Misfire on cylinder 3 on 4.6 Engine. We transplanted a used engine in this truck which we did a tune up on and I either have a bad injector or a bad wire or coil or something. The engine has had a little miss in it from time to time in the 1,000 miles I have put on it. I parked it for two weeks and started today for the first time and whammo it was missing a ton and the P0303 cylinder 3 came on. Anyone with an idea where to start?

 

Your problem has got to be somewhere around that spark plug in the number 3 cylinder. You didn't say what year so I can't tell if you have COP's or plug wires. In any event, take them off and get at the plug. Inspect the wires or COP for damage. Take out the plug and check the plug hole for grease, dirt, oil, or coolant down in the hole. If fresh oil or coolant residual is found, fix oil leak (usually around the valve cover gasket or at the PCV valve). If coolant, the leak is usually at the heater hoses or upper hose so fix those.

When you take out the number 3 plug, try to determine if it has worked loose or is tight. If tight and hole is clean, most likely the plug is bad or the COP or the plug wire. If it is loose, most likely the oil, coolant, or oversize and worn plug threads caused it to come loose and misfire.

If the plugs were new when you did the engine, the plug hole is clean, and the plug tight, you may have an injector problem. I would check the plug, COP or plug wire first since these are the most common cause of misses and easiest to fix.. When you put it back together, a little heat resistant thread locker on the plug and di-electric grease on the COP or Plug wire boot will keep the plug tight and give you a good electrical connection.

Hate to keep giving you things to check but I need to tell you it could also be a bad valve, head gasket, or piston rings. This seems doubtful since it happened all of a sudden and I trust you haven't overheated the engine before you recognized this miss problem. To test this you need to do a compression check which for starters will require you to take out all the spark plugs.

 

It's a 98 and the engine is a 97 Windsor. We haven't got the engine hot and it was running good overall before I parked it for 2 weeks. It had a little miss in from time to time. I recently replaced the throttle body that i had yet from the original engine because it had a plugged EGR tube in the throttle body. I am also chasing a P0155 bank 2 sensor 1 o2 heater circuit on this as well which I don't believe plays any part in this or the misfire with the o2 sensor for that matter. When looking at the number 3 cylinder it is a stupid question but that is the 3rd cylinder in from the front of the engine passenger side correct? We will be looking at it tomorrow night I will check the plug and wire but if I'm guessing what's wrong I believe it is an injector. After parking for 2 weeks with a slight miss I feel it is now frozen after sitting for the 2 weeks.

 

I would say that the 155 code is showing up because of your miss. The number three cylinder is the passenger side; third plug back as you said. If you think it is the injector, a simple inspection of the plug socket, tightness of the plug, and condition of the plug wire will give you a good idea that the plug and wiring is either OK or a problem so you can eliminate much of the exercise I talked about previously.

If the 155 code does not clear when you fix the misfire, you have either a bad oxy sensor or the wiring is bad (usually at the connection of sensor to harness).

 

The o2 sensor is brand new that I put in right away trying to fix the 155 code. That and the original sensor have been the culprit of the 155 code. I'm hoping I find something soon with that code it has been a PITA so far on this transplant!

 

Loose plug! What would cause it we used anti seize when we tuned it up. The threads seem good too. We will have to get high temp thread locker for now putting it back in and making driveable locally til I get the high temp thread locker.

 

Put plug back in and its still missing. New plug tomorrow and try again.

 

With a new plug that has a 1000 miles on it should it still be good after it came loose in the engine? I tightened it back up and it stool runs like crap under harder power. It idols fine but when you get up to speed and step into it the darn thing stutters and struggles accelerate? I have the old wire from the original engine that I can try if the wire is bad. It had gas around the plug when we pulled it out is it loaded around the plug with gas that is causing it to run rough? Where should I go from here?

 

If you have a round wire brush and the socket hole is dirty with grease and oil, it would be good to clean the plug hole with the brush, WD40 or equivalent, and rags to get a clean metal surface between the plug and the cylinder head. To answer your question no the plug, it should be still good after 1000 miles; just clean it up and recheck the gap before you re-install. Good plan to use another wire to check and make sure you are getting spark.

Could you tell if the plug was loose when you took it out? If it was loose, that was most likely the cause of your misfire. Regarding how the plug could become loose, the heads on your engine are aluminum and therefore, the metal is soft. The threads tend to weaken and enlarge over time with plug changes and the normal heat of the engine. Also, oil from the valve covers and the PCV valve will leak and get in the plug socket putting oil and grease between the head and your spark plug. When you add anti-seize compound to the plugs (which is a good idea), you have everything necessary for the plug to work loose.

On my 98 with a Windsor engine, I have a loose plug on the number 4 cylinder. I have been using thread locker and even with that, cleaning the plug hole, fixing oil leaks and coolant leaks from the heater hoses, it still works loose about every 6 months. The only permanent fix for this is to pull the head and replace it. The last time I fixed it I use extra thread locker and I hope it goes for more than 6 months. The truck is not worth the money and work required to pull the head and it runs great while the plug stays tight. Since it only takes me about an hour to fix the plug every few months, it will continue to be a problem that I will live with.

Final thought, if your miss fire is not caused by a loose plug, you could have an injector issue. You could also have a bad valve. The only thing I could suggest for the injector is to either run fuel injector cleaner through your fuel system or just go and change the injector. To check for a bad valve, you will need to do a compression check which will require you to pull all your spark plugs, have someone turn over the engine while you hold the compression gauge to each cylinder and read the pressure.

 

When we took the plug out we didn't even use the ratchet just the extension and wobble joint and 5/8 socket by hand. It tightened up again and we were careful not to wreck the threads by over tightening the dirty sucker. I used my shorty flex handle ratchet and I am surprised we still have a miss.

I forget what the gap is on the plug if anyone knows please pass it on. I plan on regapping the plug cleaning the hole and using high temp thread locker along with a different wire to see if the miss goes away. If not we will move onto compression check cross my fingers!