98 QX4 with random misfire code

[AudioNut]

Just joined and have a problem with my 98 QX4. Originally, it has engine code P300 and my tech checked it over and couldn't figure out what is causing it except for the ECU. He said that he checked all of the possible components like fuel pressure, plugs, wiring, injectors...etc and concluded that it might be best to reprogram the ECU to eliminate a possible bad ECU.

Just prior to take it to the dealership to have it reprogramed, I found a lightly wrecked 99 QX4 with less than 50k miles (confirmed with carfax) and was able to get the engine/trans for a good price. My QX4 has 230k miles and engine/trans was weak and leaks and thinking that this would be a great opportunity to have them both replaced.

Anyhow, after the swap, I am now still getting the same P300 code. Now, I am thinking that my tech is probably correct thinking that the ECU was the cause of the original problem. Meanwhile, the ECU from that 99 QX4 is available and I could possibly swap it with mine.

What do you folks recommend? The newly installed engine/trans runs smoothly and I don't believe it is generating the P300 code. Also, for some strange reason, I am not seeing any engine light on my dash and it was like that before the engine/trans swap.

Thanks in advance!!

[Towncivilian]

Welcome to NICO!

Try following the diagnostic procedure described starting on page EC-256 of the factory service manual.

Does the "SERVICE ENGINE SOON" light illuminate with the ignition set to "ON"? If it doesn't, the bulb is probably dead.

[AudioNut]

No "Service Engine Soon" light when the ignition set to "ON". I agree with you that it is probably just the bulb. Thanks for the reference to the service manual. My tech already went thru all of the possible causes and couldn't locate what is causing it. That's why he think it is the ECU. Do you think I should or could simply swap the ECU from the 99 QX4 to eliminate this possibility?

[Towncivilian]

If the ECU part numbers match, you can certainly try. There is no NVIS to reinitialize so you shouldn't have any problem. I would also replace the SES bulb.

[AudioNut]

Any special precaution or steps I should take when swapping it? Do I simply disconnect the battery and remove and replace the ECU? I have heard story about air bags deploying when not doing it properly.

[Towncivilian]

Disconnect both battery terminals and you should be fine. Air bags won't deploy, the air bag control module is separate anyway.

[AudioNut]

Thanks!!! I really appreciate your help on this. I am crossing my fingers and hopefully this will get me one step closer with fixing this pesky random misfire issue.

With the low mileage motor/trans installed, I am hoping to keep and drive my QX4 for another 10 years or more. With the proper upkeep and maintenance, it should be fairly easy.

I am also scheduled for the strut housing corrosion inspection on Monday but I don't anticipate any problem with it. The SUV has been driven in the west coast for the last 10 years or so.