I was driving my 1999 Pontiac Gran Prix GT on a 40 mile trip. Right when I exited the highway my car stalled. I tried to restart it and it ran very rough with no power and I had to have it towed home. I am getting a code PO300/Random/multiple cylinder misfire. I have checked the coil packs and the ECM and they seem to have fire and injector pulse. I also changed the spark plugs and it stills runs very rough and will not idle. I have to keep the gas pedal “feathered” in order for it to stay running. What could the problem be? The car ran perfect before this happened and I don’t know what to do next.
Cars are a lot like people. One day you feel great and the next day you’re sick, running to the doctor to get a shot to cure your ills. Well that what happens to cars and trucks, one day all is good and then the next day they sit on the side of the road waiting for the AAA tow truck to take them to the “auto doctor”.
So let’s look at this problem logically. You say the car has spark and the computer is working as it should. (This is because it tries to run if you feather the gas?) If so, then I suspect that the exhaust is not restricted or anything is mechanically wrong with the engine. What information I don’t have is the fuel pump pressure and volume test.
Over the years I have found that 90% of poor running conditions in vehicles can be traced back to poor fuel delivery or poor fuel quality. If it was me, I would find a shop that can diagnose your problem and fix it with a guarantee. Throwing new parts may be good for your local auto parts store, but not good for your wallet. The SWAG (Scientific Wild A@@ Guess) method of changing parts can eventually fix a car, but sometimes the cost exceeds the value of the car.
So tow it to an AAA shop using your AAA card and have them figure it out why your car does not run properly. If I had to guess what your problem is gong to be, I would GUESS that the fuel pump has died or is dying.
Here’s why, your vehicle needs 40 lbs of pressure when starting and 32 pounds of fuel pressure when it is idling. It should have a fuel volume of a ½ pint of fuel delivery in 15 seconds or a full pint of fuel in 30 seconds. The fuel pressure should be able to hold fuel pressure for up to two hours when you shut off the key. If it fails any of these tests then MAYBE all that is wrong is a dead or dying fuel pump.
This may explain the random misfire code P0300 because the fuel is not “squirting” correctly into the engine from the fuel injectors. When you “feather” the gas pedal by pumping it you are enriching the fuel delivery amount from the injectors so that the engine will continue to run.
All these tests I have mentioned above need to be performed with special equipment that the average back yard auto mechanic may not have. But any reputable auto diagnostic shop worth their salt has these tools, plus a lot more equipment to make sure what the problem is. So they can fix it right the first time!
Hope this helps and have a Happy New Year.
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