Engine Misfire

[J.J. Bishop]

About two months ago my check engine light came on on my '05 st (165,000 miles) and I got the codes pulled and it was for misfires. Put it in the shop, had all the plugs and wires replaced, as well as new coils, and ran premium gas and fuel treatment in it for a few weeks. Ran fine for about a month and a half, but now my check engine light is back on and the codes i'm getting are P0304 and P0316, for misfires in cylinder 4 and misfires in the first 1000 RPMs. Any idea what could still be causing the problem? Having noticed any performance issues besides maybe a little rougher than usual idles.

Update: Last night the check engine light began flashing while sitting at idle, would go back to normal check engine light when accelerating. Does this narrow down the problem?

Thanks,
J.J. Bishop

 

[ronixd]

Same problem with my 4.6 xlt 2003.
Waiting for advice. Thank you in advance!

 

[Thadigus]

Is that the 4.0 SOHC?

 

[SoCal2V]

P0304 as you're probably already aware is misfire at cylinder #4. The P0316 is a little trickier, and there are at least a few likely causes.

Read through the following link for diagnosing the P0316 dtc, I generally like to start with diagnosis/replacement of the easiest and least expensive items. Not mentioned in the link but definitely something I would do is have the alternator bench tested for excessive AC current ripple. Excessive ripple is indicative of bad diodes, and in modern computer controlled vehicles such a condition can definitely cause the symptoms and dtc's you are observing.

You can start with pulling and inspecting the plug at #4 first if you like, but if it were me and especially since you have already replaced plugs and wires recently I would work on diagnosing and resolving the P0316 issue first. It is likely IMO that the P0316 is causing the P0304.

www.obd-codes.com/p0316

 

[sr139fox]

I'm thinking fuel issue. Swap the injector with another cylinder. If it travels. Bad injector. Use new one rings too. Could a loose connection on the injector cause this? Dave P.

 

[ronixd]

My explorer 4.6 2003 is doing the same exact thing! I have a p0306 and a p0316.
new dorman intake manifold, new motorcraft plugs, new cop boots.
I have moved the cops and fuel injectors around but the misfire stays in cylinder 6.
Compression for 1-4 is 190psi, 5-8 170psi. I hooked up a vacuum gauge to the brake booster vacuum line, upon cold start up gauge starts at 180inhg then immedietely drops to 140inhg for around 4-6 seconds then climbs back to 180inhg and steadily stays there as it should. this only happens during a cold start or after the engine being off for around 30 min. I could post a video of the vacuum gauge but I don't know how. I am baffled and dont know what to do next! Help!

 

[J.J. Bishop]

Is that the 4.0 SOHC?

yep