Hard strating after sitting and P0300

ok so i bought a 2003 nissan maxima with 135k and it had a p0300 it had a preformace k&n filter on it so i though someone might have installed it wrong or what ever so i bought the car cheap. i have replace the k&n filter with a stock 2000 nissan maxima air box with new filter i got from the junkyard it helped the misfire calm down a little then i bought the car to a machanic and he said because of my hard starting it could be a cam senor or crank senors so ive replaced that still no luck then he pulled out the sprak plugs and they were sooty black but when i bought the car i checked and they were brand new so i swap them out with ngk platium sprak plugs then help a little but still same problem. i had a p0302,p0304,p0306 when i clear the p0300 but usally its all ways a p0300 any ideas its been a month and im still driving like this.

hard starting after 20 mins of sitting
i can smell gas from the driver seat when i start the car and open the window
exhaust air isnt smooth but in pops
getting like 14-15 mpg in city
when stoped at a stop light i can feel the car misfiring

front bay 3 ngk spark plugs cylinders 2,4,6
cylinder 4 coil
air box
cam sensor and crankshaft senor
coolant temperture sensor

P0300 is a multiple misfire code. The black plugs and poor mpg indicate the engine is running very rich.

One possibilitiy is that the oil from the K&N filter that the previous owned installed has contaminated the mass airflow (MAF) sensor. You said you switched back to the stock filter, but if the MAF hasn’t been cleaned, it will still be contaminated. A contaminated MAF will cause the engine to run rich and misfire like you are experiencing.

Try cleaning the MAF thoroughly with the correct MAF cleaner.


Here’s how to do it yourself:


+1 to jesmed’s post. That is exactly what I was thinking.

+1 to both Jesmed and cigroller.
Replacing that K & N filter was a very good idea, but until you clean the likely oil contamination from the MAF, the job is only half-done.

Misfires are caused by some combination of faulty spark, timing, fuel/air mixture, and compression. You just have to eliminate them one by one. $$-wise, besides what you’ve already done, since this car’s prior maintenance is probably unknown to you, usually it is best to start by bringing all the routine engine maintenance recommended in the owner’s manual up to date, depending on how your car is configured this might include : spark plugs, cap, rotor, spark plug wires, compression test, air filter, fuel filter, verify engine operating temp, valve clearances, valve timing, idle rpm, and ignition timing. If the problem remains, ask your local shop to test the spark system. DIY’ers usually don’t have the kind of sophisticated equipment needed for ignition system testing. Might be a good idea to have a fuel pressure test done at the same time. And a vacuum system test.

Thank you jesmed I will definitely try that and I forgot to mention the mechanic did a compression test that was OK I’ll try to do it right now thank you guys I’ll you know how it works out


Correct me if I’m wrong, but don’t you already have a thread going about this very problem?

Did it get closed?

You have misfires on bank 2 only, so I would concentrate on that

In my opinion, you have something going on that is not affecting bank 1

Personally, I would expect a bad MAF to cause problems on both banks

And I agree with the others about the plugs and the air filter. Going back to stock is a good thing, especially in a situation such as yours

I’ll go a little off topic . . .

A few months ago, I worked on a truck that had misfires on one bank only

The fuel pressure regulator was the problem. It was starving the misfiring bank of fuel. The configuration of the fuel rail, lines and regulator allowed that situation to happen. The fuel pressure regulator was sending fuel back to the tank, rather than feeding it to both banks. That is why only one bank was missing.

A good story, in any case

yes i still have a tread on the same subject ive tried so many things already like i got a fuel pressure damper for bank 2 where i was getting the p0302,p0304,p0306 and that seemed to help alittle but after 4 days im back to the same place i was before the fuel damper replacement and theres no way for me or my machanic to easliy check the fuel pressure he would have to remove the fuel pump because i dont have a fuel pressaure port and i pulled out the MAF sensor mimutes ago and cleaned it when i puled it out it had oil and dirt on it so im hopeing this will do something

This one?


Or this one?


Let me ask some more questions, please

Do you know your car’s cylinder layout?

Do you know which is bank 1 and bank 2?

Don’t be offended by my question, please. Not all manufacturers use the same layout. And assumptions occasionally lead to misdiagnosis

Did you replace the damper for bank 1 or bank 2?

Apparently, both banks have their own damper on your vehicle

The first link is the piece I replaced from another 03 Nissan maxim that I found in the junk yard but that piece rarely ever goes bad and I thought it did on mines but I was wrong and yes both banks have a damper and I replaced the damper closer to the front of the radiator its connected to the first 3cyclinders fuel rail

“i pulled out the MAF sensor mimutes ago and cleaned it when i puled it out it had oil and dirt on it so im hopeing this will do something”

I think you found the root of the problem…though I agree with db4690, it’s hard to see how that would affect only one bank.

I turned it on this morning and it turned over with no problem but let’s see today I have to leave my car park for 4hrs or more so I will know today and thanks I pulled out the maf and saw it dirty before but never thought it had such and affect on my car

OK so I was cleaning my car every where and discovered a receipt that was dated 2-15-14 for a spark plug and coil replacement and the shop wrote customer brought his own parts is it possible that when the mechanic removed the intake manifold he didn’t replace the gasket because the customer didn’t bring it with him at least I think

Many guys don’t replace the plenum gasket, if they feel that “it looks good” . . . or they forgot to include it in the repair estimate, and they don’t want to have to call the customer back

That said, I’m having a hard time seeing how that could cause problems exclusively on one bank

If the gasket was bad, I might expect to see fuel trim problems and vacuum leaks

When you say “removed the intake manifold” you mean the upper part . . . the part that is already removed in the picture, correct?

Yes the upper part its nesscary to remove it to access the back spark plugs and coils and in the picture its all ready been remove

Thanks for clarifying

My Camry V6 has the same setup, as far as the coils are concerned . . . remove the plenum to get to the back coils and plugs

Does it take some driveing before the maf sensor and ecm respond to the cleaning because this morning it started up fine then 5 hrs later it did the same thing in the video

“…he pulled out the sprak plugs and they were sooty black…”

Do you mean all 6 plugs were black?

I pulled out the first three bank 2 cylinder 2,4,6 that’s where I was getting a CEL p0302,p0304p0306

You need to pull the rear bank plugs to confirm they’re OK. Also, why to the intakes to the rear bank look black but the intakes to the front bank look clean? Optical illusion or angle of view? I’m leaning toward leaking fuel injector(s).

That’s a sample picture not my engine so you think the problem is leaking injectors