280SEL 4,5 1972 runs poorly at idle speed - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 09-27-2010, 01:32 PM Thread Starter
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280SEL 4,5 1972 runs poorly at idle speed

Hey. I have a 280 SEL with the M 117.984 engine. The engine starts fine cold and warm. But it goes poor and vibrate at idle speed, especially in gear, waiting for green light etc. Oil pressure is 45 PSI when cold, but about 15 PSI (ca 1,03 bar) when warm. When it was in gear, the oil pressure have been as low as 7-8 PSI. Is this normal? What is normal pressure?

When i accelerate, it rises to 45 PSI quite fast. And the car accelerate well. But it jerks a bit when throttle.

My mechanic said the throttle switch doesnt respond to the diagnosis aparate. There was also false air at 3 of the cylinders. The engine didnt respond when screwing at the CO. But I have now renewed intake manifold gasket, and the 8 rubber in the manifold. and also the smallest rubber part in the tip of the nozzle. So I think it should be no false air, but havend checked that. The car still goes poor it idles speed.

Last edited by 280SEL4,5; 09-27-2010 at 01:39 PM.
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post #2 of 4 (permalink) Old 09-28-2010, 10:09 PM
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Vehicle: 73 280SE 4.5, 71 300SEL, 03 BMW Z4, 72 Fiat 850, 80 Triumph TR7, 85 Porsche 911
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Sheesh, where to start ... Have you checked your plugs? What color are they? Any oil fouling?

What kind of ignition system do you have? Original points, or an upgrade?

Some things I would do first, if the car were mine: replace plugs, wires, cap and rotor, replace points (or upgrade to electronic ignition), clean fuel system trigger points, clean contacts to the sensors (air temp, water temp, manifold air pressure sensor, throttle posit sensor) and clean the grounds. The throttle posit sensor can be opened by the way - just be careful when doing so. Using an acid brush and isopropal alcohol, the insides can be cleaned (I did this and it made a huge difference).

When you say false air in three of the ctlinders, do you mean there's a bad seal?
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post #3 of 4 (permalink) Old 09-29-2010, 04:25 AM Thread Starter
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Have checked the plugs and they looked ok. But i will replace them.

I dont have upgraded the ignition system. The cap is new, but i will replace rotor. I plan to also buy points and wire.

I saw a site how to test throttle switch and clean;

http://home.cogeco.ca/~mbz/Adjusting and Cleaning TPS on R107 Mercedes.pdf

Is it enought cleaning inside the switch with electronic spray?

I meant that it was false air in the intake manifold or around the nozzles.

I will clean the contacts on the sensors.

Thanks for answer
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post #4 of 4 (permalink) Old 09-29-2010, 06:24 PM
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Two things... have valves adjusted and check all possible vacuum leaks. Grubeguy has a lot of good ideas, but you have to start somewhere. Good luck!
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