Irratic Idle [Archive] - Forum


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05-22-2010, 10:22 PM
2001 grand am 3.4

Throwing a P0305 code, cylinder 5 misfire.

The car has a very irratic low idle. it jumps from 500-1000 rpms or so. HOWEVER, when the PCV valve is removed from the motor (its a new valve) the car idles much better. There is vacuum from the PCV valve.

New plugs and wires
Coil packs are good (bought a new one and tested them all)
MAF sensor cleaned several months ago
New air filter

Basically I need to know what is causing this, and what I have to do to fix it. It started doing this out of the blue, worked fine yesterday, today not to much...

Any ideas??

05-22-2010, 11:15 PM
check the connecting line to the pcv valve maybe theres a split in the line

05-23-2010, 10:01 AM
The PCV valve and line are in good condition, there is plenty of vacuum when a finger is placed overtop. Its when its put back into the valve cover that the car shudders, shakes and has an irratic idle.

Thoughts ??

Would a leaky head gasket do this?? How would I test for this ??

05-24-2010, 02:34 PM
weird, should be the other way around, smooth when PCV valve is in, and rough when you pull it out.

05-24-2010, 07:04 PM
You said that the pcv valve is new.
Is it the right one?
Verify it is the right one and even get another to test as the orifice size might be off.
Got this from a web-site:
" The PCV valve is a spring-loaded valve with a specific orifice size designed to restrict the amount of air that's siphoned from the crankcase into the intake manifold. This is necessary because air drawn through the valve from the crankcase has a leaning effect on the fuel mixture much the same as a vacuum leak. So air flow through the valve must be controlled within certain limits. At idle, air flow is reduced because little blowby is produced. When the engine is cruising and vacuum is high, airflow through the PCV valve is at a maximum to purge the blowby vapors from the crankcase.

It's important to note that PCV valves are sized for specific engine applications. The wrong PCV valve for an application can flow too much or too little air causing driveability problems. Varnish deposits can clog the valve, so replacement for preventative maintenance is recommended (every 50,000 miles usually)."

05-24-2010, 09:29 PM
I should have said, its new about an oil change ago. So its not like i changed it and this started happening.

Also, when the car is started cold, it just shakes and misfires. But when the car is warmed up, thats when the idle starts acting up.

Possible 02 sensor?? I kinda ruled this out however, due to the car running in open loop fuel mode (taking no readings from 02 until exhaust temp reaches 600 degrees F)

Any ideas??