2000 Saturn SL1 served me well for many years. Recently started giving me random misfires code P0300. My guess is vacuum leak. Replaced spark plugs b/c it was cheap and easy. Injectors are more expensive and hard to reach. Intake gasket makes it to the very bottom of the list. Both O2 sensors stuck at 0.7. I doubt it's O2 sensors. Hard to imagine both gone wrong the same time. Intake manifold pressure reads 5psi +- 0.5 at idle. Removed EGR it looked old very old. Car rpm between 1000 and 1500 at idle, used to come down to below 1000 after a few seconds. What would be your ordered list of things to do from easy/cheap to hard/expensive? Consider the value of the car less than $500.

 

3rd gen SOHC engines have a bit of a design flaw in that The intake manifold gasket is not well enough supported around the number one and number two cylinders. What ends up happening is that part of the gasket gets dislodged and partially sucked into the manifold, creating a vacuum leak.

You can verify the location of the vacuum leak by dousing all around each of the runners of the intake manifold where they meet the head. You may need to develop new flexibility in order to get to the undersides but if you hear a change in RPM while spraying, you've found your leak. Car should idle somewhere around 7:50 to 8:50 RPM.

Also spray around where the throttle body meets the intake manifold to see if the gasket has gone bad. A good throttle body cleaning wouldn't hurt, even if it's just on the car as it will rule out any possibilities that the movement of the throttle plate is not free. Not that that would throw a random misfire code.

Other causes of misfire codes in these engines are not using the stock NGK plugs as specified by GM. The waste spark design involves analysis of energy dissipation and the PCM is looking within a certain range to determine whether or not a plug actually fired even if it's not on its power stroke. Like most engines, it has been designed with an optimal spark plug for optimal performance. I'm a DOHC guy so I don't know the NGK plug number but it is easily found on RockAuto.

High idle can be caused by a defective ECTS. It would need to be more or less an open circuit that drove the air fuel mixture crazy rich. If you decide to replace it, replace the pigtail connector as well.

 

Great help. Thank you.
Wish I had your advice before replacing the spark plugs. I removed them first then ordered the same exact part Bosch Platinum. I figured they've been working for over 10 years now so went with the same part.
ECTS is next. Good point. I've had coolant sealant in my car for 7 or 8 years now due to a bad water pump that I was neglecting for a few months. The last couple of years noticed temp gauge was delayed. Car was warmed up for sure but temp gauge wasn't right. Also tackling this first cause it's inexpensive and easy to replace.
Already done the spray test with no luck spotting the leak but I guess I wasn't thorough. Will re-do.
Thanks again.