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2003 Explorer Random Misfire


New Member
February 20, 2014
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City, State
Bronx New York
Year, Model & Trim Level
2003 Ford Eplorer 4.0
Hello all, I am new to the Explorer forum.. I was wondering if someone can give me some advice..I am getting a P0300 code.I changed the plugs, wires and coil pack..Still misfiring..The engine is the 4.0...I checked the vacuum and im getting a rather erratic reading which I will try to post..I sprayed carb cleaner around the intake with no noticeable difference in Rpms..Is there any known sensors that could cause this condition?..My friend seems to think its the injectors but I find it hard to believe multiple injectors would go at same time..I appreciate any advice or help...Thanks so much

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Could be low fuel pressure. Clogged fuel filter? Bad pump? Is the battery good, and by good, load test it.

Hello, I checked fuel pressure and it was like 60psi..not sure if thats what its supposed to be..

I've read between 56 and 72 psi.

60psi is within the range as far as I know. Having said that, although it's pretty unlikely to have several injectors go at once, there is another common element to all of your cylinders - your coil pack. Maybe there isn't a specific channel that's failing, causing a repeat misfire on one cylinder - maybe the coil pack is missing occasionally on several different cylinders. That would be my first check. Unfortunately I'm not aware of any reliable way to directly, fully test the coil pack at home, without using Ford's IDS or a similar system. Plus, electrical parts are non-returnable when you buy them, so it's an expensive way to diagnose. Do you know someone else with a 4.0 Ranger or Explorer of a similar year who might be willing to do a swap test with you (make sure you don't mess up the wire sequence!)? Another option could be to try a coil pack from the boneyard and see if that works. If it does, you could run that, or get a brand new one and keep the junkyard one as a spare, as used electrical parts are also non-returnable...

Good luck, and keep us posted. Maybe someone else here knows of a way to test a coil pack's output, in standing operation and in a stress test? I'd hate to do it but if you want to be sure, the dealer has the IDS tool which can apply a stress test, but you're looking at a minimum of $80 for the scan, and any repairs will be in addition to that.

On a side note, if it's drivable, it may be worth paying a shop $60 - $90 to do a diagnostic and tell you what it is. Then you can choose to do the repair yourself or do the shops quote. Shops often dismiss the diagnostic charge if they do the repair.

Erratic vacuum is often a sign of bad compression on 1 or more cylinders. I'd do a compression test before you continue to throw time and money at the engine.

Erratic vacuum is often a sign of bad compression on 1 or more cylinders. I'd do a compression test before you continue to throw time and money at the engine.

How could I have forgotten that part? :eek: Check all of the vacuum lines and fittings for leaks - there could be a leak ANYWHERE in the intake system, and on these engines there are a lot of lines. You can take a length of hose and hold one end to your ear, while probing around with the other end. ** make sure to stay clear of your fan and belt/moving parts! ** no loose clothing!

Good luck!

Have u tried your mass air flow sensor?