'm just hoping it isn't the vacuum pump and that it's just a broken/cracked/loose line somewhere. Any thoughts on this would be greatly apprecited.

bustedbenz · #2 07-25-2007, 10:00 AM

It ties in perfectly. She's feeling what all brakes were like before power-assist braking came along. A large vacuum reservoir of some type builds up vacuum pressure that assists in the brake pedal movement and the braking action every time you press the pedal.

To test this theory, get in a working benz (other cars seem to have larger supplies) and stomp the brake pedal five or six times in rapid succession, down to the bottom each time. You'll observe that you quickly exhaust the vacuum supply and the brakes respond the way she observed until it builds back up a few seconds later.

This is why you can only press the brake pedal a few times in a car that's not running before it becomes harder to press until you crank up again.

*EDIT* Yes, I know that "vacuum pressure" is an oxymoron. you know what I mean.

toomany MBZ · #3 07-25-2007, 10:00 AM

The vac line goes into the booster, a leak would give you brake trouble, but I would think a soft pedal.

bustedbenz · #4 07-25-2007, 10:02 AM

When brakes aren't getting vacuum, they become hard to press because you're doing all the work of compressing the brakes down on the rotors yourself without any assistance. Thus, the hard pedal. With vacuum assistance, a vacuum supply helps to do some of this work, so the pedal feels softer.

rrgrassi · #5 07-25-2007, 10:12 AM

Yep, check vacuum, vacuum leaks also affect engine shut off. Replace fuel filters first for the engine power loss.

Brian Carlton · #6 07-25-2007, 11:25 AM

The lack of vacuum clearly explains items 1 and 3. It appears that the vacuum pump is failing.........or you've got a massive leak from the line to the booster.

However, a lack of vacuum cannot explain the loss of power on a ramp. This would be a separate problem.

ck42 · #7 07-25-2007, 11:52 AM

Regarding #2, I had a brain fart

I was thinking that the ALDA was being driven by vacuum instead of pressure. That's why I tied it into all this.
I'll try to obvious stuff first for power loss. (filters, injector cleaning/purge, etc)

Thanks everyone for the responses regarding the brakes/vacuum question!
Now I just need to get in there and HOPE that it's something obvious.

Not sure how I would be able to easily tell if the vacuum pump is failing....IF it's failing intermittently (man I HATE intermittent problems

)
Only thing I can think of right now is to splice in a vacuum hose and run a gauge to the dash so that it can be monitored while driving.

Chris
'85 300SD

Jeremy5848 · #8 07-25-2007, 12:12 PM

Check this thread:

http://www.peachparts.com/shopforum/showthread.php?t=162069

My gauge is in the engine compartment. When I occasionally have a vacuum problem, it lasts long enough for me to stop and pop the hood. A dash-mounted gauge would be better but I don't have any more room for gauges.

Jeremy