I have a 1999 Sonata which just started misfiring. We brought it to the
dealer with the check engine light on and they wanted to replace all the
spark plugs and wires (which requires removing the air intake plenum) and
then see what the problem was.

I just replaced the fuel filter and I have read on other sites about
similiar problems with misfire codes and dealers performing the same
repair only to find it wasn't the problem. I remove the spark plugs and
they don't look that bad, neither do the wires. The car has about 50,000
miles on it.

Under a load, starting from a stop the car will misfire or ping. It idles
a little rough and eventually the check engine light will come on and the
computer will, according to the dealer, show 3 cylinders misfiring code.

I find it hard to believe that 3 spark plugs and wire failed at the same
time out of 6.

Anyone have any ideas?

Thanks

Dave

 

Unfortunately, this is probably the easiest place to start. While the
plenum is off, it's probably a good idea to check out all six fuel
injectors visually. I've seen the injectors on this engine cause misfires
(although I've only been barely able to feel them and only when cold, with
the exception of one). The fact is, the wires are usually the culprit.
If you have a misfire on one of the even cylinders, you can try to address
that one first, since those cyliders are in the front and the plugs, wires,
and injectors are all reasonably accessible. You can try swapping plugs
and injectors between misfiring and nonmisfiring cylinders to see whether
the misfire changes cyliners with the change.

 

Do you replace the injector or what would you recommend to do with the
wiring? From taking the spark plug wires off the coils I have narrowed it
down to the back middle cylinder. When that plug wire is off it does not
misfire anymore. It still runs rough, since it's running on 5 instead of 6
cylinders but it doesn't misfire with the cylinder out of the loop.

 

Ok, I did more probing today. The plugs I just put in days ago are already
black. I am leaning towards a rich mixture and the backfires are from too
much fuel causing detonation.

So I am looking at the oxygen sensors. Then I have realized that there are
4 on this engine. The two on the top are reasonable and the two on the
bottom are expensive. They are 4 wire sensors. black/white/green and
yellow wires. My understanding is that two wires are the heater and the
other two send the signal.

Does anyone know which is which and how to test the sensors?

 

If you don't have an oxygen sensor dtc, look elsewhere first. Even if you
do, you may still want to look elsewhere first. The most common problems
are ignition (plugs, wires, and to a lesser extent, the coil). A misfire
will cause the oxygen sensor readings to be incorrect.

What do you mean by not misfiring any more but still running on five
cylinders? Do you mean the car actually runs better with the #3 wire
(center rear) disconnected?? What is the misfiring you describe if it's
not the failure of the #3 cylinder to fire?

I wouldn't do anything right off with the injectors unless one looks
clogged. If it's clogged, replace it.

 

I was wrong on that. This is my analysis.

I replaced the spark plugs. All black. The wires test ok. The fuel filter
has been changed. The removed the front fuel injectors. All visiable ok.
Look clean as a whistle.

The engine backfires when starting from a dead stop or under power,
accelerating. Rough idle. No power.

I think that one of the oxygen sensors is bad telling the computer that
the engine is running lean and the computer is running the engine rich to
compensate. The back fire is the rich exhaust gas pinging on the hot
convertor. The computer is interpreting this as a misfire.

The dealer did state on the ticket that the computer read 3 cylinders were
misfiring. Since two sets of oxygen sensors read each side of the engine 3
+ 3 cylinders = V6, it appears that one of the oxygen sensors on one side
is completely dead reading lean.

This car has never had an oxygen sensor replaced.

 

How did you test the wires? The typical mode of failure is arcing out the
side of the plastic tower that goes down into the hole in the head.
Usually, you can see carbon growth, but sometimes the hole is so tiny you
can't see it. I've seen cases where the plug wire had to be pulled off
the plug a little with the engine running to see the arc to the head.
(You cannot do this on the rear bank of the v6, of course).

Which 3 were misfiring? If they were the back three (#'s 1, 3, 5), then
you may be on the right track with the oxygen sensor. This should be
easily discernable with a scan tool since the fuel correction will be much
different between the banks. I still think you're likely to get an o2 code
if you have a sensor problem.

If your engine is still apart, you can move the injectors from the rear
cylinders in question to a cylinder on the front bank and see if the
misfire moves. This way, you won't have to pull the plenum again to
replace the injector.

The computer is not reading anything that happens in the converter as a
misfire. The computer detects misfires by deviations in expected
crankshaft speeds. Anything that happens in the converter won't be able
to affect that. The backfiring you're hearing in the converter (or
intake) is a result of the actual misfire.

I had one car with a bad injector that I had difficulty finding. It
didn't look bad, but replacing it fixed the problem.

I had another car with bad fuel, and it turned the plugs black like you're
describing, and the car really wouldn't run worth a crap. I had trouble
just driving it up the hill to get into the shop. I went through a lot of
stuff before I eventually figured out what the problem was.