Joe... I suppose it's often the simplest of things... small oversights... that cause some of the symptoms/problems. The brake symptom Luis describes
quote:
2.-When you press the brake pedal parking the car at really low speed (withouth pressing the gas pedal) it is really hard( Shifting to neutral causes to have no motor force and you can stop the movement of the car)At normal driving it brakes correctly. He told me this was caused for the motor fault or for an pressure loss in the barke circuit .
and earlier
quote:
At the same time when riding at low speed it seems as if the servobrake is not working and the brake pedal is really stiff and does not brake . You must change to neutral and then it is possible to stop the car.
both describe a lack of braking power at low speeds or effectively lower rpm range, but back to normal braking power at idle level rpm "parking... at really low speed (without pressing the gas pedal)" and "You must change to neutral and then it is possible to stop the car." which indicts a lack of boost somewhere between the brake pedal and MC. The alternate situation is that there's good braking at normal speeds (which I translate to also mean normal driving range engine rpm), but poor braking at some intermediate rpm level between higher driving range rpm and idle rpm.
If there were a vacuum leak at the booster, rpm would increase a 200 - 400 rpm on braking at stop (in neutral)... Luis doesn't mention anything like this... not to mention that the braking power is only negligably reduced with a minor leak. So it doesn't sound like the classic brake-booster oil or gasoline damaged internal rubber.
The vacuum check valve may indeed be installed backwards (All he has to do is just remove the valve from the hose (hose is actually in 2 halves.. valve inserted in middle or so), reverse it's direction (or blow on it on one side to see which side is to "stop air flow", and which to allow it), and reinstall, tightening the clamps.
Another check is to see whether the vacuum line at or from intake manifold to the check valve is plugged or clogged ... remove the line from the manifold and check vacuum level at manifold first. If that's drawing at spec, then reinstall the hose, remove the check valve, and check the hose line between intake and where the check valve goes. Finally, if that's ok, vacuum magnitude in the line between check valve and input to booster.
There's a gasoline collector (small plastic bottle) in the line from intake manifold to booster also... this should be below the level of both the intake manifold and booster so that any gas condensate collects in the bottle and doesn't make it's way to the booster input (which will wreck the rubber inside the booster in short order). The connections to the bottle, and the bottle itself should be checked for vacuum leaks also.
Basically all the symptoms simply point to an inefficiency of the booster's ability to apply force to the MC at some intermediate rpm range at some level slightly above idle/near idle, and normal driving range rpm (say > 2000 or 2500?)... which either means
- booster is damaged internally such that both a lower level of vacuum and a higher one overcome the internal problem in the booster, but an intermediate vacuum causes a problem. OR
- insufficient vacuum getting to the booster at the intermediate rpm range, but sufficient vacuum getting there at lower rpms and higher driving range rpms
Sounds funny, since one would expect the reverse... higher rpm, more vacuum, lower rpm less vacuum being pulled on the booster.
But therein also lies the clue... If cyl 6 isn't drawing from the intake on the intake stroke at the intermediate rpm range where braking is a problem, then one possibility is that the exhaust valve's remaining open too long during the intake stroke... so the intake stroke's simply getting
some of, or too much of it's intake from the exhaust instead of the intake side.... and that this occurs more obviously in terms of a vacuum being drawn from intake manifold at cylinder 6 end at an intermediate rpm... but which is not a problem perceptibly at lower rpm and higher ones.
In fact, since the intake manifold side of things is open to all cylinders the intake manifold vacuum should be equal no matter where the take-off is located on the intake manifold... but the venturi at cyl 6 is the location at which the higher vacuum level is achieved.... so the venturi may be operating ineffectively at the intermediate rpm range where braking's a problem... dropping ability to create the vacuum at that rpm range... but not at either higher or lower rpm range... at least not perceptibly in braking at the higher normal driving rpm range.
The vacuum from the venturi should behave nearly linearly with rpm... and there should be an MB spec for the rate of vacuum increase with increasing rpm.... or the rate can be checked by using a normal functioning SL model with same engine as Luis's.
So, the problem could be with the venturi, or with valve timing at Cylinder 6... which problem might account for both braking symptom and the spark plug color at cylinder 6. If the valve timing on cylider 6 is off on the exhaust (or intake to exhaust valve lag), this might not show up in a standard compression test (cracking over the cylinder a few times with starter motor)... and if that's the case, then it's more than likely related to a root cause being the cam shaft lobes on cylinder 6.
All the above purely from an engineering point of view... not from an experienced mechanic's expertise. I wonder if anybody's checked the cam?... timing chain streched? perhaps?
Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport
