I have a '77 450SL. When trying to start it I must pump the accelerator heavily. Once it starts it runs fine. I would appreciate any suggestions on where to start looking for this problem.

 

Without consulting my manual, I am hazarding a guess here that the FI 450 is similar to the 560... Obvious checks are the fuel pump, but more likely the fuel accumulator, assuming the 450 has one. On the 560, a fuel accumulator assists the start-up by retaining fuel and pressure on tap, ready for the ignition to by switched to "start"

I don't mean to be critical to this board... In most respects it is a very good source of info; however, I am amazed at the lack of replies issued to posts. I see topics, like yours, with 20, 40 100 or more viewings, but only 1 or 2 replies... Sometimes none, even. The other two boards I view are experiencing the same... Just a curious observation, I guess.

Good luck with your starting problem If you haven't yet, navigate to
www.mercedesshop.com and register. It seems to me there is more "TECH" help available on their board there...

 

It could be a leaky frequency valve, located on top of the manifold - driver side - just in front of injector system - under air cleaner. A little over a hundred for this part at any parts store. Or, it could be leaky injectors. If it is the accumulator, located to the right of the rear end just above the fuel pump, the vehicle would experience hard starting while hot but good start while cold.

Just some of the problems I have encountered over the years with my '81 380. Hope this helps.

Yes it is a shame that there are not more replies to the posts here. I got fed up with the attitude of a couple of the techs and the moderator over on merecedeshop forum. The forum moderator used to (maybe still does) berate some of the members and always sides with a tech even if they are condescending to other members. YMMV.

 

1989 300E

I recently acquired a 300E and encountered difficulty starting the vehicle. It had been sitting for approx 6-8 weeks. The car would crank over but the engine would not engage. Only after numerous attempts and moderate use of the accelerator pedal it would actually start. After the engine started, the idle seemed rather low - 550-600 RPM, and the engine would keep running only with encouragement from the accelerator pedal. Once the car was driven, it would run very well. After only several minutes of running, I would notice the engine rpms would climb to 900-1000 then would go back down to 700 once it was warm.

This problem generally occurs after the vehicle has been sitting for 3+ hours and overnight. I have just had a fuel pressure test done and the mechanic indicated after the test that there was a slow pressure leak occuring. He rationalized that it was in the front half of the car and not the check valve by the fuel pump (the reason for this was that the check valve leak would be more rapid).

After making this determination, he then removed the one of the injectors to inspect the spray pattern. At this point, he noticed that it was spraying in an irregular patter. He also noted that the throttle body showed some minor residue on the inside. It was then concluded that since the aperture of the injectors were not closing properly, it was releasing some of the pressure from the fuel system.

Since this car had been sitting for 2 months prior to starting with very low mileage, it was suggested that the engine would be attached to a "motorvac" machine and cleaned with a combination of fuel and detergent. This would clean the injectors as well as the O2 sensor and other vital components. It was further explained to me that this may clear some of residue that may be impeding the injector from spraying properly and causing a potential vacuum leak in the fuel system.

After the process was completed, I drove the car home and did not find a significat change in performance. Secondly, after shutting the engine down and attempting to restart it again, it seemed to show the same reluctance to start. It will turn over when cranked but will not engage without depressing the pedal. It would seem that it does not have the extra kick (or infusion of gas) to allow it to start on its own.

The mechanic suggested driving the vehicle over the weekend and observind whether it would start. If this is not resolved, he suggested replacing the injectors. My thoughts are that if the cleaning did not work, would it be advisable to replace the injector since we cannot conclusively prove that they are at fault.

Am I overlooking something simple like the OVP (checked fuse and it is fine - no ABS warning light on ) or the coolant temp sensor?

Thank you in advance for any additional insight that any of the members of this forum are able to offer.

 

There is also the possibility of 'false air'. Those 8 little 'boot sleeves' on top of the manifold deteriorate with age and may allow air into the intake system. One might spray a starter fluid around the top of the intake manifold area while the engine is idling to see if the RPM picks up. If so, you have an air leak somewhere, (boots, gaskets, loose fitting....)