Cyl 4 Misfire

Jamessandlin

New Member
Aug 17, 2020
24
0
1
22
Kentucky
Changes all plugs and wires and also the fuel injector, still get a code for a misfire on Cyl 4, the engine light goes off for a week and comes back on for 1 day but then goes away, I do not feel a misfire one bit, there is a stutter at stops from the upstream o2 sensor, could the o2 sensor not getting the new fuel injector the fuel or could it be something else?
 
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08GT500

Active Member
Jul 12, 2018
898
139
53
50
Massachusetts
Hi,
What year is your Pony?
The multilingual connector on the single Coil pack is notorious for making bad connections, pull it & have a look, clean it up with CRC electrical parts cleaner & reattach.
Do you have an OBD2 Scantool that can watch live data? They also Datalog and freeze frame exactly what occurred right before, during, after it threw a CEL, frame by frame.
Yes, if you have a flaky 02 Sensor it can affect the air/fuel mixture and cause a code for a misfire to be thrown. Because is says misfire doesn’t mean it’s that, don’t interpret Scantool readings so directly, they help locate what the EEC’s believing the cause is.
How did you determine a bad 02 sensor exists?
Does the CEL flash under acceleration but not remain lit?
Fill up your tank at a new gas station or recently change octane levels?
Best!
John
 

Jamessandlin

New Member
Aug 17, 2020
24
0
1
22
Kentucky
It’s an 09 , when I bought it there were tons of codes, Cyl 4 misfire, random misfire, Cyl 4 fuel injector and both o2 sensors, replaced the downstream sensor and fuel injector already and the code for misfire went away for a month, previous owner had same problem, at red lights and stops it studders and shakes really bad, replacing upstream sensor and fuel filter tommorow
 

08GT500

Active Member
Jul 12, 2018
898
139
53
50
Massachusetts
It’s an 09 , when I bought it there were tons of codes, Cyl 4 misfire, random misfire, Cyl 4 fuel injector and both o2 sensors, replaced the downstream sensor and fuel injector already and the code for misfire went away for a month, previous owner had same problem, at red lights and stops it studders and shakes really bad, replacing upstream sensor and fuel filter tommorow
Hi,
Is the motor and supporting systems stock, or all aftermarket parts installed, running an aftermarket Computer Tune?
Car run the same whether just started at idle, or only when engine is up to operating temperature?
Spray your MAF with CRC MAF cleaner and reinstall.
Loom up your plug wires so they are separated and not contacting valve covers, etc, run at night and look for arc’s that may cause an intermittent Misfire.
A Scan-tool that can read streaming data (PID’s) or perform freeze frame or drive cycles helps greatly, inexpensive & will catch many issues such as this.
Check the basics, any vacuum leaks in all your vacuum lines, air filter & leak free intake tube connections to rule out unmetered air from entering the motor, as well as exhaust leaks between the exhaust manifolds to block and any leaks between?
Check for 02 sensor wiring melted to the exhaust as well as any bad sensor wiring connections up into the engine compartment.
The upstream 02 sensor(s) is what the EEC references for fuel calculations correction, lean or rich. Downstream 02 sensors measure improvements the Cat(s) make in emission reductions.
Be certain the 02’s wires are landed to the correct sensor.
Checking the streaming data will tell you if an 02 sensor is bad, though it seem it’s likely bad, replacing it now may be a shot in the dark. A Scantool will tell you if there’s an issue with it.
Random issues are the most difficult to pin down.
The batch fire ignition coil/Module is vulnerable to bad connections of the large plug entering it, check for bad connections or moisture, corrosion & repair.
Start with the easy testing & unless a part is proven as bad, don’t replace or it becomes more difficult (and expensive) to troubleshoot.
Fuel filter replacement is not a bad idea, check your fuel pressure at the rail and remove the fuel pressure regulator line & see if fuel is present by applying a light vacuum, pressure should be 38lbs running & 40lbs with the regulator vacuum line disconnected. If fuel is found in the vacuum line, replace the regulator.
Check for 12VDC at the battery, and 13.7+ V at the Alternator, this May cause some idle issues.
Was the car backfiring when you first bought it, the only code you now have is a cyl#4 misfire at random, and the idle being rough?
John