2006 G35 idle stumble/misfire
 

I'm having a drivability issue with my 2006 G35 coupe 5AT with 150,000 km.

While idling it will stumble/misfires. It doesn't always do it, but it has been increasing in frequency. With the engine under load and cruising around 65-70 km/h you can definitely feel an unevenness and the car will shutter/stumble slightly. Other times it is perfectly smooth and there is no issue at all.

No check engine light is illuminated. Spark plugs were changed around 45,000 km ago. I cleaned the throttle body today as there was quite a bit of carbon built-up. I had an assistant depress the throttle pedal with the ignition on/engine off so as to not move the throttle plate manually by hand, as I heard they can be quite finicky. I also cleaned the MAF sensor.

After I put everything back together I had a check engine light and the code was P0113, intake air temp circuit high. I am not sure if this is due to the MAF being unplugged while I was cleaning it or something else. I cleared the code and it has not come back on, but the idle stumble/misfire is still present.

I am not sure where to go from here. I have a basic code reader that provides live data, but I am not sure how to interpret the data. From what I can understand the intake temp sensor is built into the MAF sensor. A new MAF is only $100 but I want to carry out some other basic tests before going that route.

I am thinking also maybe a bad coil pack, but shouldn't that throw a code for cylinder misfire? From what I can hear, the stumble is more pronounced from the left side exhaust. Is there an easy way to test/inspect the coil pack?

At your mileage I would replace the cam and crank sensors. Misfires can also be caused by oil leaks in the valve cover gasket at the spark plug holes. Pull the coil packs and check for oil leaks.

Pull the spark plugs and inspect and/or replace. You'll probably find something wrong with one of them.

EGR can also cause similar issues. On Honda engines, it is very common for the passages to the EGR to get clogged and cause those exact symptoms. Not sure if it is an issue with the VQ engines though. Easy check, cap off the port to the EGR and put a screw or golf tee in the hose to disable the system. You'll get a CEL but if the stumbling stops you know where the problem is.

Thank god these engines don't have exhaust gas recirculation lol. It's a death sentence for a lot of engines out there.

I would start replacing coils you can get import direct from O Reilly’s for about 50 each lifetime warranty. I had a misfire at idle for a while not bad enough to throw code. Replaced several parts ended up being bad coil replaced all of them cause 170k and they were original.

You are probably right. I'm pulling the plugs and coil packs this weekend to inspect them.

If it does end up being a bad coil pack, is Hitachi a trusted brand? They are about $50 each from Amazon.

Hitachi is original equipment
 

I would be careful ordering car parts from Amazon. You can get Hitachi from dealer or have them ordered at your local parts store. They are a bit pricey.

Verify the part number and verify the SELLER RATING on Amazon before ordering stuff from them. If their seller rating is good then I'd buy it.

I like Hitachi for a lot of electronics stuff so I'd be comfy using them.

These ones are sold and shipped directly from Amazon, and are listed as from Japan.

I looked at the FSM and it says to test resistance between centre and outer pin of each coil. If 0, that is good, correct? Is it advisable to replace all coils at the same time instead of individually as they fail?

Yes and No, mostly depends on how many miles are on them. Typically I won't go replacing all of them unless they have like 150k on them or you have had multiple failures.

Or if they show ANY signs of damage, blistering on the epoxy resin encapsulation (black shiny part on top), scorching of the spark plug well tube shaft, corrosion on the contact spring inside the spark plug well tube shaft, etc.

I checked the coils today. Cylinder 2 and 6 had oil in the wells.

I checked the resistance between inner and outer terminals. I checked all the coils on the driver's side and all were all showing resistance between 1.700 - 2.400. I think that means they are bad? FSM isn't quite clear but states it should either be 0 or infinity. Didn't bother to pull the passenger side coils. I am just going to replace all 6, but looks like I will be doing valve covers as well.

Are there any aftermarket covers that work okay? Factory ones are pricey. With the covers and coils I am going to be in it for over $1000 in parts alone.

NGK sells a set of 6 coils for $115. The 350z guys swear by them. The cheap valve covers work if you discard the gasket that comes with them and use a Gates gasket set.

Take the leads on your digital multimeter and touch them together, whatever the meter reads when you do that is what you should have for a resistance reading on the coil packs.

Some DMM's don't actually display ZERO, some only display 1 ohm resistance or 0.50 or whatever. If you have resistance (looks like you do) then that affects the coil packs ability to deliver proper voltage to the spark plug. Basically you end up with weaker spark the more resistance you have.

Surface corrosion on the pins in the harness can GREATLY affect this reading though, use electrical contact cleaner (read the can make sure it's safe for plastic) on the pins and try measuring resistance again.

Quick update. I checked the coils again, confirming the terminal were clean and still got resistance on a few of the coil packs.

Since I had oil in two of the spark plug tubes, I went ahead and replaced the valve covers and gaskets along with the coil packs.

Unfortunately, I am still getting a stumble at idle and when the engine is under load around 1500-2000 RPM. I pulled the MAF and although it still stumbled, it was noticeably smoother. Is this an indication of a bad MAF or is it just the ECU compensating to base value due to no MAF?

I got codes P102, P103 and P0113 which I was able to clear. I do remember seeing a P0113 for intake air temp sensor circuit high a few months back when I cleaned the MAF sensor. No CEL was lit at the time so it was not a hard code. Just wondering if the IAT has something to do with this. I looked at the FSM and it shows the IAT is part of the MAF and is not a separate sensor.

I have a basic code reader that shows live data but I am not sure how to interpret or further diagnose since there is no CEL currently being set from the misfire/stumble at this time.