Mechanics

RANDOM MISFIRES

2000 Ford Explorer

Engine Performance problem
2000 Ford Explorer 6 cyl Four Wheel Drive Automatic 81500 miles

After sitting all night, the engine will run rough for a minute or so, but it clears up after 30 or so seconds of driving.

Also having random misfires that may or may not be related. Check engine light was on, code indicated Cylinder 3. Pulled the plug and checked, looked OK so I replaced it. Cleared the code, a week later it was Cylinder 4.
Plugs also have less than 15k miles on them.
Misfires happen very infrequently at any speed, but are definitely more consistent in the first 30 seconds after starting the engine cold.

Perhaps completely unrelated, my idle speed in park and neutral is 1000-1500RPM, but drops to ~900 when put into gear. That seems abnormal to me.

Any ideas?
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Eitri
August 8, 2008.



Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.

High Idle

Too fast an idle speed. If an engine without computerized idle speed control is idling too fast and refuses to come down to a normal idle speed despite your best efforts to back off the carburetor idle speed screw or air bypass adjustment screw (fuel injection), air is getting past the throttle somewhere. Common leak paths include the carburetor and throttle body gaskets, carburetor insulator spacers, intake manifold gaskets, and of course, any of the engine's vacuum fittings, hoses and accessories. It is even possible that leaky O-rings around the fuel injectors are allowing air to leak past the seals. Another overlooked item can be a worn throttle shaft and a defective idle speed speed control motor/valve stuck in the extended (high idle speed) position/throttle position sensor. Also the throttle plate could be binding in its bore and kinked accelerator cable, coolant temperature sensor might not be operating properly misleading the computer that the engine is still cold and computer throwing fuel at it raising the idle speed.

Rasmataz
Aug 8, 2008.
Thanks for the quick reply.

I'm reasonably sure my plugs and wires are OK, when I pulled #3 it looked normal, and I had plugs/wires replaced less than 15k miles ago.

What would explain both reasonably frequent running rough when the engine's first started and random misfires on multiple plugs at speed? Is it likely the two are related in some way?

I know it could be a vacuum leak, I'm also thinking it might be a bad PCV valve, but I'm not sure that fits the symptons.

Regarding the idle, would a dirty/faulty Idle control valve potentially cause the high idle in park/neutral?

Tiny
Eitri
Aug 8, 2008.
Regarding the idle, would a dirty/faulty Idle control valve potentially cause the high idle in park/neutral?

Idle air control valve could be stuck in the extended position or sticking-

Rasmataz
Aug 8, 2008.