1997 QX4 P0300 Code

[papagayo]

Misfire on # 3. All is good while the engine is warming up, but once it has been running for a while the misfire starts until the engine is barely running and then stops.
The distributor ignition coil, spark plugs, distributor cap, and rotor were changed about 7 months ago. Any ideas as to why it happens when the vehicle is hot? Thanks.

[mdmellott]

P0300 is a multiple cylinder misfire DTC. P0303 is a #3 misfire code. I know the ECM will ignore the O2 sensor readings until everything is warmed up and then it will take those readings and do what it must to balance fuel trim correctly. It sounds like that may be what is happening but why it is happening is a harder question to answer without more data. Do you have a scan tool with a live data feature or is it just a code reader you used to get the DTC? Is the DTC a P0300 or P0303? Are there any other codes?

[papagayo]

I have a small code reader that gave me a P0303, then I purchased a BliuDriver pro-scan that interfaces with the iPhone, and I got the P0300 and P0325. I was thinking about how the problem got worst as the car engine run longer and temperatures got higher until the engine stops running. Thanks

[VStar650CL]

The P0325 is a knock sensor code, it won't cause rough running. Degraded performance when warm often indicates a vacuum leak or a dirty or lying MAF sensor. I doubt you're looking at an individual injector, that would cause a dead miss but not a P0300 or near-stalling. There are a couple of peculiarities on the 3.0/3.3 that you could also look for. One, since it was just worked on, it's possible on most of them to accidentally swap the front O2 sensor connectors. That will cause all kinds of crappy-running issues. Second, the ECM uses the CMP in the distributor as the sole source for piston position, the crank sensor is for misfire-diag and nothing else. When the distributor bearings go bad, the thick oil when cold will often keep them stable but then they screw up when the oil thins out.

[papagayo]

I think I read in this forum or somewhere else that when changing distributor components people should purchase OEM parts because the after-market components would eventually cause problems. I purchase an inexpensive distributor cap and rotor so I am wondering if these components could be affected by the changes in temperature. It is odd that the engine runs well when first started then after a while it runs rougher and rougher until it dies.

[VStar650CL]

Some rebuilt distributors and some (I'll be kind) "budget" new ones are very fly-by-night. There's generally nothing wrong with a reputable reman, but the cheapest s#%@ is generally just that, s#%@. So yes, it's entirely possible that a cheap replacement has chintz silicon or opto components that are malfunctioning when the temperature rises.

[papagayo]

I am going to change the distributor cap and the rotor using OEM parts, then I will post the results on a later day. Thanks

[papagayo]

Update: I changed the distributor cap and rotor with OEM parts and it did not fix the problem. I decided to take a risk and changed the distributor and that was the solution. One of the parts inside the distributor failed when it got hot and caused the problem. The QX4 is now running perfectly.