4.6L engine misfiring. Causes?

First thing I do to older cars is a compression test on the cylinders. If they aren't up to par, I advise customers to not invest money into it. Unless they want to replace/rebuild the engine.

1. The first thing you should have done is have the engine scanned to see what fault codes the computer has generated because with a misfire there will be a fault code generated, possibly more than one. Just changing parts hoping to stumble on the cause is not the way to go.\

2. Assuming the plugs and coils now fitted to the engine are serviceable the possible cause of the misfire may well be a vacuum leak. But there can be other causes so I suggest you have the engine scanned and once you have the data you can diagnose the possible cause from there.

a Hyundai Terracan????????
what year, must we guess that too, and country? and engine size option too?

i think that is random misfire.
what is the question,
how to diagnose it?, read the FSM.
or who can fix it, answer any ASE mechanic on earth can.

open the FSM what does it say about 300s.
all say the same thing. MISFIRE GROSS and random cylinders.
and can be bad engine compression bad ,bad spark or bad fueling.

top causes. (not a guess, )
1: never ever doing the 60k mile service, not EVEN new spark plugs.
2: lean AFR. (1 example, low fuel presssure) for dirty MAF if it has one.
3:rich Air fuel ratios. (or flooding) causing misfire, high fuel pressure.
too high, (horrible clogged up air filter? extreme)
4: cam timing way off, cam belt slip or even a chain, if deaf.
#4 cause huge drop in compression.

That is the why, there are more.
why not get the car scanned and check out for if the engine
needs help or sensors read wrong using a SCAN TOOL.
does cooling system (engine) hit 180F or more. hold there.
if not it will rich misfire.
using a real scan tool shows all DTCs codes stored. other wrong or cheap tools will miss lots of factory code.s

good luck.

Could be injector or possibly intake gasket leak.
see misfire section at link below.
Do It Yourself Diagnosis and Repair

When your engine light flashes about 95 percent of the time (f not more) means that your vehicles computer (PCM) has detected a misfire and that the misfire at the time it is flashing that the misfire is bad enough to cause damage like to the catylist converter(s), oxygen sensor and a number of other things. You need to find out which cylinder has the misfire. You can go to your local auto parts store and most of them will pull diagnostic trouble codes (DTC\'s) for free. Now once you get the DTC(s) you can narrow down whcih cylinder(s) has the misfire by what DTC they pulled. For example a DTC of P0301 means a misfire in cylinder 1, P0302 would be cylinder #2 and so forth. A P0300 means multiple random misfire a much further testing needs to be done in order to diagnose. Say you have a P0301, depending on your engine size you will need to isolate cylinder # 1 to diagnose. If you have a 3.8 liter engine in most cases it\'s the fuel injector ot a bad spark plug wire for that cylinder that is bad but if you have the 4.6 liter most common cause is a bad ignition coil for that cylinder as they have a seperate coil for each cylinder but it could be an injector too instead of the coil. I\'m just giving you the most common causes but diagnosing needs to be done in order to replace the faulty part(s). Other causes of a misfire are crank or cam position sensors, a vacuum leak, Faulty spark plug wire (if you have the 3.8), engine problems (internal), electrical and/or wiring problem or even the PCM. Many other things can cause this, these are the most common. Gatting that DTC is vital in order to diagnose. If you need further help as far as testing procedures once you get any DTC\'s I\'ll be happy to helps you diagnose the problem. Hope I was able to help you understand what\'s going on...

Stop using the car

You have serious cylinder misfires,so severe
you maybe ruining the exhaust converter,thus the smell

I won't write a book on all the causes or fixes,have a
repair shop diagnose

There are MANY things that can cause an engine to "shake and sputter".
Shake and sputter in the auto repair world is called "cylinder misfire". There is not enough room on this page to list all the different things that can cause a cylinder misfire. Diagnosing a misfire can take several different turns depending on things like how many cylinders are misfiring, at what engine RPM(S) is(are) the misfire(S) occuring, what engine temperatures, vehicle speeds, etc. etc. etc.. Certain possible causes can either be verified or eliminated due to the answers to these questions.

The first thing to do is to scan your engine control computer to see if there are any stored Diagnostic Test Codes (DTCs - commonly reffered to as "fault codes"). This can have a HUGE influence on which turn to take. It is kind-of like when your thumb is smashed to pieces and turning black, you know when you check the data that your pain was not caused by a razor blade...it was the hammer that did it. If you do not look at the data, all you know is that your thumb hurts.

Misfire codes (including P0301 (THIS WOULD BE YOUR CODE), P0302, P0303, P0304, P0305, P0306, P0307 & P0308) indicate that there is a misfire in a cylinder, the last number indicates which cylinder has the problem. If something is causing a miss in random cylinders or is affecting all of them, a P0300 misfire code will be stored in the car's computer. It's always smart to begin with the basics. Also consider the mileage and service history when diagnosing a misfire. Higher mile vehicles are more inclined to have mechanical issues with the engines, like low compression from worn valves or rings etc. Any accompanying codes should also be considered in case they may be related. If the spark plugs are worn (excessive gap) or the car is past due for a tune up, it may be smart to go ahead and start with spark plugs and spark plug wires and go from there.
The most common misfire causes on the cars I've worked on have been:

1. Spark plugs
2. Spark plug wires
3. Ignition coil
4. Fuel injector
5. Wiring to fuel injector
6. Timing Belt (IF EQUIPPED)
7. Vacuum leak or stuck open EGR
8. Contaminated fuel or bad fuel pump
9. Weak compression
10. Blown head gasket

Obviously there are many different types of cars, so a service manual for the specific one that you are working on may be required to help pinpoint the misfire you're looking for, but hopefully this will direct you to some of the most common causes of misfires in cars of today.
When diagnosing a SES light, it's best to start with a quick scan with a code reader. If checking the engine's mechanical integrity specialty tools, like a compression tester, an exhaust back pressure gage or cylinder leak down tester may be needed.

Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example).

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust.

What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad spark plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak.

Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it's likely the head gasket is the culprit.

Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine load or operating conditions. They seem to occur for no apparent reason. The engine may only misfire and run rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent reason. Intermittent misfires can be a real challenge to diagnose, so let's start with a steady misfire in one cylinder before moving on to intermittent misfires.

Hope this help.

Even with low miles, the truck has some years on it. Is the gas old? If the truck seemed to run fine, I would clear the code and see if it came back. Sometimes these things are just a one-off occurrence. If it continues, I would make sure it isn't running on old gas (if the truck has been sitting a while), then check the spark plugs, and go with the usual tune-up items.