Not sure what to do next

[jcwecker]

Hello. Somewhat of a newbie here. I have lurked in these forums for a few years and usually can find someone who has had a similar problem that posts and then I learn from that. Have a 1997 SL2 that seems to have developed an odd issue and I haven't been able to figure this one out. Car had a head job done about a year ago. Replaced the usual stuff and at that time also replaced the ECTS, thermostat, wires, plugs,coil packs and it ran pretty well for about a year. Now I keep getting the code P0300. I have replaced the plugs (new wires about a year ago), the TPS, IAC, ICM, fuel filter, throttle body gasket, tried blocking the EGR. Still getting the P0300 with a high idle that gradually goes down to normal but the idle seems high while driving. It has an automatic and while in gear at a stop, feels like it is going to die. Not sure what to try next. The multiple misfires has got me. The temperature guage is where it should be in the middle--think I should try replacing the ECTS again? Getting tired of throwing money at this.

[Ramics]

Im in the same boat as you. Rebuilt last year completely with the same dreaded P0300.
Mine runs fine but at upper RPM and in industrial areas and in traffic it starts up with absolutely no hesitation at all.

Stumped here as well.

[Ramics]

Anyone else with recent head work experiencing a P0300?

[Inky]

Have either of you checked for a vacuum leaks around the throttle body?

[rockorager]

How about the valve cover gasket? You could be leaking oil down into your plug wells and causing random misfires. Have you pulled the plugs to see if the threads are wet with oil?

[jcwecker]

A couple of days ago I changed the plugs and didn't see any oil in the spark plug wells. I sprayed carb cleaner around the intake manifold gasket (as best I could) and didn't see or hear any change. This is a college car for my daughter and I was really hoping it could make it a couple more years. Now I have poured a lot of money, and time , into it and I am stuck with a poorly running car with a lot of new parts.
This weekend I may try taking off the intake manifold, replace the gasket, and see if this helps. Any other suggestions that ANYONE has will be appreciated.

[fdryer]

Originally Posted by jcwecker

............. I haven't been able to figure this one out. Car had a head job done about a year ago. ....... replaced the ECTS, thermostat, wires, plugs,coil packs and it ran pretty well for about a year. Now I keep getting the code P0300. I have replaced the plugs (new wires about a year ago), the TPS, IAC, ICM, fuel filter, throttle body gasket, tried blocking the EGR. Still getting the P0300 with a high idle that gradually goes down to normal but the idle seems high while driving. It has an automatic and while in gear at a stop, feels like it is going to die ..........The multiple misfires has got me. The temperature guage is where it should be in the middle--think I should try replacing the ECTS again?

The temperature gauge needle at half way position doesn't seem right - it should be at the 3/8ths position (between 1/4 and 1/2). As long as the brass coolant sensor is used then replacing it won't help. Is the connection clean and free of corrosion? Is the radiator/condenser coil free of any debris blocking air flow?

What's high idle? Actual rpm? Normal warm idle should be around 800-900 rpm. Was the throttle body removed, tps and iacv removed to clean the throttle body and iacv?

[jcwecker]

Temperature Needle varies between 3/8 and 5/8 (when fan kicks in). Used a brass ECTS. I will double check the connectors. Forgot to mention, new radiator about a year ago---should be fine.When you start the car (no flashing SES, codes cleared) rpm goes to 1800-2000 for a few seconds, then slowly goes down to about 800. Then when you put the engine in gear, idle goes to about 1100 and slowly goes down to 500. Somewhere in between the 1100 to 500 journey, the SES starts flashing and the car starts to feel like it is going to die. While driving, it feels like the car is accelerating itself sometimes. I will admit, the last time I took off the throttle body, I didn't replace the gasket and I know this car likes fresh gaskets there. Tough to spray carb cleaner on the intake mounting area to check for gasket leaks there. I am thinking that is my nest move, to replace the intake manifold gasket. Hate to waste time pursueing this as I know it is a bear to remove and replace the intake manifold. The real disaster is that school starts soon and this is my college daughters car. I have so many new parts in this car that I am thinking of having a long block put in. Expensive solution and I have tuition due early next month-----frustrating.