Misfire Cylinders 3 and 4, Running Rich

2003 Double Cab V6 227000 Miles

I have a misfire in cylinders 3 and 4 (P0303, P0304), its running really rough. I also have had a persistent P0172 code. I have replaced all sparkplugs, both O2 sensors, MAF (its a denso), and one leaking fuel injector. I tried switching the cylinder 3 coil pack to 5 and its still popping the same codes for cylinders 3 and 4.

Coolant levels are good and Compression is 150 in all cylinders. I have used my code reader and gotten the following data:

Short Term Fuel Trim1: 1.5%
Long Term Fuel Trim1: -35.4%
Short Term Fuel Trim3: -100.6%
Long Term Fuel Trim3: -100.6%

Kinda at my wits end here, have no clue where to begin. My mechanic suggested it was a bad ECM but that seems unlikely to me. I also have a really rotted fuel filter but I don't know if that would fix it. Any advice is appreciated.

 

Those fuel filters are like $30? Definitely should replace that. No clue if it will resolve your issue but that thing looks quite toast and that is mostly a simple job, though the rust makes it difficult. Take your time freeing up those nuts.

 

Disconnect the front O2 sensor and see what happens. Check the coolant temp reading on the scanner as well as the Intake air temp sensor.

I have never seen fuel trims above/below 35%. I didnt think it was possible. I would put a set of known good injectors in her and go from there.

 

So my mechanic swapped the ECM and the same misfire still happened. I do not know if he replaced the fuel filter yet I will ask. Would swapping a fuel injector from a working cylinder to one of the misfiring ones be okay to tell me if its the injector thats faulty?

 

I wouldn’t go that far yet. Start with the coils. Swap the wires on the coils to see if you get a different code and that will tell you if it’s the coil or not. Then move deeper into it.

 

I already swapped a coil pack and the same cylinders misfired

 

hmm. Do a leak down test next then so you can see where the pressure is escaping to?

 

My mechanic did a compression test, all cylinders are around 150, would that suffice for a leak down?

 

150 is great. My cylinder number 3 has 60 psi. Bad valves. I’m running her till she dies and put a new head or motor in. She still idles and runs fine though so that’s why I’m still driving her.

 

So my mechanic swapped ECM's and nothing changed. I took the truck back home and erased the codes. It is still misfiring badly. The engine light popped back on and I got a P2196 and P0172 at first. I took the freeze data that I could with my scanner:

Coolant Temp: 190F
Intake Air Temp: 68F
Short Term Fuel Trim1: -12.1%
Long Term Fuel Trim1: -30.1%
Short Term Fuel Trim3: -100.6%
Long Term Fuel Trim3: -100.6%

I checked most of my grounds that I could find, all seem good. P0303 and P0304 both came back after starting the truck a couple of times. No milky oil, no white tail pipe smoke. The engine is still shaking very badly. I do not know what to do next.

 

Would definitely work on that fuel filter, start spraying the fittings now to help get them unstuck. I can't imagine it's helping anything.

 

After you switched coil packs did you clear the codes and wait for new codes? And how does it start? Easy, long start, hard start. Get as much info out as you can.

 

How did all of this start? Just one day it ran like shit? Could also be a faulty cam or crank sensor. Who replaced your spark plug wires? Are they in the correct firing order?

 

I replaced the crankshaft position sensor and got no change, still all the same symptoms. Plugs are in the correct order. I really have no idea where to go next

 

Are the new 02 sensors oem? An did you try the disconnect method on front 02,as mentioned earlier? Those readings are way off onthe 2nd readings for STFT an LTFT..
Id swap 2nd 02 sensor out an see of it balances out..or changes dramatically..
I just checked mine ..an a good steady low - % at warm idle.
I also checked the readings for voltage at 02 sensord an did the other tests the scanner can do to test them. Theres also a graft that can be monitored on them on most scanners.

 

I cleared codes and unplugged front O2 sensor, got P0031 as well as P0300 P0303 P0304, this time the engine light was flashing. (Although it flashed once before I did this as well). As for stats here’s what I got off the scanner:

Coolant Temp: 141F
Intake Air Temp: 59F
Short Term Fuel Trim1: 0%
Long Term Fuel Trim1: -32.7%
Short Term Fuel Trim3: -100.6%
Long Term Fuel Trim3: -100.6%

 

Try it on 2nd 02 sensor ...the trim 3

 

Negative fuel trims and misfire means it's detecting rich and pulling fuel. Could be actually too much fuel and misfiring, or actually lean now and misfiring. Fuel pressure? Injectors leaking/need cleaning? Spark plugs gapped to spec?

I'm inclined to think your "#3" fuel trims are fake and just a result of the scanner you're using. AFAIK there are no first gens with multiple trim banks, there should be one short and one long, which I thought capped out around +/-20-40% each.

 

I just re did a compression test (twice) and leak down test. I believe my pressure gauge might read a little high. Here are the results:
Passenger Side from Front; The slash indicated each compression test
Cylinder 1: 190/190 PSI......Leakdown: 10/11%
Cylinder 3: 180/185 PSI......Leakdown: 10/15% (Slowly climbed to 15% and stayed there, I figured because my air compressor hadn't recharged itself)
Cylinder 5: 200/190 PSI......Leakdown: 10/12%

Driver Side from Front;
Cylinder 2: 185/185 PSI......Leakdown: 10/11%
Cylinder 4: 155/165/190 PSI (Did this one three times because the readings were low)......Leakdown: 10/12%
Cylinder 6: 170/175 PSI......Leakdown: 10/11%

I have no idea what to do next.... I am thinking this is some wierd electrical problem. I have a set of new spark plugs (

$7 NGK BKR5EKB-11 NGK Spark Plug Stock # 3967

NGK BKR5EKB-11 3967) ready to go, but I don't know if its worth it to install them.

 

Are you still having a rich condition? If so that needs to be addressed first because you will have misfires if the PCM can't properly control the air/fuel ratio.

You can ignore the long and short term fuel trim 3, they aren't used.

Long and short term fuel trim 1 are what you need to look at.