Intermittent misfire problem

[fourwindsoh]

I have a Mustang GT 2003. I just took it to a shop to get a series of things done for standard maintenance, 66,000 miles: new brake pads, brake fluid, coolant refill.

I asked them to check out an intermittent sluggishness or rough running while going uphill and during acceleration. Its not terrible and doesn't happen all the time but its annoying.

Previously, I replaced the fuel filter to try to fix the problem. It didn't help at all. The problem has not gotten worse and I've driven this car little over about 1 1/2 years since it started having the problem.

I was told it is likely an ignition coil problem, but because the check engine light is not on it is not going to tell them the cylinder with the problem. Are there other ways to tell which cylinder is the problem? Is it work checking the fuel pressure, replacing the spark plugs or troubleshooting or replacing the MAF?

Any suggestions would be appreciated because I don't know where to continue looking for the intermittent issue.

- Andy

 

[BurningRubber]

Sounds like a coil pack may be on its way out.

I am having the same problem. But its usually when I first start the car up after its been sitting overnight, also when I take off slow the car will bog. At times it has randomly started mis-firing breifly.

I have replaced my spark plugs and fuel filter, no improvments. I have reason to beleive my coil packs are on the way out (age, mileage, appearance...).

They arent hard to check for visual problems (cracks, splits...).


If you do decide to replace them, you are better off shopping around online than going through a dealership. Motorcraft (OEM) coil packs are expensive... Accell and MSD both make quality parts and will sell a full set from $230-$380. I checked with a local Ford dealer and they wanted $90 something PER pack, and there are 8 of them... no thanks...

 

[GettinThere]

...I was told it is likely an ignition coil problem, but because the check engine light is not on it is not going to tell them the cylinder with the problem. Are there other ways to tell which cylinder is the problem?

I don't know if this is true, but I've read that if you go uphill in 5th gear (almost bogging it), that will trigger a check engine light. Then you could get the code read, and hopefully that will lead you to the source of the problem. Good luck.

 

[fourwindsoh]

Spark plugs replaced; works fine now

I had the spark plugs replaced. The problem doesn't appear to be there anymore. Because it was intermittent, I'll watch it closely. I did some tests and couldn't recreate the problem under load going up hills.

Thanks for all your advice.

 

[BurningRubber]

No problem. Hopefully it was just that simple.

 

[ForPointSex]

I had a simular problem. One of my coil packs was bad. I bought one on ebay and swapped them out until I found the bad one. There is no way to check which one is bad.

 

[kylebgt]

I think I am having the same problem. Did you guys ever get stumbling/bouncing rpm's? Mine will sometimes bounce between 900-500 up and down and then smooth out for a little while? Possibly bad coil pack? It has newer spark plugs so thats not the prob. It does need a fuel filter change

 

[BurningRubber]

I think I am having the same problem. Did you guys ever get stumbling/bouncing rpm's? Mine will sometimes bounce between 900-500 up and down and then smooth out for a little while? Possibly bad coil pack? It has newer spark plugs so thats not the prob. It does need a fuel filter change

Yeah mine does that a bit. Try cleaning your IAC sensor out, thats normally the culprit.

If it is dead then cleaning wont help... it could be another issue but I would start there.

 

[fourwindsoh]

Spark plugs / ignition coil testing

Looks like the spark plug change did it for me. The car runs very smooth now.

By the way, I read the Chilton Repair Manual for 1996-2003 Ford Mustangs. They claim that an ohmmeter or multimeter can be used to check the primary and secondary circuits of an ignition coil. They have resistance figures published in that book and they claim that if the resistance is off by even a little it indicates a problem.

By the way, the procedure for removing and testing coils is different depending on the year of the car and wether its DOHC or SOHC and if it's 6 vs 8 cylinders. They describe the different ways in the book.

I didn't bother trying because it looked a little difficult to actually remove the coils to check the secondary circuit for each one. On the Ford 2003 GT the air intake is in the way of the left 4 and there's also a bracket over each group of 4. Looks like you don't have to remove the coil to test the primary circuit, just unplug the power cable.

Maybe some day in the future I'll get to have fun playing with the ignition coils.