p0300, ruh roh!

**cheezus** · 09-04-2014, 06:54 PM

You know the deal, SES light comes on, starts flashing.... ruh roh! I get out the handy dandy oblink mx bluetooth obd II connector, plug it in, and crack open the laptop. On the laptop I'm using scantool pro with the ehanced gm codes. Less than $300 for the whole kit including the obd II connector. You could do scantool standard software trial license with enhanced codes for $120 less, hint hint.

Anyway, random misfires shown in cylinder 2/3 and appearing as I rev the engine (yup, you can see the live data with this setup). I check the plugs and wires, they look good. I swapped coil 2 around, misfire didn't follow, still showing cylinder 2. I swapped the injector from cylinder 2, misfire didn't follow, stayed on cylinder 2.

Over about 30 miles of "spirited" driving I'm seeing close to 1000 misfires logged in both cylinders 2 and 3 each. I guess next is swapping a plug and wire around see if it follows. I would have done that first but they looked fine. Guess I will try it now as a last resort. Anyone have other suggestions before I have to go exploring in the top end?

**pajeff02** · 09-04-2014, 07:01 PM

I just posted a reply to your question in the other thread. I presume you are clearing codes between tests?

**cheezus** · 09-04-2014, 07:02 PM

Originally Posted by pajeff02

I just posted a reply to your question in the other thread. I presume you are clearing codes between tests?

... and I was just replying to your reply!

Yup, cleared them in between.

**cheezus** · 09-04-2014, 07:07 PM

I'll try swapping plug/wire around and see if it follows. If not I guess my next move would be a fuel pressure tester and compression / leak down test? Harbor Freight has that stuff really cheap iirc

Edit: Also cleaned MAF while installing new lid and smooth bellows.

**pajeff02** · 09-05-2014, 02:10 AM

If swapping plugs and wires does not yield a diagnosis then unplug your MAF and see what happens. A malfunctioning MAF, or one that has been modified by deblading, descreening or porting, will misreport airflow to the PCM and can cause a stumble like you are experiencing. By unhooking the MAF, the PCM will default to run the engine in speed denisity mode and estimate airflow using the MAP and engine rpm. It will also set a MAF code which you can simply clear.

**cheezus** · 09-05-2014, 06:31 PM

Swapped plug/wire today but misfires didn't follow. I will try the MAF tomorrow as well fuel pressure, cylinder pressure and leak down test. Not looking good

On the plus side maybe I'll do those top end upgrades sooner rather than later

**pajeff02** · 09-05-2014, 06:35 PM

If I understand it correctly, misfire detection is based upon sensing rotational force on the crank. When the sensor sees a momentary drop in torque, it equates that with a specific cylinder in the firing order. Any sign that the intake is leaking and leaning out those cylinders? How is the plug color compared to the others?

**cheezus** · 09-05-2014, 06:42 PM

That's a good question. I only looked at the plugs in the misfiring cylinders for obvious signs of fouling. I never thought to compare two plugs to each other. I'll check that tomorrow as well. Thanks!

**kensz28** · 09-05-2014, 07:47 PM

I have the same code in my 2004 chevy 3500 van with a 6.0 litre. No ses lights though. Really dont know where to start. Also idles like it has a blower cam. Runs pig rich too.

**cheezus** · 09-05-2014, 08:06 PM

Originally Posted by kensz28

I have the same code in my 2004 chevy 3500 van with a 6.0 litre. No ses lights though. Really dont know where to start. Also idles like it has a blower cam. Runs pig rich too.

I started by using an obd II code reader to pull the exact cylinders the misfires are reported on. Afaik advanced auto or others only read the code and not these enhanced codes for the powertrain. You can do this for ~$200 (mentioned above). Then I pulled the plug/wire and checked them for damage. They looked good. Next I swapped the coil pack from a misfiring cylinder to a good cylinder. Using the same reader I checked the misfire counts while rev'ing the engine and the continued on the same cylinders. Next I swapped the injector around but the misfire didn't follow. Today I swapped the wire/plug from a misfiring cylinder to a good one but the misfire didn't follow (should have done this first). Tomorrow I will unplug the MAF and see if the misfires continue. Then hook up to the fuel rail using my $15 harbor freight pressure tester to check the fuel pressure. Next is a cylinder pressure and leak down test ($30 from harbor freight). My vaccum system looks fine but I'll double check. I doubt it's the o2 sensors because the misfires are on opposite sides. Otherwise swapping the o2 sensors is something I would check to see if the misfires switched cylinder sides. I've read other posts about ground connections but my grounds look good. There's a mention of a body control module and my windows are slow so I might check that. In summary the most usual causes are plug wire, plug, coil, injector. Swapping them around and seeing if the misfire follows will be a smoking gun. After that air systems, vaccum and o2. Lastly, it's a bent rod or busted spring. A fouled plug is likely evidence of such a problem. So if you do all these things and don't find the problem then pulling the valve cover and checking springs/rods might be in order (I may have missed something).

