I had a P0304 engine error code that occurred when driving up hill or accelerating out of corners with multiple symptoms that weren’t leading to an obvious cause. Symptoms included an inconsistent check engine light, sluggish acceleration, unusual vibration at idle, unusual vibration while driving/accelerating (20-60km/hr), and emission warnings (not error codes but warnings).
Back story:
Hunting to buy a 2012 or 2013 Discovery LR4 with the rear locking differential (Heavy Duty) package, lead me to one for sale in a neighbouring province. It turns out the optional rear locking differential is very hard feature to find. This LR4 had 240,000 km’s and the options I was looking for, but there was one problem. I wasn’t able to view it in person. So I fully expected it to have an issue or two.
After having a pre-inspection and video of the vehicle starting and running sent to me, I made an offer. Importing a vehicle requires a provincial vehicle inspection. I had it shipped to an inspection facility and fortunately, my new LR4 passed and was registered. The interior was in much better condition than I expected. Once insured, I hopped in and drove it to a nearby gas station to fill up the tank and give it a car wash.
It drove smooth. Hitting the highway I accelerated up a hill. The engine felt great. Then without warning a check engine light appeared. Followed by a restricted performance warning icon. After scanning the engine I discovered a P0304 misfire code. Meaning there was a misfire on cylinder 4.
Because I had purchased it sight unseen, I wasn’t surprised to see an issue. So I set to work to solve it.
Diagnosis:
A misfire is caused by either air, fuel or spark.
Air comes in from the air filter(s), through the intake into the engine. Air is then compressed by the cylinder. So you just have to check your air filter is clean and sealed. Your air intake manifold is free of leaks (easy enough with soapy water). The Mass Air Flow Sensor(s) is clean. The throttle body is clean. And there is compression in the cylinder. If it’s all good, then the issue will be fuel or spark.
Fuel is pumped from tank to the fuel rails and into the fuel injectors. So you can check the fuel pressure with a scanner. You can check the wiring connectors to the fuel injectors. And you can check the injectors by listening to them with a long screw driver while running.
Spark can fail because of a fouled spark plug, a bad ignition coil, or a bad electrical connection. Again very easy to check.
Here’s what I did to diagnose the misfire.
Initially I wondered if the carwash was correlated to the misfire. As the check engine light appeared shortly after the carwash. Had water shorted a wire? Or had water gotten into the fuel?
Step 1:
Quick spark check.
First I swapped the ignition coils and spark plugs on the cylinder. If either the plug or the coil were the issue, then the misfire would follow it. But the misfire remained on cylinder 4.
Step 2:
Quick fuel check.
My next question was if it was an issue with the fuel injector or fuel pump. I purchased fuel injector cleaner to see if the engine ran any different. And there was an improvement, meaning it more likely an issue with a dirty fuel injector than the fuel pump.
Step 3:
Quick air check.
I checked the air intake system for leaks. There were none. I cleaned both Mass Air Flow sensors and cleaned the throttle body. I also cleaned all electrical connectors to the MAF sensor and throttle body. I did not check the cylinder compression as it seemed unlikely that the symptoms would be related. The engine started fine. It ran well (except when the misfire occurred). I did later check the live data of the engine with a scan tool and all valve timing, position sensors, and values were well within the ideal range.
So far all checks lead me to conclude that it was most likely a faulty fuel injector.
Step 4:
Test drives to recreate the issue.
After clearing the code, and taking the LR4 for test drives, I learned I could consistently cause the misfire code and performance restriction by accelerating out of a roundabout. But what I then noticed were emission code warnings on the scanner for the oxygen sensor and the catalytic converter. While they weren’t throwing error codes, they were still rather concerning, and needed to be solved. As replacing a catalytic converter is expensive and I would rather avoid that. So I investigated further.
Step 5:
Documenting unusual symptoms to rule out other issues.
One of the symptoms I had (that didn’t always cause the misfire code or check engine light) was a rough running engine when driving up hill at speeds of 20 to 60 km’s per hour. Driving up hill puts your engine under load. This could be a symptom of bad compression, a weak electrical connection, a leak in the air intake or a faulty PCV valve. So I began to suspect there may be another issue related to the emission warnings and engine performance.
A random YouTube video about the PCV valve, or Positive Crankcase Ventilation valve appeared on my feed. This turned out to be very helpful. The PCV valve is there to improve emissions. It vents excess oil fumes back into the cylinders to burn in with combustion – rather than venting directly to the atmosphere. However, a bad PCV valve vents too much oil into the combustion chamber, clogging fuel injectors, fouling spark plugs, and damaging the catalytic converter – even causing check engine lights and misfires. And most importantly, affecting engine performance under load.
