Recently took my 1999 maxima, which was working perfectly, into the shop to get new spark plugs, oil and filter change, wheel alignment, and some rust issues fixed in the inner front door hinge.

Picked up the car, and noticed that the car had an intermittent rough idle when in gear, almost like its stalling. The car shakes as it loses power and then shakes again as the power is restored. It seems to drive fine and idles perfectly in park. Also it doesn't happen when the car is cold and it idles at a higher RPM. Putting my foot on the gas to get it up to 1000rpm seems stop the 'stalling'.

Here is a video:
https://1drv.ms/v/s!Ap8Ze8GqHWPZ4xtvRzkReS94cpB3

I took it back to the shop, the guy confirmed the problem and thought it could be a misfire. They had it for the day but couldn't replicate the problem. They also pulled the spark plugs and thought that they were fine.

I picked the car up and once I had driven 500m I noticed the problem. I've spent a few days to try and diagnose the problem and this is where I'm at:

• I disconnected the spark plug wires for each cylinder to see if I could isolate it to a cylinder. The issue continued regardless of which cylinder was not firing. I noticed that the problem was a lot worse when disconnecting one of the lower 3 cylinders rather than the top 3.
• I switched the coils between cylinders to see if I could isolate it to a single coil.
• I replaced all the spark plugs. They put in ngk platinums, I put in some cheap autolite copper core plugs (yes I know that I should have just done the plugs myself in the first place!)
• My lights don't dim when the RPM drops so I don't think it's an electrical supply issue.
• We noticed that the problem almost seems to get worse when the car is parked on a slope, but not sure if this is real yet, or its just that I notice it more when I'm idling in my driveway which has a very mild slope.
• I replaced the air filter.

I've moved on past the ignition system and wondered about the oil change. I've noticed that the oil level is above the max fill mark. In fact it's a little bit above the letter H.


Could this be the problem? Could too much oil cause frothing, and put the engine under strain when it's idling in gear? That might explain why the severity is different when the car is angled differently.

The other usual suspects I think I need to check are the MAF sensor, IAV and PCV (which may have been fouled by age + too much oil)

I need to check for a vacuum leak too. I haven't noticed or heard anything leaking, and my investigations with soapy water hasn't yielded leaks.

My next step is to get the shop to take a look at the oil level and syphon a bit out. They might also need to clean the above sensors if the oil is overfilled.

Any thoughts would be most helpful.


​​