2004 Alero 3.4L Idle and Missing Issues.

Greetings all,

My 2004 Alero 3.4L is having issues.

Here are the issues:

Trouble starting (won't start unless I push gas pedal)
Rough Idle
Car dies unless foot on the gas (park, neutral or drive)
Bad exhaust smell
Missing
Seems to run better for a few minutes after car sits all night.


Error Codes from computer:

P0300: Multiple Cylinder Misfire Detected
P0122: TP Sensor Circuit Low Input
P0442: Evaporative Emission (EV AP) System Small Leak Detected (0.040")


Here's what I've tried:

New Spark Plugs
New Wires
Tested resistances of Ignition Coils--all within spec (visual spark verification as well)
Replaced TP Sensor
Charging battery. Battery reading 12.1V. A/C, lights, radio work fine. No dimming is experienced when accelerating.


Concerning P0122, I'm getting varying readings from power running to the TP sensor. According to the interwebs, it should be ~5.0V. I've gotten 5.0V, 0.00V, 3V, 0.5V (and other readings) when testing it over the past few days. I don't get it... How the hell can I keep getting different readings? I'm taking the readings from the power cable before it's plugged into the TP sensor so I can see the actual supply. These readings are all with the electric system on but engine not running.

I also tested the other circuit's on the TP sensor (this time power cable plugged in) and the 11V return passed as well as the 0.5 - 4.5V for the throttle being at start position and fully open.

I'm not a mechanic and I'm very confused. I really don't have the money to take this thing to a professional but don't really have anymore ideas. I'm also stressing because I have to get to work next week somehow. Anyone have an idea on this? Thank you!

There should be three wires on the TPS. One is ground, one is the constant 5V reference, and the last is the signal back to the PCM. Verify that the signal from the TPS is correct first, since it can affect some of the other potential problems.

Your description of the symptoms is helpful. It may indicate a rich/flooding condition. This could be a result of an incorrect TPS signal or other problems, such as a leaking injector(s), leaking fuel pressure regulator, or other possibilities.

Opening the throttle to start the engine admits more air, counteracting any rich or flooded condition. The fact that it starts "better" when cold is another indication, since a rich mixture on cold start is more conducive to starting.

The rough idle and misfire are other symptom of excessive fuel, as is the exhaust odor.

Thank you for the reply. I keep getting mixed readings from the 5 V reference. I listed some of the readings I've gotten above in my other post. I know that that's strange. I'm wondering if it could be a problem with whatever supplying power to the TPS?

Strange things can happen. I've had wire harnesses chafe and fail inside the convoluted protective tubing, with no external indication of a problem:

http://www.wwdsltd.com/files/PCMWireHarnessWear.jpg


The only way to trace that is monitor circuit voltages.

Thank you again! I also wanted to add another symptom but you probably guessed it would be here: I smell gas when working on the engine. Never smelled before like this. So I should trace the wires back to the source and check for damage? Could it not be th car computer misbehaving?

Something odd I noticed was that if I disconnect the car battery and reconnect it, I get a higher voltage to the TPS. It does off eventually.

Blue has given you some good advice.......

You mention 11 volts when discussing the TPS harness? Nothing should be 11 volts......

For that tps code to happen, I am thinking you either have a bad TPS or, one of the wires in the harness to the tps is broken, causing the low signal....it can be broken inside the insulation, making an intermittent contact.......what you want to do is back probe the tps harness on the signal wire....at closed throttle, it should be 0.4 volts, key on, engine off......now open it part way to about 2.5 volts for a reading, and gently pull and flex on each wire going into the tps, and see if the reading fluctuates...it shouldn't....if it does, bad wire or contact in the harness connector...

Here is what I would do....first pull off the vac line to the f/p reg, and see if there is any fuel in the line....if ok, plug it....next disconnect the harness to the injectors, and remove the fuel pump fuse.......now crank over the car for a few seconds to get rid of any residual fuel, if an injector is leaking......now take some carb cleaner, and have someone spray into the intake while you crank......if idle is good, as long as you spray, then we know we have a fuel delivery problem......

Next, I would reconnect the injector harness, and reinstall the f/p fuse, and get a fuel gage attached......you want to get close to 40 psi, and it should hold, after you cycle the key three times to "on"(without starting).......If it drops, then you have a problem.....

Thank you for all the replies. It did help me finally figure out the issue. It was the fuel pressure regulator. I don't know why I was receiving bogus voltage ratings, but replacing the fuel pressure regulator cleared up all issues and it runs great now. Frustrating but glad it's fixed.