My '76 450SEL has a cold start problem. When Cranking the engine, it will start instantly, but then immediately die when I release the ingition key from the "start" position. If I hold the key in the "start" position (with starter continuing to crank the engine, even though it is running), the engine will continue to run until I release the key. I think this W116 may have a similar cold-start system as the earlier fuel-injected M-B's (W128 220SE for example) where there is a cold-start relay that signals the system to inject a spurt of fuel when the starter is cranking the engine. I'm fairly convinced that if I could locate the correct relay on this 450SEL and install a manual switch to override the key-activated system, my problem could be solved, at least temporarily. Can anyone out there point me to the physical location of this relay? OR- tell me if I'm on the wrong track.

Your problem is no fuel delivery excpet the cold start valve (it operates when the starter does), not failure of the cold start.

Most likely you have either a binding air meter flap in the flow meter (visible just under the air filter) or a large vacuum leak. It may also have a seriously out of adjustment fuel mixture, much too lean.

Major vac leaks sources are the boot between the air flow meter and the throttle body (check for a loose clamp -- if it is, the rubber is shrinking and leaking air), hoses to the idle control valve if so equipped (and I'm not sure, I believe it has computer control, but I'm not positive), leaking seals between the manifold halves, bad brake booster line or booster.

I suspect ALL the vacuum line rubber parts are shot by now, unless replaced in the last few years.

It is also possible that the fuel distributor is bad (very expensive), but diagnosing that requires the correct guage setup to test.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!