Good evening!

First time poster here and I have searched/read a couple posts with similar problems but I have a couple additional questions. My sons car suddenly lost all acceleration and the CEL began flashing. I plugged my scanner into it and it showed the P0300 code. I changed the spark plugs and while doing so I found the connections to a couple of the coils with what I can only describe as being "loose". I ensured all the coil connections were secure and took it out for a test drive. It went great for nearly 20 miles then the problem came back. I got it home and plugged the scanner in again. The P0300 code was back and it also showed specifically random misfires on all three odd cylinders so I decided to replace all six coils and since I was there I changed all six fuel injectors too. Took it out for another test drive and it drove great for the 10 or so miles I was out. Thinking it was fixed I told my son he can drive the car again only to have it recur again after driving a few minutes. Tonight when I plugged the scanner in I'm showing P0300-P0306, random misfires on all six cylinders now!

This brings me to my question: What should I be looking for since the misfire doesn't happen right away? After clearing the codes it will drive perfect for 10-20 miles across all speed ranges then they come back.
I have replaced the upstream O2 sensor on the rear manifold a while back but not the downstream one. I also ran a bottle of Cataclean through it in hopes that if one or more of the three catalytic converters is going bad (I know there's no such thing as a fix in a bottle but one can hope!).

Tomorrow I will try some of the trouble shooting suggestions I've found on other posts (vacuum leaks, O2 sensor wires rubbed bare, etc) but if anyone has a suggestion or two I would be greatful.

Thank you in advance!

 

What we need to look at is , what is common to all cyls to give the 300 code.
The first thing that comes to mind is the crank sensor. It is possible that when it heats up it is becoming intermittent. I would suggest consider making sure the connection and wiring are good there and if nothing is found , replace the crank sensor.

 

I was finally able to spend some time on the car this morning, I changed all six spark plugs, coils and fuel injectors (I know they have nothing to do with the misfire but I figured what the heck). I also changed the intake plenum gaskets, crank position sensor and battery which turned out to be bad. I then started it up and sprayed carb cleaner all around and found no signs of a vacuum leak so I headed out for a test drive after clearing the misfire codes. I made it about a mile and a half then the misfire happened again! I'm absolutely at a loss! I've read that the issue could be tied to the timing belt tensioner, does this sound like a possibility??? If I have to dive into that area of the engine I may as well do the timing belt while I'm in there. I just don't know what else to be looking at.....

 

You covered quite a bit. There is one check you might consider, if the problem is coming from the ECM/powertrain computer, make sure you can get access to it, start the engine and apply some heat with a hair dryer to the outside of the ECM. If the ECM is crapping out after getting warm , this will bring out the problem without taking it out on the road.

 

I haven't had a chance to do the hair dryer test yet, but I went ahead and ordered a used ECM for $30 just in case (part number matched OEM module). I know that ECM's are coded with the VIN, can these be re-flashed for my vehicle? If the hair dryer test shows the OEM module to be bad, am I better off sending that one in and have it repaired?

Thank you for your help, it's truly appreciated!

 

Because of the electronics involved, it would be better if you had the dealer or equipped shop handle any work involving the PCM. Sometimes problems like the following appear,
You install a PCM that is not programmed to your particular vehicle, the PCM doesn't recognize your key and now the vehicle will not start.
So you reinstall the original PCM and it gets confusing info and now will not start with the original PCM, and now you might need to have a tech come to your home to reprogram everything back to "As built". Because you don't know the condition of the replacement PCM now what?
Or you wind up having your vehicle towed to a shop equipped to handle the problem(S) Not a good idea to go switching electronic parts these days.

 

Just a note:
The down stream O2 sensor detects the efficiency of the catalytic converter
The up stream O2 sensor detects the efficiency of the fuel air ratio burn
And will adjust the fuel according to the results.
If you have a scanner then it might help to look at the fuel trims while good and with misfires
O2 sensor is not used during open loop or cold engine
O2 sensor is used when it goes into closed loop
With V6 engines, there are one O2 sensors on each bank

O2 sensors are not cheap.
And, as difficult it is to do, should always try to use OEM with the electronics.
Always check the wiring before replacing parts

 

Thank you for the advise, I opted to send the OEM module in for analysis and repair (if needed). I figured it wouldn't hurt to have it checked by someone that knows what they are doing. If that doesn't work I'll do more digging into the O2 sensors that was suggested.

I truly appreciate all of the advise you guys have provided.

 

Yeah, you mentioned that it would run fine for the first few miles then misfire.
When the engine is cold the engine runs in open loop.
Meaning is that it doesn't use the O2 sensor.
Instead it uses a air/fuel ratio map.
Then in closed loop, when the engine is warmed up, the PCM will use the O2 sensor data to tune the fuel injectors
This could be the cause of the misfire. But it is only a guess.
Looking at fuel trims can help determine if that is the problem

 

It’s been a while since I posted, I sent the ECM off for testing/repair. I was told that the ECM main processor was bad so I purchased a repaired module that was cloned to my OEM module. I finally got it back a cold days ago and installed it, then took it for a 20 mile test drive and it ran perfect. I thought it was finally fixed but the next night the random misfire came back on all six cylinders! I’m being frustrated.... I know there was mention of the O2 sensors, I can watch real time data with my code reader, but I do not know what to look for or where to look for it. Can anyone point me in the right direction?

Thank you