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post #1 of 5 (permalink) Old 10-02-2016, 08:09 PM Thread Starter
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Po303 Assistance Requested

Hi, I'm a first time user/poster.
I have a 2007 lt 2.2, 5 speed.
Chronic po303.

It sat unused for about 3 years (no codes/running great) and just last week I decided to plate it and drive once again.
It now throws po303 however it did not when I last started it up a few months ago.

I've installed new oem plugs gapped at 45, swapped all coils around, compression is 150 on all cylinders, dumped a whack of seafoam in before and after oil change/filter and to gas tank and it runs considerably better but still throws po303. After reading posts here for several days, I became aware that the 07/08 had gm software issues that threw po303 erroneously and that it may be a mas or map sensor instead....so, changed both with new ones and it runs even better still....but throws the po303 eventually.

When initially started, the engine seems to be smoother (but that may be because of the higher revs - 1200 or so - until it drops to 800 and the slight miss can be felt. It also chugs/misses around 2000 rpm when under acceleration.

From what I read here, it seems that the two issues left that are suspect are either the ecm or plugged/fouled injectors.

A few questions;

I've found an ecm on kijiji from a 2006 lt, 2.2 5 spd., can it be installed directly to operate my 2007, or does it need a dealer "flash"? (any info on what ecm's are compatible? Will an automatic ecm operate a 5 spd model car?)

As this is not a pristine vehicle, I do not want to throw 400 in to it for injectors......can the injectors be "freed up" by an injector servicing while on the car? (seafoam made it run much better so far, but if there is solids lodged unlikely that is going to change with seafoam) And, I can pick up cheap injectors on amazon, that indicate they are compatible, (I realize that oem are designed for their specific application and are better, but I would rather spend 100 for 4 injectors then 400 on oems on a problem that I am not yet sure is responsible.)

Thanks for any input, it is appreciated.

Last edited by pioneer; 10-02-2016 at 08:33 PM.
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post #2 of 5 (permalink) Old 10-03-2016, 01:44 AM
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06 has a PCM and is not compatible with your 07. Did you clean your throttle body yet?

The P0303 is a bit of a mystery code. I doubt you have a misfire and the cause has often been one of the HOs going out of range. You can eliminate combustion issue by putting a scanner on the OBDII port and checking for misfire counts. This will also eliminate your injectors.

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post #3 of 5 (permalink) Old 10-03-2016, 04:55 PM Thread Starter
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I did clean the throttle body.
Helped it run a little better, as did all the other attempts.

I have only the autel 319 for codes, and cannot seem to determine if it will provide a misfire count or which cylinder it is. May pick up a different setup tomorrow.....some bluetooth ones say they provide a misfire count. ??? Any preferences?

I've changed the oil/air filter, plugs, maf/map, cleaned out throttle body, ran seafoam several times, put the coils on all different cylinders and all efforts have made it run so much better/crisper, but it still chugs/lurches (almost like an electrical short, as it "breaks free" and will lurch ahead without the slowing of the chugging).

Odd, as it had no codes before sitting....I'm thinking it may have been some crud in the fuel line that got lodged in an injector.....
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post #4 of 5 (permalink) Old 10-03-2016, 05:06 PM
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Or moisture in the exhaust causing the HOs and cat to start going bad.

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post #5 of 5 (permalink) Old 10-04-2016, 06:53 PM Thread Starter
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Would issues with the HO2 sensor cause the lurching/chugging instances? It will run a little rough, then seem like it's "breaking loose" and drives smoothly/good acceleration.

Thanks for the replies.
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