what causes and engine miss?

Normally an engine miss wouldn't be something to boggle the mind, but the cars has a fairly new timing chain, spark plug wires, and distributor (less than 15k miles), new spark plugs and fuel injectors and fuel injector seals
(around 2k miles or so). All fluids are fresh (less than 10% of their useful lives have been consumed). I can't think of what else could need to be done (this might not even be everything I've done). Any ideas? Thanks, Richard
BTW - It misses at idle
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seals
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done
A compression check should have been made long ago, but it's never too late. It won't fix the problem, but it will help to identify it.
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I don't recall what year this car is, but a bad O2 sensor causes a rough idle (newer V8s have four of them, two ahead of the engine, two at the exhaust system for ECU comparison and fuel adjustment).
I passed a car uphill over a mountain pass without a problem last summer, but when I got off the freeway a little later, the car started suddenly shaking at idle. It accelerated fine, but refused to idle smoothly. When Roadside Assistance pulled up and connected the laptop to the car, it indicated one of the front Oxygen sensors burned out. According to the laptop, the sensor overheated at approximately 102 mph some twenty-five minutes earlier. Ha.
Another culprit may be the MAS (Mass Air Sensor) on the intake; that also controls idle and fuel mixture.
Annoying problems, but you seem to be fairly close to solving it. Hope it's simple from here out.

(less
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Here is some more info in response to the responses. It is a 1985 380SE. A compression check was performed when the timing chain was replaced, and it checked out perfect (according to the guy who was doing the work). The O2 sensor is also very new (maybe a couple thousand miles on it). Basically ever maintenance item has been done to the car (even diff fluid, tranny fluid, and oil is pretty fresh). I should note that there was no miss before all this stuff was done (well, ok the car shook at idle before the timing chain was done, but that fixed everything...also a camshaft was replaced at the same time). But inbetween the stuff that was done 15k ago and the stuff done 2k ago, the car ran perfect. After the stuff done 2k ago was done, the car ran even better. Only recently did it develope a miss. Richard

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could you make out any connection to engine temperature? does it miss hot and cold? (cold in after a whole night)

the
Richard
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I'll have to find out. You see, I'm 600 miles away from the car, so I'm diagnosing this from afar through my dad. I do know that the external temp where the car is is about 20F. Richard

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had similar problem...when car sat over night...ran great next morning...when engine reached about operating temp...it would miss a cylinder or 2....took long time to finde the prob it was a spark plug wire...which measured out fine...but I changed them all (costy) an now it runs great again
good luck
alex
ps this happens a lot after steam cleaning a gas engine..connections start corroding....

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My first thought was also spark plug wires.
With as much troubleshooting as this car seems to have had, it's not out of the question that a cable got yanked out of the connector and the recently replaced set of cables was damaged. It certainly has all the symptoms of a bad set of cables.
You might try experimenting, one by one, with a known good spare cable to see if one of the existing cables has been damaged. Those caps/connectors can be aggravating to get off at times, and it's natural to just yank on the wire instead of the connector in a fit of frustration. Rather than go replace and expensive set of cables, I'd suggest using one nice long good cable, and going one by one around the engine to test-replace each wire, seeing if it eliminates the problem. Of course, be careful in removing the existing cables so you don't create new problems that will confuse the troubleshooting you've already done.
Spark plug wires, no matter how new, clean, or visually perfect they may seem, can be damaged internally and cause exactly this problem. I had a car burn out the rear-most wire every six months (bad design, a GM product) because it should have had a heat shield; the wires always looked good, they were never more than six months old, and the idle problem always reoccurred. It's amazing how delicate the internal wires can be on a set of spark plug cables. I'd suggest another look at the plug wires, as the above contributor suggests.

to
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With these cables, if the cable gets yanked out of the connector, you can just plug it back together (right?). So if this is the case, I would be very happy. Thanks, Richard
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No, you can't just slide the cable back into the plug and then press the plug onto the spark plug in the engine. If the cable was yanked out of the connector at the end of the spark plug cable, it's probably damaged. Some spark plug cables have small Phillips screws to secure the internal wire to the connector clip, which is then pressed onto the spark plug. Most cables are pressed into the connector plug, which is then pressed onto the spark plug. If the cable has been yanked out of a pressed connector plug, it's shot.

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Really? The parts guy (who is far more than a parts guy-when a trunk lock broke, he was able to rekey the lock so that I wouldn't have to change all of the locks) said the cables where good to go. And why wouldn't I have had any problems immediatley after? Richard

out
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If the cables are new, professionally installed, it's probably fine. If the cables are several weeks old, and they've been yanked and put back by someone who removed them incorrectly, then they're probably shot. The point is, spark plug cables should only be removed by the end connector, and never by pulling on the wire itself. That alone will damage and ruin a set of cables that look and seem like they should be fine. The kind of damage they cause is more of an idle problem than an outright failure to run or start; it's the quality of the electrical connection that suffers, not an absolute failure to do anythingt at all.
As the previous poster and myself have pointed out, when our cars have had identical symptoms of missing idle, the source of the problem has been faulty spark plug cables, even when they have looked new. The previous poster also mentioned that he had his cables electrically tested good, but even then replacing the set of cables eliminated the missing idle problem he was experiencing. The same experience goes for myself, several times on several different cars.
Good luck with your parts guy, then, if he knows it all and still doesn't have a smooth idle.

symptoms
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I got a price on wires for my car...and thought it was a bit rich. An old 380
My solution, which may horrafy some purists. After some thinking and looking. The little knobs on the top of spark plugs screwed onto the distributor cap. I screwed the knobs back on the spark plugs on the car, and got some for a regular HEI set up of an American Car.
I went to the parts store, and with a list of lengths, and had the whole job done for about $35
Then I found out the miss at idle was a vacuum line came off the back of the injector, such is life.
Alex Hemmerich wrote:

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I put a new set of plugs, leads dizzy cap and rotor arm in my 77 280E. It still missed bad. Checked everything again. Compressions...the lot. Guess what it was. TWO duff brand new plugs. Change the plugs again for another new set. They cost bobbins, and can be fitted easily. It may just solve the problem. Alternatively Does your model have the Bosch K-Jetronic (CIS) mechanical fuel injection?. Could it be that one of the injector O-ring seals has cracked or been trapped/damaged during fitting. I believe the test is to spray gas (perhaps 'easy start' [that's a UK product name, sorry don't know the USA equivalent]) around the injectors whilst its idling (being careful not to get it to ignite and take our eyebrows off), and see if the idle smoothes out. I'm sure someone who's done this can give you the finer points on not frying yourself (or Pater dearest at the moment) or the car. By the way, if the car does go up in flames, be sure to push it out into the street. The car insurance cover (at least in the UK) is better there (off private property).
Just a thought (or two)... Rob.

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The missing didn't occur directly after the injectors were replaced. It is a more recent, as in last 100 miles, issue. Richard
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Most anything in a spary can will work for finding vacuum leaks. (except paint, well that works tooo but makes a mess) One I use is fly spray, as its always easy to find in the pantry, when the engine smooths out when U spray in that area.... thats where you have a closer look.
Rob. Smith wrote:

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