I keep misfiring in #7 cylinder

Asked by Apr 19, 2016 at 12:55 AM about the 2000 Ford F-250 Super Duty XL 4WD Extended Cab SB

Question type: Maintenance & Repair

My  truck is running good at high rpm's but
at lower it shakes I have changed the coil
pack and the injector in number seven and
it hasn't changed.  What else would cause
these symptoms? Sometimes my exhaust
smells really rich and we Had it hooked up
at O'Reillys the guy said that my truck is
trying to dump off fuel?

18 Answers

20,195

so hows the spark plug??? did you check/spray for a vacuum leak?

1 out of 1 people think this is helpful.
51,305

Check the compression of that cylinder... P0307 FORD - Cylinder 7 Misfire Detected... Possible causes - Faulty spark plug 7 - Clogged or faulty fuel injector 7 - Faulty ignition coil 7 - Fuel injector 7 harness is open or shorted - Fuel injector 7 circuit poor electrical connection - Ignition coil 7 harness is open or shorted - Ignition coil 7 circuit poor electrical connection - Insufficient cylinder 7 compression - Incorrect fuel pressure - Intake air leak

The plug keeps failing badly and I have replaced the injector I have replaced the coil pack

The first time I change the spark plug it had Blawn the entire and off the plug the second one it was within two weeks all brownish black and burn like fried chicken

51,305

Coco brown is a good color but the black is not. Should be coco brown to white in color. There may be a vacuum leak somewhere around that cylinder, but check the compression also.

When I shut off my truck I can hear what I think is the vacuum reservoir for a little while? Still pulling air? It is a box that is around at the vacuum lines going Into

How do I spray for a vacuum leak?

51,305

Get some carburetor cleaner, make sure the engine is cool as to not start a fire, have it at idle and spray along the intake manifold gasket, under the throttle body, anywhere there is vacuum. The rpm's of the engine will pick up if there is a vacuum leak.

I hooked up a pressure tester to the fuel rail and it says I should be at 18 psi with the key on but the motor off and 35 to 45 with it running! I read 6 PSI with the key in the motor off 18 while running. I replace the fuel filter and wondering how to decide whether it's the fuel pump or fuel pressure regulator or I need to monitor next? I googled symptoms of a bad fuel pressure regulator And it damn near diagnosed my symptoms to a T but I don't want to buy the part and it be something else

51,305

So then you check the fuel pressure before the fuel regulator. A chart that I look at says fuel pressure should be 30 to 45 lbs. You should have no less fuel pressure than this. If so then the fuel pump would need replacing.

I did not know I could check the pressure bypassing the fuel pressure regulator ! Where would I look for the service port?

51,305

There is none. You need the proper fittings to tap into the fuel line or just tap into the line that runs into the fuel regulator.

So if it was you would you think it this point it would be the fuel pressure regulator or over the fuel pump? I got under my truck and had somebody turn the key on and off and it sounds like everything is working properly on the fuel pump but those are the only two things that would cause me To get low-pressure out the service port right?

But even then it makes me wonder how come only seven is the one miss firing but I have changed out the plug the coil pack and the injector in seven?

51,305

Ya I know. But proper fuel pressure is very important. Actually if you are not reading the proper fuel pressure at the Schrader valve, then it is the fuel pump.

Hello Shane have you been able to fix the problem you had with your truck on cylinder 7 yet?? My truck has the same problem???

Does any one have an idea what could it happened to my truck by ny chance ??

51,305

Luis, check the compression in the engine, compression values should not vary more than 15% between each cylinder.

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