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fuel lines inside the intake plhenum may be cracked and leaking inside the plhenum. same thing happened to my 1992.sonoma. they are made of a plastic material and tend to crack right in the bend of these lines.
Do a compression test on all of your cylinders when it is cold. Backfiring is a sign of a worn valve seat allowing mixed fuel and air (lit by spark) to be forced back into the intake. You will probably see compression loss on one or more cylinders.
Also... there is a chance that the EGR valve is not totally seated when the engine is cold with just a small blow by. The EGR valve seats coke up over time with black gunk.
There are a couple of things i would check for.The first would be a real good check for vacuum leaks all around the intake manifold.2nd i would verify that ignition timing is set where it should be.If these 2 check out ok i would check or replace the idle control motor on the throttle body.Let me know if this helps or you need more info.
According to my book, the first code is the brake control rough road sensor, and P0302 is a cyllinder #2 misfire. Check the gap on your plug, and make sure the plug wire is on all the way. Give the boot an extra good push. Backfiring or misfiring under load is always caused by poor ignition. You may also have a faulty plug or wire.
HI MY NAME IS JUAN:
I HAVE A 93 BUICK SKYLARK CUSTOM
AND I HAD THE SAME HESITATION.
I CHAGED GAS FILTER, AIR FILTER, AND USE SEA FOME IN MY GAS TANK AND USE IT ALSO TO CLEAN THE INTAKE SYSTOM, AND IT HELPED ALOT. TWO MONTHS DOWN THE LINE HADE THE SAME HESITATION AND IT WAS ONE OF THE INISION COIAL PACK WAS NOT GIVING THE ELETRICAL SPARK TO THE PLUG. I CHECK IT BY PULLING THE CABLE OFF THE SPARK PLUG WITH THE CAR RUNNING AND FOUND THE ONE THAT WAS NOT GIVING SPARK. I REPLACED THE PACK FOR THAT CILINDERS PLUG AND THE PROBLEM WAS GONE.
HOPE THIS CAN HELP YOU.
I'd run a code check first, just to give you an idea if anything is leaving footprints in there. Transmission performance is somewhat related to engine performance but if you haven't serviced trans in a long time, do it.(if there is a lot of debris in there it may be time for a rebuild) Backfiring may be caused by an air leak in the exhaust manifold, allowing gasses in front of the cat to re-ignite. But, this could be caused by other things as well...that's why code read is important.
Did you check the timing? Also make sure the injectors are putting out the correct amount of fuel. By oxygen sensor I'm assuming you replaced the MAF (mass air flow) sensor? Are all the vacuum lines in good shape and is the EGR vavlve actuating correctly (they can get gunked up)? Is air getting into the fuel lines somewhere? Is the engine not getting enough air when the turbo isn't on? Maybe your intake is clogged up but this isn't extremely likely since you'd notice power loss at higher speeds. What was done recently to the vehicle that might have caused this?