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560SEC
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Discussion Starter #1
SOLVED: 1986 560SEC fluctuating RPM and manifold vacuum at idle

Hi

Some weeks ago my SEC started having the following symptoms: Sometimes when stopped at a traffic light, the idle RPM would decrease noticeably and a couple of cylinders might have not fired. As the light turned green and I applied throttle, the car would hesitate for a moment but then regain normal operation. As it transitioned from idle to part throttle driving, no problems whatsoever.

Now, the symptoms are worse. The idle is quite poor and every now and then, when I apply throttle, however carefully, the engine will shut off. I restart and quickly apply throttle and off I go. Once again, transitioning from idle to part throttle and I have no problems.

Two factors are ALWAYS present when I have the trouble described above:
1) The engine is not up to full operating temperature. After driving for like 10 minutes, everything is just perfect. No problems idling or taking off whatsoever.
2) The manifold vacuum gauge indicates drop in vacuum. I know that the manifold vacuum (=economy) gauge should indicate maximum vacuum at idle, but when I have the problem described above it certainly does not, instead pointing at a reading which is perhaps 1/4 way to the right.

I would certainly appreciate you guys sharing whatever experience you might have on the issue. Thank you very much in advance.
 

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Things to look for... throttle linkage lubrication, idle control valve to see if not gunked up, idle control relay (common circuit board failure), possible vacuum leak sealing itself once expanded by engine heat.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,787 Posts
I'm leaning with

"possible vacuum leak sealing itself once expanded by engine heat. " (G.T.)

and its getting worse..

I bet you can find by spaying carb cleaner… WD40… NOT Silicone spray .. Not Brake Cleaner… something flammable with one of those little nozzles..

When you reach the vacuum leak the engine RPM will change.. smooth out something along those lines..

Hey good luck man,

MBL
 

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Works both ways, doesn't it?

Depending where the crack is, heat will either close it or pull it further apart. You need to be Colombo sometimes...


 

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560SEC
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Discussion Starter #5
Solved

SOLVED!

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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Premium Member
1991 560 SEC 1994 E500 2014 E350 Cab
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Aah, another true life vindication for the coil suggestion! Thank you fellow Texan!
 

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Congrats and thank you for bringing a resolution to the thread.
 
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