**35th-ANV-SS** · 09-05-2014, 08:08 PM

^^^your issue sounds like a vacuum leak. I would start there. Maybe the intake or one of the vacuum hoses has a crack or has deteriorated. This response is for Ken.

**pajeff02** · 09-06-2014, 10:19 AM

Agree. Ken's problem sounds like air is entering the combustion chamber or exhaust after the MAF so it is not accounted for. The O2 senders see this as a lean condition are telling the PCM to add fuel.

**cheezus** · 09-06-2014, 04:16 PM

Originally Posted by pajeff02

If swapping plugs and wires does not yield a diagnosis then unplug your MAF and see what happens. A malfunctioning MAF, or one that has been modified by deblading, descreening or porting, will misreport airflow to the PCM and can cause a stumble like you are experiencing. By unhooking the MAF, the PCM will default to run the engine in speed denisity mode and estimate airflow using the MAP and engine rpm. It will also set a MAF code which you can simply clear.

It's the MAF! I hooked up my laptop and monitored realtime data for per cylinder misfires. Then drove my local loop with the MAF hooked up to confirm I was generating misfires, which I was. Then I shut the car off, disconnected the MAF, drove the loop a few times. No misfires. I hooked the MAF back up, ran the loop, misfires (cylinder 2/3).

Then the sprinkles started so I zoomed back home into the garage. Tomorrow I'll try some MAF cleaner from autozone and see if the problem goes away. I should note that the po apparently never changed the air filter in the 10 years (10k mi) he owned it because it was caked with dirt in the grooves when I changed it. During the purchase process I opened the lid and could see the air filter was clogged up, bad. I disturbed it when opening the lid. So it's very likely I knocked some dirt loose back into the MAF. The MAF itself looks fine and nothing has been ported or modified by the po or me. But this would explain, with me disturbing the dirty air filter, why I tripped the code after I bought it (he swears no codes existed and I logged 25 mi on it before buying it with no ses light).

**pajeff02** · 09-06-2014, 04:30 PM

If you end up having to replace the MAF, I recommend you simply get one through GM Parts Direct. It will be an 85 mm MAF with a plastic housing and have a short adapter harness, but it is calibrated correctly for our cars. The stock MAF on our car had been debladed, descreened and ported by a second grader with a hammer. I swapped in the GM replacement shortly after buying the car and have never had another problem.

**cheezus** · 09-07-2014, 04:23 PM

Originally Posted by pajeff02

If you end up having to replace the MAF, I recommend you simply get one through GM Parts Direct. It will be an 85 mm MAF with a plastic housing and have a short adapter harness, but it is calibrated correctly for our cars. The stock MAF on our car had been debladed, descreened and ported by a second grader with a hammer. I swapped in the GM replacement shortly after buying the car and have never had another problem.

Pulled the MAF off and wow was it dirty. I used the MAF cleaner from autozone but that wasn't cutting through it just spraying. Next I tried some cleaner on q-tips but that wasn't getting it all off. Then I remembered I had some radio shack electronics cleaner and that worked great. The bottle had a brush tip on it which cut right through the crud and let me get around those tiny electrode wires. I put it all back together and set out. Throttle response was much better. The misfires were worse at first but quickly settled down as the computer relearned. Once it settled down I was only logging a misfire on cylinder 2/3 about 1 misfire per mile per cylinder. I drove just over 50 miles and logged ~50 on cylinder 2/3 and then anywhere from 1-4 on the other cylinders. I'm debating if I should just get the replacement.

LOL sorry to hear about your MAF abuse by a second grader with a hammer. Did he also pnp the throttle body with his hammer?

**pajeff02** · 09-07-2014, 04:42 PM

CRC Spray MAF cleaner is the ticket. Seems to work well and dries without leaving any residue.

Thankfully, the prior owner left the throttle body untouched.

**cheezus** · 09-07-2014, 04:46 PM

Yeah the CRC brand is what I used. Used a whole can and it wouldn't take off the crud by itself. I'll pick up another can and spray it off again just in case the radio shack electronic cleaner left any residue.

**pajeff02** · 09-07-2014, 05:48 PM

If you run a K&N or similar style filter that is what happens. The oil sticks to the MAF wires and then dirt and other crud adheres to them. This buildup acts to insulate the wires and results in the MAF reporting less than actual airflow, hence the lean condition you were experiencing.

**cheezus** · 09-09-2014, 09:07 PM

Originally Posted by pajeff02

If you run a K&N or similar style filter that is what happens. The oil sticks to the MAF wires and then dirt and other crud adheres to them. This buildup acts to insulate the wires and results in the MAF reporting less than actual airflow, hence the lean condition you were experiencing.

No K&N filter. I got another can of crc maf cleaner, still misfires on 2/3. I was going to order a maf from gmpartsdirect. The website said to email to confirm before buying or you void the return policy. So I sent the year/make/model. A day later they asked for the vin, I sent it. A day later they sent me a link to order the part (same part number) but the link didn't work. I gave up and went to Autozone and bought a reman'd maf. And I still have the misfires, even with the new maf. I disconnected the maf again and confirmed the misfires DO stop. I'm a bit confused at this point lol.