Stumped by backfire and shaking problem - Ford F150 Forum

James_TX · 06-22-2009, 11:55 AM

Hello everyone, longtime lurker, first time poster.

This one has had me stumped for weeks, maybe someone here can help.

2001 F-150 XL 4.2L V6 with 99k miles.

About 3 months ago, I'd get an occasional shake when accelerating. I replaced the fuel filter, air filter, spark plugs and wires. Dumped a bottle of fuel injector cleaner in the gas tank. I noticed that the connector on the ignition coil for valve 3 had some corrosion, but after I cleaned it the new wire was still able to snap on and hold a connection.

Trouble free operation for about a month and then it started shaking again with an occasional backfire (that sounded more like a pop so I didn't think too much of it). By playing with the gas pedal, I could get it to stop.

This past weekend, I drove it around the neighborhood, manipulating the gas pedal to make it shake as much as possible. I finally got an error code - 303. It was my old friend the ignition coil with the corroded post. The ignition coil has been replaced, computer reset and the check engine light has not come back on.

I have replaced the rear brakes, checked the sway bar, checked the transmission and motor mounts. The emergency brake engages and releases correctly.

The truck idles very smooth, no unnatural noises when you move through the gears. On hills or when accelerating to highway speeds, I get violent shakes and backfires. I can usually back off of the pedal or give it more gas and they will clear. When maintaining a constant speed, there are seldom any shakes.

Anyone got any ideas?

James_TX · 06-24-2009, 10:25 AM

I ran it as rough as possible yesterday to see if I could get it to throw out a code. I got the 303 error again.

This morning, after I started it, I checked the code again and got 181 7E5 E3. After I drove 10 miles I rechecked and just got 303.

James_TX · 06-24-2009, 02:28 PM

Cleaned and pinched the connector tighter on the spark plug wire, that seems to have taken care of the 303 error.

I unplugged and removed the MAF assembly and had a significant power increase, so this may be the culprit. I will clean it tonight and post a followup.

James_TX · 06-25-2009, 04:44 PM

Nope, that wasn't it.

Good old Bill · 06-25-2009, 06:04 PM

Have an 02 with the same engine, I know you replaced the plugs and wires, its happened to the best of us, swap the 2 & 3 plugs just to see if the problem mooves. Hoping you either got a bad plug or it was damaged during installation. The 303 code points to #3, lets see if it changes before a coil is purchased.

James_TX · 06-26-2009, 01:41 PM

Hey Bill, thanks for the response.

I've already replaced the coil, the old coil had a corroded post that eventually broke off flush with the housing.

I took it into the shop this morning, will post their diagnosis.

20024x4 · 06-27-2009, 03:13 PM

i have the same problem w/ shaking and a lag of accelorating. the backfire i have is b/c i have straight pipes but idk.

i would like to c wat the shop has to say

James_TX · 06-29-2009, 12:27 PM

I took it into the shop, they think it may be a problem with the associated fuel injector. We'll see.

James_TX · 06-30-2009, 01:57 PM

Now they think it's a leak around the intake manifold gasket. Spraying carb cleaner around it makes the engine rev.

Also said it's taking it a little while to get the fuel pressure up at idle, but it's good once it gets there. Think it's a blockage at the fuel filter, even though I replaced it 10k miles ago. We'll see.

James_TX · 07-07-2009, 12:37 PM

Talked to the shop, they still have no idea what the problem is and every mechanic in the place has taken a crack at it!