Mercedes-Benz Forum - View Single Post - 1986 560SEC fluctuating manifold vacuum when cold
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post #5 of (permalink) Old 01-31-2015, 05:49 AM Thread Starter
BenzWorld Junior Member
Date registered: Sep 2013
Vehicle: 560SEC
Location: Plano
Posts: 13
Mentioned: 0 Post(s)
Quoted: 0 Post(s)
(Thread Starter)


I am so happy right now. Without further ado: it was the ignition coil.

My best bet was the problem is a vacuum leak. Maybe in the air conditioning system, I thought, since it has a number of small vacuum lines that when leaking into the manifold, would be enough to cause a disturbance at idle RPM but not when the engine is really breathing.

My second bet was the onboard "computer" that does idle rpm regulating, or one of its inputs.

My third bet was ignition, since the roughness in the idle did sound a bit like a misfire. So perhaps those misfires were enough to bring the engine to a near-stall and cause manifold vacuum to spike.

While I really did think ignition was not the most likely cause, it still was the easiest to isolate. Replacing plugs and plug wires did help just a little bit but definitely did not fix the problem. I was almost certain it would not help but still wanted to replace the coil, just to make sure I did not miss something so easy to replace before diving into a massive project to replace all rubber under the hood.

Bingo, after replacing the coil with a OEM one, the engine positively snapped into life as I turned the key and the tach held a firm 750rpm at idle, economy gauge glued to the left. A quick test drive proved the problem was defeated!

Replace your coils as they approach 28 years of age, boys and girls.
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