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Misfire - P0303 Code - Did the basics. what next?

Posted: Sun Aug 13, 2017 12:46 am
by dmilamj
So I have a '04 Astro AWD with about 110k miles on it. I've owned it for ~2 years or so, and it has never gotten very good gas mileage - typically about 14mpg and has always had a rough idle.

A few weeks ago, I was driving it and the check engine light started flashing and the engine began running very rough. I had Autozone try to pull a code, but there was no code set. I went on vacation, bought a code reader, came back, and there was a P0303 code on start up.

I've pulled the #3 cylinder spark plug, it was dark and sooty. I replaced it with a new plug and installed a new plug wire. Tested spark and the spark strength looks the same on the #1 and #3 cylinders. Fired up the van, cleared the code, and it came right back after a few minutes.

I did a compression check on Cylinders 1 and 3, and I've got about 150 psi in both. I don't think compression is the issue but I'll check some cylinders on the other bank to see what they look like.

I was using the code reader to monitor the fuel trims while idling and then at 2500 rpm. I don't really know what I'm looking for, but the Bank 1 short term fuel trim seems to oscillate a great deal at idle, and can range from 0 to -30%, which seems extreme. The Bank 2 short term fuel trim also oscillates a good bit, but is usually quite a bit closer to zero than the Bank 1 trim.

However, I don't know if I trust the reader, as the long term fuel trim seems to change way too fast. The short term trim also fluctuate pretty quickly, and will occasionally just reset to 0%.

Here are the ranges I'm seeing for the following parameters at idle and at 2500 RPM
700 RPM / 2500 RPM
Engine Coolant: 195 deg F / 204 deg F
Shirt Term Fuel trim bank 1: -2.3% / -13.3%
Long Term Fuel trim bank 1: -14.1% / 10.9% (how did this change so much in just a few minutes of engine operation?)
Short Term Fuel trim bank 2: -2.3% / 7.0%
Long Term Fuel trim bank 2: -16.4% / -3.1%
Intake Manifold Pressure: 11.4 inHg / 9.6 inHg
TIming advance for #1 cylinder: 17.5 deg / 42 deg (does 42 degrees of timing advance seem odd?)
Intake Air Temp: 111 deg F / 109 deg F (It was hot out)
Air flow rate from mass air flow sensor: 0.9 lb/min / 3.2 lb/min
O2 Sensor Bank 1 Sensor 1: 0.58V / 0.710 V
O2 Sensor Bank 1 Sensor 2: 0.57V / 0.85 V
O2 Sensor Bank 2 Sensor 1: 0.735V / 0.065 to 0.825 V (The 2500 RPM reading seems off)

Can anyone give me some idea of what ranges are typical for these parameters? I have no idea.

It seems like it is probably an injector on the #3 cylinder. I would think that an O2 sensor would lead to problems with the whole bank, while a distributor gear would lead to a random misfire condition.

Is there anything I can do to verify before I crack open the intake and replace the spider? Any other diagnostics I should do?