Car shakes in low RPM, check engine light on, diagnostic says misfire

Right after refueling and adding fuel injector cleaner and gas
treatment, while washing the car, the engine started shaking and the
check engine malfunction indicator light (MIL) came on. Next day MIL
started flashing. Took it to Auto Zone. The 1 minutes test said
misfire on cyl 4. So, I changed all spark plugs. Disconnected the
battery to clear MIL error. Started engine. It still shakes but no MIL
light. Next day MIL light came on again but not flashing.
I also spotted a very thick resin-like substance around the top of the
fuel pressure regulator. I do not see any gas dripping or leaking from
the fuel pressure regulator.
I have a full tank of gas.
Can it be caused by just bad gas?
Should idle the car for hours and burn all the gas and refuel with
high grade gas?
Hyundai Elantra 2000 with 59K miles.

You could have bad gas. Gasoline additives in high concentration tend to foul spark plugs; it is best to use them when driving at highway speeds. Because it still shakes with new plugs I would bet on bad gas. It will take a long time to burn the gas at idle; better to burn it on the highway.

wrote:
>
> Right after refueling and adding fuel injector cleaner and gas
> treatment, while washing the car, the engine started shaking and the
> check engine malfunction indicator light (MIL) came on. Next day MIL
> started flashing. Took it to Auto Zone. The 1 minutes test said
> misfire on cyl 4. So, I changed all spark plugs. Disconnected the
> battery to clear MIL error. Started engine. It still shakes but no MIL
> light. Next day MIL light came on again but not flashing.
> I also spotted a very thick resin-like substance around the top of the
> fuel pressure regulator. I do not see any gas dripping or leaking from
> the fuel pressure regulator.
> I have a full tank of gas.
> Can it be caused by just bad gas?
> Should idle the car for hours and burn all the gas and refuel with
> high grade gas?
> Hyundai Elantra 2000 with 59K miles.

Thanks Mike Walsh for your response.
I just found out that I have a bad spark plug wire/cable on cyl 4.
Pulling the #4 spark plug cable out while the engine idling does not
change the engine performance at all. Performance change dramatically
if I pull out 1 of the other 3 cables.
Cyl #4 wire shell is cracked, and visibly discharges through the side
of the shell into the body of the engine. Since electricity travels
the path of least resistance, the spark plug misses the power to
create the spark at the tip of the spark-plug and ignite the gas.
The engine has been running on 3 cyls since the car wash 2 days ago.
I only drove a few miles though.
I guess the water was pushed through the engine cover with the help of
the high speed dryer at the car wash. I guess a drop of water caused
the premature death of the cable.
I hope this is the only problem I have with the car.
If not, I will update tomorrow.
However, I am disappointed with Hyundai. They advertise so much about
their 10 year 100k mile warranty. In reality, after the 5th year or
60k miles, only the very internal core parts of the engine and
transmission are covered. Everything else is only covered for 5
years / 60k miles or less.
I just checked the local part store's web site, they only sell sets of
all 4 spark-plug wires. I only need the #4 wire. Where can I find
the #4 wire only?

Spark plug wires on most cars are much better than they used to be and rarely go bad, but anything is possible. Back when bad wires were common I always had a few wires in my junk box I could use in a pinch. I also had a reel of copper wire, which was much more reliable than suppressor wires.
You can get a reel of wire from AutoZone, or individual wires from PartsAmerica.

wrote:
>
> Thanks Mike Walsh for your response.
> I just found out that I have a bad spark plug wire/cable on cyl 4.
> Pulling the #4 spark plug cable out while the engine idling does not
> change the engine performance at all. Performance change dramatically
> if I pull out 1 of the other 3 cables.
> Cyl #4 wire shell is cracked, and visibly discharges through the side
> of the shell into the body of the engine. Since electricity travels
> the path of least resistance, the spark plug misses the power to
> create the spark at the tip of the spark-plug and ignite the gas.
> The engine has been running on 3 cyls since the car wash 2 days ago.
> I only drove a few miles though.
> I guess the water was pushed through the engine cover with the help of
> the high speed dryer at the car wash. I guess a drop of water caused
> the premature death of the cable.
> I hope this is the only problem I have with the car.
> If not, I will update tomorrow.
> However, I am disappointed with Hyundai. They advertise so much about
> their 10 year 100k mile warranty. In reality, after the 5th year or
> 60k miles, only the very internal core parts of the engine and
> transmission are covered. Everything else is only covered for 5
> years / 60k miles or less.
> I just checked the local part store's web site, they only sell sets of
> all 4 spark-plug wires. I only need the #4 wire. Where can I find
> the #4 wire only?

The criminal was the cyl 4 spark plug wire. After replacing, the car
is running very smooth. So, if your car starts shaking after driving
in the rain or going through the car wash, check the ignition wires.
I wish they sold single spark plug wire AKA ignition cables. They
only sell them by the set of 4 or more. So, I had to buy all 4 and
throw away 3 of the old but working wires to accommodate the brand new
ones. What a waste! There should be an agency looking over the
possibility of reducing waste by forcing part manufacturers to sell
single wires.
To test bad wires, start the engine. Use rubber gloves and insulate
yourself from the ground by standing on a dry piece of wood. Very
carefully remove one of the spark plug wires. If that wire was
working properly before removal, you will hear and feel a significant
performance drop in the engine. Put that wire back and the engine
will start running smooth again. Do one wire at a time.
Do not disconnect more than one wire at any time. Make sure you take
a digital photo of the engine before starting. So, you will know
which cable goes where, in case you mess up. You may see sparks
flying from the end of the removed wire to the engine body if you are
not careful pulling it out. Make sure you do not have a gas leak or
anything flammable around. The sparks may ignite them. Also check
the resistance of the wires in kilo-ohm to make sure they are within
your manufacturer's recommendation. Rule of thumb for many passenger
cars is: the resistance has to be less than 10 kilo-ohm per foot of
spark plug wire to be acceptable. The ignition wires are made from
carbon impregnated nylon strings. They carry less current than metal,
but are more flexible, significantly reduces radio and electro-
magnetic interference. Common sizes are 7mm to 8.5mm in diameter.