Engine Puzzel That Dealership Couldn't solve

[joshuaD]

I have a great car. Nissan Sentra Spec-V 2.5.
I love it. It has 114K miles and never had any trouble with it. Plugs are iridium and were added at about 30K. The crankshaft position sensor was changed out as it failed like is common. Other than that never had a starting or running issue. I had the dealer service it for air bag inflator recall and ECU reprogramming. After that I had a few random misfires at higher revs. Then randomly it would start running rough and misfiring. It will usually start that way. I will shut it down, wait a bit and then it will start and run fine. Sometimes it will take 10 times of restarting before the problem will go away. I took it to the dealer who couldn't find anything wrong with the engine. About two weeks after taking it to the dealer it started running rough again occasionally. Also sometimes it will take 3-5 seconds to start. From reading the forums here I got a few ideas:

1. Head gasket in cylinder is leaking coolant into the cylinder which causes it run rough for a while.
2. Ignition coils need to be replaced.
3. A sensor such as the knock sensor is sending bad data.

Any ways I can determine if any of these are happening?

I appreciate your help.

Thanks,
Joshua

[AZhitman]

Welcome aboard, Josh! You're in the right place, our Sentra guys are very talented.

Here's my quick feedback:
#1 - Do an oil change. If the oil is milky, tan, or otherwise abnormal, that's likely coolant in the cylinders. Also, have a peek at your plugs - now's a good time to replace them anyway.
#2 - Switch out with a known good set - These can be scored at a junkyard (get more than 4) and are great for testing.
#3 - Have you pulled ECU codes? Easily done with a OBD2 reader and a smartphone app like Torque.

Lastly, I'd pull the battery cable, leave it off for 30 min, and reconnect - then, do the idle re-learn procedure from the Factory Service Manual found here: www.nissanservicemanual.com

Let us know what you find!

[joshuaD]

Thank you for responding! I appreciate the help.
I did an oil change after this started happening and didn't find any milky tan or abnormal oil.
I've checked the ECU codes and the dealer did and couldn't find any code. If it threw a code it would help in debugging the process.
I read the service manual and did the idle relearn. It did seem to help for about a week or so but I don't know if that was related or not.
The only thing I've not done is replace the plugs, ignition coils but the dealer looked at them and didn't see a problem but I can't know how good that inspection was. Right now I'm also thinking it could be inconsistent pressure from the fuel pump. I'm going to test this by when it happens again to leave the engine in the on position until the system should have pressurized and then see if that is a consistent way to fix the problem. Any other ways to test that?

Again thanks for your help.