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
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.
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.
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
ps this happens a lot after steam cleaning a gas engine..connections start
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
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
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?
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.
I got a price on wires for my car...and thought it was a bit rich. An
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
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:
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.
Does your model have the Bosch K-Jetronic (CIS) mechanical fuel
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.
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:
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.