450 SL in Denmark

I went for a drive and both before, during and after the trip I took off the fuel cap and only heard a very subtle "swoosh". I have tried to upload small video clips. Seems pretty normal to me.

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Well, here is an update on the almost ever lasting (same) problem. As a recap, the engine starts up fine and idles fine. There is an occasional miss here and there at idle. Once in gear it misses quite significantly and is difficult to get moving from 0-60 km/h. Once up to speed, it pulls nicely and evenly. Ocassionally, I experience that the engine is behaving normally and pulls as it should from 0 km/h, but then a day or two later, the same symptoms appear again. I feel I am chasing a ghost.

Here is what has been checked/replaced over the past several weeks:

• Fuel filter replaced
• Fuel strainer in fuel tank replaced
• New fuel hoses at fuel pump assembly
• Fuel venting system checked
• Fuel pressure OK
• Capacity check OK (1 L in 30 sec)
• New ignition points
• Dwell OK
• MAP sensor holding vacuum 
• Smoke test to check for vaccum leaks - my mechanic says there are no leaks
• Timing OK and the centrifugal mechanism is working as mechanical timing advance is working at 1,500 and 3,000 rpm
• When pulling the vacuum hose from the vacuum can at ignition distributor, the timing advances 5-7 degrees - so seems to work
• Tried to connect another working ECU - no difference
• All injectors are ticking steadily and evenly (although two injectors have "softer" pulses than others (have tried to switch them with other injectors to verify if it was the trigger points, but the "soft" pulsation followed along, so likely not trigger points)
• When I connect a T-piece to the MAP sensor and measure vacuum at warm idle I get steady-ish (perhaps sligtly fluctuating) 13-14 inHg or 340-350 mBar - that seems low. 
• When stepping on the throttle, the vacuum goes to 0 and then jumps to 20 inHg or 500 mBar - after that it settles again around 13-14 inHg. - I have inserted photo 

I am beginning to suspect that I might have a vacuum leak after all, even though my mechanic says that he has checked it and found nothing. What else can explain the low vaccum in the manifold?

Any thoughts?

Best, Peter

Hi Peter,

have you ever done a compression or better a pressure loss test on your engine. If piston rings or valves are not tight, you might also have low level of vacuum in manifold. That of course makes MPS to signal wrong load to ECU and enrich mixture. That would fit to low vacuum and problems in low rpm.

Also check all hoses connected to manifold whether they sit tight or loose.

Hi Volker,

Thank you. I will start by checking valve clearance. I have bought the correct wrench Hazet 329-4 on Kleinanzeige which will hopefully reach me when I'm in Hamburg next week.

The strange thing is that the behaviour of the engine is totally arbitrary. The other day it ran flawlessly across the speed range and during acceleration only to be totally unpredictable the next day where it did not want to accelerate but ran fine when up to speed. This afternoon, I just took it for a short trip to see how it behaved today and here it also missed and lacked power even when acceleration from 70 km/h onwards. This erratic behavoir makes it extremely difficult to diagnose the real underlying problem. I would suspect that if the rings are worn or the valves are out of adjustment or there is false air, then the symptoms would be the same all the time and dependent on load, speed, etc. and not erratic.

Best, Peter

Could the fact that I have two injectors whose pulses are softer, yet still regular, suggest that they are not providing enough fuel into the combustion chamber, hence leading to too low a manufold vaccum? I have not yet performed a volume and spray pattern test.