Step 6:
Replacing the PCV Valves.
PCV valves are inexpensive and easy to replace. It’s generally a good idea to replace these every 50,000 to 80,000 kilometres. There was no record of these being replaced so I ordered two and swapped both on the LR4. Sure enough, one of them had a crack in the rubber valve. I was making progress. And immediately there was a noticeable change. The engine idled better and it had more power under load.
New PCV valves would prevent future emission issues, but I needed to do something about the current emission warnings displaying on my scanner. So I ordered 2 bottles of Cataclean.
Step 7:
Cleaning the fuel system, exhaust system, and emissions sensors.
Cataclean is a fuel system cleaner, but it’s actually does a lot more than that. It will clean your fuel system, your fuel injectors, your valves, your 02 sensors, and also your catalytic converters. After following the directions religiously and using both bottles the emission warnings went away and the fuel mileage improved. At this point I did go on one trip without a misfire code. But… the check engine and P0304 cylinder 4 misfire light returned. Leading me to believe it was still a faulty injector.
Step 8:
Oil change (and engine oil cleaning)
Cataclean recommends doing an oil change after using the product. So I did that. But first, I replaced 2 litres of the oil with transmission fluid and Marvel Mystery Oil. This is an old school mechanic’s trick. With 240,000 kilometres on the clock, it was possible that some of the oil passages were clogged, possibly leading to a sticking lift rod. It’s unlikely, but this too could cause a misfire. As I was changing the oil anyway, I used this as an opportunity to clean out the engine.
I was curious if there would be any sludge build up in the engine oil cavities. Transmission fluid and Marvel Mastery oil is smaller than regular engine oil, so it gets through tight spaces and is able to clear blockages (if there are any). I let it idle for about 40 minutes. You don’t want to drive it with this stuff in it. The engine did sound quieter after. I took that as a win. I drained the oil, and checked the oil filter, but there wasn’t very much sludge, debris or strange discolouration. I had expected much more. This was reassuring. Another win. Someone had taken care of this engine. I let it drain extra long, to get everything out, before filling it up.
Step 9:
Changing the fuel injector.
I ordered a new Bosche fuel injector.
After removing the ignition coils and spark plugs, disconnecting the wiring harness that runs across the cylinders, I then pulled the fuel rail off using my pry bar. It was pretty stuck on, but with enough even pressure/force it popped off.
(Out of an abundance of caution I ended up doing this job twice as I then ordered new rubber seals for the other injectors and then pulled and cleaned all the injectors. Then replaced their rubber seals before reinstalling the fuel rails. I should have done this the first go around.)
Of note: There are many diagrams online of the cylinder positions in the AJV8 engines. They’re almost all wrong. Here’s the correct one for North America. Cylinder 4 is on the drivers side directly after cylinder 2.
I pulled the fuel injector out using a slide hammer. It had a lot of carbon build up. I then swapped the cylinder 4 fuel injector, put it all back together. And was pleasantly surprised that the issue was solved… almost.
Step 10:
Assessing the situation.
I was no longer getting the P0304 misfire code. It had worked! This was progress.
But the symptoms weren’t completely resolved. When driving the engine at low speeds it was occasionally running more rough that I would like. But it wasn’t a consistent issue. It was possible that the ECU was still learning, but I began to suspect an issue with the wiring harness.
Step 11:
Extra inspection pays off.
I inspected all injector wires and coil wires, looking for potential shorts. A couple had exposed metal. Exposed metal on ignition coil wires can cause arcing and a rough running engine. So I used liquid tape to seal those. But the issue did not go away after test drives. I checked the wiring harness again. And then I noticed something that I had missed before. All the ignition coil connectors were on, but cylinder 4 had a slightly bigger gap than the others. A millimetre or 2. But, no matter how hard I tried, I could not get it to fit tighter. So I pulled it off and looked inside. There was some sort of grey sealant in it. None of the other connectors had that – I checked. So I cleaned the sealant out, reconnected the connector and voila – the intermittent engine vibration problem was solved.
Solved:
It has now been running smoothly for 3 months.
Conclusion:
The P0304 misfire error code was caused by a failing fuel injector.
The injector likely failed because of carbon build up caused from the leaking PCV valve.
It’s possible that this was previously diagnosed as a bad wiring connection to the ignition coil, hence the sealant in the connector. But, ironically, that sealant then caused a weak connection. As a result the combustion was poor and the carbon build up in cylinder 4 was worsened.
The fix required:
Replacing the Fuel Injector
Replacing the PCV valves
Running Cataclean to clean the fuel system, catalytic system and emissions sensors.
Sealing exposed wires and removing the old sealant in the connector
