2008 H3x I5 with 56k miles with a P0305 code on hand scanner.Intermittent P0305 code with an occasional dip in RPMs/miss fire on a warm start up.
PP TB, AirDoc intake installed
All plugs changed, new coil pack on #1 and #5,TB is clean, MAFS is clean, clean air filter.
Swapped coil packs around trying to chase the issue...stayed on #5.
Took H3 to dealer...tech noted miss fires on cylinder 3, 4 and 5 (my hand scanner on picked up on #5). Tech moved coils around and checked plugs..all good and continued to get same miss fires.
Tech recommendation: remove cold air intake and PP TB and reinstall factory set up and see if problem continues.

Truck runs strong except for the occasional issue on a warm start up. I have tried top tier gas on several tanks with bottle cleaner (snake oil right...lol) and still have this code come and go.

Thoughts?
Thanks in advance.

 

Same thing happened to me last year until I replaced ALL of the coils(previously replaced the plugs & insulators). I didn't get any hard codes(cel) after that, however I scanned the computer multiple times, and still received a P0300(which came up only when sitting at idle for a long time). Since then I have ONLY been using Top tier gasoline(Shell) & Lucas fuel treatment, periodically. The coils cost under $100.00, for all of them, shipped. and the fuel treatment cost around $25.00 a gal.(and lasts a lonnnnnggg time).
The I5 is a sensitive engine, however it has some serious power, if you take care of it! GL

 

Good idea on the coil packs. I will order some and see what happens.
Thanks!

 

I picked up a set of 5 Acdelco D1935E coils from autopartstomorrow, last winter. Had a coupon plus free shipping, total was under $100.00. The coupon code I used no longer works, you can check the net for a coupon (retailmenot.com) or sign up for emails from them and they will send you coupons.. GL

 

I'm guessing bad head...h3's with i5 are notorious for that...especially if you swapped coils and changed plugs already...I suggest compression test...

 

An engine misfire occurs when one or more cylinders in the engine fails, which causes the entire engine to misbehave. A cylinder fails when the air/fuel mixture fails to ignite in the combustion chamber. When the engine is misfiring, it will shake enough that the driver can feel vibrations throughout the vehicle, especially in the steering wheel. Misfires can lead to stalls while idling and can also make it difficult to start the car. A misfire can be constant, making it hard to ignore, but intermittent misfires can be tricky to diagnose. The occurrence of a misfire can vary depending on the driving conditions such as speed and outside temperature, and it may just happen at random. It is important to address an engine misfire as soon as possible, as this issue will cause a loss of performance and fuel economy. It will also increase the vehicle's emission levels due to an increasing number of hydrocarbons in the exhaust. The first step in assessing the damage is to determine which cylinder is problematic, which can be done easily with a diagnostic scanner. The last two digits of the code readout typically indicate the cylinder where the onboard computer detected a misfire, but it is a good idea to consult the owner's manual to make sure you are reading the code properly. There are four main causes of engine misfires - a loss of spark, an unbalanced air/fuel ratio, a faulty fuel injector and a loss of compression. The loss of a spark could mean that the spark plug needs to be replaced, which is a relatively simple process. It may also be caused by a faulty spark plug wires. In either case, if the spark plugs or wires are failing in one cylinder, they will need to be replaced. When one spark plug or wire goes bad, there's a good chance the others are not far behind. It is a good idea to replace all of the others when replacing one of these components. If the mixture of fuel and air is too lean - meaning there is more air than fuel - then it will be unable to ignite, even if the spark plugs are working. There is also a possibility that the mixture is too rich - too much gasoline - even though this tends to be far less common than a lean mixture. A gasoline-heavy ratio of air and fuel can occur if the fuel injector leaks, but this tends to occur in all cylinders, not just one, if it happens. An improper mixture can be caused by leaks in the fuel lines, a weak fuel pump or a faulty fuel injector. It is important to check the fuel injector on the problematic cylinder. It should spray a fine mist of fuel, but if there are drips, it may indicate that it is clogged, dirty or otherwise damaged. Low compression can also contribute to engine misfires. Pressure in the combustion chamber is just as important to a healthy cylinder as the fuel mixture and the spark that ignites it. If a chamber is losing compression, it means that the gasoline and air are leaving the cylinder before they can be ignited. This can be caused by leaky vacuum hoses, a busted exhaust valve or a blown head gasket. When two adjacent cylinders are misfiring, chances are that the gasket between them is cracked or damaged.

 

Update: Replaced all spark plugs again, all new coil packs...no more P0305 but now has P0300 and rough idle. Then swapped out PP TB with OEM and no change...same P0300 and rough idle. Going to swap the MAF sensor next and see what that does....after that it looks like either injectors or an internal issue...any other suggestions?

 

try some lucas & check for vacuum leak.. GL


check here for more info:

Automotive Technical Service Bulletin - TSB | Engine-Codes.com


P0300 Hummer Cylinder Misfire Detected Random Cylinders OBDII Engine Light Trouble Code | Engine-Codes.com

 

Additional update: dumped in 2 bottles of Techron a bit ago and poof the poor idle issue went away. Granted it was only a few miles driven since the bottles went in but i let it idle for awhile and she was purrrring. Time will tell....

 

Even for a short time, you found the source. Nice work...


I also picked up a set of Acdelco 217-2915 (GM# 12599025) fuel injectors from Amazon for under $120.00 shipped, for the set of 5, last winter. Haven't needed them after switching to shell & lucas. They are still sitting in my garage..