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1. replaced the air filter
2. replaced the PCV valve
3. inspected the PCV hose and connectors (nothing wrong)
4. ran a tank of gas with fuel injector cleaner
5. tested the EGR valve by pulling the vacuum hose off the top, and blocking the hose. The idle didn't smooth out. I applied a vacuum to the EGR valve manually and the car immediately stalled which tells me that the EGR valve itself is working fine.
Bad throttle position sensor (especially if the "Check Engine" light is on or flashing)
Bad oxygen sensor (especially if the "Check Engine" light is on)
Water in fuel or bad fuel
What to do:
Start by removing the air filter to see if the problem persists. If the air filter is dirty, replace it.
Next, verify that the Check Engine light is operating by turning the key ON but not starting the vehicle. The Check Engine light should be illuminated. If not, replace the Check Engine light bulb.
If the Check Engine light is ON when you start the car, try feathering the accelerator pedal to see if the base idle stabilizes or changes. If it does, replace the TPS (Throttle Position Sensor). This is a fairly inexpensive part.
If the Check Engine light is FLASHING, or ON, next you will need to have a local mechanic or your local auto parts store attach an ODB scanner to determine what error codes are present. This will help you pinpoint the issue.
Once you have pinpointed and corrected any issues present using the ODB codes, you will need to attach a FUEL PRESSURE GAUGE to the fuel rail test point to determine if the fuel pressure is correct. If it is too high, the FUEL PRESSURE REGULATOR has failed. Again, this is an inexpensive part but the fuel system pressure must be released before servicing this part. Best done by a qualified mechanic.
HAS THE CAR SAT UNUSED FOR A PROTRACTED AMOUNT OF TIME WITHOUT FUEL BEING REMOVED OR ADDED? If so, the condition you are describing may be a result of bad fuel. Modern fuels contain ethanol, which causes moisture to condense in the tank when it is not agitated. This can cause water to build up in the fuel, and causes the fuel system to rust as well as an idle issue that resembles what you are describing. Try adding new fuel, and add a full bottle of FUEL SYSTEM WATER REMOVER. Remember: IF FUEL IS ALLOWED TO SIT IN YOUR VEHICLE'S TANK (without new fuel being added) FOR MORE THAN SIX MONTHS, IT WILL GO BAD.
Did you test any of those parts before you replaced them ? Have you checked the cap, coil, and distributor yet ? The distributor has a module and a pickup coil inside that could be faulty. Is the check engine light on ?
With the age of the vehical it might be a good idea to do a compression test first. I would check the timing. Sounds like the engine is advancing for some reason, if that checks out then recheck for vacuum leaks( it could be leaking past the base gasket, vacuum booster, or intake), The vehicle has a carb. not a throttle body injection? Also if you can retrieve that engine code it may be helpful.
the p0300 car's computer has detected that one of the engine's cylinders is not firing p0301 In this case it's cylinder #1.p0302 In this case it's cylinder #2.and so on i would check the three listed above first then check
Defective computer Random misfires that jumps around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leakthat may be a broken hose or in the intake manifold (gasket,spary aruond it with wd40 or starer fluid if the engine idle goes high theres your problem) or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open. hope this helps