Hey Buds,

My car is acting up....and randomly misfiring (but only on occasion). IT doesn't do it until the car has been running for about 15min.

I'm coming up with the P0300 code "multiple misfire." I DID some searching on the forum and Obviously it could be a coil (but which one is the mystery). I've already tested the front three and no changes in the code or the way the car runs. Before i remove the IM and go through the effort of changing the back plugs and coils, I thought i would pose these questions.

1. Is it possible this code be a sensor (cam shaft) or another issue?
2. Would the Mass Air Flow Sensor cause multiple misfires ?


Thanks for your input...!


~M

 

P0300 is hard to diagnose. I would start by checking coils, intake leaks, and also for possible exhaust system clogging. INMHO, these are the most likely cause of your problem. However, be aware that the FSM (pg. EC-384) lists the following as possible causes:

• Improper spark plug
• Insufficient compression
• Incorrect fuel pressure
• EGR volume control valve
• The injector circuit is open or shorted
• Injectors
• Intake air leak
• The ignition secondary circuit is open or shorted
• Lack of fuel
• Drive plate or flywheel
• Front heated oxygen sensor

Good luck, and let us know what comes out.

 

Start by cleaning MAF and checking for vacuum leaks.

Look for vacuum leaks in the intake boot and rubber vacuum lines.

Those are the cheapest and easiest things to rule out first.

Do you feel it misfiring or is that when it throws a code?

Also, it helps to add the year, package and mileage of the car when asking questions like this.

 

Hey Buds,...thanks for the Tips.

I got a 2003, vq35, 102k miles

1. I'll clean the MAF sensor ASAP.
2. Vacuum LEAKS in the intake (not sure where this is located)?
3. I'm thinking that oil is leaking from back Value Cover and into the spark plug chamber...is this possible? (i never changed the VC to the 04 upgrade)?

~M

 

Same issue here same code, but a noticeable mis-fire, especially with the solid mounts. Really only at idle though. I unplugged coils while the engine was running and noticed when unplugging cylinder 3 the engine did not really run any rougher. The other two when unplugged the engine got noticeably rougher. My plan of action is ordering 3 coils for the rear since you have to pull the upper IM and 6 new plugs. If I have to order 3 for the front easy to replace but hopefully the thing will be happy with three new ones. But it probably wont happen. Just gotta decide on Hitachi or Denso coils. Any recommends? Good luck with your DIAG.

 

Hitachi is the OEM coil.
The original OEM coils must have had a design problem as most of them failed within the first 4 years or so (mine did). The redesigned coil has a GREY dot on it. Rockauto.com sells them (http://www.rockauto.com/catalog/more...id=450&jpid=20 ) for $49.79.
I have installed the "grey dot" coils early in 2005 and they have been working without any problems ever since (that's 11.5 years).
The Denso coils are a bit more expensive but Idon't know much about them.

 

How old are the plugs? The coils in the 3.5 were much better than the ones that came with the 3.0 so I wouldn't immediately blame them.

I'd at least pull the front plugs to see what condition they're in. Ideally, it would make sense to look at all the plugs and maybe swap the coils around around to see if one is bad.

I don't know how long you've had the car but if the previous owner was lazy, or had a lazy mechanic, they might have only changed out the front plugs. More than once I've pulled plugs on a car to find the difficult ones were original while the rest had been changed.

 

The rubber boot between the throttle body and the MAF housing can split and cause leaks. There are also some vacuum lines on the other side of the intake. Basically, any rubber lines coming off the big aluminum intake (also under the engine cover) are vacuum.

 

Thanks Derrick,

I've noticed that the engine has LESS MIS-FIRES when it is cool?

Weird?

~m

 

I still haven't figured mine out. I through Iridium's in it about 30k-40kmax ago along with rear valve cover and new pcv hose that goes valve cover to valve cover, that thing fell apart during the swap. Bought the car from my father in-law who is very good with maintenance. Have two of his cars now. Probably gonna hit up my buddy who has a smoke machine see what I can find. I hate throwing parts at cars and need to find the root of this problem. Throttle body boot does have some cracks but no all the way through. Have sprayed intake cleaner/brake clean all over the place looking for leaks. Kinda stuck right now, but hopefully will figure it out soon. Pulled the iridium plugs and replaced with resistors for now, was hoping that was the problem but runs the same. The old plugs are caked with oil deposit nastiness since this thing does burn quite a bit of oil.

FlaMark, mine was the same way, it didn't really start noticeably misfiring until it go to operating temp, and the computer throws the code consistently every time. 192 degrees temp, rpm 612-720, vehicle speed 0. Same garbage 300 code. But now it's gotten more consistent where it is misfiring first thing in the morning. Not as noticeable cause the rpms are higher but I can tell the Miss has gotten worse.

FRUSTRATING no doubt!

 

Import,

1. Did you ever get that smoke detection test done?
2. How would a smoke detection test work anyway---what would it tell you?
3. Did you ever change the back VC, coils, plugs ?

Anyone?

Thanks!

 

Smoke test is a way to find vacuum leaks. You cap the intake and plug the smoke machine into a vacuum nipple in the intake. The smoke machine pressurizes the intake with smoke that will escape anywhere there's a leak. It has to be done indoors or with no wind. I've found it best to do in the dark with a flashlight.

A fast and easy way to search for a vacuum leak is to spray carb cleaner of brake parts cleaner around the intake and listen to the RPMs. When you hit the leak, the RPMs will surge or drop.

My favorite way is to watch the short term fuel trims on a scan tool. If one or both are +1 or more you MAY have a vacuum leak. (The ECU will add fuel based on the o2 sensors detecting the lean condition caused my the vacuum leak) Spraying the leak like above will cause the fuel trim to go into the negative. This way is more sensitive than listening to the RPM.

 

Nice Trick...no chance of damaging parts this way?

 

No, those cleaners are mostly harmless. Just don't soak the exhaust manifolds with it and watch the fumes.

 

I have done the smoke test and didn't find anything, except for the usual VIAS leakage that is normal, which I always think is excessive. The rear valve cover was replaced a while ago and I got six coil packs but I haven't put them in the rear yet. The front three made no change, even though I have gotten a cylinder 4 misfire rarely. I'm moving on today with testing internals of the engine. I think this thing is on it's way out. Having trouble finding a decent engine in my area so a 6mt sedan G35 might be in my future.

We shall see. Today is a big day.

 

Hopefully I didnt miss it, but if you havent yet, try changing the intake manifold gasket as thats what mine was.

 

For me it's an engine. LOL! Only have 90psi in cylinder 4. So not good news. 2 and 6 were at 180 though. Didn't even bother checking rear cylinders. Just put it in the garage and wanna keep it movable when I need it out of there. It may be time to move on, it might get an engine. At this time I don't know. Good luck with your condition FLAMark. Didn't mean to jack your thread.

 

No worries.

How did you blow your engine?

 

LOL! Well I wouldn't call if blown. The thing is still perfectly drive-able, just drove it today cause I didn't want to move my kids car-seat into my other vehicle.

As to why it happened I have no idea, just wear I'm assuming. There wasn't a particular event that happened with it. Noticed the pinging getting worse and worse and eventually started getting a semi rough idle then started getting the 300 code then a rough idle.