It's misfiring,then not, misfiring then not. Usually when...


It's misfiring,then not, misfiring then not. Usually when the is off then when I try to start. Mechanic's Assistant: How many miles are on the car? Is it a steady misfire? Or random? Random 210 thousand 3.7 v6 e85 Mec...

It's misfiring,then not, misfiring then not. Usually when the is off then when I try to start. Mechanic's Assistant: How many miles are on the car? Is it a steady misfire? Or random? Random 210 thousand 3.7 v6 e85 Mechanic's Assistant: What is the make/model/year of your car? 2011 f150 Mechanic's Assistant: Are you fixing your F150 yourself? What have you tried so far? Yes myself Changed the sparks plugs. Visaulli inspected for vacuum leaks but not 100% sure so right now I'm building a smoke machine so I can find out for sure tomorrow

Automotive Expert
Hello and thank you for your question. My name is James. Have you pulled trouble codes from the pcm yet?

p300 in all banks
Automotive Expert
Ok give me just a moment please.Ok does the [engine](/topics-engine-ford/) misfire at idle or when driving? Any particular speed or condition when it is worse?

BothNo particular speed but the ride is bumpy and rough
Automotive Expert
Ok the self diagnostics on a 2011 are pretty decent. If you’re just getting a random misfire it’s possible at that [mileage](/topics-mileage-ford/) there is a vacuum leak as you stated or the Evap purge valve may be leaking intermittently. Making a device to use as a smoke machine is an excellent idea. You can also try disconnecting the Evap line where it attaches to the intake manifold and plug the port of the intake to see if it makes a difference.

What would that do if plugg the portAlso I don't know if it's coincidence but this all started when I switched from regular glass to e85 gas I was told that it cleans the system and in my clogged up the fuel filter
Automotive Expert
This will isolate the evaporative emissions system from the engine. If the purge valve sticks a bit, which is common, the vacuum of the intake will draw in fuel vapor from the tank and the pcm will not know this is happening so it cannot accurately adjust the air fuel mix to run correctly.

Oh I see. Will it run normal or rough when I plug the port?What will determine if it is vacume leak or bad purge valve ?Ok I understand now.
Automotive Expert
It will run normally. The Evap system basically monitors fuel tank pressure and when cruising it will open the purge valve which allows excess fuel vapor from the tank to be burned through the engine. Older vehicles did not use this system so the engine runs fine without it. It’s designed to keep fuel vapor from the tank from being released into the atmosphere. Basically you can disconnect the Evap line from the engine and seal off that port. If the misfiring stops, we use a smoke machine on one side of the valve and see if smoke comes out the other side when the electrical plug is disconnected. It’s just a free quick check you can make.

I will try that tomorrow . I was focused more on the e85 ethanol cause. But this makes more sense because it misfires randomly
Automotive Expert
that also may be a cause. The pcm takes a while to recognize e85 and adjust to it. I haven’t tried using e85 myself so I’m not sure exactly how long it takes the pcm to make the changes. If you don’t find an Evap or vacuum leak it may just need to be driven more for the adjustment to take place.

I think I got a better picture now. I will try all things you suggested. Hopelly I'll get lucky and find the cause in my first attempt . Thank you. You've been very helpful !

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