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?

22 Answers


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

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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

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

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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


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?


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


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?


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?


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 ??


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

Having same problem with 6 and 7 multiple misfires and pipe on other side is glowing red hot and help?


Check the catalytic converter to see if it glowing red. May be a plugged cat.


I am having trouble with 7 and 8 misfiring I have changed the coil pack and changed the plugs several times have had licensed mechanics not understand what is going on this is to much of a coincidence everyone having the same problems

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I know this request is a year old.. I dont see if you ever got to the root of the problem. For anyone reading this looking for a similar condition. When you have a specific misfire code, P030x (1....8) You must start the diagnosis with a compression test If the engine has a mechanical malfunction such as a burned or stuck (open) exhaust or intake valve, no amount of spark plugs, coils, injectors, fuel pumps or new tail lights are going to fix this!. Its equivalent to putting new running shoes on a broken leg and expecting a fix! Mechanical issues could be; Burned valves, stuck open valves, worn camshaft lobes. Broken valve springs. Broken pistons, broken or worn out piston rings, bent connecting rod. These are all cylinder specific failures. If the compression test, and a dry and wet compression test should be performed, does not show a failure (low compression) then move on to the accessory items that function to allow the engine to operate.... Its mentioned that the spark plug is black, soot covered. This indicates that there is fuel entering the cylinder! But it's not burning it to create combustion. A vacuum leak at the intake manifold will be more prevalent at idle and low speed operation The amount of unmetered air being introduced at the leaking intake of vacuum line is a fixed sized opening. At low engine speeds the volume of unmetered air will adversely affect the fuel stir ratio for that specific cylinder. At high RPMs the amount drawn in from a small intake leak isnegligible. However it will show up in Long term fuel trim as positive 20+% trim readings.. Mind you though... low mechanical compression will create similarfuel trim readings. That's why a compression test is first order of diag! As to a fuel pump issue.. If all the sinks in my house work good(water pressure) but one, Then why would I suspect that the main water supply pressure leading into my house is bad.. Same with the maf sensor on your truck.. The F. pump, F. press regulator, mass air sensor, throttle position sensor, even the timing chain alignment. All deal with the entire engine and can not differentiate between cylinders I'd never have even begun starting with any of these items on a p030x code.... If you think a coil may be the issue, which was very common on Ford engines. Swap coils with another cylinder. If the malfunction code moves to the cylinder you moved the suspected coil to..its your issue. If the same po30x (7) code remains it's related to that cylinder ... mechanical or vacuum or injector or ignition related. So in summary.. Engine mechanical integrity (aka compression) must be established prior to any attempt at diagnosis of a specific misfire code. And general rule is 150.175 lbs dry test... no cylinder readings can be less than 75% of the average test result. 150, 150 140, 150. Ok 150, 170, 60, 150. NG!!!

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