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Engine misses at idle

06-22-2008, 09:44 PM
I have a 2002 Taurus SES 3.0L Duratec engine, (89,000 miles). The car has a very bad and annoying engine miss at idle, most noticeable when in drive. The miss can be felt through the entire car. The engine miss is not very noticeable or non existent when driving. However, I sometimes can feel a misfire when I accelerate hard.

The Service Engine light came on with the error codes P1504 (Idle Speed Control) PO303 (Cylinder 3 misfire) and PO316 (Misfire in first 1000 revolutions).

I had the spark plugs replaced along with new Motorcraft spark plug wires. These changes did not resolve the problem. I was then told to replace the coil pack. That seemed to fix the problem for a short period of time, but the problem recently came back along with all three error codes. I removed the number three plug and replaced it again even though it looked like new. The car drove fine for a couple hundred miles but then the error codes and engine miss came back. I then cleaned out the Idle Speed Control Valve, removed, tested and cleaned out the EGR Valve. I also changed the air filter along with the in line fuel filter, but the problem continued.

I then noticed an interesting fact. The engine miss would occur whenever I was idling and also when I had the Selector for the Heat/Vent/AC/Defroster on. It did not make any difference which position I had turned it on, the miss almost immediately came back whenever I turned the knob to an on position. However, the miss was somewhat worst when I put it on AC or Defroster. When the selector was in the off position, the miss immediately went away for the most part, but would occasionally misfire, but it was significantly better. I have tested this theory by turning the selector on and off many times and it is very consistent. It runs much better when the selector is off. I have the standard selector, not the electronic version.

Based on this new fact, I figured that the selector had to be losing or leaking vacuum or something like that because the problem was amplified and occurred whenever I turned the selector knob to any other position. Therefore, I decided to replace the actual selector part. I felt pretty good that I diagnosed the problem this time, but my smile turned to a frown when the replacement part did NOT fix the problem. The problem was the same and the engine misfired the same as it did with the original part installed.

The problem sounds like some sort of vacuum leak, (I think) because the symptoms were somehow tied into the selector switch. However, I looked all over the engine but cannot locate a leak. I also don’t understand why the number 3 cylinder is the only cylinder that keeps coming up on as an error code. If it was a vacuum leak, wouldn’t the other cylinders miss as well? Also, when I replaced the coil pack and later replaced the plug on cylinder three, the car ran better for a few days, but the problems and the error codes always came back. These items have nothing to do with vacuum. I am totally confused.

Anybody have any suggestions. I really need some help on this one.

Thanks in advance.

06-23-2008, 06:42 AM
A misfire is not always caused by the ignition system. A faulty/dirty fuel injector can also cause a misfire. Try moving the #3 fuel injector to another cylinder and see if the misfire code follows the injector. Be sure to pick a cylinder that is not fired by the same coil pack.


03-11-2009, 09:27 PM
I have the same symptoms on my 2003 Wagon with the Duratec engine. I have the automatic climate control and the misses get worse when I select the control on for heat - its winter here- and get better when I turn the system off.
What was the outcome of your troubleshooting for the vacuum leak?
Thanks, J.

03-12-2009, 02:32 AM
Shorod may be right on the injector, but to follow up on your vac leak hunch, try finding the place under the hood where the vac line splits to go to the heat/ac controls and disconnect the line to the controls and plug the T. That should rule out a vac leak or not inside. Along with what Shorod said, you could put injector cleaner in your gas at double the suggested rate while you can still run it. I wasn't so lucky to have that opportunity. I have to replace the injector. :-( I'm not partial to any brand. Sea Foam and Berryman's B-12 Chemtool are popular.

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