3800 Engine High Idle

I just installed a brand new (remanufactured) 3800 series II engine with 0 miles in my 1999 Monte Carlo Z34. New EGR valve, spark plugs and wires, upper intake, lower intake in great condition, new gaskets, water pump, idler pulley set up, Catalytic converter is new less than a year old and I just installed a remanufactured set of fuel injectors. Fired it up and it sounds great but in about 2 minutes when it warms up the idle gets up to 2000 to 2100 rpms and stays there.

Code only comes back P0300 random multiple misfire. Please any Ideas as to what else it could be. I want my Monte back.

Assuming this 3800 engine ran ok before replacing it with a remanufactured engine, several possibilities come to mind.

If you have code P0300 then the engine should also be running rough and misfiring. A high RPM would not cause the code P0300. That may just be a result of the way it is running.

Does the engine rev up OK or does it stumble and shake?

Check for a vacuum leak. Look for a hose left off, possible gasket problem on the intake manifold, O' ring missing on the PCV valve when engine was replaced. You can check that last one by removing the oil cap and see if it runs any different. If the RPM gets higher or much lower, then that is your problem. This is commonly missed on the 3800 engine. There is an O' ring on the valve itself and one on the cap. Either one will also cause the engine to such and burn oil at an alarming rate.

Make sure the throttle or cable is not actually mechanically bound up holding the throttle plate open. Pull the air duct off and look at it closely. Also try an idle relearn. Let the engine run until warm, shut it off for one minute, turn the key to the ON position for about 10 seconds, start it up and put it in gear and let idle for 2 minutes. The idle should start to come down if there is not another problem.

Also, unplug the MAF (mass air flow) sensor connector to see if that straightens it out. This will of course set more codes, but it is a quick diagnostic check. A MAF can be bad without setting any codes. The resistors inside the air stream get coated with dirt and can give the PCM innacurate readings so the computer does not know it is bad.

The thing about the remanufactured fuel injectors is a bit troubling. If they are not flowing properly or one or more is stuck open then that would cause a very rich condition and P0300. Also check the O rings on the fuel injectors for vacuum leaks at the intake runners. Are these injectors the proper flow rate, is the resistance of each one correct, is one shorted ? Never sure about an aftermarket remanufactured injector. Not sure why you would want to do that anyway.

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