Help

I have a 1974 450SEL with electronic fuel injection. Car starts fine, runs at 2000 rpm, blip throttle revs drop but dont settle to steady idle. Engine revs up to 2500rpm then cuts, drops to 500rpm and catches and revs to 2500 rpm again. If I blip the throttle again the engine will rev to about 3000rpm and stay there. Have I lost a temperature sensor? Is the cold start valve stuck on? How do these things measure air flow? The Haynes manual is as much use as a chocolate fireguard. Any ideas of what to check first would be gratefully received. Any ideas of where I can get an engine manual with ignition info would be a bonus. Thanks .

Colin Whyte

Check for a large vacuum leak, broken hoses, or a bind in the linkage.

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Benzmac:
Donnie Drummonds
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS

A: they don’t actually measure intake air volume – this setup uses a vacuum signal to indicate load on the engine. It is an approximately baseball sized unit with a fairly large diameter hose from the intake manifold running to it. If yours is a 4.5, it should have a blue color code on it (3.5s are yellow)

B: This could be a combination of factors, one of which sounds like the (timing advance) weights inside the distributor are sticking. This really seems unlikely due to the propensity of the distributor shaft seals to go bad – this is why/how the injector trigger points get fouled with oil. You may want to put a timing light on it to see how the advance curve looks – does it move smoothly with increasing RPMs, or does it jump around? Also look for consistent readings at the same RPMs

Good Luck!

RTH