I have a 2003 Cavalier. I have a p0300 random misfire code. If I turn on the ac or heat the check engine light will go off.
I have swapped tons of parts... and still no fix.
When I look at my scan tool what should me readings be?
I noticed my throttle body was completely closed, Idle was 500rpm. My fuel trim was -18 and ignition was bouncing +6/+12
When I manually opened the throttle body a little, raised idle to 750rpm, fuel trim went to +2 and ignition advance went to 0. Runs smoother.
What should these numbers be?
tps%
st fuel trim
rmp
bla bla bla. what should I look for. I have replaced.....
Ignition module
Coil
Boots
Plugs
ECU
Fuel Injectors
IAC
MAP
Crankshaft Position Sensor
Knock Sensor
Oil Pressure
Timing chain and timing belt
water pump
thermostat
purge valve
Fuel Pressure Regulator
The only thing I haven't replaced is the fuel pump.... that will be my final step. My fuel pressure checked good, and car runs fine driving. If I floor it plenty of power......
I don't really want to buy a fuel pump if it is not needed..... At this point I feel I have thrown alot of money away. I did compression check and all 4 cylinders were good.
Hi Carl,
I would take off the intake manifold (on the '05 it is made of plastic) and would check if the mating surface to the head is flat.
You could have vacuum leak there.
Peter
Ive sprayed a couple cans of carb clean and never any idle change.
When I look at my scan tool Peter, it shows Short term fuel trim at -18 at idle. If it were an intake leak it would be adding fuel to make up for the additional air.
Just installed the new TPS sensor. No change at all. So I basically have all new sensors. My idle is at 500 rpm, and I feel a miss and hear a sputter out the exhaust. If I manually raise the idle on the throttle set screw to 750 it runs 99% better. On occasion I feel the miss but way better.
People say it could be the fuel pump.... but once again I'm gonna say the computer is pulling fuel away at -18. If it were fuel pump the computer would be trying to add.
Ive also read it could be a clogged cat.... but that would bog the engine down because the exhaust has no where to go... this car runs like a champ driving.....
I'm getting close to throwing in the towel. Cars headed for the junk yard when its time to smog. I'm completely out of things to try unless someone has any ideas.
You shoos the be swapping parts. -18 means the ecu is pulling fuel. I would add propane or carb cleaner to the intake while it is running and see if the fuel trim goes full negative, fore example, -25%. It it does that means the oxygen sensor should be working. Then I would hook up a pressure gauge to the fuel rail and check for fuel pressure to leak down when the key is off. If it does, it would then use an injector pulse genertor and check for injector balance or leakage. This if you do not have any missfires.
On the inside my car looks like a fighter jet.
Hi Carl,
Just noticed that you have not changed the upstream ox-sensor yet. (It is at the end of the exhaust manifold, where the down pipe with cat converter bolts up.)
What is the mileage on that sensor?
If the fuel pump supplies enough fuel at high speed, then it hardly can cause any trouble at idle!
But do measure the fuel pressure if you are suspecting it!
The fuel pressure will change about 10PSI, as the vacuum changes in the intake manifold.
As suggested before, detach the IAC connector to see if that circuit/system is working.
Peter
